Paleo Profile: Everybody Walk the Archosaur! - Poposaurus · 11:07pm May 4th, 2018
The Late Triassic, over 225 million years into the past. The Great Dying that prematurely ended the reign of the mammal ancestors and nearly destroyed all multicellular life on the planet was dim memory upon the landscape which it ravaged 25 million years earlier. The air was thick despite the lower oxygen levels than the present, still dense with humid moisture spurred by the hotter morning sun.
New creatures called Dinosaurs and Pterosaurs are getting their start, and have been quite successful for the past few million years, but they are far from alone. A lone, bipedal form calmly walks through the steamy, misty woodlands bordering the river. Careful to watch and keep alert in case the local apex Rauisuchian predator, Heptasuchus were to show up as well as being wary of the larger herbivore herds of mammal-like Dicynodonts moving through, the tall figure strides up to the river shallows and paces down them.
It could hunt, but an easier source presents itself to its powerful sense of smell. A fresh carcass, probably from one of the tusked Dicynodonts who perished of age or illness. If the latter, nothing the lurker’s stomach couldn’t handle. Several early Theropods were already picking at the carcass, but they soon cleared back when the form clad in thick, shiny scales and a boxy head emerged from the brushline and muttered a faint rumble from deep inside its throat. A reptilian hiss seeped out from tall, fanged jaws as the carcass was claimed. There was enough to go around, but there was a pecking order in the Triassic. And dinosaurs were not at the top of it.
In the Southwestern USA, the animal likely on pecking order was something that looked just like a dinosaur, and yet was born from their cousins. Walks like a dinosaur, looks like a dinosaur, but that's a croc!
Strap yourselves in folks, time to learn about the crazy time called the Triassic were evolution for one family of animals was on steroids!
Profile
Species: Poposaurus (Poe-Poe-saur-us) gracilis
Name Meaning: “Popo (formation) reptile”
Clade: Suchia, paracrocodylomorphia
Location: North America, USA southwest
Time: Late Triassic period 237-201(?) MYA
Height: 1 meter at the hips, 1.5 meters reared up
Length: Just under 4 meters
Weight: 90-100 kilograms
Habitat: forests and scrublands bordering rivers and lakes
Notable features: Powerful legs with potentially adaptive stance, long tail, sharp teeth and jaws
Poposaurus is a crocodilian that didn’t necessarily look nor behave the part we think of today. In essence, this is what happens when you build a Theropod Dinosaur out of a crocodile.
Modern crocodilians have a long, flat, broad skull filled with peg shaped teeth. Poposaurus, based off fossils from close relatives and the fragments available, had a shorter snout, taller and less broad skull, and serrated, backwards pointing teeth called ziphodont teeth similar to those in carnivorous dinosaurs.
Modern crocodilians chiefly live in the water, Poposaurus was almost certainly primarily terrestrial.
Modern crocodilians have short legs and don’t pursue prey for more than a few seconds, Poposaurus had very long, well muscled legs and a springy step that almost certainly could obtain a higher speed with less energy.
Poposaurus was also living in a dryer environment than one might think crocodilians would live in. While there were numerous water sources, as indicated by the fossils of large amphibians found nearby, Poposaurus’ build with a less broad tail and long limbs would not bode especially well for an aquatic existence.It is unknown if the feet were webbed like modern crocodilians, as this is soft tissue that wasn't preserved in the fossils. However given the species was very likely entirely or at the very least chiefly terrestrial, this is unlikely. Still its tail was quite long and powerful, so it was at the least a reasonable swimmer even if it was thinner than in modern crocodilians. Thus it might not be unreasonable to think that, like some monitor lizards, Poposaurus might cross passages of water from time to time.
However, the very long hindlimbs and well-developed hips make it clear the species was a land-based predator. It might be odd to think of a land-based crocodile, but it is important to remember the group was far more diverse than it is now. In fact, unless you knew where to look on the internal skeletons if you traveled back to the Triassic, you would probably have a very hard time telling the dinosaurs apart from the crocodiles.
One such way is the hips. Both Poposaurus, its relatives, and dinosaurs all have erect legs situated directly under the body in a straight path without the sprawling gait lizards and most other reptiles have. This is the “push-up” pose one might see modern lizards in.
However, how they achieved this differs slightly. Dinosaurs and mammals both have an offset femur head. What this means is the round connection point that fits the femur into the hip bones’ sockets are off to the side of the bone with the legs slightly off to the sides of said hips. In Poposaurus and relation, the femur “head” is instead directly on top of the femur and the connection between the femur and the hips is set directly below the body with the hips stretched out to the sides. This allows the legs to be held straight directly below the hips.
This diagnostic trait is one of the major ones that finally allowed paleontologists to tell that Poposaurus, despite having extremely close similarities in its skull, arms, joints, and overall skeletal structure to Dinosaurs, was in fact not one. Both groups seem to have independently evolved an erect gait style of walking, which might have derived from the “High-Walk” modern crocodilians can do with their shorter legs as a sort of marginal erect-limbed walk that is not at all the sprawling gait lizards have. The high-walk is a stance modern Crocodilians adopt while moving on land, which straightens the legs out and helps them support their way to move more efficiently. It or something very much like it was probably the locomotion form most early Archosaurs used.
Still, Poposaurus was so extremely dinosaur-like it was completely mistaken for one for years upon years until the error was finally cleared up and was placed with its own group nested near the ancestors of modern crocodiles, gharials, caimans, and alligators.
Archosaurs as a group are a distinct family of reptiles or reptile derivatives that include Pterosaurs, Dinosaurs (and birds), Crocodilians, and relation. Yes, if you took a DNA test of an alligator, a monitor lizard, and a chicken; the chicken and the alligator would both come up as closer genetic matches and relatives to each other than either would to lizards or any other animal group. Just because a crocodile looks like a lizard, doesn’t mean it actually is a lizard.
Outside of birds, Crocodilians are the only surviving Archosaurs and the closest cousins to the dinosaurs. Archosaurs as a family all share many features across all their members. Thus Crocodilians share numerous features unique to them with both dinosaurs and their feathery descendents. Four-chambered hearts, advanced circulatory and respiratory systems, much more advanced brains, erect limbs that don't sprawl like a lizard's, and avian-styled scales different from those found in other reptiles.
But importantly, Poposaurus and it’s kind help dissuade an old labeling of Archosaurs. It used to be stated that Pterosaurs and Dinosaurs are “Advanced Archosaurs” and Crocodilians with their relation are “Archaic/Primitive Archosaurs”. There were definite differences between the two, but it was judged that these differences amounted to one group being out right more advanced and implied “better” than the other. But numerous discoveries about Poposaurus, its relatives and biology demonstrate many traits that were once thought to be exclusive to so-called “Advanced Archosaurs”, were fact well distributed across the whole family. In essence, no Archosaurs were truly distinctly more advanced than the others; just that they advanced in different means.
Even the often prized trait of Endothermy or “Warm Bloodedness” now seems to have found a home in Crocodilians and their relation, including Poposaurus. It might be the group independently evolved it and then subsequently modern crocodilians traded it for Ectothermy (“Cold Bloodedness”) because that trait was more advantageous to the situation they found themselves in; or, that Endothermy was ancestral to the last common ancestor between Dinosaurs, Crocodilians, and Pterosaurs. Exact data and information is lacking at the time.
So why does Poposaurus look and act so much like a dinosaur? Well, again, take into account the family and times.
Remember, the Triassic was the first period Archosaurs had their prominence after potentially emerging in the Permian. The group did emerge in the Permian, but they were largely fairly small, rare, and not very diverse. Go back to right before the Permian mass extinction that kicked of the Triassic and pretty much all Archosaurs looked much like this Protorosaurus.
It is from a small, generalist like this that everything from mockingbirds to alligators to Supersaurus to Pteranodon came from. The Permian mass extinction was simply devastating and I'd love to get into it later, but the important thing for our little Archosaurs is the massive proto-mammals who had gobbled up almost all the diverse ecological niches from sabretoothed carnivores to browsing herbivores were all gone. Almost no land animals bigger than a house cat survived to the Triassic from the Permian, so it was basically back to square-one for everyone and the race to become megafauna was on. The Therapsid proto-mammals did survive, but when they started to grow larger again they didn't diversify all that much. The Archosaurs meanwhile, able to grow in tandem, tried out every body type and niche under the sun! It is in the Triassic you find the first Pterosaurs, Dinosaurs, modern Crocodilians, and a slew of relation that could look like everything from a shell-less turtle with shoulder spikes and teeth with Desmatosuchus-
-to non-Crocodilian Crocodilian mimics with seemingly bent noses like Proterosuchus-
-to this four legged T.rex-look alike called Fasolasuchus!
With this rapid diversity, Archosaurs swept the board and gradually replaced the Therapsids. But even with this huge divergence in forms, especially after the Triassic, relation can be determined.
And in general, the further back you go on the evolutionary trees of two related animals that don't look alike, the more those two will resemble each other. This process can be observed in living animals. The order Carnivora includes animals in two main branches, Caniformia or "Canine Forms" (Canines, bears, weasels, seals, sea lions, raccoons, and relation) and Feliformia or "Feline Forms" (Felines, mongooses, civets, hyenas, and relation). If you look at them in the present day they look markedly different and are not fertile with one another.
However if you track back in the family trees of both branches you wind up with something that looks a lot like the modern day genet. Now genets aren't the direct ancestors of all Carnivora, but they are a member that hasn't ended up needing to change all that much from the original Carnivora who was the great-great-grandma of them all. This is thus why when you look at a genet, you see something that looks like a raccoon, fox, and cat thrown into a blender. Because the animals the genet was derived from and didn't change much in form from, was the ancestor of raccoons, foxes, and cats (and more)!
Different branches diversify in different ways, this is how evolution works as different populations will go under different pressures and adapt differently. The same happened with the split between Dinosaurs, Pterosaurs, and Crocodilians. It you traveled back and saw the first Crocodilian, Pterosaur, and Dinosaur, they'd all look extremely similar because a couple dozen generations back they were the same.
It's also important to understand we can have ancient Crocodilians looking so relatively weird by modern standards because early true Crocodilians didn’t look like they do now and they experimented with many forms and niches before arriving at the final state we see today. Some were terrestrial vegetarians with beaks, some strut around with armadillo-like shells, and some experimented with a variety of terrestrial predatory forms both before and after the Triassic. So having an early Crocodilian very similar to a dinosaur to the point it was mistaken for one is hardly surprising as that and the inverse happened quite a lot.
Modern Crocodilians are very much derived from their ancient kin. It is incorrect to say Crocodilians (Suchia) haven’t changed since they first evolved. Over time, compiling traits unique to them that made them superficially distinct from both their predecessors and kin resulted in how Crocodilians look and act today. And with 200+ million years separating birds from Crocodilians, it's only natural those 200+ million years resulted in gradually increasing numbers of differences even if they are relatives.
But if you traveled back to the beginning of the Triassic, during the competition for dominion of the planet between the Archosaurs and mammalian Therapsids, the boundary lines of the Archosaurs family begin to blur. It certainly fooled scientists as Poposaurus was misclassified as a dinosaur multiple times and then bounced around much of the Archosaurs family before being tested near the next branch over from the ancestor of true Crocodilians. The Triassic was a time some crocodiles looked like dinosaurs and other miscellaneous Archosaurs looks more like crocodiles today then their ancient ancestors did.
Poposaurus was chiefly or entirely bipedal, had a floating tail for balance that didn't drag on the ground, walked either entirely on its toes or was capable of running digigrated to add more ‘spring’ into the leg movement, very likely had a tall skull instead of a flat one based off skull fragments and close relatives, and sported serrated teeth better suited for slicing than the cone shaped teeth modern crocodiles have for crushing and holding. It also has a good chance of even being at least partially endothermic or warm blooded, if not entirely so. Distinguishing body features aside, Poposaurus and its relation were functionally near identical to early Theropods. It’s one of those odd cases were convergent evolution and actual relation granting similar features converge.
Convergent evolution is when two animals of different families evolving a similar body type or feature independently of each other, regardless of if they are related much at all or not. A good cases is the Thylacines resembling placental canines, even though the Thylacine is a marsupial related to quolls and the Tasmanian Devil whereas canines are placental mammals related to bears, raccoons, and seals. However because of environmental conditions favoring the right circumstances, both ended up with remarkably similar skulls.
Poposaurus’ resemblance to Theropods could thusly be a case of convergent evolution occuring and acting on still-shared traits between the major Archosaur families.
Heck, given it would lack some birdlike behaviors Theropods had and sported crocodile-like, thick scales, Poposaurus looks more like some movie dinosaurs than said movie species’ real life counterparts did.
And this certainly was a successful body type. Lean enough to be agile and out-maneuver or dodge most of the larger crocodile relatives of the time as well as large, aggressive herbivores while chasing down more agile prey. Simultaneously it was also large and strong enough to able to out-muscle most of the Theropods or mammalians at the time away from kills to scavenge from, cover ground, or bring down bigger game. If one were to compare it to a modern predator, I feel a leopard would be a good counterpart in at least an ecological niche rather than behaviorally. Leopards are big, but not the biggest predators in their range. They're fast, but not the fastest predators in their range.
But a leopard can easily intimidate or outfight a hyena or wolf and outrun or avoid a lion or tiger, while scavenging or hunting everything from large game to small prey. Jack of all trades generalist.
Poposaurus was a member of the likewise called, Poposauroidea branch of the Suchia family tree; because as with many taxonomic names the first well described member becomes the name of the group. The whole of Poposauroidea was extremely diverse, ranging from robust, toothless quadrupedal herbivores, some of which even had sails or raised humps on their backs such as Lotosaurus
Other were bipedal, beaked herbivores like Sillosuchus
There were quadrupedal, likely piscivorous predators that looked like a mash-up of an otter and Spinosaurid like Qianosuchus.
And then, of course, the bipedal, sharp toothed carnivores like Poposaurus. Given they showed just as much variation as early dinosaurs, many having comparable forms, and arose at the same time with such a degree of resemblance to the point many were mistaken for dinosaurs; I sometimes jokingly call the group “Pseudo-Dinosaurs”. However it’s important to acknowledge them as a successful and ever-growing group which capitalized many niches. These weren’t dinosaur imposters or the poor-man’s version, but competent competitors who might even predate the dinosaurs by a narrow degree and were quite successful through the Triassic.
This group which arose at the very start of the Triassic saw it through to the end of the period, disappearing in the Triassic-Jurassic extinction event that claimed many Archosaur groups to the point the only ones to survive in good numbers were Pterosaurs, Dinosaurs, and the ancestors of modern Crocodilians. Exactly why the Poposauroidea and Poposaurus itself died out is still a matter of vagueness and speculation. Triassic period fossils are much more poorly understood than the other two Mesozoic periods and non-Dinosaurs or mammals get little notice. It might have been mass ecological changes just made the group unable to keep up. It might have been competition with newer forms that applied stress to their populations. It might have been one or more of a number of mass climate shifts, which are known from the end of the period, that hit the regions the group lived in especially hard with no safe havens other animals might have had.
For whatever reason, the Poposauroidea doesn’t seem to persist into the Jurassic Period, where their dinosaur look-alikes and cousins really hit their stride.
Still, to fancy a bit of speculation; it always is fascinating to think what might have happened in situations like these. What might have happened if it was the dinosaurs who perished in the Triassic and not their Poposauroid kin? What might have happened if both groups persisted? We do know they did cohabitate, so might a similar situation arise as the one that befell South America after the Cretaceous extinction? On that once island-continent, both placental mammals and the twin kin of marsupials and their Sparassodonta cousins flourished; however in largely different roles. Placentals were primarily omnivores and herbivores, where is the other two groups became their predators.
Could have something similar occurred if the Poposauroids survived? Maybe some ecosystems or continents might have Dinosaurs in one role and Poposauroids in another? Or might one group outcompete the other almost entirely on one continent and we get the equivalent to Australia in the pre-human contact past, where marsupials largely took all the major niches with placentals relegated to less numerous and diverse roles? Or might one group inevitably outcompete the other and either the Dinosaurs or Poposauroids would be doomed to extinction in the Jurassic anyways after surviving the previous mass extinction event?
For whatever reason to their extinction, Poposauroids to stand as a remarkable chapter in Archosaur evolution. Where, for a time, there was something that looked like a dinosaur, acted like a dinosaur, lived like a dinosaur, and had dozens of scientists convinced it was a dinosaur; but was actually a crocodile.
Another excellent lesson.
Very thought-provoking!
Thanks for the profile, Tarb. Really brightens my day to see updates on the Bridge, one of your film/book reviews, or one of your paleontology blogs.
Quick question, have you ever used what you wrote here for your paleontology papers or did you used the info from your paleontology papers to make this blog? The way you wrote this blog really makes me think that I’m looking at a research paper for paleontology, so I feel curious about it.
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My pleasure to supply a smile to a friend :)
And no papers of my own no, but I have used actual papers and literature for my sources, either directly or using their ciatation to find such. So for instance I'd use something like this work cited page.
https://www.realskeptic.com/2011/11/29/short-essay-the-evolution-of-endothermy/
To end up on an actual research paper like this
https://www.researchgate.net/publication/229181586_The_Bipedal_Stem_Crocodilian_Poposaurus_gracilis_Inferring_Function_in_Fossils_and_Innovation_in_Archosaur_Locomotion
First off an excellent choice of the opening song my friend, truly a classic. Furthermore, as someone who adores crocodiles, currently attempting to work with them, I love how strange and different prehistoric crocodilians and archosaurs got. Until you brought it up I didn't even know this guy existed but now he might be one of my favorites!
Yet again, a well researched and well written Paleo Profile. Do keep up the good work.
I've noticed a trend, that the greatest jumps in diversity follow the most dire of mass extinctions. This is hardly surprising, as the new forms diversify to replace the lost species, but as this blog post points out even "lesser" extinction events can allow groups to diversify in amazing ways.
That said, I do wonder.... is there a definitive cause for the Triassic-Jurassic extinction, or were there multiple issues like with the earlier PT "Great Die Off"? Also, how bad exactly was this extinction event? You mentioned that no animal larger then a house cat survived the PT event, and I've read similar things about the later CT event, but how bad was the TJ extinction, and was there a similar event between the Jurassic and Cretaceous periods, or was there instead a more gradual shift in populations and environment similar to the shifts that occur during the periods?
I know my focus on wider scale, global shifts may seem odd as a response to the profile of a single species, but I've always felt every species is the product of their environment, and to understand a species you must understand their environment and how changes in one affect the other. Thus my interest in the environment and how it changes. Since this is the first time you have looked at a Triassic species, it seemed the best time to ask about how the world changed during the Mesozoic as a whole.
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Well one thing to keep in mind with the Mesozoic is that nothing was consistent just given the sheer enormity of the timeframe involved. The Triassic Period or Jurassic Period alone were almost as long as the entire Cenozoic era and the Cretaceous Period was almost 20 million years longer than the entire Cenozoic. One area that might be desert climate in one geologic age might be swamps and then rain forest in the next ages. Every environment you could imagine today did exist at points in the past and even the modern age, the Holocene, has seen drastic changes without human intervention. Many iconic climates and ecosystems we see today such as the Greenland ice sheet, Amazon rain forest, Sahara desert, and Australian outback would be virtually unrecognizable 1 million years ago, let alone 60 million years ago. Nothing was really consistent in the Mesozoic other than continent movement. One period for instance might have lower oxygen levels, another might have higher. Global temperature was higher on average, but not significantly in the some times and most climates were very comparable to existing ones found on the same latitudes.
The Triassic-Jurassic extinction event is very poorly understood, though the process was likely abrupt only geologically speaking. It would still be gradual and since it probably took thousands of years to happen, just like Cretaceous extinction (which took even longer), it was just a relatively quick in geologic terms. As for the cause it was likely a combination of factors revolving around rapid drying of many climates. Isotope changes suggests an abrupt, rapid expansion of CO2 content in the atmosphere alongside other gases that might have caused a runaway greenhouse effect. This might have been spurred by factors like a mass volcanic activity and volcanic gas traps being unleashed, possibly be shifting tectonic plates (as this was right before Pangaea broke up) or from meteor strikes; with the end result being massive spikes in CO2, SO2, and aerosols. This spike in turn could cause a rapid fire heating and cooling effect until the world stabilized.
Gotta agree with Moongaze, these Paleo Profiles are simply a delight to read. So much fun, and so educational as well. Heck, I wasn't even aware of poposaurus and its Triassic kin before reading this, and now all of a sudden they're a major, compelling "what-if" for ancient fauna in general.
...Also, that Fasolasuchus looks crazily cool. I never knew I wanted a quadruped T. Rexodile, but I do now. '3'
I've found the Triassic era to be a period of time that fascinates me the most out of all of them, with the Cretaceous and Jurassic following suit. In fact, my favorite segment of the documentary, "Walking with Dinosaurs" was "New Blood" set in the Triassic, showing the birth of a new dynasty that was to follow. It also made me realize the sheer variety of life that helped create the species we know today, and if things were different, how vastly varied life on earth might have been. It makes our mere existences, which were pretty much up to chance, all the more AMAZING.
Though, imagine if the Poposaurus kept it's bipedal form and continued to evolve today: A quick crocadilian that could both hunt in the water, AND sprint on TWO legs to chase down prey.........the mind boggles.
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Since you've mentioned so many of these extinction events across multiple epochs, it makes me wonder if we as a species may subvert this trope HOPEFULLY in the future of our kind. Perhaps I'm speaking more of a cultural standpoint rather than a paleontological one, but considering how much of an influence we've had on this planet, in the arts, the sciences, and our cultures, I would hate to see all that just blown away like dust in an extinction event. The Roman Empire may have died, but at least it's legacy still lives on and we have a pretty good idea what it was like at least, but hopefully another extinction event won't wipe out nearly everything that makes our culture understandable to the next one, if there is another species that survives us.
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Well now, this is a thing to consider: One million years ago, our ancestors were not human. One million years from now, I doubt any decedents of ours will still be human. At some point, our species will go extinct.
Will our history endure? Perhaps. But how well? The history of Babylon has been shrouded by the passage of time, and that's only a few thousand years ago. Rome rose less then 3000 years ago, and fell around 1500 years ago, and already many records of that time have been lost.
I expect thousands of years from now the people of this time will be known for worshiping the mysterious Big Mac; why else would there be so many remaining plastic tablets with that name upon it? (Once upon a time, Big Macs were served in Styrofoam containers. Styrofoam does not biodegrade. Ten thousand years from now, intact Big Mac containers will still be in old land fills, waiting for future archeologists to dig up. So, yes, this is a possible conclusion.)
Something to think about, yes?
And in an alternate evolutionary timeline, this fella and their kin probably would've given rise to a group known as... THE RETROSAURS!!
But seriously, I've been meaning to recommend the stuff of fellow monster enthusiast & writer William "TyrantisTerror" Cope (apparently a descendent of well-known paleontologist Edward Drinker Cope) to you for quite some time, and I figured the ending of this article gave me the perfect excuse to do so. The man's even working on publishing his own original kaiju story all about those previously mentioned Retrosaurs, "the Atomic Time of Monsters."