Beyond the Light of Dawn: Prehistoric Life After the Silurian Gamma Ray Burst

[Eotyrannus]

Spoiler: A.N

Eyooo! I'm posting this from another forum, because I'm bored and need more likes for the like throne.

I'm not quite sure if this fits over here, seeing as it contains... precisely zero history, seeing as it's four or five orders of magnitude older, but I booped a mod and apparently history goes in War and Peace, which I assume to mean here. Even if it technically isn't history.

But anyway! This is in the vein of Walking With Dinosaurs or The Future Is Wild. Basically, a gamma ray burst- a type of supernova that produces two gamma beams capable of remaining destructive even on a galactic scale- hits the planet about 420 million years ago. At the time, the first jawed fish were appearing, the oceans were ruled by crocodile-sized scorpions, and the tallest forests were a couple of centimetres tall.

So yeah. Big changes. Hope you all enjoy!

In a familiar scene, a swarm of insects twirls and dances over the water.

There are no trees here. There are no birds, either. No aeroplanes or helicopters cross the sky, and there is no sound of roads, engines, or people. By all accounts, it is picturesque and serene. The river is slow-flowing, and normally quite placid, but the shapes at the surface have drawn the attention of aquatic life.

There are fish here, to be sure. Even from the surface, a small shoal is visible. Their bodies are the shape of a jester's shoe, and this shoal possesses spines where one might expect to see fins. They are not the ones interested in the flying insects, though- these fish are jawless, sucking microscopic food from the water. In fact, all fish here have a similar diet, or sift through the mud for sustenance.

Instead of fish gulping down insects from the surface, another predator is hunting here. Just below the surface, there are creatures hovering. They adjust their position in the water with flattened, paddle-like fins, and when an insect comes too close, scorpion-like claws dart out to catch them. Though conventionally known as sea scorpions, this particular species- like many- is an entirely freshwater variety of somewhat lobster-like invertebrate. As a whole, they are called eurypterids. These striped individuals have sleek, striped exoskeletons and a long, curling tail behind them- a larger, mud-brown individual appears for a brief moment, plucks an insect from the surface, and disappears, going to the bottom to enjoy its catch while it waits for larger prey.

There are a few more creatures in the gauntlet- there are other species of eurypterid here, and a lone freshwater horseshoe crab is paddling upside-down to try its luck with more adventurous game than its standard fare of river worms. But despite the risks, many still make it to the riverbank.

A carpet of vegetation exists here. Calling anything here a shrub would be generous- but their reproduction gives these flying insects the food they need to sustain their mating flight. Many of them have sent up long stalks their liverwort-like bases. These stalks are forking and leafless, and bear many spores at the top- a delicious, protein-rich treat for the insects that will fuel the growth of their eggs.

Or it would have, if this were our world.

Instead, the flimsy fluttering of wings is interrupted by a flash of light that envelops the sky- and lingers. The seconds pass, and the world remains blinded by what must seem like the rapture.

In a way, it is. But as the light dims, no sudden disappearance is revealed- just the actions of a myriad of startled animals. Unanimously, the inhabitants of this once-calm river have dived for the riverbed.

A plume of mud marks the delight of the larger mud-brown eurypterid, having grabbed one of the striped insect hunters in the chaos, pulling the smaller animal apart. The luckier striped eurypterids warily surface, and they too find food to eat- many of the insects were snared by the water in the confusion, and their struggles draw up their hunters again, though the horseshoe crab wisely refuses to return to the surface.

Life seems to go on as usual for the eurypterids. The bottom-feeding fish, too, return to grubbing around in the mud, temporarily safe as the mud-brown eurypterid digests its catch. But not all the animals here are so unperturbed.

The insects seem disoriented. Many of them are blind, and in the next few days even the lucky survivors begin to die of a strange, sweeping sickness. And the filter-feeding fish move on, tasting decay in the water- but other shoals meander in the opposite direction, all of them seeking new feeding grounds and none of them able to find it.

In that first nightfall after the flash, the world remains in twilight, lit by the last flickers of a long-dead star.

Thousands of years ago, and thousands of light years away, an alien sun was destroyed in a catastrophic explosion known as a gamma ray burst. Its solar system is nothing but celestial ash, scattered to the interstellar void, but from its poles it released a jet of energy that took all this time to reach the humble planet Earth.

The Silurian period has come to a startled halt. When that jet of cosmic radiation struck, the molecules there were sundered. Ozone, a gas that once protected the world from being bathed in harmful ultraviolet rays, is one of the casualties. And like methane from a corpse, the shattered molecules will begin to reform in new and dangerous ways, until in half a year's time the planet is blanketed from the sun by a tinted, smoggy haze.

Despite the serenity of the scene, this could not have come at a worse time for the inhabitants of this blooming planet. Already suffering from cool temperatures and a sudden sharp drop in sea level, the flora and fauna now has much worse in store for them than poor breeding seasons or lost reefs. Now, their very existence hangs in the balance.

Soon, their oceans will be left starving as their planktonic food- pelagic natives, single-celled sunlight harvesters and microscopic floating larvae- is irradiated to the breaking point and beyond. Breeding creatures find that the warm light of the sun is a bane, killing rather than nurturing new life. And as the oceans grow acidic and the world enters the grip of a stellar winter, entire ecosystems will be left on the brink of annihilation.

But it is not the strongest of species that survive, nor the most intelligent- it is the ones that are most adaptable to change. And there are already creatures here that show the potential to survive.

There always are.

The question, then, is not if life will survive. It will. Today marks the start of a new era in life's history, a chapter that never had the chance to be written. This alien sun has risen over the landscape for the first and last time. Before it, was the old and familiar world. Now, it is the start of a newer, stranger one.

So, if it is not a question of if life will survive, then the real question is...

What will Earth and its strange, ancient creatures look like, beyond the light of dawn?

 

[Eotyrannus]

The End-Silurian Mass Extinction: Winners and Losers

The end-Silurian mass extinction was a protracted event. The gamma ray burst was the primary cause of the extinction, but it hit during a variety of lesser environmental crises that were already having catastrophic effects on the ecosystem. Though the potential causes range from change in sea level to volcanic brine eruptions, what is certainly known is that the oceans at the time were being hit by sudden lack of oxygen- anoxic events- for its creatures to breathe.

The supernova itself only intensifies these issues. Normally, sea water is stirred by millions of tiny animals and plants, plunging down to the depths on a daily or bi-daily basis to take advantage of changing light. Large, active predators such as eurypterids, orthocones or the shark-like acanthodian fish are the main force behind this movement, forcing these massive vertical migrations by hunting those that remain behind.

The gamma ray burst had already killed these creatures off, immediately before the worst of these events. The lack of plankton stirring the water intensified the effects of these extinctions compared to our own timeline- leaving many creatures choking in unbreathable depths even after making it through the initial catastrophe.

The whole series of events- irradiation of planktonic juveniles and food, choking smog, extreme temperature crashes, acid rain, sudden sea level drop, and those final intensified periods of anoxia- created a whole cascade of one-two punches, leaving even major groups struggling for survival. In conditions such as these, it could be considered astounding that so many creatures survived- as they always do after mass extinction. But for every winner, there were many more that lost the game of life.

Losers

The most shocking loss, perhaps, is the loss of the insects.

While in our timeline they would go on to become the most diverse terrestrial animals to ever live, in this world, they were significantly less prepared to face the catastrophe. It was their flight- the very ability that made them so successful- which proved their undoing. Reproduction was one of the primary reasons to engage in flight, but the small size and relatively feeble physiology of these pioneers compared to the modern diversity left them reeling in the wake of the post-supernova shower of ultraviolet light on the landscape.

Flying insects were the only creatures on the planet that could end up so hopelessly vulnerable to the deadly rays- reproductive females engaged in flight were not only being harmed by the light themselves, but their much more vulnerable eggs were being affected by the penetrating rays as well. Combining an end-of-year temperature crash with such a catastrophic loss of breeding individuals made them some of the first animals to enter critical condition, and the devastation to both the freshwater ecosystems of their nymphs and the miniature forests of the adults ultimately spelled their doom.

Those miniature forests, too, were affected catastrophically. Both the plants and their spores were bombarded by deadly rays, and with no natural defences- such as a forest canopy or hardened seeds- wind-dispersed plants were effectively wiped out. Though they would soon reappear from the survivors, it would have immense implications on the future of terrestrial life on Earth, turning the fate of life on land in a strange new direction.

However, the majority of life affected was not terrestrial, but aquatic. And one group in particular suffered catastrophic losses.

All creatures were hit by the extinction, but cephalopods turned out to be uniquely vulnerable. At the time, instead of being soft bodied, cephalopods were shelled- relying on large shells for buoyancy, physical structure and protection. They ranged from pelagic cruisers to benthic scroungers, filling marine niches across the globe as they had done since the Cambrian. And yet nothing could have prepared them for this extinction event.

The simultaneous acid and cold shocks from the smog-filled post-blast atmosphere were deadly for all animals with carbonaceous shells. Calcium carbonate, the mineral that forms this type of shell, dissolves in acid and is much harder to lay down in frigid temperatures- and every single cephalopod was completely dependent on their shell, just as we are dependent on our skeletons. This alone would be a catastrophe for them.

Worse, though, was that they had effectively been caught in a trap. Seafloor cephalopods were already vulnerable to the anoxic events that happened later, but the radiation pouring through the shattered atmosphere killed off many sea-going cephalopods first. The orthocerids, the most widespread and prolific of the cephalopods, were also the first to be lost- their unique, buoyant larvae were helpless against the cosmic radiation. Their close relatives, the pseudorthocerids, were wiped out in the second wave- their larger larvae were precisely the sort most vulnerable to the anoxic events, and they perished too, still reeling from the last wave of extinction. Only the oncocerids- a conservative group, and the distant ancestor of the nautilus- pulled through, and even then they were in no position to reclaim their old kingdoms.

The fish, too, suffered immensely. Most groups of jawless fish were wiped out entirely. Their often filter-feeding habits and low range were a poor option for making it through. Conditions directly after the gamma ray burst was recovered from would have been perfect for them, but the reliance of many of them on microscopic prey made the intermediate time immensely difficult due to the radiation killing off those cells and small creatures they ate. The heterostracans- most successful of the early Devonian jawless fish- were annihilated utterly, as were almost all of the jawless fish. Even the thelodonts, a group of lightly-armoured fish that temporarily dominated in our own timeline, were cast down by the gamma ray burst before they truly had the chance.

Trilobites- with their greater reliance on the same carbonaceous minerals that brought such pain to the cephalopods- were also hit hard by the extinction event. The lichida, perhaps recognisable as the spiny trilobites, were wiped out. Once they had been the kings of trilobites, but now, their co-rulers- the small, smooth phacopid trilobites- would have to go on without them, putting an end to the height of trilobite sovereignity.

Finally, the reefs were shattered by the same series of extinctions that affected all other creatures. Anything calcareous and stuck to the floor was left at the brink of annihilation. Almost all corals and hard sponges died out, with all but a few of the survivors lacking in calcium carbonate exoskeletons. The bivalves and brachiopods were similarly hard-hit, both groups being left on the brink of death. The free-floating graptolites (a type of colonial branching animal) died out too, leaving only their bottom-dwelling relatives.

In addition to these major ecological players, a whole host of other unique fauna was destroyed, never to be seen again. A whole host of ancient creatures that had been happily trundling on were suddenly and irrevocably destroyed- such as the anomalocarids. Once the largest life forms on the planet, they join a planetary mass grave of uncountable creatures. The early Paleozoic is over- a new reign is about to begin.

Winners

By all standards, anything that survived was- by definition- a winner. But surviving doesn't necessarily mean going on to bigger or greater things. The eurypterids, for example- though they lost many of their most charismatic forms, such as the gargantuan, crocodile-sized hyperpredator Pterygotus, their swimming capabilities and varied forms would carry them safely from the Silurian to the Devonian beyond. Many creatures would neither diversify nor decline, simply returning to the ecosystems they had lost and continuing their evolution.

Some, though, were perfectly-adapted for extinction conditions- or evolved to fill them. Creatures like these that are already common during the extinction have a leg up on other varieties. By diversifying in the wastelands before others can even start to return from their refuges, they secure a place in the ecosystem and give their descendants the opportunity for a very bright future.

The acanthodians, or spiny sharks, were one of these creatures. Though the conditions were initially as cataclysmic for them as they were for all other creatures, their efficient locomotion and respiration put them in the perfect position to adapt and fight back against the hostile conditions. This type of creature, that adapts to the conditions of a mass extinction, is called a crisis progenitor- and as a crisis progenitor, the spiny sharks have ensured themselves a major role in the world to come.

Surprisingly, the jawless fish have also produced such a success. The deaths of both plankton and competing jawless fish left one group- the osteostracans- with a chance to claim their place in the world. The descendant of this group was a fish similar to the famed Cephalaspis, with a sucking mouth, filtering gills, keen senses and a head shield hardened by the same material as teeth. These animals sucked up mud for their food- and mud was a resource the post-extinction world certainly wasn't lacking in, giving them the chance they needed to transform from local scum-suckers to a hardier, wider-ranging creature that stood a chance against the Devonian world.

On land, a completely different armoured creature has enjoyed unparalleled success- the humble millipede. At the time, these creatures dug protective burrows from which they could feed, and as both herbivores and detritovores, they could find more food than feebler soil inhabitants such as springtails or mites. In addition to this, a burrow was much more protective from extreme cold than simple shaded crevices and soils that kept arachnids and primitive hexapods going in the irradiated post-blast world- they were in the perfect place to coast comfortably through both events, and experience a burst of diversity unparalleled in the history of life on land.

Along with these millipedes came another survivor- the liverworts upon which they fed. As counterintuitive as it may seem, being fed upon by these voracious herbivorous arthropods turned out to be one of the very best strategies to survive, for a very simple reason- spores carried on the cuticle or within the gut of a millipede were just as protected from freezing or radiation as the millipedes themselves, while wind-dispersing spores suffered the full effects of both. As the extinction went on, liverworts began to exist in ever-closer association with millipedes, creating a legacy of mutualism that would send echoes throughout the future of the plant kingdom.

Finally, one more notable animal was in a perfect position to thrive in the wake of the extinction. Unlike the others, though, it was not a charismatic example of megafauna (by Silurian standards, at least)- it was a member of a group of tiny, centimetre-long armoured worms. The machaeridians are a type of annelid with mobile, calcareous armoured plates- but unlike the groups that suffered, machaeridians were much more flexible. Perfectly content to reduce their calcium carbonate skeleton to almost nothing, and then bounce back to full armour with new species once the effects of the gamma ray burst were over, these unassuming worms suddenly found themselves with an extraordinary opportunity to radiate like worms had never radiated before.

With these creatures came a whole host of other species that began to radiate into the shattered world that the dead had left behind. Many of them would go on to leave countless descendants, but of all of them, these five groups would perhaps be the most significant in shaping the face of the Devonian world beyond.

 

[Eotyrannus]

The Early Devonian: The Millipede-Industrial Complex

It is a few weeks after the seasonal rains, and a dry Devonian savannah is blooming.

Nestled in the dry shadow of the Appalachian mountains, this landscape is a testament to the growing fortitude of terrestrial life. The plants here have developed true leaves and stalks, though the largest plants are still herbaceous, as woody trees are yet to evolve.

The plants here are strange. Some seem to have strange, antler-shaped fruits so disproportionately large that their delicate stalks droop all the way down to the floor. Others present what seem to be slimy, tumorous flowers to the sky. And many seem to grow in strange patterns- in rings, or in oddly regular intervals. This is a virtual explosion of diversity compared to what one might have expected for this period in the history of life.

Perhaps it is easiest to observe their life cycle at the start of the rains, when the savannah is dominated by the brown and gold of dead and dormant plants.

Alvuphytes

When investigating plant life, the obvious place to start would be with whatever is already growing.

In sheltered conditions- growing out from under rocks, for example- a few hardy plants can remain moist enough to stay green throughout the year. A cluster of boulders provides shelter for both these plants and a variety of microfauna- indeed, there seems to be even more burrows than there are plants.

A tiny terrestrial eurypterid wanders towards the rocks- though food and water are scarce, a few hardy arthropods remain active throughout the year, and the boulder's microforest is worth investigating. Normally, it would be looking for other wanderers, but today it is lucky- one of the locals has emerged from its underground aestivation to lick moisture from the boulder's surface.

The false scorpion's bulbous eyes watch its target keenly as it carefully moves in. Its target is a long, slender millipede. Most of its prey is rounded and hard to grab, a protection both from predators and from exposing too much surface area to the heat- this burrower is less a beach ball and more a pool noodle, making it easy to grab if only the roaming predator can get to it.

It makes its approach. The millipede's eyesight is poor, and the dry exoskeleton of the scorpion leaves little scent- but when the scorpion lunges, the millipede reacts just in time, throwing itself into a fit of spasms to startle the predator and give it a moment to react.

The herbivore's spacial awareness is good- it shoves its head into a burrow that is almost completely concealed by one of the local plants growing directly atop it, and begins to disappear. When the scorpion moves to follow- it is no stranger to burrow-raiding- it finds itself flummoxed by a perfect vegetable seal.

It's no coincidence- the plant is actively caring for its pet millipede, just as the millipede did for it a long, long time ago.

Two different species- and this bryophyte is very different to its arthropod guest- are called symbionts when they live together for mutual benefit like this. There are many examples of this- bees and flowers, cleaner fish and larger reef animals, or even humans and dogs- and this unassuming meadow is full of it. But this plant, a type called an alvuphyte ('womb plant' or 'hive plant'), is probably the most extreme example.

Primitive land plants have a two-stage life cycle. As adults, they release spores. These spores grow into male and female plants called gametophytes. The males release sperm in wet conditions, and hope that a female gametophyte is near enough that the sperm can swim to it- after which an adult plant will grow from the female to start the cycle again.

Obviously, if the males lack wet conditions or a nearby female, they won't fertilise the females- and the females share this issue. The alvuphytes have developed an ingenious solution to the problem- hiring millipedes to create the conditions they need on their behalf. Inside a burrow, the sperm can hitch a lift on the millipede's feet or back to reach females without water being necessary. Millipede holes are also dug in damp places and are fertilised by the millipede's excrement, making them perfect for the adult plant as well.

To grow in a millipede burrow, though, alvuphytes first need to get to the millipede burrow in the first place. A wide variety of options exist. Many of these rock-dwelling alvuphytes simply create sticky globs of spores anywhere a millipede is likely to walk- often, their spores actually require frequent brushing to develop spores in the first place, to ensure they only grow over well-treaded paths. The antler-fruit on drooping stalks are also from alvuphytes- the shape gives a lot of surface area to release millipede-attracting scent, and while eating them, spores are stuck to the millipede and are carried back to its home.

Once in the burrow, the gametophytes will likely be surrounded by other gametophytes of varying species. If they are lucky- or if their parents possessed toxins to keep non-symbiotic species from picking up their spores- they will find an accepting millipede to protect them by savaging other herbivores that try to enter the burrow, but they will still be faced by stark competition. Some species, like the ones that grow in rings, avoid the burrow entirely- they grow outwards on runners as fast as they can, to properly set down their roots in less competitive real estates. Others are red in tooth and claw- or green in shoot and tendril, rather- and compete violently to be the sole symbiont of the millipede, as most of the rock-growers do. Most alvuphytes have very large spores, to provide a headstart for competing in the burrow's dark recesses.

The millipede need not have the same plant over its head for its entire life. Some alvuphytes are short-lived and can only reproduce in small burrows, effectively a baby's-first-alvuphyte, while others can only grow alongside well-established millipede burrows. Some even parasitise existing alvuphytes, burrowing into their tissues en-masse to sap their strength and eventually replace them entirely. This means that populations of alvuphytes evolve from basic colonisers to a final, stable 'climax community' of plants that are perfectly-adapted to a plant-filled savannah- a process called succession.

Florescophytes

If millipedes live on the ground, though, why are there flowers facing the sky? And why are they most frequent in places without millipedes at all- where the ground is too dry, perhaps?

The answer is that some millipedes have developed bolder solutions, and that some plants have solved the problem of mating in a dryer climate in a much more familiar way- those slimy, tumorous flowers may not biologically be flowers, but in other ways, calling them flowers is not misleading at all.

A strange arthropod rests on one of them, basking in the morning light. It is small, perhaps, but anyone familiar with the events of this world yet unfamiliar with its ecosystems would be startled by its presence- it bears a striking resemblance to an insect. It possesses an obvious head, thorax and abdomen. Its eyes are large and bulbous. And, most strikingly of all- this arthropod has wings.

But insects are long-dead. And if that isn't enough, it possesses not two, but four pairs of wings- long and bristly, more like feathers than an insect's hard, filmy wings.

Closer inspection reveals that, in a myriad of ways, it is wholly unlike an insect save for the barest resemblances. Its eight wings- it flutters them every so often, not yet warm enough to take to the air- are legs, and it seemingly possesses a hunchback to increase the space for flight muscles and reduce the gap in its legs those two segments leave. And that ratio of segments to limbs- one pair of legs per segment- is damning evidence of this tiny animal's true nature

This, too, is a millipede- but one wholly unlike any millipede the world has seen before.

At first glance, it seems to have lost all save a few of its legs. It possesses two pairs of walking limbs in front of its wing-legs (and they certainly are legs, rather than simple bristly or carapaceous extensions, as the millipede is capable of furling and unfurling them as it prepares for flight), and another four pairs behind. Its abdomen seems entirely absent of legs- but on further inspection, its rear end is composed of a whole variety of fused legs, and it possesses more at the front as well.

Four of its rear's legs are small and bladed- this animal is a female, and she uses them to cut holes in plants to lay her legs. Males use them as claspers, and other species have other uses, such as digging, oviposition or simply ensuring the eggs are clumped up neatly. And four are large and powerful. The millipede grabs part of the flower it has been resting on with them, and coils up like an inchworm or mating dragonfly- and with a powerful flex, it throws itself into the air, catching the morning air on its fluttering wings.

It flies, briefly- other members of its kind are also taking to the air, but this one quickly lands on a nearby flower. And it lowers its head to feed, revealing the purposes of the last of its non-walking legs.

Two segments in these animals are not quite head or thorax- rather, they form a neck, and the eight legs that hang below have developed into mouthparts. As they aren't fused to the head, they are a type of limb called a maxilliped- which can roughly be translated to 'jaw foot'. The lower ones serve to pierce and open up the various tiny pods on the flower, while the upper ones are for fine manipulation of food into the mouth.

Ultimately, it would be safe to say that they deserve an entirely new name. Scientifically, they are the ferramentupedes- the 'tool legs'. More conventionally, they are known as myrawings.

However, this advancement in the chewing efficiency of myrawings is no good for many of these false flowers, a type of plant called a florescophyte ('bouchet plant'). After all, this destroys many of their spores- and some are beginning to enter a much closer relationship with myrawing partners.

Primitive florescophytes, like ordinary bryophytes, simply have male and female flowers on male and female plants. Pollenating myrawings, be they specialised for it or simply looking for an easy meal, feed on the male flowers- attaching sperm to their mouthparts in the process- and fertilise the eggs when they feed on the female flowers afterwards. But novel innovations in the most recent few million years are bringing new diversity to these methodologies.

Some, rather than attracting myrawings with male and female flowers, have developed what could generously be called a berry. These edible spore sacs contain spores resistant to stomach acids, and many of these spores contain toxins to drive off non-frugivorous animals. Others have hard casings that must be chewed open to suck out the spores inside. Each bouchet of berries produces only a few ripe berries at a time, but rapidly cycles through them during the 'flowering' period- meaning a myrawing will have to alternate between plants to get a good meal. The male and female plants then mate within the droppings of the myrawing- many berries contain diuretics to make the myrawing defecate small amounts very frequently, ensuring that the spores don't have too much competition with other spores wherever they may grow.

Others have taken their male and female plants, and ran with it. They produce a liquid alternate foodstuff- nectar, for all intents and purposes- and simply lined the flower with eggs or sperm, encouraging specialised nectarivores that will carry a much higher proportion of gametes (sex cells) on their exoskeleton than before. The sperm fertilise the eggs on the myrawing's shell- fertile eggs simply drop mid-flight, usually growing into a simple stalk with wind-dispersed male and female spores on the end. Going through a simple, wind-dispersed asexual stage provides these plants with an advantage over both primitive bryophytes and the alvuphytes- they now need neither millipedes' presence nor a film of water to mate, and once male and female plants are established in an area, their myrawing pollenators can simply fly out to meet them.

With florescophytes to tread new, dry ground, and alvuphytes to form a thriving ecosystem alongside them, it's no wonder that the invasion of the land is proceeding full-steam-ahead. As the Devonian drags on, however, they will have to face new challenges- but also new opportunities. The sea level is starting to fall, temperatures are rising, rain is growing more common with the addition of water vapour coming off of these primitive plants' leaves, and new living space is opening for terrestrial life… and with that new space will come a new explosion of diversity as evolutionary pressures intensify amongst a thriving community of plant life.

This ecosystem is only a prelude. Soon the age of herbs will end, and the age of forests will begin.

 

[Eotyrannus]

The Early Devonian: The Worms With Worlds On Their Backs

In the realm aquatic, the most vulnerable ecosystem to the extinction was- as always- reefs.

This may seem odd- after all, weren't plants the most vulnerable group to the extinction itself? Indeed, many plants and land animals did go extinct- but they also bounced back much faster than the reefs did, despite a much greater initial punch.

To understand this, we must return to the concept of succession, and what this means for reefs.

Succession is how different species appear and replace each other as an empty space is colonised by new life. This is a relatively rapid process in the millipede meadows of the continents. Ecosystems on barren terrain start out from florescophyte seeds, which then draw in millipedes, which draw in more specialised florescophytes and new alvuphyte arrivals, ultimately leading to a rich, alvuphyte-dominated ecosystem. When they appear, forests will follow a similar system- weeds colonising barren lands, which are replaced by shrubs and herbs, which are in turn replaced by trees and forest floor plants that could never have supported them back when it was still a wasteland.

A similar process could be applied to the ecosystems of the dinosaurs. Insects and weeds arrived in barren wastelands, shortly followed by mammals and beaked birds that could feed upon them. These would allow for shrubs, toothed birds and small dinosaurs, ultimately leading up to open plains or forest dominated by gargantuan herbivorous or carnivorous dinosaurs.

The keen reader will notice a pattern- those organisms that relied upon more advanced ecosystems were the ones that were wiped out.

During a mass extinction, one must either be able to repopulate in extremely damaged ecosystems, or one must be able to cope with them well enough until better conditions let you replenish your species and last through the next time of hardship. This is why crocodiles and beech trees could survive- either waiting months between meals on the brink of starvation, or surviving dormant as seeds for the next opportunity to send up twiggy trees in poor health- and this is also why dinosaurs, who could do nothing but starve in the shattered world, could not.

Take into account how long it takes for the mere colonisers of a coral ecosystem to arrive, and the mystery of why reef ecosystems become so decrepit after times of crisis becomes clear as day. In a disturbed ecosystem, there is simply no reef at all, unless one considers a few polyps growing on the bleached skeletons of the previous generation to be even vaguely comparable to the crags and towers of a reef in its prime.

And yet, if one looked at the post-extinction world from space, one would not see a reduction in reefs compared to our own timeline.

In fact, the new reefs would be even larger- all thanks to one peculiar creature.

Orbirursids

Look at the skeletons of these expanded reef systems, and one might notice something peculiar- they seem to have started out simultaneously on a muddy sand bank.

On a normal sand bank, there is simply nothing for a reef to colonise. And yet, find a comparable muddy sand bank in the modern day, and one would see a whole host of polyps and sponges feeding. If the site is just right to grow an entirely new reef, many of these would be corals. In other areas, perhaps one might see a host of anemones.

Strangely, though, all but two things sticking up from the otherwise-sandy seabed would be a very bad idea for a theoretical dimension-travelling diver to touch. Both would be tall, frilly and capable of retraction, and both would be placed precisely opposite from each other.

These two are both the same creature- the filter-feeding tentacles and the splayed gills of an extremely resourceful creature. These entire tiny ecosystems are growing on the back of a filter-feeding worm.

In the Paleozoic, deep-sediment filter-feeders were yet to evolve. The ancestors of the orbirursids, or worldback worms, lived buried in shallow sediment or with their armoured backs camouflaged as pebbles. In fact, around the worldback worms, many more primitive orbirursids still live like this today- descendants of the armoured marine worms called machaeridians.

While machaeridians have many mobile, overlapping plates, orbirursids only leave one plate remaining. The others form muscled spines with which to anchor themselves into the mud, or were lost entirely. Around this plate, they can extend their front and their back- the former to feed with, the former to breathe with. And, atop the plate, the worm farms.

The reason these worms only have stinging species on them is because the worm needs them for protection. By chewing off anything that doesn't taste painful to it, the worm ensures that anything trying to eat it will find its potential meal hiding beneath a nasty stinging barrier.

Worms in potential future reef ecosystems have evolved to take this defence one step further. They, like most hard-shelled marine animals, use calcium carbonate to create their back plate. However, they are at a disadvantage to corals- not only do corals have a larger surface area, the algae that live inside their tissues and provide them with sugars also help them to form their skeletons, and much faster than a similar worm or mollusc could.

The worm, however, can circumvent this by hiring the corals themselves.

By growing the bare minimum to trick the coral polyps into considering its back a stable surface, reef-building worldback worms can create new, metabolically-expensive skeletons while paying a metabolic dime a dollar. This can be reinvested into the worm expanding its shell and body- in fact, some species have even returned to the reefs themselves, growing multiple plates like their ancestors to wedge themselves into gaps in the reef. Some even just chew a coral until it's bare, then let the coral regrow atop itself, as if an epiphyte (a plant that grows on the branches of trees) started growing epiphytes of its own.

The appearance of these worms is beginning to have an evolutionary effect. By barricading crags, selecting for more aggressive species and expanding the reefs themselves, these ingenious worms are actually starting to reduce the diversity of any particular patch of reef even as the reefs themselves grow larger. Particularly affected are reef-dwelling cephalopods and eurypterids- the former are becoming rarer, while the latter have disappeared entirely from this ecosystem, competition from the closely-related, open-habitat horseshoe crabs being a primary factor.

Acanthodian fish, meanwhile, are having a grand time- their powerful jaws and mobile frames are much better-adapted to take advantage of the changes in habitat. Other machaeridian worms are also starting to have increased presence thanks to their armour and powerful, extendable jaws helping them feed on stinging polyps or bite through protective shells. In the more open habitats where these worms originally evolved, meanwhile, the increased shelter has been a boon for the diversity of small invertebrates- surprisingly brachiopods have been aided by the disruption of water currents, helping keep sediment from drowning them.

Yet more creatures have had mixed fortunes from the introduction of this new group. The armour and keen senses of osteostracan jawless armoured fish help them to take advantage of anemone-covered plates similarly- but the presence of biting juvenile worms in the sediment they filter is driving new changes in the morphology of their gills and mouths.

Whatever the final consequences of these worms will be, it's undeniable that their appearance is having major ramifications in sunlit marine waters across the globe.

 

[Eotyrannus]

The Early Devonian: A Clash of Spines

Some say that in our timeline, the Carboniferous was the age of arthropods. Others might say it began in the Cretaceous, with the rise of flowering plants.

The Silurian, on the other hand, is a little-known contender- and perhaps the best one of all.

With the fall of the giant orthocones at the end of the Ordovician, and the jawed vertebrates being yet to appear, arthropods had a brief period of global dominance in all facets of the ecosystem. Both on land and in the planet's vast waters, they reigned supreme- from the smallest bottom feeders to the apex predators, they were profuse and diverse, with only a few groups daring to join them in their dominance.

Chief amongst these rulers of the world were the eurypterids- aquatic relatives of scorpions.

The key to their success lay in a variety of features. Their non-calcitic exoskeleton could be shed easily and replaced quickly, their paddle-based style of swimming was the most hydrodynamic form of movement in the oceans, and their chelicerae allowed them to take on a wide variety of prey.

These chelicerae- despite actually being the mouthparts on their modern scorpion relatives, rather than the claws- took a wide variety of shapes and forms, letting them adapt to new foodstuffs as they appeared. Even in small families, such as the famous pterygotid family of crocodile-sized river eurypterids, different members often had different chelicerae for entirely different niches.

With this in mind, it's only natural that the rise of sharks brought about the beginning of the end for them.

Though not closely related to modern sharks- indeed, a ratfish would be a closer relative to a great white than a Silurian acanthodian- spiny sharks possessed all of their advantages (albeit in a more primitive form). And those advantages were precisely what had brought eurypterids to power- the same story, but with a new, more powerful player.

The first advantage of the eurypterids- their ease of shedding compared to the other most notable Paleozoic arthropods, the trilobites- was invalidated by the fact spiny sharks lacked an exoskeleton entirely, meaning they had no periods of vulnerability at all, while shedding became ever more difficult for eurypterids. Their paddling movement was also inferior to the undulations of fish, with greater hydrodynamics, more efficient muscle attachments and reduced movement for vision to detect. And their highly-adaptable chelicerae were invalidated by teeth- a system of modular and replaceable blades that could vary just as much, if not more so, as the cutting mouthparts of their eurypterid rivals.

With the acanthodians dominating the oceans, eurypterids had to leave, adapt or die. Some have managed to adapt to life on land, establishing a foothold while the true scorpions were yet to recover from the extinction. Many died, as victims of ecological turnover are wont to do.

Adaptation, though, was harder than it may have first appeared- because other arthropods were already filling roles that had once been theirs.

Taurocarcina

A weary eurypterid glides over the muddy seafloor.

While this individual may look impressive, with a wingspan half a foot wide, it is a far cry from the might of its ancestors. As sharks grow bigger, more agile and ever more dominant, eurypterids grow smaller and smaller, driven to moult at smaller, safer sizes and in more distant moulting grounds to cope with the new threats. A recent spike in temperature- growing more common with recent changes in climate- has put this species under stress, and while his ancestors may have moved in small groups for mutual protection, this one has been tense and stressed for weeks ever since his partner was attacked and killed by a giant armoured worm (which have been growing faster and more voracious with the warmth to help build their shells).

Looking for a place to rest, he touches down beside a boulder. But his instincts serve him well- before he stops completely, he probes around the boulder with his claws, to ensure he won't be ambushed while he dozes.

The scorpion pauses- it has tasted danger.

It prods at the soil. It's unlikely that it realises the implications of its texture- it is littered with discarded fragments of exoskeleton, both from jawless fish and from other arthropods like himself. His chelicerae pick up a claw just like his own- much smaller, perhaps, but an obvious sign that whatever lives here, it is a killer of scorpions much like himself- and though he is not intelligent enough to know this, his instincts recognise the taste of death in the water.

As he investigates further, he comes across the entrance to a burrow- and a large one, at that. Another animal has excavated a pocket under the boulder. However, his hunger overcomes his nerves- on top of the taste of death, there is a much fresher one that draws him in.

Vulnerability.

Though his own moulting is a time of great danger, he is wired to consider other moulting arthropods as an easy source of food. Trilobites and horseshoe crabs also share his weakness- trilobites even more so, as they molt irregularly even within species, and their haphazard shedding is followed by a slow regrowth of carbonate compounds to harden the shell. While their advanced vision and other advanced features have kept them going, and they are following a trend towards more standardised and efficient methods, trilobites' ancient means of moulting are putting greater and greater strain on their kind as well.

The voracious scorpion reaches in with his chelicerae- and then, after a moment, sets down his walking legs and begins to push his way inside the burrow. He squeezes in, his large size making it a tight fit, as he pushes himself forwards, and for a brief moment his claws touch soft, freshly-shed meat.

The scorpion is startled, and recoils- and his equally-terrified prey shoots straight over his head, evacuating the safety of the burrow.

His target is a strange-looking one. Instead of large chelicerae, it sports a set of maxillipeds, and it breathes not with book lungs but with a set of gills beating underneath its abdomen. Two antennae stick out from underneath a large, rounded head shield, and it has two large, beady eyes with which it warily regards its predator. This animal is not a trilobite, a scorpion or even a horseshoe crab- it is a recently-evolved type of arthropod called a bull crab.

In the Silurian, his ancestors were small and fed on detritus or filter-fed algae from the water. They were a common member of muddy sea floor communities, and still are to this day- but after the extinction came new opportunities, and those little arthropods were some of the first to seek them out.

The bull crab is part of an entirely different lineage of arthropods to millipedes, trilobites or eurypterids and other chelicerates. He is a type of animal called a maxillostrace- which today includes crabs, barnacles and insects. More specifically, though, he is an ancient type of maxillostracan called a leptostrace.

Most familiar crustaceans had yet to evolve by the Silurian point of divergence- for example, prawns and mantis shrimp only appeared (or became common enough for fossils to be found, at any rate) during the Devonian, and true crabs appeared as late as the Jurassic. Leptostraces, meanwhile, were already an important part of Silurian ecosystems- though they didn't preserve well, sites of exceptional preservation show they were a major component of soft floor ecosystems at the time, and their kind had already existed in forms similar to modern Nebalia shrimp since the Cambrian. With the enormous hit to trilobite diversity after the gamma ray burst, leptostraces have joined horseshoe crabs in expanding out from soft muddy sea floors into a much wider range of ecosystems.

The bull crabs, or taurocarcinans, are simply the largest and most charismatic of a much larger explosion of leptostrace diversity. This big bull crab has a variety of features that makes him much less vulnerable to predation by fish- his short legs, large head shield and flexible abdomen are all features that make his body sturdier and his moults faster than that of the eurypterid he has encountered, and- in an emergency- that same sturdiness gives him more ability to escape even at his most vulnerable.

With a flick of his tail, the sensitive bull crab flees the scene, while the scorpion tries to extract itself from the hole it has got itself stuck in. And just in time- a large acanthodian has been attracted to the commotion, and the struggling eurypterid makes for a perfect distraction.

As the shark closes in for the kill, the bull crab escapes, eventually settling beside a worldback worm's mini-reef as an impromptu shelter from the dangerous open sea in order to finish hardening its shell after its shed. Both trilobites and eurypterids are following its lessons- the simpler, less spiny exoskeletons of the phacopids and a trend towards fewer thorax segments are both adaptations to make moults a safer process.

This happened in our timeline, too- and it wasn't enough to save their reign. The future may be bright for the leptostracans, horseshoe crabs and the jawed fish- but with two extinctions to go, it's anyone's guess as to whether these two more ancient dynasties will make it to the end of the Paleozoic.

 

[Eotyrannus]

The Late Devonian: Welcome to the Crucible

For thirty million years after the extinction, the climate of the Devonian was characterised by a slow decline in temperature. Though in our timeline this intermission of cool temperatures was dominated by the stromatoporoid sponges- a major reef former, and a very distinctive fossil characteristic of Paleozoic reefs- in this world corals, such as the long-extinct honeycomb corals, were the reigning group of the time.

This mild, stable climate allowed for a profusion of new forms to evolve in the wake of the Silurian mass extinction. Though some were modified radically- such as bull crabs and terrestrial millipedes- others radiated into new niches while retaining their old forms.

Perhaps most notable were the sharks. Immediately after the extinction, they were small carnivores restricted solely to marine ecosystems. As they adapted to fill new niches, though, they went from primitive and generic animals to the true rulers of the Devonian seas, with the largest reaching up to four metres in length. Even fresh water habitats began to open up to them, putting pressure on the last holdouts of eurypterid dominance.

Similarly, life on land has been growing in intensity. Although no true trees have appeared due to a lack of wood, herbaceous plants are growing ever more significant. Many florescophytes- the group of plants most well-adapted to becoming these larger members of the ecosystems- grow to the size of small shrubs. They tend to possess flower stalks in the centre of the plant, with long, slender leaves that are resistant to predation from the herbivorous nymphs of their pollinators.

However, cosmic events are once again putting the inhabitants of this lush planet under pressure.

Of Orbits and Oceans

This time, it is nothing so extreme as irradiatory annihilation. This time period is defined by simple physics, and the steady contest between the mass of the planet and the immense gravity well of the Sun.

The Earth's orbit is not perfectly stable- over the course of its eternal journey around its parent star, its tilt and orbit change slowly over time. The most famous of these are the three Milankovitch cycles- the Earth's orbit reaching its most elliptical every hundred thousand years, its tilt reaching its most extreme every fourty thousand years, and its precession (the amount of wobble in the axis itself) reaching its peak every twenty-five thousand years, each of these causing increased seasonality at their height and a more stable climate at their lowest.

Like many such phenomenon, however, these cycles are just minor, short-term effects compared to much longer ones, and the world reaching the peak of one of these deep-time orbital cycles has wrought havoc on the global ecosystem.

Somewhere during the Frasnian epoch- a part of the Late Devonian spanning from 385 to 375 million years ago- the Earth's orbit reached a point of extremely high eccentricity, meaning that its orbit was much more oval-shaped than it is today (though a map of the solar system would still show the planet's orbit as looking like a circle anyway). Though the planet didn't recieve more or less sunlight, it did make the planet's seasons much more extreme than normal, and in West Gondwana (near the South Pole, in regions that today form South America and Africa), the more intense summers caused a literal meltdown.

Though not especially notable in the grand scheme of things, small polar glaciers did exist at the time- perhaps a little larger than in our own timeline, as a relic of the effects of the gamma ray burst. However, these glaciers couldn't survive in the new climate- and their slow reversion to a liquid form flooded the lands with oceanic water, starting a catastrophic chain of events that would inevitably lead to the start of one of the most protracted times of suffering in the history of life on Earth- the Devonian Mass Extinctions.

As the lands were flooded by rising sea levels- a phenomenon called marine transgression- the oceanic creatures suddenly had to deal with an unexpected phenomenon. Rather than windswept landscapes, amenable for colonisation by marine life, they encountered a whole host of soils, muds and carbon-rich deposits created by the activity of plant life. More importantly, the microbial decomposers that fed on such organic matter found it.

It was a feast like they'd never had before, and life on Earth suffered for it.

The carbon, once sequestered by plant life, found itself being released all at once back into the atmosphere. The climate spiked, the hottest temperatures that the planet had known since those early, nurturing Cambrian waters- but this time, rather than a tropical paradise, conditions were leading to a cesspool.

But that carbon dioxide wasn't being made from nothing- as carbon dioxide increased, the marine oxygen being used to form it was being lost. And perhaps it would have been temporary… but at this point, the planet chose quite possibly the worst time possible to undergo major geological events

Life Ore Death: Sedex Events

In the modern day, ore-rich Devonian rocks exist throughout the countries of Germany, Canada, Mexico, Kazakhstan and the United States of America. These mineral ores mainly consist of barites and sulfides, together constituting a major source of metals including zinc, lead and barium that together are used in everything from storage tanks to paints.

They may seem utterly harmless today- and even at the time, these particular metals were. However, the key word here is 'deposits'. The minerals that weren't deposited- that is, those that escaped from these sedimentary-exhalative (or Sedex) events- were the real killers, pushing a sensitive time for the biosphere into its next collapse.

The continent of Euramerica, at the time, existed near the equator. At the very start of these events, perhaps there might not have been anything wrong at all- some sediment floods and coral bleaching from the marine transgression crisis that would be about to rear its ugly head, yes, but things would look normal in the reefs and shallows.

Exactly what you'd find if you went deeper is more debatable. But you'd certainly find something. Looking at similar geological phenomenon in the modern day, perhaps you would see something with a rather whimsical name- an ocean floor, covered in what look like underwater lakes. Today, such a location is known famously as the Hot Tub of Despair.

In a sedex event, it's a reasonable guess that you'd see a basin almost flooded with them.

Each Hot Tub represents the top of a hydrothermal vent, with temperatures at around sixty celsius inside and forty where it meets the ocean. At the surface, a visible boundary between the hypersaline brine- the 'hot tub'- and the sea water would be seen, fringed by deposited material and beds of chemosynthetic brachiopods, joined to each other with interlocking spines. At the deepest source of the brine, meanwhile, one would find the carcass of a much older basin- a deposit called an evaporite, formed from a dried-up sea, buried under a gargantuan weight of rock.

Rather than draining into the rock, being superheated and then being forced out, as is the case for the famous 'black smoker' variety of hydrothermal vent, this brine is fossil water that originated in those long-gone waters. As the sea dried out, trace amounts of sea water- perhaps as water-containing minerals, or perhaps trapped between the pores in the rock- were retained, until they were buried. A combination of immense pressure and a geothermal heat source, much deeper than the water itself, ultimately lead to fractures in the rock above. The brine escaped from these fractures… and though the zinc, lead and barium became trapped in various places between the seafloor's very surface and the deep-down rocks it originated from, other metals and dissolved minerals made it into the ocean above.

At first, these sedex events might have been a wellspring of life. The rich chemical soup would have been plentiful food for a long-lost Paleozoic alternative to the Hot Tub of Despair, on a much grander scale. Mussel beds would be replaced by the aforementioned spiny brachiopods, and on those brachiopods an entire ecosystem would subsist.

Down here, the pressures of worms or bull crabs are absent. Eel-like spiny sharks and slow-moving open-water eurypterids would slowly glide over the underwater lakes, preying on the trilobites feeding on both the mussels and with their own arrays of internal chemosynthetic bacteria. But the deep seas of the past are a great mystery- perhaps long-lost creatures from an earlier age would still survive in those arcane depths. Perhaps an anomalocarid might be seen, fluttering on paired fins to extract tube worms with delicately-curled mouthparts. Or maybe the embalming waters of the pool would be prowled by the last descendants of what in our time we would know as placoderms, but in this time were just an interesting curiosity from the earliest attempts at jawed fish.

Ultimately, though, this ecosystem would not last long- smothered by the weight of death above it.

At the surface of the brine, some minerals would slowly diffuse into the sea water beyond, mixing as new, hot water seeped out from below. These minerals were a witch's cauldron of dissolved nutrients, containing elements such as sulphur, nitrogen, reduced carbon, silicon, magnesium and iron, as well as rare isotopes of strontium. These strontium isotopes give a picture of what exactly was going on- as each Middle-to-Late Devonian sedex deposition occurred, the rare isotopes spiked. And they still hadn't returned to normal when the next event happened in a series of five different periods of major sedex deposition- and each spike marked a phase of sudden death for the various animals present.

This alone would have been bad. When combined with the sudden richness of carbon dioxide and assorted organic matter in the oceans caused by the flooding of the plant-enriched continental surfaces, it became catastrophic.

Animals Vs The Microbial Reconquista

In the present day, oceans are- for the most part- deserts. Minerals are rare- but where upwellings occur and deep-sea nutrients are delivered to sunlit surface waters, life is present in abundance. This is one of the reasons why pelagic animals are so successful in the open ocean compared to other, less physically-complex forms of life. The capacity of animal life to allow any individual cell to travel vast distances ensure that they are disproportionately dense in nutrient-rich areas, letting them make the most of the scarcity of resources.

This, however, relies on one key concept- that nutrients are the limiting factor.

The immense capacity of animal life to locate and harvest sources of nutrition has led them to become utterly reliant on oxygen being freely-available. And this is perfectly fine. Apart from in extremely rare scenarios, an ocean without oxygen is like an ocean without water- it simply doesn't happen, so pelagic animal life being able to withstand anaerobic conditions is just as absurd as a whale in the Mediterranean retaining its legs just in case the ocean dries up.

In the Late Devonian, however, this was suddenly no longer applicable. The combination of sedex events and terrestrial organic material meant that nutrition was freely available throughout the ocean. Microbial activity skyrocketed- algae, bacteria and a whole range of other microscopic organisms were able to flourish, and though not capable of movement, their various adaptations towards floatation ensured that these nutrients were recycled frequently. It might have been a boon to animal life initially- but as the ocean became more crowded, and nutrients continued to flood in, the oceans finally reached a critical point.

That point was when oxygen became scarcer than food.

Suddenly, the positions of animal life and microbial life had been reversed. No longer were animals the most competitive organisms in the ocean- now they were desperately seeking comparatively vast quantities of oxygen, a dissolved compound they didn't even have the capacity to detect, in an ocean full of microbes that were often quite content to simply wait out periods of deoxygenation in much the same way that animals, with their ability to retain nutrients as fat or yolk, once waited out periods without nutrition.

On top of this, this organic activity would have created vast quantities of carbon dioxide, ammonia and organic acids- each one an additional poison to affect oxygen-starved pelagic life. The sudden lack of animals also affected oceanic circulation. The mass migration of life to the depths to evade diurnal predation is a massive contributor to the mixing of the ocean, and as those predators became ever-scarcer in an environment transitioning from delicious and nutritious to toxic and anoxic, the lack of need to perform this movement made the oceans ever more stagnant and brought pre-existing anoxic layers nearer the surface.

And this process was capable of self-sustaining. In addition to the minerals from the vents, anoxic waters are actually more capable of dissolving vital metal nutrients than well-oxygenated waters. Many school lab experiments mix dissolved metals with other dissolved compounds to make brightly-coloured and easily-identifiable metal oxides fall out of the water, in a process called precipitation, and when oxygenated water oxidises pure dissolved metals they also precipitate in those scenarios as well. Anoxic water, on the other hand, actually can dissolve and retain those same metals- most notably magnesium and iron, both of which are vital limiting nutrients, both of which already existed as precipitated compounds that only anoxic waters could reabsorb, and both of which were also released in significant quantities by the sedex events.

As oxidated sea beds were replaced by anoxic ones, they effectively removed the cap of oxygen that kept iron, phosphorus and magnesium compounds contained. Slowly, these compounds diffused back to the surface and were leached out, fuelling the microbial activity that caused the anoxia in the first place. This reached a point where the anoxic waters became metallic to the point of toxicity, and as they swept into new ecosystems and removed the oxygen seals on more mineral-rich sediments, they caused catastrophe. Even in our own timeline, the trauma is visible- the sheer amount of dissolved metal was enough to leave a record of malformed, metal-poisoned fossils at the time.

To be an animal actually present- not just damaged in the fossils, but in the tissues, the neurochemistry, and suffering every other hardship besides- is almost unimaginable.

Eventually, as the carbon sources from the marine transgression ran out, the greenhouse would end. Glaciation would return to South American Gondwana five million years before the end of the Devonian, and the last sedex event before conditions could finally return to normal would occur just as the Carboniferous dawned. But that wouldn't matter much to the animals that had to live through this horrific period of time. Marine life is about to enter a crucible of evolution- an extended fight against the forces of nature, like none it's ever known before, and like none it will ever know again.

Forty million years of suffering is about to begin.

This is the story of the Devonian Mass Extinction- the longest-lasting extinction event of the Phanerozoic.

 

[Baron Steakpuncher]

Love this evolutionary speculation.

 

[xa na xa]

Epic mass extinctions.

Surreal new lifeforms.

Subscribed.

 

[Jake]

Alt-Paleontology? That's a new one.

 

[TGUT]

*Sees thread title and immediately thinks: Oh Gawd somebody is trying to dredge up the stupid gamma ray hypothesis again...*

*Notices that thread title actually says Silurian instead of Ordovician*

*Realizes that it is a alt-prehistory timeline!*



In all seriousness, great work thus far but dear lord the Paleozoic is going to be seen as a nightmare in this timeline with no less than four of the presumably six mass extinctions happening in one dreadful two hundred million year spree.

Just to make sure I've understood correctly, apart from the obvious loss of the insects and the success of the spiny sharks, it looks like vertebrates haven't managed to get onto the land by the time of the Devonian mass extinction?

 

[Eotyrannus]

Just to make sure I've understood correctly, apart from the obvious loss of the insects and the success of the spiny sharks, it looks like vertebrates haven't managed to get onto the land by the time of the Devonian mass extinction?

Click to shrink...

Not by the time of its start, no- in OTL, it's likely that the anoxic conditions caused by the extinction were a significant factor in driving the evolution of air-breathing vertebrates and their later terrestrial descendants.

 

[Memphet'ran]

I don't have any comments on this right now, but it's nice to see some environmental alternate history; it's an interesting idea that I don't see very much of. Personally, after reading Brian Fagan's The Long Summer I think it'd be interesting to see an alternate history exploring how civilization might have developed if the ~6000 BCE dry period didn't happen or the Green Sahara never dried up.

 

[Eotyrannus]

The Late Devonian: Biocrust Reefs and Vermiform Variations

Conditions as the Early Devonian came to a close were some of the most perfect reef conditions the planet has ever known. A combination of various factors- ideal climate, the evolution of new biologically-generated reef habitats via orbirursid worms, and a mass diversification of coral and stromatoporoid rocky sponge species generated reef ecosystems on a scale unheard of, before or since.

As the anoxic water swept in, though, the ecosystems ground to a halt. Unlike other mass extinctions, the Devonian Mass Extinction is characterised not by how many species died out, but by how few evolved to replace them. As the immense reefs began to shrink and fracture, species began to disappear- and in such hostile conditions, nothing was evolving to replace them, resulting a slow breakdown of the ecosystem's ability to function.

These weren't the only problems faced by these fragile ecosystems. The stability of the planet's climate had, for a long time, meant that extreme weather conditions were a rarity. With the increased seasonality of the planet, though, storms began to batter the shorelines with growing frequency and intensity. In addition to this, biological changes to the terrestrial landscape were also having an effect. Forests' ability to create their own rain systems is well-documented, and the influence of increased terrestrial humidity created a double-pronged assault of hostile weather. In the grand scheme of things, it wasn't much, but in the middle of an extinction it certainly didn't help.

The corals were the hardest-hit. With nothing but simple diffusion to support them, they were extremely vulnerable to the deoxygenated waters, let alone the rising heat and acidic waters. As they grew scarcer, hardier species stepped up to take their place. The stromatoporoid sponges fared better- sponges are superb at feeding on single-celled plankton, but ultimately it was their spongy texture (though perhaps that isn't quite accurate when describing a sponge intertwined with its own limestone skeleton) that worked to their advantage. By having water flowing through their entire body, and using flagellae rather than muscles, stromatoporoids were able to make the most of the oxygen available while corals were forced to the fringes to survive.

Worldback worms are another group that are still able to form a significant part of the reef-building ecosystem. Worms of all sorts have a remarkable capacity to cope with low-oxygen conditions, and though much of their biology is alien to us, their solution is remarkably familiar- they possess hearts, a circulatory system and red blood. In our world, this is how lugworms and other such creatures survive in tidal flats- they can squeeze the very last bits of oxygen from an environment and continue to survive.

Despite these two groups' ability to survive in the harsh Late Devonian world, though, they are failing to keep the biosphere's decay fully at bay. The reefs' structures are no longer dominated by animal life, but by the same microbes that filled the world after the last extinction event. Calcium-secreting bacteria and algae, growing in concert, have created a strange and waterswept landscape. Enormous pillars of microbialite rock rise up from the lumpy, flattened seabed in places, interspersed by a large minority of reef-building worms and sponges, with a few other rare inhabitants such as crinoids, tube anemones and various types of coral clinging on alongside them. Many worldback worms can no longer find a stinging array of defences- while some still find toxic sponges, others simply allow the living rock of the microbe reef swallow them up to hide beneath its surface.

Where reefs are entirely unable to exist, though, other animals are stepping in. Though they still lack the adaptations or the ecosystem dynamics to exist as far below the sediment as modern marine worms, more conventional bristleworms are starting to transition to living below the ooze that has smothered the bones of the old reef systems. In addition to this, beds of sediment-living brachiopods are covering vast expanses of the sea floor. Normal conditions would disrupt their growth frequently- they rest upon the sea floor, in shapes like little baskets- but as long as they have areas without excessive sedimentation that could bury them, these immobile little seashells are growing virtually everywhere.

And that brings up another problem- the world is becoming ever more homogenous.

The extreme suddenness of the rising sea levels has unexpectedly linked seas across the planet, putting different species into contact where they might never have met before. This has caused an epidemic of invasive species- where once different reefs were diverse and unique, now you find similar-looking animals on completely opposite sides of the Euramerican landmass. With most of the world's tropical seas being separated by this landmass- the warm, wet subcontinent of Siberia on the Northern side, and Gondwana only being separated by the tiny, ever-shrinking Rheic Ocean- this same mixing of marine species is spreading across the world.

In the context of an extinction, this is worse news than ever. Organisms that are able to range far and wide to find strongholds or other, more temporary refugia are more resistant to virtually every type of extinction, but this global interchange is one of those few exceptions. Animals with similar niches spread similarly, and in this scenario, it means that what would ordinarily be survivors are being killed off in a winner-takes all deathmatch. Though the last man standing will be more likely to survive than ever, this will be little comfort to the dead.

However, this same highly competitive climate is driving new adaptations- ones that could have profound impacts long after the extinction itself is over.

The Rise of the Bugsquids

A strange-looking organism floats serenely over a hard microbe reef.

At first, one might be forgiven for thinking that the anomalocarids had returned from their graves or the crushing depths of the oceans. It paddles through the water with a series of lobed fins, running in segments down its body. It has a paddle-shaped tail, with streamers stretching out behind it. And it does have two eye stalks- but on closer inspection, these eyes aren't the dragonfly-like eyes of the planet's first predators. Instead, they possess distinct pupils, perhaps like a snail or octopus.

On seeing that, perhaps one would think that- rather than an armoured arthropod like an insect or crab- that this animal is a cephalopod. Perhaps those fins on its side are an advancement on the frill of a cuttlefish. And looking at its face, it does have a set of tentacles- twelve in total, with eight sticking out like barbels and four concealing its mouth. The odd animal pauses, stretching- and as it contracts its muscles, a powerful beak is squeezed out from between those four feeding tentacles.

But again, on closer inspection, this certainly isn't a cephalopod. After all, no mollusc has segments- and this animal has clear segments running along the length of its body. And rather than two eyes, it has four- a second pair exist just above its tentacles, facing forwards instead of scanning the horizon for prey or threats. Its fins are well-differentiated from each other, and down the centre of its body is a pulsing cavity lined with blood-red gills.

It turns its attention back towards the seabed- and spots movement. Carefully, the animal glides towards its target.

A small trilobite is scuttling over the seabed. With open environments continuing to spread, the horseshoe crabs and bull crabs are tightening their grip as the dominant arthropods of the sea- but trilobites are nothing if not hardy, and this individual is from a species that spans Euramerica in its entirety. Unfortunately for this individual, though, the foot-long predator floating above it has no knowledge of the trilobite species' achievements- and probably wouldn't care even if it did.

Slowly, it maneuvers into place, crossing the path of the trilobite- and then, quite suddenly, it thrusts its jaws out from its head entirely on a long, fleshy tube.

The startled trilobite is caught in the animal's maw, and it immediately begins to writhe, trying to escape the horizontal deathgrip. It's no use, however- the eversible pharynx that the jaws shot out from is retracted, and the flailing little trilobite is pulled inevitably towards the tentacles. As the animal's grip closes, it can finally use the full strength of its jaws, and the chalky shell of the trilobite is useless as it is hewn apart by the predator with a satisfying crunch.

As the animal feeds, it doesn't notice another animal come in to scavenge on the dropped fragments. It is a small, wriggling bristleworm- and it is a juvenile of the very same species.

This animal is a bugsquid- the most advanced type of worm that the world has ever known.

Originating from the rise of worms in the earlier reefs, and forged in the harsh extinction habitats, bugsquids are a clade of omnivorous or carnivorous macrofaunal polychaete. Specifically, they are machaeridian worms- the armoured worms, as odd as it may seem for such a distinctly fleshy animal. They originated when these small worms were able to grow to larger sizes in the habitats of the Early Devonian reefs.

With their growing size, the ancestors of the bugsquids needed better leverage- the simple muscles of the polychaete 'leg' (a fleshy lobe called a parapodium, only capable of extending, retracting and moving forwards or backwards) were not doing good enough. By compressing their bodies with the muscles of their shells, though, they were capable of creating different shapes that allowed for simple walking gaits to appear- and once certain clade's chaetae (the titular bristles that give the parapodia strength) joined up with the plates, and those plates in turn worked as a paired concert, suddenly these worms had the beginnings of a skeleton.

The pressures of the extinction turned things up a notch, but the answer to their woes was simple. By coiling the intestines to produce a robust body, and by creating a cavity for the 'gill' half of the parapodia on the underside of their bodies, these worms transitioned from a long, vulnerable worm to a streamlined, robust predator of the sea in a single stroke- and so the bugsquids were born. With the seas effectively all being connected, this newborn clade has spread across much of the globe- becoming yet another invasive species.

These animals are very similar to cephalopods in many ways- their extendable beak, decent vision and squishy bodies put them at odds with the much older tentacled clade, but the bugsquids also have advantages over them. They possess red blood, flesh-protected calcium carbonate skeletons and forwards-swimming bodies- all of which are much better in the current extinction climate. Cephalopods are being put under more stress than ever by the bugsquid competitors, though thanks to their ability to be hatched fully-formed instead of going through a larval worm stage (as bugsquids need to do in order to develop a full array of adult segments) they do have a head up in some of the most vital aspects of extinction survival.

Bugsquids are one of the most charismatic organisms of Late Devonian reefs, living alongside sharks and a few hardy coiled nautiloids. Echinoderms such as sea urchins and starfish are common prey here- the hard, munching beaks of the former and the external digestive capabilities of the latter make them good at exploiting the reef for food. A variety of polychaetes and arthropods also live here, though the last of the flippered sea scorpions have finally faded into oblivion, leaving only their crawling stylonurine sisters to carry on their legacy.

The future looks bright for this strange and adaptable new type of animal, even as so many other species are being lost around them. But that is the tale of every species with the grit to survive extinction.

 

[Halocon720]

How large are worldback worms? I don't recall ever seeing any specific measurements.

 

[Eotyrannus]

How large are worldback worms? I don't recall ever seeing any specific measurements.

Click to shrink...

Probably similar in range to most other reef filter-feeders- though, being worms, they tend to be long rather than fat. Single-plated species would probably reach the side of a dustbin lid at most, while the reef-living multi-plated species might be multiple metres long.

 

[Shevek23]

Is the goal here to work up to the present day and beyond? Do you already know what lineages will take the places of OTL insects and chordates on land, and what strange alternative plant types might form the basis of the ecosystem there?

I assume the geology will remain pretty fixed to OTL patterns on rails, though I am mindful that alternative ecological developments can have effects that feed back on geological outcomes and even deep driving forces perhaps. You've already noted how conditions that formed deep mineral ores today differed.

If say some fluctuation makes Earth too warm for the glaciations of the past ten million years or so to happen despite the gross configuration of continents being about the same, then the geological impact of miles thick sheets of ice on half of North America and the northwest end of Eurasia about 90 percent of the time would not be present and that would not only prevent direct knock on effects (I believe the mountain range separating Norway from Sweden, the Jotunheims, are effectively just the popped up fringe of land on the periphery of the great European ice sheet, for instance, and without the glaciations, I doubt they would exist at all...I might be mistaken about that, but anyway the broad gulf that is Hudson's Bay is I am much more confident a depression created by the ice sheet and again a warmer Earth or one that for some other reason lacked these continental glaciers would have higher elevation there, and therefore lower elevation somewhere else since there is a fixed volume of rock to work with. Same thing goes for the Baltic and indeed Sweden and Finland's terrains are entirely governed by being under the ice more often than not. So plainly we'd have to edit some details in a world where the glaciers never formed on that scale...but I would think that beyond that, their OTL existence could have diverted deeper mantle currents and thus shifted centers of uplift, directions of plate drift, and other factors to make the overall layout of the planet, or anyway the regions directly affected by glaciers OTL, notably different across the board.

But as you've already alluded to, there are lots of factors that might not be apparent to beings with such short life spans as us, whereby ecology feeds back into geology.

But if you assume that cascading butterflies will randomize geological outcomes then things will start going astray for you pretty early and you'd wind up with pure fantasy.

So are you simply assuming that barring the single variation of the gamma ray burster, Earth's geological outcomes must remain essentially the same, either because honestly the biologically based variations in masses of material feedstocks, changes in acidity, oxygenation, etc can't really change the gross outcomes much, or because strange and alien as these lifeforms appear to be to us, by and large they will play much the same role as OTL ecosystems did and so while on paper there is room for much strangeness geologically as well as biologically, in fact geologically the same global and local fluctuations as OTL will be replicated by parallel means in this timeline, and so in fact the geology will remain close to OTL--so we get the glaciations, the formation of the English Channel and the Great Lakes, all the geology is still as OTL?

You've already thrown one big monkey wrench into it with your recent posts about the Devonian, which mean at the very least the mineral deposits of the present era will be quite different. Many key events as relevant to modern human geography and economics and strategic concerns still lie in the future though I believe, such as the Carboniferous Era. If that happens! I'm guessing that if plants do not develop wood, they will develop something else that serves broadly similar function, perhaps meeting some needs by remarkably different means, and not meeting other needs as well.

Fungi I gather use chitin rather than cellulose for structure. Can plants too adopt some form of chitin that will serve to enable tall trees and so forth? But if they do, will we have a failure to form the vast coal deposits of OTL and thus have more CO2 persist in the atmosphere (this would be an example of how the ice ages of modern OTL eras might be sidestepped despite the continents being in the right configuration to foster glaciation!) Might massive carbon sequestration happen by other means in another era? Or will chitin thick and strong and refined enough in chemical detail and in structural shape turn to carbon deposits over time the same as cellulose would?

Some sources tell me that the important thing underlying the formation of the coal deposits was that between the adoption of cellulose as a structural material in cell walls enabling wood to form and plants to get tall, and the end of the Carboniferous, organisms simply had no means of breaking down cellulose and digesting it as food, and so in the right climatic conditions anyway, dead plants simply accumulated in great big mats; their other biological components would be eaten up and restored to the ecosystem somehow albeit with some delay, but the cellulose was untouchable. So over time it formed really thick layers, then got buried under soils and rocks and eventually deep down heat and pressure cooked it into coals of various kinds. So if chitin for instance, or perhaps some other polymer unknown to OTL biology, were to fill cellulose's role instead, perhaps the alternative could more readily be digested and thus there never would be this massive sequestration of carbon, not by that means anyway. Or vice versa some alternative compound could be even harder to digest than cellulose, and the ATL version of the Carboniferous last much longer with even more carbon sequestration than we inherited OTL!

It is a puzzlement to me just how far life could stray from the OTL broad patterns, and how drastically the alternatives would change climate history and even the basic geography of the planet's surface! And at least technically speaking if we can shuffle continental masses around, we are changing Earth's behavior as a rotating object projecting a not quite ideal gravitational field into the Solar System, so eventual butterflying of even cosmic events that are due to Solar system orbital dynamics and occasional meteor impacts is theoretically in the cards. In practice though I can't imagine it would be easier to roll the dice than to just assume that nothing beyond Earth's upper atmosphere is significantly affected.

 

[Shevek23]

I don't have any comments on this right now, but it's nice to see some environmental alternate history; it's an interesting idea that I don't see very much of. Personally, after reading Brian Fagan's The Long Summer I think it'd be interesting to see an alternate history exploring how civilization might have developed if the ~6000 BCE dry period didn't happen or the Green Sahara never dried up.

Click to shrink...

Plausibility of that would tend to depend on the exact causes of the 6000 BCE dry period. I understood that the Sahara and South America are in some kind of subtle cyclic relationship, and if that is so then presumably the people saying so have observed dryings in prior Interglacials. As I understand it the previous interglacial was significantly warmer than the current one, but to estimate the probability the Sahara will in fact go dry during the interglacial, aside from theories of why it happened in our current instance, knowledge of whether this happens more often than not in Interglacials even if it did not in the previous one, versus the current condition of the Sahara being a not too common fluke, would bear heavily on the question.

My assumption is that our interglacial followed a similar pattern, barring possible significant though until recently subtle effects of human activity (alteration of ecosystems and lots of fires), to most prior ones, and my guess is that for the past several million years the Sahara has typically been about as dry and desertified as it is today in the majority of elapsed time since the glacial cycle started our planet has spent in Interglacials. That of course is only 1/10 the overall elapsed time, and my guess is that the drying happens about 1/3-1/2 the time span between the melting of the continental ice sheets and the next Fimbulwinter signaling their gradual eventual return. (I have no idea whether the Sahara region, or Arabia, is more or less likely to be a desert during glaciations than in the interglacials...but one thing I have learned about glacial periods is that climate fluctuation is more rapid and chaotic during glaciations, so my guess would be it jitters and judders a lot between being moderately arid savannah and relatively brief crises of deep drought).

On the other hand I am blissfully ignorant of actual facts; those are just my guesses. If you can show that actually Sahara as a desert is relatively rare in the modern (since glaciations started I mean) era, and thus our current situation is something of a fluke or even human caused, a world where the Sahara is more or less arable and moist enough to support grassland and maybe some light trees but inhabited by human beings who somehow don't cause the OTL collapse would be pretty amazing. Would a less lush Amazonian climate be the necessary corollary of a less blighted north Africa, or are the two completely unrelated?

Anyway yeah, vast grasslands just over the Atlases and south of the coasts of north Africa generally would be a heck of a big change!

At AH people have attempted TLs with the Earth flipped over or with the polar axis running through different pairs of surface points. In the most careful attempt to work out a turned upside down Earth I have seen yet, the author concluded that flipped, the Sahara region would never get as dry and in terms of human societies attributed a vast super-Chinese size unified empire there.

It is possible to dig up maps of the rivers that would flow through the Sahara if it only had the rain to produce rivers--which it did as recently as 4000 BCE apparently!

 

[Eotyrannus]

Is the goal here to work up to the present day and beyond? Do you already know what lineages will take the places of OTL insects and chordates on land, and what strange alternative plant types might form the basis of the ecosystem there?

Click to shrink...

I tend to research things on the spot, or jot down smaller ideas for the next few posts that I can research properly later. Since evolution isn't a planned phenomenon, this tends to work quite well for creating a reasonably realistic outcome.

I assume the geology will remain pretty fixed to OTL patterns on rails, though I am mindful that alternative ecological developments can have effects that feed back on geological outcomes and even deep driving forces perhaps. You've already noted how conditions that formed deep mineral ores today differed.

If say some fluctuation makes Earth too warm for the glaciations of the past ten million years or so to happen despite the gross configuration of continents being about the same, then the geological impact of miles thick sheets of ice on half of North America and the northwest end of Eurasia about 90 percent of the time would not be present and that would not only prevent direct knock on effects (I believe the mountain range separating Norway from Sweden, the Jotunheims, are effectively just the popped up fringe of land on the periphery of the great European ice sheet, for instance, and without the glaciations, I doubt they would exist at all...I might be mistaken about that, but anyway the broad gulf that is Hudson's Bay is I am much more confident a depression created by the ice sheet and again a warmer Earth or one that for some other reason lacked these continental glaciers would have higher elevation there, and therefore lower elevation somewhere else since there is a fixed volume of rock to work with. Same thing goes for the Baltic and indeed Sweden and Finland's terrains are entirely governed by being under the ice more often than not. So plainly we'd have to edit some details in a world where the glaciers never formed on that scale...but I would think that beyond that, their OTL existence could have diverted deeper mantle currents and thus shifted centers of uplift, directions of plate drift, and other factors to make the overall layout of the planet, or anyway the regions directly affected by glaciers OTL, notably different across the board.

But as you've already alluded to, there are lots of factors that might not be apparent to beings with such short life spans as us, whereby ecology feeds back into geology.

But if you assume that cascading butterflies will randomize geological outcomes then things will start going astray for you pretty early and you'd wind up with pure fantasy.

So are you simply assuming that barring the single variation of the gamma ray burster, Earth's geological outcomes must remain essentially the same, either because honestly the biologically based variations in masses of material feedstocks, changes in acidity, oxygenation, etc can't really change the gross outcomes much, or because strange and alien as these lifeforms appear to be to us, by and large they will play much the same role as OTL ecosystems did and so while on paper there is room for much strangeness geologically as well as biologically, in fact geologically the same global and local fluctuations as OTL will be replicated by parallel means in this timeline, and so in fact the geology will remain close to OTL--so we get the glaciations, the formation of the English Channel and the Great Lakes, all the geology is still as OTL?

You've already thrown one big monkey wrench into it with your recent posts about the Devonian, which mean at the very least the mineral deposits of the present era will be quite different. Many key events as relevant to modern human geography and economics and strategic concerns still lie in the future though I believe, such as the Carboniferous Era. If that happens! I'm guessing that if plants do not develop wood, they will develop something else that serves broadly similar function, perhaps meeting some needs by remarkably different means, and not meeting other needs as well.

Click to shrink...

Realistically, there'd probably be quite a lot of geological change. The geochemical composition of rocks is important to various processes- for example, more hydrated minerals change the types of volcanism, thicker and cooler plates caused by glaciation or reef growth might subduct sooner, and so on and so forth.

However, these changes- though they'd have a cause- would be too arbitrary to actually say anything interesting about, possibly barring minor generalisations from obvious scenarios such as expanded chalky-soil ecosystems or increased volcanism in localised areas. Since making changes would detract from being able to make comparisons between Blueworld and OTL, I'm putting a butterfly net over deep geological processes. Similarly for the Chicxulub meteorite- though a big honking laser beam a few hundred million years beforehand could probably cause enough variations for it to miss, it hits at an interesting time in Earth's history, and breaks up what would otherwise be about two hundred million years of peaceful stability into two more interesting chunks.

Fungi I gather use chitin rather than cellulose for structure. Can plants too adopt some form of chitin that will serve to enable tall trees and so forth? But if they do, will we have a failure to form the vast coal deposits of OTL and thus have more CO2 persist in the atmosphere (this would be an example of how the ice ages of modern OTL eras might be sidestepped despite the continents being in the right configuration to foster glaciation!) Might massive carbon sequestration happen by other means in another era? Or will chitin thick and strong and refined enough in chemical detail and in structural shape turn to carbon deposits over time the same as cellulose would?

Some sources tell me that the important thing underlying the formation of the coal deposits was that between the adoption of cellulose as a structural material in cell walls enabling wood to form and plants to get tall, and the end of the Carboniferous, organisms simply had no means of breaking down cellulose and digesting it as food, and so in the right climatic conditions anyway, dead plants simply accumulated in great big mats; their other biological components would be eaten up and restored to the ecosystem somehow albeit with some delay, but the cellulose was untouchable. So over time it formed really thick layers, then got buried under soils and rocks and eventually deep down heat and pressure cooked it into coals of various kinds. So if chitin for instance, or perhaps some other polymer unknown to OTL biology, were to fill cellulose's role instead, perhaps the alternative could more readily be digested and thus there never would be this massive sequestration of carbon, not by that means anyway. Or vice versa some alternative compound could be even harder to digest than cellulose, and the ATL version of the Carboniferous last much longer with even more carbon sequestration than we inherited OTL!

Click to shrink...

Cellulose as a whole was already present- the evolution of lignin was the major change in the Carboniferous, though I don't remember which one was the indigestible compound at the time. Both of these compounds, or ones with reasonably similar properties, are probably the most likely form simply due to the fact that they're carbohydrates.

Plants, by photosynthesis, can generate an effectively-unlimited amount of carbon, hydrogen and oxygen for use in biological processes. Thus, any structural compound in a plant is likely to use as much of these and as little of anything else as possible. Chitin, on the other hand, requires nitrogen to form- earlier iterations of the project did include blue-green lichens as the most significant terrestrial photosynthesisers after the extinction of plants in the GRB, thanks to their ability to cyanobacterially fix nitrogen and negate that problem, but lignin is probably going to be convergently evolved in whichever plants appear. (In fact, it's convergently evolved even in seaweeds in OTL- though seeing if it was evolved independently on multiple occasions is something I'm too lazy to check right this second.) In addition to this, lignin is waterproof- giving it a good reason to evolve specifically in the superheated Late Devonian climate/

 

[Septentrion]

Some Illustrations would be nice as a lot these terms are rather technical, at least of the animals that actually existed if nothing else, so I can better picture the Silurian.

 

[Eotyrannus]

Some Illustrations would be nice as a lot these terms are rather technical, at least of the animals that actually existed if nothing else, so I can better picture the Silurian.

Click to shrink...

That sort of thing would be quite useful, actually- I might work on some informational posts for that sort of thing.

 

[Eotyrannus]

Bestiary: Eurypterids

The eurypterids, often called sea scorpions, are a type of aquatic arthropod that first appear in the fossil record about 470 million years ago in the Ordovician. When the largest of the orthocone cephalopods- bus-sized monsters with conical shells and a myriad of grappling tentacles- were wiped out at the start of the Silurian, eurypterids rapidly took over, coming to dominance in a variety of predatory niches. Indeed, the Silurian was their heyday, with 90% of all OTL eurypterid fossils being of the foot-long Silurian paddling scorpion Eurypterus.

Despite their common name, eurypterids are not scorpions, and the vast majority were not marine- possibly due to the competition of predatory fish, which came to dominance both in the alternate timeline's extinction event and the minor mass extinction at the end of the OTL Silurian. The giant OTL Early Devonian scorpion Jaekelopterus was the pinnacle of eurypterid evolution, a freshwater animal the size of a saltwater crocodile and the largest arthropod to have ever lived. Most eurypterids were native to the continent of Euramerica, but exceptional members such as the late-surviving Adelophthalmus and the giant Pterygotus achieved global distributions.

The eurypterids are the closest relatives of the arachnids (which include spiders and true scorpions), with the horseshoe crabs being distant cousins in a group called the chelicerates. Like all of these chelicerates, eurypterids have a head and an abdomen. They have six appendages- the chelicerae (the same as the pincers of a camel spider or true spider), and then five other appendages that acted as mouthparts, movement appendages, or- for most eurypterids- something in between the two. Curiously, how they breathed is not known- their abdomen has spongy chambers similar to the breathing chambers found inside the legs of woodlice, but these don't appear to have been big enough for breathing underwater unless these animals possessed a far more advanced circulatory system than expected. They had two major eyes and multiple smaller ones, similarly to spiders and scorpions, and more active species had very good eyesight.

Eurypterines



Eurypterines are what comes to mind when most people think of a sea scorpion. Virtually every famous member of the group- Eurypterus, Pterygotus, Jaekelopterus- is an example of a eurypterine. Eurypterines are easily distinguished by their back legs appearing like paddles- while some species lived entirely in open water, others crawled along the seabed with an insect-like gait and only used their paddles for sudden bursts of speed to escape or hunt.

In OTL, these animals thrived from the Silurian to the Early Devonian. Their reign was effectively over by the Middle Devonian, though- a single genus called Adelophthalmus survived up until about 280 million years ago in the Permian.

Stylonurines



The less-famous relatives of the eurypterines, stylonurines are the finless eurypterids. Famously, the 'giant spider' Megarachne actually turned out to be a freshwater stylonurine on closer investigation. Stylonurines often have extremely long back legs. These were the last eurypterids to go extinct, finally disappearing at the end of the Permian some 250 million years ago, and formed the bulk of eurypterid diversity post-Devonian.

In the alternate timeline, a few of these eurypterids successfully terrestrialised in the barren ecosystems after the Silurian Mass Extinction.

 

[Shevek23]

Yay Art! I am rotten at drawing things and shudder in sympathy for any author who is demanded to produce pictures, but of course we love them if we can get them! Can you keep doing that without it bogging you down, especially for the weirdest ATL stuff? I could actually picture scorpion-related forms pretty well, but some of your critters are just kind of hard to visualize. If it's all the same to you, if they are clear in your own head and you can sketch so skillfully, I'd love to have a better idea what you are describing.

 

[TGUT]

I hope nobody minds me replying to this thread again but a rather interesting person question has occurred to me. Suppose at some point in the future, a sentient species develops and starts studying the deep time, how and indeed will they be able to decipher what caused the Silurian mass extinction? Because unlike an asteroid, direct evidence of a gamma ray burst will be slight. The list of victims might give some clues but probably not enough to make a definitive judgement. This has dogged the theories about the Ordovician event which put forward gamma rays as the cause. I suppose the speed of the extinction will help - unlike the Ordovician and Devonian extinctions, this die off will happen instantaneously. In addition, if they have identified the cause of the Permian and Cretaceous mass extinctions, then they could by a process of deduction deduce that it wasn't mass volcanism or an asteroid, which might make some scientists think about supernova. Maybe. The problem is that whilst the Siberian traps couldn't be hidden, an asteroid could - the hypothetical opponents of the gamma ray hypothesis could argue that it might have landed in the ocean and therefore the crater will have been subducted long ago. And while iridium is more common in asteroids, it isn't a universal feature. I can't help but feel that the end-Silurian mass extinction might well remain a true enigma that will fascinate the hypothetical sentients as much as the Cretaceous extinction, albeit for its inscrutability rather than simple drama.

 

[Shevek23]

On another site, someone recently suggested a scenario where a planet similar to Earth existed in Mars orbit, and I am right now trying to work out just how cold it would likely reach equilibrium at. Conventionally this would be addressed by having some kind of greenhouse heat trapping through the wazoo (five times the current level of trapping that Earth's atmosphere currently does would bring the planet into the same average temperature ballpark as Terra I think) justified via the Gaia hypothesis.

But I have now been wondering...if indeed the average temperatures on the surface, and even the high extremes, are as far below freezing as attempts to estimate without prejudice lead me to think, perhaps as cold as 200 K (much chillier than that and the carbon dioxide would all snow out of the air being frozen into dry ice), with extremes of warmth rarely exceeding 270....

Might we not have organic life very similar in broad biochemistry to Earth's OTL, but adapted to very cold temperatures, so that cytoplasm is some mix of water and other stuff that serves as antifreeze?

On the surface of such a planet, water is basically a kind of rock. Liquid water would exist in abundance, but only under skins of frozen ice, as on Europa (but far less salty water than I gather that moon is believed to have). The surface temperature would be far colder than needed to freeze Terran sea salinity ocean water, but I believe a few tens of meters of ice at most would slow down heat loss over the ocean areas to levels mostly accounted for by geological tectonic heat, so that the water below reaches equilibrium just above freezing temperatures... sea life could not much develop photosynthesis, being shaded from sunlight already reduced to between half to a third Terran levels by sheer distance from the Sun. But if thermal vent chemically fed species were to diversity enough for some variants to make a go of surface life, eventually photosynthesis would be developed; it has less than half the power available versus Earth but that is still a lot!

So gradually a class of plantlike genuses I call "snowcrop" (not that it could really snow much on such a world; the frozen surfaces would only produce little water via sublimation to saturate the cold air, and so little precipitation as such would ever happen) after a think in Ursula LeGuin's Planet of Exile would develop, finding niches on the icy surfaces.

If ambient temperatures remain enough below freezing, might not frozen water serve as a structural material? Per discussion of the development of lignin (defining the dawn of woody plants OTL) an advance beyond making cell walls out of ice as such might be to reinforce the ice with some sort of protein strands equivalent to spider silk or the like, making a sort of organic Pyrkrete. Could animals use ice composites as a kind of shell or bone?

Snowcrop would tend to raise the albedo, and perhaps tip the snowball planet over toward partial melting and set off runaway feedback melting all the ice. If the base of the food pyramid is life forms adapted to subfreezing temperatures, this could be quite catastrophic.

Perhaps Gaia hypothesis type feedback loops might evolve to head off such crises? Say repeated incidents of successful snowcrop lineages melting the ice shelves they are adapted to survive on result in a lineage being favored that dies off when temperatures rise to a few degrees below freezing, so the albedo lowering is checked that way and the ecosystem tends to maintain temperatures below 0 C.

I don't mean to derail this thread, just offer up examples of how many possible paths there are, and I seem to recall some discussion here to about snowball Earths.

 

[Halbeard]

A speculative evolution thread on SV. My life is complete.