Okay now this is fascinating. We need more speculative evolution threads on SV.
Beyond the Light of Dawn: Prehistoric Life After the Silurian Gamma Ray Burst
- Thread starter Eotyrannus
- Start date
The Late Devonian: Linnorms Conquer the Land
- Location
- England. Tea and scones, wot wot!
The Late Devonian: Linnorms Conquer The Land
It is midway through the Devonian Mass Extinction. After a few million years' reprieve, another wave of death has swept over the planet- and on a warm, tropical beach off the Iowa Sea, a stranding of enormous fish marks the edge of the flood of toxic waters.
Each one is bigger than a man, with bulging eyes near their snout, each filmy-eyed animal lying motionless on their side in scattered directions. They swam up here while the tide was high, at first to escape the bitter taste of the flood of rotting sea water, and remained there as the waves' white horses kicked oxygen back into the surf. They retreated, following the tide- but when the tide went out, they settled into a small depression in the sand rather than pass the ridge into the toxic ocean beyond. By the time their mistake became obvious, the water level was already too low for them to see their escape route, and their sense of smell told them that going beyond the ridge would be worse than staying.
Various gouges in the sand and bleeding bellies tell the story of their frantic struggles. Only a single trace crosses the sandbar into the sea- the rest end with a carcass. Carnivorous myrawings are already buzzing around, laying their eggs in splits on the fish's sunburnt skin, and a number of bull crabs have exited their burrows to investigate the enticing scent of death.
As the arthropods tussle, though, there is a loud exhalation by the beach's edge.
Something serpentine is swimming in the water. A fish with a long, slender thorax and a bulging abdomen is moving in to investigate the terrestrial commotion. Like the dead, beached creatures that attracted its attention, this is a spiny shark- but it is an incredibly odd one. Its hindfins are enormous, with incredibly muscular bases for an aquatic animal, while its pectoral fins have a number of forwards-pointing spines jutting out like a eurypterid's claws. Their shape is odd, too, more like fins tipped with hooves than any conventional fin shape.
The fish exhales, releasing a small spout of mist, and then breathes in again. Then, incredibly, it swims straight for the shore to beach itself as well- and when its tail can push it no further, it pushes its belly off the floor, and begins to swagger across the surf to join the other scavengers at their feast.
Vertebrates have hit the shore.
Air-breathing Fish
For the past few million years, fish breathing air has not been unusual at all.
Despite its life-giving properties, water has a major flaw compared to air- how fast oxygen can diffuse through it. In air, molecules whizz past each other at great speed, only rarely encountering each other (at least, compared to water), letting individual molecules spread far and wide with only minor interruptions.
In fact, in the comfortably-warm room you're likely reading this in, oxygen molecules are bombarding your body at over a thousand miles per hour- and despite having collided with ten magnitudes of other molecules in a single second, has still travelled a vast distance compared to what that same molecule could manage in water.
Lungs and other air-breathing organs extract it by using haemoglobin or other proteins, which react with oxygen when it's rich and then release it in oxygen-poor tissues. This system lets them sap the oxygen from a thin film of fluid- the oxygen-deprived fluid is then used to catch new molecules of oxygen from air. This remarkable adaptation lets air-breathing organisms to tap into a nigh-infinite supply of oxygen, one that has never ran out since long before the dawn of visible life.
Many different types of spiny shark have discovered this, and have discovered a wide variety of other applications, too. The swim bladder is one- while marine fish can fill a swim bladder with their blood alone, they are actually the remnants of ancient fishy lungs, which have since drifted backwards in their body to the point where many modern fish are capable of 'singing' via expelling air from their swim bladder through the anus. Many Devonian spiny sharks now possess swim bladders, though in this harsh world they generally retain air-breathing as a secondary function.
But it is the relatives of this strange hooved fish that have truly embraced the lung for what it is- a method of breathing entirely divorced from the aquatic world.
Linnorms
The fish, by now, has reached the nearest carcass. It rears up with the front of its body to rest it atop the corpse of the giant fish. Then it begins to drag itself down the fish's length, nosing around the partially-dessicated skin and headbutting a few unfortunate arthropods out of the way as it does- before finally, it finds what it was looking for. The gills are not the most convenient orifice, but it pushes its way inside the carcass nevertheless, and once the front of its thorax is inside it swings its barbed fins forwards.
After a number of horizontal tugs, its tears out a chunk of gill filaments, and throws its head backwards to bring the food to the back of its mouth. Rather than repeat the procedure as a crocodile might, though, the animal almost delicately feeds the rest into its mouth using its forefins- there is a sound of sloshing as it pumps the muscles under its jaw a few times, and then it goes in for a serving of the more delectable meats deeper within the skull.
This may be an amphibious vertebrate, but it is a most distinctly different animal from our own ancestors. And yet, in many ways, it is remarkably similar- and though its two eyes are bulging and far-forwards, more like a frog or toad than a crocodile, the fact that they sit atop its head gives a good indication of its niche.
The animal is a linnorm- a group of aquatic and semi-aquatic fish that range in behaviour anywhere between dolphin and crocodile.
Evolving such similar lifestyles is no coincidence. While in the modern day such active, obligate air-breathing aquatic vertebrates tend to have evolved from terrestrial ancestors, the fact it's evolved so many times speaks to just how successful a lung can be underwater- in fact, if fish didn't have to contend with their land-lubbing tetrapod relatives, it's quite possible that fish like these would have evolved over and over again. In this timeline, the linnorms are the first to stake their claim- and now they've established it, it's unlikely their descendants will relinquish it.
An underwater lung's key advantage is in the lack of necessity to move any volumes of water. In the case of dolphins and deep-water linnorms, this manifests in their unmatched speed and stamina. While a tuna or shark can be equally fast or enduring, they have to make one critical tradeoff- as their bodies demand more oxygen, their gills need a larger surface area in contact with the water to supply it; and yet expanding their gills means an ever-larger surface area to drag through the water by force, a conundrum that air-breathers avoid entirely.
For crocodiles and shallow-marine linnorms, this manifests in unmatched stealth. While the scavenging linnorm may seem slow and cumbersome, in the water he is a predator with reflexes like lightning and a jaw that can snap up prey in milliseconds. The seabed is low in oxygen for most, and he uses special sensors in his snout and in a line on his flank to detect the water being forced in and out of the gills of his prey- he, on the other hand, can move in any slow and careful path he likes as long as it occasionally intersects the surface.
However, his form is distinctly unlike a crocodile. Some- such as his frontwards eyes and the lack of anything resembling digits- can be attributed to his acanthodian ancestry. But there are too many differences to be coincidence, and that can be explained by the differences in how his kind solved key barriers on the way to a life of feeding and living on land.
The first, and perhaps most obvious, is his lack of a neck. Rather than raising his head and bending down to go for food items like a salamander or mudskipper, this linnorm is behaving more like a freshwater eel- as well as his head, he lifts his pectoral region too as he probes deeper for more meat. He is also like an eel, or rather like normal fish as a whole, in that the act of opening his mouth levers his pectoral girdle forwards too- his ancestors exapted this ('to exapt' refers to using an adaptation for something completely unrelated to its original function) to help corral terrestrial prey into their mouths, and by this point his forelimbs are equally as important for feeding as they are for walking on.
But why lack a neck in the first place, when it is such an easy and useful adaptation? This can be explained rather simply- he simply doesn't have the room. His throat is a multi-purpose machine that combines multiple unrelated functions, and in doing so it has left it too heavily-built to bend any more than we could bend our own ribcages.
The animal rears back with a chunk of an enormous, coelocanth-like spinal cord. Following the chunk of meat down reveals exactly how that machinery works.
Immediately obvious- the animal lacks a tongue. The back of its mouth has much larger lips than its outwards appearance would suggest, and its throat is sealed off not by a sphincter (any purely muscular closing mechanism in the body) like our own esophagus, but by what looks to be a second pair of jaws.
After the animal fits as much as it can in its mouth, it tilts its head back- and then the rear mouth opens, flushing the oral cavity with water. This is not uncontrolled by any means, and rather than acting like a flushing toilet or draining sink, the animal's tongue is actually the flow of water itself. This is seen in the modern mudskipper, and like the mudskipper, this animal possesses fine enough control of the water he could press his lips together and use it to lick a millipede right off a sandy surface. His real tongue is more like an antenna- it is long and with a small bulge on the end, letting him taste the water without unduly interrupting its flow.
The meat disappears past these second jaws (the pharyngeal jaws, so called because they're jaws within the pharynx, also known as that little uncertain bit between what's definitely your mouth and what's definitely your throat). A few morsels don't quite make it, but the second jaws have tiny teeth of their own, letting them bite well enough to avoid having to use his hydrostatic tongue a second time.
The meat travels further down, and encounters… a third pair of jaws. Like the original fishy jaw, both pairs of pharyngeal jaws were once gills. This second pair of pharyngeal jaws is much less well-developed than the first, and mostly exists to help mitigate how much fluid is sent to the stomach. While he can regurgitate stomach acid to replenish his tongue reserves, and he does possess bile glands here to neutralise the fluid if it gets too acidic here, it's preferable to avoid both that and replacing the microbially-acclimated tongue water entirely.
However, this pair of jaws is strange- they bulge at the sides, with two orifices flanking them. Suddenly, the animal tilts its head back, and its back pair of throat jaws kisses the inside of the front- its throat bulges as its tongue fluid is forced to the side, and the two orifices open.
Then the fish inhales, in a vastly-streamlined version of how its earliest air-breathing ancestors first accidentally caught bubbles around their gills. And anybody familiar with the biology of certain much-loved aquarium fish will recognise- this animal breathes not like a lungfish, but a labyrinth fish.
Gouramis and betas are the most famous types of labyrinth fish, but what they all have in common is a complex chamber actually found inside the bone of their first gill. The structure that linnorms possess is not quite the same- the orifices at the second pharyngeal jaw actually lead to a sort of bellows between that jaw's bones and a structure that was once the fourth set of gills- but ultimately, it's the closest known comparison available to the strange-looking lungs of a linnorm.
The linnorms are just the first radiation of a group called the edurocollans- though nowhere near as snappy and self-explanatory as the 'four-footed' tetrapods, the 'rigid necks' are a group taking its first, tentative steps into conquering the world beyond the waves. And though they might lack tongues, they might lack necks, and they even lack enough limb segments to give themselves more than a shoddy hoof-like imitation of feet, the linnorms have a set of incredible adaptations and the immense power of the evolutionary process behind them.
With a full belly, the scavenger slips back into the toxic ocean waves- unaware of what his kind will achieve in eras hence. But as he does, he leaves a trace of his legacy- a trackway.
A glimpse into what the future holds for life beyond the sea.
It is midway through the Devonian Mass Extinction. After a few million years' reprieve, another wave of death has swept over the planet- and on a warm, tropical beach off the Iowa Sea, a stranding of enormous fish marks the edge of the flood of toxic waters.
Each one is bigger than a man, with bulging eyes near their snout, each filmy-eyed animal lying motionless on their side in scattered directions. They swam up here while the tide was high, at first to escape the bitter taste of the flood of rotting sea water, and remained there as the waves' white horses kicked oxygen back into the surf. They retreated, following the tide- but when the tide went out, they settled into a small depression in the sand rather than pass the ridge into the toxic ocean beyond. By the time their mistake became obvious, the water level was already too low for them to see their escape route, and their sense of smell told them that going beyond the ridge would be worse than staying.
Various gouges in the sand and bleeding bellies tell the story of their frantic struggles. Only a single trace crosses the sandbar into the sea- the rest end with a carcass. Carnivorous myrawings are already buzzing around, laying their eggs in splits on the fish's sunburnt skin, and a number of bull crabs have exited their burrows to investigate the enticing scent of death.
As the arthropods tussle, though, there is a loud exhalation by the beach's edge.
Something serpentine is swimming in the water. A fish with a long, slender thorax and a bulging abdomen is moving in to investigate the terrestrial commotion. Like the dead, beached creatures that attracted its attention, this is a spiny shark- but it is an incredibly odd one. Its hindfins are enormous, with incredibly muscular bases for an aquatic animal, while its pectoral fins have a number of forwards-pointing spines jutting out like a eurypterid's claws. Their shape is odd, too, more like fins tipped with hooves than any conventional fin shape.
The fish exhales, releasing a small spout of mist, and then breathes in again. Then, incredibly, it swims straight for the shore to beach itself as well- and when its tail can push it no further, it pushes its belly off the floor, and begins to swagger across the surf to join the other scavengers at their feast.
Vertebrates have hit the shore.
Air-breathing Fish
For the past few million years, fish breathing air has not been unusual at all.
Despite its life-giving properties, water has a major flaw compared to air- how fast oxygen can diffuse through it. In air, molecules whizz past each other at great speed, only rarely encountering each other (at least, compared to water), letting individual molecules spread far and wide with only minor interruptions.
In fact, in the comfortably-warm room you're likely reading this in, oxygen molecules are bombarding your body at over a thousand miles per hour- and despite having collided with ten magnitudes of other molecules in a single second, has still travelled a vast distance compared to what that same molecule could manage in water.
Lungs and other air-breathing organs extract it by using haemoglobin or other proteins, which react with oxygen when it's rich and then release it in oxygen-poor tissues. This system lets them sap the oxygen from a thin film of fluid- the oxygen-deprived fluid is then used to catch new molecules of oxygen from air. This remarkable adaptation lets air-breathing organisms to tap into a nigh-infinite supply of oxygen, one that has never ran out since long before the dawn of visible life.
Many different types of spiny shark have discovered this, and have discovered a wide variety of other applications, too. The swim bladder is one- while marine fish can fill a swim bladder with their blood alone, they are actually the remnants of ancient fishy lungs, which have since drifted backwards in their body to the point where many modern fish are capable of 'singing' via expelling air from their swim bladder through the anus. Many Devonian spiny sharks now possess swim bladders, though in this harsh world they generally retain air-breathing as a secondary function.
But it is the relatives of this strange hooved fish that have truly embraced the lung for what it is- a method of breathing entirely divorced from the aquatic world.
Linnorms
The fish, by now, has reached the nearest carcass. It rears up with the front of its body to rest it atop the corpse of the giant fish. Then it begins to drag itself down the fish's length, nosing around the partially-dessicated skin and headbutting a few unfortunate arthropods out of the way as it does- before finally, it finds what it was looking for. The gills are not the most convenient orifice, but it pushes its way inside the carcass nevertheless, and once the front of its thorax is inside it swings its barbed fins forwards.
After a number of horizontal tugs, its tears out a chunk of gill filaments, and throws its head backwards to bring the food to the back of its mouth. Rather than repeat the procedure as a crocodile might, though, the animal almost delicately feeds the rest into its mouth using its forefins- there is a sound of sloshing as it pumps the muscles under its jaw a few times, and then it goes in for a serving of the more delectable meats deeper within the skull.
This may be an amphibious vertebrate, but it is a most distinctly different animal from our own ancestors. And yet, in many ways, it is remarkably similar- and though its two eyes are bulging and far-forwards, more like a frog or toad than a crocodile, the fact that they sit atop its head gives a good indication of its niche.
The animal is a linnorm- a group of aquatic and semi-aquatic fish that range in behaviour anywhere between dolphin and crocodile.
Evolving such similar lifestyles is no coincidence. While in the modern day such active, obligate air-breathing aquatic vertebrates tend to have evolved from terrestrial ancestors, the fact it's evolved so many times speaks to just how successful a lung can be underwater- in fact, if fish didn't have to contend with their land-lubbing tetrapod relatives, it's quite possible that fish like these would have evolved over and over again. In this timeline, the linnorms are the first to stake their claim- and now they've established it, it's unlikely their descendants will relinquish it.
An underwater lung's key advantage is in the lack of necessity to move any volumes of water. In the case of dolphins and deep-water linnorms, this manifests in their unmatched speed and stamina. While a tuna or shark can be equally fast or enduring, they have to make one critical tradeoff- as their bodies demand more oxygen, their gills need a larger surface area in contact with the water to supply it; and yet expanding their gills means an ever-larger surface area to drag through the water by force, a conundrum that air-breathers avoid entirely.
For crocodiles and shallow-marine linnorms, this manifests in unmatched stealth. While the scavenging linnorm may seem slow and cumbersome, in the water he is a predator with reflexes like lightning and a jaw that can snap up prey in milliseconds. The seabed is low in oxygen for most, and he uses special sensors in his snout and in a line on his flank to detect the water being forced in and out of the gills of his prey- he, on the other hand, can move in any slow and careful path he likes as long as it occasionally intersects the surface.
However, his form is distinctly unlike a crocodile. Some- such as his frontwards eyes and the lack of anything resembling digits- can be attributed to his acanthodian ancestry. But there are too many differences to be coincidence, and that can be explained by the differences in how his kind solved key barriers on the way to a life of feeding and living on land.
The first, and perhaps most obvious, is his lack of a neck. Rather than raising his head and bending down to go for food items like a salamander or mudskipper, this linnorm is behaving more like a freshwater eel- as well as his head, he lifts his pectoral region too as he probes deeper for more meat. He is also like an eel, or rather like normal fish as a whole, in that the act of opening his mouth levers his pectoral girdle forwards too- his ancestors exapted this ('to exapt' refers to using an adaptation for something completely unrelated to its original function) to help corral terrestrial prey into their mouths, and by this point his forelimbs are equally as important for feeding as they are for walking on.
But why lack a neck in the first place, when it is such an easy and useful adaptation? This can be explained rather simply- he simply doesn't have the room. His throat is a multi-purpose machine that combines multiple unrelated functions, and in doing so it has left it too heavily-built to bend any more than we could bend our own ribcages.
The animal rears back with a chunk of an enormous, coelocanth-like spinal cord. Following the chunk of meat down reveals exactly how that machinery works.
Immediately obvious- the animal lacks a tongue. The back of its mouth has much larger lips than its outwards appearance would suggest, and its throat is sealed off not by a sphincter (any purely muscular closing mechanism in the body) like our own esophagus, but by what looks to be a second pair of jaws.
After the animal fits as much as it can in its mouth, it tilts its head back- and then the rear mouth opens, flushing the oral cavity with water. This is not uncontrolled by any means, and rather than acting like a flushing toilet or draining sink, the animal's tongue is actually the flow of water itself. This is seen in the modern mudskipper, and like the mudskipper, this animal possesses fine enough control of the water he could press his lips together and use it to lick a millipede right off a sandy surface. His real tongue is more like an antenna- it is long and with a small bulge on the end, letting him taste the water without unduly interrupting its flow.
The meat disappears past these second jaws (the pharyngeal jaws, so called because they're jaws within the pharynx, also known as that little uncertain bit between what's definitely your mouth and what's definitely your throat). A few morsels don't quite make it, but the second jaws have tiny teeth of their own, letting them bite well enough to avoid having to use his hydrostatic tongue a second time.
The meat travels further down, and encounters… a third pair of jaws. Like the original fishy jaw, both pairs of pharyngeal jaws were once gills. This second pair of pharyngeal jaws is much less well-developed than the first, and mostly exists to help mitigate how much fluid is sent to the stomach. While he can regurgitate stomach acid to replenish his tongue reserves, and he does possess bile glands here to neutralise the fluid if it gets too acidic here, it's preferable to avoid both that and replacing the microbially-acclimated tongue water entirely.
However, this pair of jaws is strange- they bulge at the sides, with two orifices flanking them. Suddenly, the animal tilts its head back, and its back pair of throat jaws kisses the inside of the front- its throat bulges as its tongue fluid is forced to the side, and the two orifices open.
Then the fish inhales, in a vastly-streamlined version of how its earliest air-breathing ancestors first accidentally caught bubbles around their gills. And anybody familiar with the biology of certain much-loved aquarium fish will recognise- this animal breathes not like a lungfish, but a labyrinth fish.
Gouramis and betas are the most famous types of labyrinth fish, but what they all have in common is a complex chamber actually found inside the bone of their first gill. The structure that linnorms possess is not quite the same- the orifices at the second pharyngeal jaw actually lead to a sort of bellows between that jaw's bones and a structure that was once the fourth set of gills- but ultimately, it's the closest known comparison available to the strange-looking lungs of a linnorm.
The linnorms are just the first radiation of a group called the edurocollans- though nowhere near as snappy and self-explanatory as the 'four-footed' tetrapods, the 'rigid necks' are a group taking its first, tentative steps into conquering the world beyond the waves. And though they might lack tongues, they might lack necks, and they even lack enough limb segments to give themselves more than a shoddy hoof-like imitation of feet, the linnorms have a set of incredible adaptations and the immense power of the evolutionary process behind them.
With a full belly, the scavenger slips back into the toxic ocean waves- unaware of what his kind will achieve in eras hence. But as he does, he leaves a trace of his legacy- a trackway.
A glimpse into what the future holds for life beyond the sea.
Last edited:
Baron Steakpuncher
Hopeful Idealist
- Location
- Australia
A primitive but efficient start to land vertebrates.
But with so many disadvantages (the limbs in particular) I do not think his kind shall be the masters of the land.
But with so many disadvantages (the limbs in particular) I do not think his kind shall be the masters of the land.
- Location
- Illinois
Hey! Speculative Evolution is moving to this forum, awesome! I haven't fully read this yet but I think I recognize your username from Speculative Evolution Forums?
I'm actually pretty happy you started this because I've been sitting on a few SpecEvo projects for--as the kids say--a long ass time. I definitely will get to reading this once I get a good sitting to do so!
I'm actually pretty happy you started this because I've been sitting on a few SpecEvo projects for--as the kids say--a long ass time. I definitely will get to reading this once I get a good sitting to do so!
Halbeard
Ship ALL the things!
- Location
- USA
To be fair, they do have a place where they perform quite well; they might be most successful staying right where they are, and maybe they'll find an advantageous spot in fresher waters as well. Whether they'll be able to adapt when something else rolls around that does the whole semiaquatic thing better, that's a different story, but that's how it goes. Not that I expect them to be short-lived, but I don't see them taking the place of tetrapods either.
- Location
- The great frozen north
This thread is a bit hard to follow without much visual aid, but still Speculative Evolution is really interesting! It might be a while before I catch up, but this is a unique thread!
- Location
- England. Tea and scones, wot wot!
I drew a pretty picture. It is now in the Linnorm page.
Redditredcoat
American Loyalist
- Location
- Long Island
ooooo interesting premise, watched.