Crocodiles are very TV-genic – scores of documentaries have focused on them, and most start off with a variant of the following: “Crocodilians are some of the most successful predators on the planet. They survived the dinosaurs, and have remained unchanged for millions of years.”
Today I wanted to write about why that’s not entirely true, and why crocodilian evolution, far from monotonous, is instead dynamic and, like crocodilians themselves, full of surprises.
There are only 23 species of non-extinct crocodilians, compared to 327 turtles and tortoises and more than 9,000 lizards and snakes. The fossil record, however, is full of crocodile relatives, the crocodylomorphs, dating back to at least 225 million years ago (Bronzati et al. 2012; Russel and Wu 1998).
The first crocodylomorphs looked nothing like the crocodiles alive today. Instead they were small, nimble creatures that ran on four long legs. Their sharp teeth reveal a predatory lifestyle, but instead of waiting in ambush at the water’s edge, they were adapted for wandering about over land in search of prey.
These animals, the sphenosuchians, appeared during the Late Triassic period when dinosaurs were just beginning to evolve. With legs pointed down under the body (rather than splayed to the sides), the sphenosuchians were probably terrific at chasing down prey. One such reptile, Hesperosuchus, was about the size of a domestic dog.
As the dinosaurs began to diversify, so did the crocodylomorphs. Although many stayed small, some got larger, and began to experiment with different lifestyles. I say “experiment” euphemistically, but evolution is blind — circumstances and natural selection simply allowed crocodylomorphs to fill a wider range of niches. There were large, land-dwelling predators, like Protosuchus:
There were also marine crocodylomorphs, the thalattosuchians. These, too, were predatory and looked vaguely like modern crocodilians in that they had long jaws, a muscular tails, and laterally flattened bodies (Young et al. 2014). Here, from Europe, are three species of Machimosaurus:
Others looked nothing like any modern animals, and had fin-like feet and tails. Dakosaurus, also from Jurassic Europe (Young et al. 2012), had such an un-crocodile-like skull that their fossils were at first mistakenly attributed to dinosaurs.
Most of the sea crocodiles lived during the Jurassic Period, when dinosaurs on land were becoming truly massive (e.g., spike-tailed Stegosaurus and long-necked Brontosaurus*). Despite an uncanny resemblance, sea crocodiles did not “evolve into” modern crocodiles. Instead they represent an off-shoot of crocodylomorph evolution, a kind of “dead-end” with no modern descendants.
There’s plenty more to say about fossil crocodylomorphs in the Jurassic, but we’ll skip to the Cretaceous, the last period of non-bird dinosaurs with Tyrannosaurus, Triceratops, and the like. Crocodiles, too, were reaching a magnificent crescendo with on of their most diverse lineages, the Notosuchia.
The Cretaceous saw crocodylomorphs occupying all sorts of unusual ecological roles. There were, of course, the usual land-dwelling predators like Baurusuchus:
Others had turned over a new leaf, so to speak, and adopted a life of plant-eating. Simosuchus, one such herbivore (or omnivore — see Buckley et al. 2000), hardly looked like anything you might call a crocodile:
There were still weirder crocodylomorphs. Armadillosuchus had armor plates over its back — it looked, and quite possibly behaved, very much like a modern armadillo, only 6 feet long and with sharp, fang-like teeth.
Meanwhile, some of the predatory crocodylomorphs had returned to the water, where they began to take full advantage of the lifestyle that would someday become the trademark of crocodilians. Many could be easily mistaken for true crocodiles — they had long snouts, flattened bodies, and spent their time ambushing visitors to the water’s edge. Some were colossal, and scientists have found evidence that some of the largest species (Sarcosuchus and Deinosuchus) preyed on dinosaurs (Boyd et al. 2013), just as Nile crocodiles today pick off unwary wildebeest and zebras.
Modern-day crocodilians belong to the Neosuchia (“new crocodiles”). This group also evolved in the Mesozoic and includes such strange beasts as Stomatosuchus, a 30-foot-long behemoth with a broad, spoon-like snout. Paleontologists aren’t sure exactly what it ate, but some suspect the lower jaw supported a pouch-like membrane for gulping fish, just like a pelican (Nopsca 1926).
Neither Sarcosuchus nor Stomatosuchus were “true” crocodilians, but crocodylians start to appear in the fossil record at around the same time. The oldest crocodilian fossils date back around 80 million years, during the Late Cretaceous (Russel and Wu 1998).
Just 15 million more years — a blink of an eye, really — until Earth’s most recent apocalypse. The same mass extinction that wiped out the dinosaurs (except birds), likely caused by a meteor, also wiped out most of the crocodylomorphs. Perhaps owing to their aquatic habits and (relatively) small size, crocodilians were one of the few survivors. A few other groups, like as the sea-faring dyrosaurids, lingered into the so-called Age of Mammals, but these too gradually fell away until only the true crocodiles, alligators, caimans and the gharial remained.
So we arrive at the present day, with 23 living species of crocodilians. Those 23 are the survivors survivors, here today because they are adaptable, tough, and a little bit lucky. They bear witness to an ancient lineage of incredibly diverse, versatile, and often bizarre animals, most of which have died. They are the ones that lived through mass extinctions, competition with dinosaurs, and climate change. They may even survive us.
*Brontosaurus may or may not be the same as Apatosaurus. If they are the same, then Brontosaurus is not a valid name. I’m not a paleontologist, so I have no opinion on the matter — I used the name Brontosaurus here since I think it is more recognizable, although I may be wrong about that, too. To learn more, check out this article by zoologist and writer Darren Naish.
I’ll be writing more about crocodilian evolution tomorrow, this time with an amazing species that is alive today. If you want to read more about extinct crocodylomorphs, read this article, also by Darren Naish. If this kind of stuff interests you, I recommend following his blog Tetrapod Zoology at the same link.
Finally, if you are interested in a more technical review of crocodylomorph evolution, some good papers to read are Bronzati et al. (2012) and Russel and Wu (1998), cited below.
Boyd C.A., S.K. Drumheller, and T.A. Gates. 2013. Crocodyliform feeding traces on juvenile ornithischian dinosaurs from the Upper Cretaceous (Campanian) Kaiparowits Formation, Utah. PLOS ONE 8(2): e57605. doi: 10.1371/journal.pone.0057605
Bronzati M., F.C. Montefeltro, and M.C. Langer. 2012. A species-level supertree of Crocodyliformes. Historical Biology 24(6): 598-606.
Buckley G.A., C.A. Brochu, D.W. Krause, and D. Pol. 2000. A pug-nosed crocodyliform from the Late Cretaceous of Madagascar. Nature 405: 941-944.
Nopcsa F. 1926. Neue Beobachtungen an Stomatosuchus. Centralbl. Min. Geol. Palaontol, B212-215.
Russel A. and X.-C. Wu. 1998. The crocodylomorpha at and between geological boundaries: the Baden-Powell approach to change? Zoology 100(3): 164-182.
Young M.T., S. Hua, L. Steel, D. Foffa, S.L. Brusatte, S. Thüring, O. Mateus, J.I. Ruiz-Omeñaca, P. Havlik, T. Lepage, and M. Brandalise de Andrade. 2014. Revision of the Late Jurassic teleosaurid genus Machimosaurus (Crocodylomorpha, Thalattosuchia). Royal Society Open Science doi: 10.1098/rsos.140222
Young M.T., S.L. Brusatte, Marco Brandalise de Andrade, J.B. Desojo, B.L. Beatty, L. Steel, M.S. Fernández, M. Sakamoto, J.I. Ruiz-Omeñaca, and R.R. Schoch. 2012. The cranial osteology and feeding ecology of the mentriorhynchic crocodylomorph genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. PLOS ONE 7(9): e44985. doi: 10.1371/journal.pone.0044985