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The Surprising Diversity of Crocodilians

There’s a common perception that crocodiles and alligators are fundamentally ancient creatures, unchanged since the time of the dinosaurs. They have been the same for millions of years, and they will be the same for millions more, should they survive. This seems to check out; there’s more diversity in modern crocodilians than most people recognize, but they all fill a similar role as semi-aquatic ambush predators.

However, our modern crocodilians are only a very small branch of what once was a massive group that lived in every possible environment, filling thousands of different niches. The modern branch of crocodilians is known as “Eusuchia” and only emerged in the Cretaceous, and even they have undergone many unusual variations that we will tackle later in this article. To explore the diversity of the crocs, let’s go way back to the start. Where did they come from?

Crocodilians are a part of the group of reptiles we call the archosaurs, the “ruling reptiles”. This group emerged in the wake of the Permian extinction, the greatest mass extinction in history. Nearly all life had gone extinct, leaving the world’s niches open to exploitation. In the middle of the Triassic, the archosaurs split into two branches. The first branch later became the dinosaurs and pterosaurs. The second became the crocodiles and their relatives, a group known as the Pseudosuchia (which means “false crocodiles”, despite containing the true crocodiles).

Hesperosuchus, a crocodile relative from the Late Triassic. Notice the longer back legs, allowing it to run on its hind legs. Nobu Tamura

The earliest relatives of the crocodiles were very similar to the ancestral dinosaurs of the time and very unlike any modern crocodiles. Most of them were very small animals, and strangely, many of them were bipedal, if not fully, at least part of the time, something called “facultative bipedality”. Their legs were not sprawled, like a modern crocodiles, but instead directly underneath them. These were not amphibious ambushers; they were active land animals. At the time, an entirely different group of animals took the niche now associated with the crocodilians.

Rutiodon validus, a phytosaur. Note the blowhole-looking nostrils up by the eyes. DiBgd

The phytosaurs were once considered close relatives of today’s crocodiles, but now are thought to be far more distant, outside of the ruling reptiles entirely. To the layman’s eye, they were nearly identical to a modern crocodile. They had the same general bodyplan, with an elongated snout, semi-sprawled legs, and paddle-like tails. To a layman, their only obvious difference from a modern crocodile was the placement of their nostrils. Modern crocodilians have their nostrils at the end of their snout, like most animals, while phytosaurs had their nostrils placed more like a whale’s blowhole, up near the eyes. Like modern crocodiles, they had a variety of shapes and sizes specialized in hunting different prey.

The Triassic crocodilians, in turn, also filled roles that would soon be taken by other animals. One of those was that of large, terrestrial carnivores. Multiple lineages of pseudosucihians took grew to significant sizes, and some even retained their bipedal natures and closely resembled theropod dinosaurs like the famous Tyrannosaurus and Allosaurus. Collectively, these erect-standing large-bodied crocodiles were called the rauisuchians. The rauisuchians are not a proper group; multiple different groups of crocodile produced animals that were called rauisuchians, making it a less than accurate label.

A Postosuchus skeleton, showing off their low bipedal stance. Dallas Krentzel

Some of the rauisuchians, such as Postosuchus, were the apex predators of their environments. While the phytosaurs lived in the valleys, dominating the rivers and lakes, Postosuchus was a beast of the hills. Between 13 and 16 feet long, it was the biggest predator around, capable of feeding upon the small dinosaurs of the day, alongside the varied herbivores of the environment, including several pseudosucians, because yes, many of the early relatives of crocodiles ate plants.

A bronze statue of an aetosaur at Appalachian State University. Note the armor and the broad beaky mouth. Dr. Lauren Waterworth

Convergent evolution is a process by which unrelated animals take on similar forms to fill a similar niche. The aetosaurs were a varied group of crocodile relatives that resembled the armored dinosaurs that emerged later in the Mesozoic. Like those dinosaurs, most were heavily-armored herbivores, often outfitted with long spikes near their shoulders, either for display or further protection. They were in some ways the “boars” of their environment, with many species sporting upturned snouts for digging in the dirt. Some may have also been digging for grubs, furthering the boar comparison with omnivorous tendencies.

A model of Arizonasaurus at the Museum am Lowentor in Stuttgard, Germany. Ghedoghedo

There were dozens of other small offshoots of the crocodilian family tree in the Triassic, but we’ll wrap this one up with the ctenosauriscids, a small branch of pseudosuchians who were defined by having tall neural spines that formed a sail or a hump, similar to more well-known animals like Dimetrodon and Spinosaurus. The most well-known genus among this group is Arizonasaurus, a mid-sized carnivore from a few million years before the aetosaurs and Postosuchus. Like the other crocodiles so far, it would have been thoroughly terrestrial rather than amphibious. Like the other ancient sailed animals, it’s uncertain just what Arizonasaurus would use its sail for, but it was likely for display.

Sadly, the end of the Triassic was also the end for all of these diverse crocodile lineages. An increase in volcanic activity poisoned the seas and heated up the planet up, killing off every branch of pseudosuchia but for the crocodylomorphs, the ancestors of modern crocodiles. Their old roles were quickly taken over by the dinosaurs. Their niches as top predators were taken by generations of increasingly massive theropod dinosaurs like Dilophosaurus and the later allosaurs and tyrannosaurids. Armored dinosaurs like Scelidosaurus quickly replaced the lost aetosaurs. The true age of the dinosaurs had begun, and crocodiles were pushed to the sidelines.

However, that didn’t mean that they weren’t incredibly diverse for the rest of the Mesozoic. They may not have held the variety of major roles they did in the Triassic, but they continued to diversify and claim strange niches the world over. The earliest crocodilomorphs were small, slender terrestrial creatures, represented best by Hesperosuchus above, the perfect start to radiate into other roles newly vacated by the recent mass extinction.

One of the stranger habitats the crocodiliomorphs took to was the ocean. In the modern day, crocodiles have salt glands that allow them to tolerate brackish or salty water, and the saltwater crocodile can spend nearly a month out at sea, travelling hundreds of miles at a time. If they continued to adapt further to the ocean, they might one day look quite similar to the thalattosuchia.

An old but decently accurate depiction of Metriorhynchus. Graham Rosewarne

The thalattosuchia were a diverse array of marine crocodiles, adapted to spend most if not all of their lives in the ocean. Some, like the telosaurids, were likely coastal creatures, still retaining limbs and bodies with terrestrial features. These would have been similar to a modern saltwater crocodile, spending much of their time on land. The metriorhynchoids were a branch that was entirely aquatic, with their legs turned into paddles.  They quite closely resembled the later mosasaurs, although none of them reached the immense sizes of those great aquatic lizards.

The metriorhynchoids might also have been the only crocodiles to ever give live birth, freeing them from clambering onto land to lay eggs. If this was the case, it is likely that the metriorhynchoids spent their entire lives at sea, never coming ashore.

A more modern Metriorhynchus. Dmitry Bogdanov

These fully marine crocodilomorphs were not ancestral to our modern crocodiles. Indeed, they went extinct early in the Cretaceous, leaving only terrestrial forms, which had been diversifying in their own ways. By the late Cretaceous, they again filled hundreds of different roles across the planet. Native to what is now South America were the sphaegesaurids, crocodilomorphs distinguished by unusually mammalian teeth.

An Armadillosuchus mount at a museum in Rio de Janero, Brazil.

Most well-known of these is Armadillosuchus, a mid-sized terrestrial crocodile made famous by its armor plating, an unusual elaboration on the osteoderms (bony plates within the skin, an easy term to remember if you know that oseteo=bone and derm=skin) that all crocodiles have. Also like a modern armadillo, it had elongate claws on its front limbs, likely allowing it to dig for food. What food it was digging for, its teeth might explain.

Our modern crocodiles all have homodont dentition, meaning every one of their teeth is basically the same. However, when I say the sphaegesaurids had mammal-like teeth, I mean they had heterodont dentition, meaning they had multiple types of teeth. Armadillosuchus was outfitted with a set of incisor-like front teeth, sharp fangs, and sheering teeth along their cheeks, possibly allowing an ominivorous diet.

Armadillosuchus might have fed on whatever it could dig up, whether it be roots or burrowing animals, while deflecting larger predators with its tough shell. Sadly, Armadillosuchus was gone before the end of the Cretaceous, which as most people know, was another mass extinction. While the end-Triassic extinction opened the path for dinosaurs to dominate the planet, the end of the Cretaceous nearly drove them to extinction. Their positions were now reversed, with the remaining crocodylomorphs taking dominant roles in several ecosystems in the earliest parts of the Cenozoic.

If any small mammals thought that their days of being eaten by giant reptiles were over with the end of the Mesozoic, they were wrong. With no theropods dominating the niches of terrestrial predators, multiple groups of crocodylomorph harkened back to their ancient raisuchian relatives. Most prominent among these were the planocrania, a group of killer land-crocs found all over the world.

A Pristichampsus hunts some early horses. John Sibbick

Being closer related to modern crocodilians, they were much more familiar creatures than their Triassic equivalents, looking mostly like a modern crocodile adapted to the land. They had long legs with claws blunted and widened into what were effectively hooves, like an ostrich’s claw. Their teeth were also different, outfitted with teeth more like knives, useful for inflicting deadly slashing wounds on prey, rather than the tough, prey-holding spikes that modern crocodiles have. Despite how strange these land-running crocs would have looked in person, they might have actually been closer related to crocodiles, alligators, and caimans than the bizarre gharials are.

The modern crocodilian bodytype dates back to the late Cretaceous, so animals very similar to the ones we know today lived alongside all these unusual terrestrial forms. Over the course of the Cenozoic, these modern crocodilians became more and more dominant, and after about 50 million years ago, the terrestrial crocodiles were gone, totally replaced by mammalian predators. And then, millions of years later, they came back.

As an isolated island, Australia has been free to take its own odd little evolutionary paths, such as most mammals on the continent being marsupials rather than the placental mammals that dominate the rest of the planet. The crocodilian equivalent of this was the mekosuchinae, a branch closer related to crocodiles than alligators, placing them well within the modern crocodilia. Their ancestors were rather standard as modern crocodiles go, with the earliest known member, Harpacochampsa, having a long, thin snout for catching fish. However, with little competition, several species of mekosuchids took new roles unlike any of their modern relatives.

Quinkana timara skull.

Sometime in the late Miocene (about 5 million years ago), something strange happened in Australia. Many of the dominant mammalian carnivores were cleared away, and in their wake were large terrestrial reptiles. It seems that a new wave of droughts and extended grasslands and deserts favored dry-adapted reptiles over mammals on much of the continent. Most famous of these was the giant monitor lizard known as Megalania, a relative of today’s Komodo dragons. However, there were also multiple species of mekosuchid, with the most likely terrestrial one being the genus Quinkana. Like other terrestrial crocodiles, they traded in their spike-like teeth for knife-like slashing teeth.

Unfortunately, the rest of Quinkana is rather poorly known. Enough of the jaw has been found to separate it into two species, one with a relatively broad snout and another with a narrower one, but most of their bodies are poorly known. It’s not even known just how large they are, with most sources agreeing they were about 400 pounds, a middling crocodile, while an email from a paleontologist I found promised remains that suggested an animal 26 feet long and weighing in at over a ton. I can’t find any evidence of these measurements, and I have my doubts about them for various reasons, not least that this would make them far larger than even the largest estimates of Megalania’s size. People don’t just sleep on the largest carnivorous reptile to walk the land since the end of the Cretaceous.

Quinkana and the other Australian mekosuchids survived until about 46,000 years ago at the latest, when all of Australia’s megafauna perished. Only Mekosuchus, the genus the clade was named for, survived, having spread to the nearby islands known as New Caledonia. Mekosuchus was smaller than most of the other crocodile relatives mentioned in this article, most species only reaching about three feet in length. They had a neotenic look to them as well, with stocky, short snouts and extremely large eyes.

Mekosuchus inexpectatus. Apokryltaros

Their lifestyle isn’t well known, but there’s a likelihood that they were nocturnal and, much stranger, possibly arboreal. This conjecture is based on their arm bones resembling those of modern monitor lizards that regularly climb trees, possibly suggesting a similar role. By now you are probably sick of terrestrial crocs, but tree crocs, at the least, are brand new to everybody.

The age of the strange crocodiles finally came to an end as recently as 1,000 years ago, when humans reached New Caledonia. The days of land crocodiles were over, only their amphibious relatives remaining. As the only ones to reach the modern day, they became emblematic of their kind, symbols of unchanging perfection as the stranger journeys their relatives took were forgotten. While that staying power is admirable, I hope you all remember their much wilder history from now on, and think about the galloping land crocs of the past once in a while.

Enjoyed the article? Want more prehistory? Check out the links below!

Sivatherium

Was There Ever a Kraken?

The Fossil Record of Bigfoot

Dubay’s Dinosaur Denial

Web Flotsam

One Comment

  1. Ah yes, how to make crocodiles and alligators scarier. Make them giant and let them primarily roam around on land. Good work, nature!

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