The Crocodylomorphs

the Late Cretaceous Deinosuchus attacking a tyrannosaur

The crocodylomorphs are a group of archosaurs that continues today with modern crocodiles. Because our interest here is in the crocodylomorphs of the Mesozoic – often called ‘proto-crocs’ to distinguish them from modern species – we won’t examine their evolutionary path far beyond the Cretaceous-Tertiary Extinction Event. The Mesozoic was the heyday of crocodilian ancestors, as they reached greater diversity and abundance than they currently enjoy. It was during the Age of the Dinosaurs that they reached titanic sizes that make modern crocodiles look like puny wimps just begging to be beat up on the crocodilian playground: Sarchosuchus reached thirty feet in length and Deinosuchus forty feet, dwarfing their modern counterparts who clock in around fourteen feet (the Nile crocodile), seventeen feet (the saltwater crocodile), and sixteen feet (the American alligator). This isn’t to say all Mesozoic crocodylomorphs reached epic proportions, for Knoetschkesuchus grew only to fifty-five centimeters in length!

not all crocodylomorphs reached epic sizes

Modern crocodilians include only twenty species, and most of them look similar and lead similar lifestyles; this prompts one to imagine that the crocodilians of the past were similarly like-minded, but this would be a mistake. As one paleontologist puts it, “There are no long-legged running crocodiles or deep-diving ocean crocs [today]; no crocs the size of Chihuahuas and none the sizes of T. rex, either; and no living croc has blade-like teeth or eat plants. But that was not always the case.” He continues, “Living crocs are not big, slow-moving, semi-aquatic animals because they are primitive. Those characteristics are evolutionary adaptations for their current way of life.” As we will see, the crocodilians of the Mesozoic were a diverse and interesting lot that, in many ways, put their modern descendants to shame! As we explore the crocodilian lineages of the past, the cladogram below can help make sense of the various cul-de-sacs and detours the Mesozoic crocs took in their development:

Hesperosuchus

The first crocodylomorphs appear in the Late Triassic, and they looked nothing like crocodiles as we know them today. They were small, lightly-built, agile, and quick-footed creatures whose limbs were built directly beneath their bodies (they didn’t ‘sprawl’ like modern crocs). Paleontologists have often described them as ‘reptilian wolves’ built like greyhounds. One scientist put it this way: “Imagine a greyhound dog with scales and a long tail, and you’ll have a pretty good picture of most sphenosuchians.” These early proto-crocs probably ate insects and other small animals, and they survived into the Middle Jurassic before being supplanted by their larger evolutionary descendants. The earliest species was Hesperosuchus in the Late Triassic, and one of the last known sphenosuchians was Macelognathus in the Late Jurassic. Hesperosuchus grew four to five feet in length, was lightly built and agile, and it had strong hind-limbs with smaller, slender front limbs. Some hypothesize that it may have been bipedal (or at least able to move about on two legs, perhaps when chasing prey). It’s been discovered in Arizona and New Mexico, and during the Late Triassic that area was cut by streams and peppered with lakes and ponds. Though Hesperosuchus was fully terrestrial, it prowled these waterways hunting supersized amphibians, perhaps snagging fish, competing with the burgeoning fast-running theropod dinosaurs and dodging the tyrannical phytosaurs of its day.

Redondavenator from New Mexico is another sphenosuchian that has acquired lots of attention. It was a large creature with a skull at least two feet long. It lived at the very end of the Triassic Period, when the rauisuchians – the predatory heavyweights of much of the Triassic Period – had gone extinct. Because the predatory theropod dinosaurs had yet to bloom in size to fill the ecological niche left vacant by the demise of the rauisuchians, Redondavenator may have filled that role before the evolution of large theropod dinosaurs such as Dilophosaurus.

The sphenosuchians didn’t last past the Late Jurassic, but crocodylomorphs did. First to evolve were the ‘protosuchians’, or primitive crocodiles, which were the least like the crocodiles we know today. The protosuchians reached between three and nine feet in length, and they were adapted for life both on land and in the water. None of them got too big, and they survived through the Jurassic and into the Early Cretaceous Period. Next came the ‘mesosuchians’, or middle crocs, which had more in common with modern crocs but would still look out-of-place if we came across them today. The mesosuchians have been subdivided into four different groups, each with their particular characteristics: the ocean-dwelling thalattosuchians, the heavily-armored notosuchians who decided to go vegan, and the ‘eusuchians,’ or true crocs, which consist of all living crocodiles and their closest extinct relatives. 

You may have noticed, however, that only three groups were just mentioned; that’s because the fourth is an odd anomaly. Because it doesn’t pertain to the Mesozoic, a brief quip will suffice: Sebecus – named after the crocodile-headed god of ancient Egypt – lived during the Eocene epoch of the Tertiary Period, and it had theropod-like teeth. When its skull and teeth were discovered, it prompted a mad scramble: “Did some theropods survive the end of the Mesozoic?” False alarm, for these theropod teeth belonged to an odd breed of crocodile. Sebecus had a thin, blade-like teeth and a long, tall, narrow snout (much like a theropod dinosaur). Sebecus lived in South America during the early Cenozoic and competed with few large mammalian predators; instead if competed with large flightless birds with hooked beaks. Paleontologists don’t know if Sebecus evolved in the early Cenozoic to adapt to a new post-apocalyptic environment or if it was a survivor of a currently unknown lineage from the Cretaceous; if the latter, its skull and teeth would be examples of convergent evolution that made it appear similar – at least in respects to the skull – to heavyweight theropod dinosaurs. This wouldn’t be too surprising, as another crocodile lineage – that of the herbivorous notosuchians – ended up looking a lot like ankylosaurs and playing similar ecological roles. 

the Eocene crocodilian Sebecus; note the 'theropod' look of its skull

But let’s get back to the crocodilians of the Mesozoic. We begin with the ocean-going thalattosuchians that thrived in the Jurassic Period. These crocodilians lived full-time in the ocean, unlike modern saltwater crocs that will swim the ocean but remain ‘at home’ along shorelines. Thalattosuchians were long- and slender-bodied with paddle-like limbs; a few even had shark-like vertical fins at the end of their tails. Many had porous bones like modern animals – such as whales and seals – that dive deep into the ocean. Though fully aquatic, these crocodilians likely came onto land like sea turtles to lay their eggs. The thalattosuchians are divided into two groups: Teleosauridea and Metrirhynchoidea (the teleosaurids would become the largest-bodied and most successful of the two). Teleosaurids are known mostly from lagoons and coastal marine environments, but some remains have been found from prehistoric estuaries and freshwater ecosystems. They were highly successful and diversified quickly. In most ecosystems there were three types of thalattosuchians: long-snouted forms with lots of small, pointed teeth designed for grasping small fish; long-snouted forms that had a shorter snout and fewer teeth than the first form, though these teeth were larger and capable of taking down larger prey; and short-snouted forms that had even fewer teeth, but in which the teeth were robust with numerous ridges on the enamel. All three types occurred side-by-side in many ecosystems, indicating that they maintained their diversity by occupying different yet overlapping ecological niches. By the end of the Middle Jurassic, a fourth ‘type’ would appear: these ‘Machimosaurini’ were the giant teleosaurids, reaching between sixteen and twenty-four feet in length to become the largest crocodylomorphs of the Jurassic. Though they started off similar to the third type given above, with short snouts and robust teeth, they underwent evolutionary development in the Late Jurassic and Early Cretaceous to have short snouts, huge jaw-closing muscles, and blunt teeth with serrated edges – perfect for feasting on sea turtles. The Metrirhynchoids – the second group of thalattosuchians – is split into two subgroups: ‘true’ metriorhynchids that became dolphin-like with a tail fin, flippers, and loss of bony armor; and ‘basal’ metriorhynchoids that are immediate between the teleosaurids and the dolphin-like metriorhynchids. The ten-foot-long Metriorhynchus had a streamlined body, paddle-like forelimbs, and a finned tail like fishes; the fifteen-foot-long Dakosaurus had a strong skull and killer teeth designed to kill and eat large marine reptiles like ichthyosaurs or plesiosaurs. 

Dakosaurus, a Jurassic thalattosuchian

the boxy skull of the herbivorous Simosuchus

The notosuchians were small, heavily armored crocodilians that adopted an herbivorous lifestyle. They had short heads that resembled bulldogs, and their jaws were filled with multicusped, mammal-like teeth. They lived fully on land and ate plants. In a case of convergent evolution, these armored crocodilian tanks looked a lot like the armored ankylosaurs. It’s interesting that no ankylosaur fossils have been found in South America, Africa, or Madagascar – but notosuchians thrived there! It seems that the niche filled by ankylosaurs in the northern hemisphere was filled by notosuchians in the southern part of the planet. Like ankylosaurs, they went extinct at the end of the Cretaceous. Notable notosuchians include Uruguaysuchus from South America, Chimaerasuchus from China, Malawisuchus from Africa, and Simosuchus from Madagascar.

The fourth group of mesosuchians continues to this day. The eusuchians – or true crocodiles – first made their appearance in the Cretaceous Period and have survived until the present day in the twenty species of crocodiles we can visit in the wild or see at zoos. The current crocodiles don’t differ much from their Cretaceous ancestors, so to say that they look ‘primitive’ as relics from the Mesozoic Era isn’t far off the mark. Eusuchians, ancient and modern, are designed for a semi-aquatic lifestyle. The most notable difference between modern eusuchians and their Mesozoic counterparts is their size: the largest eusuchians from the Mesozoic had skulls between four and five feet in length and reached up to thirty-three to forty feet in length (Deinosuchus topped out around forty feet head-to-tail). 

an artist's rendition of Deinosuchus from the Cretaceous Period