Some clades get all the love. Dinosaurs are at the top of the list. For the past century and a half they have been the icons of lost worlds and extinctions, and the even greater variety of life that lived alongside them is often treated as a motley aggregation of “also-rans.” Crocs, in particular, have reason to be jealous. When a dinosaur impersonates a croc, it’s news, but when crocs steal a few pages from the dinosaur’s evolutionary playbook, no one seems to notice.
In 1998 news sources heralded the discovery of Suchomimus – the dinosaurian “crocodile mimic.” One of the bizarre spinosaurs, this fish-eating predator had a long, low skull full of conical teeth vastly different from the deep, knife-toothed skulls of other large predators like Tyrannosaurus, Allosaurus, and Ceratosaurus. So unusual was this dinosaur that it seems to have been the inspiration for two bare-bones b-movies – DinoCroc, and, of course, DinoCroc versus SuperGator. What few people know, however, is that one group of crocs also imitated fearsome dinosaurian predators.
Paleontologists Douglas Riff and Alexander Kellner described the characteristics of the true dinocrocs in a paper recently published in the Zoological Journal of the Linnean Society. In particular, they focused on a complete specimen of the crocodyliform Stratiosuchus maxhechti from the approximately 90 to 83 million year old strata of Brazil. This creature was unlike any croc alive today.
All modern crocodylians – alligators, crocodiles, gharials, caimans, and the like – are aquatic ambush predators that have been doing their thing for about 84 million years or so. But take a step back and look at their larger family – the crocodyliforms – and the diversity and disparity in the group is fantastic. There were entirely marine forms like Dakosaurus, genera like Armadillosuchus which developed complex armor shells before mammals did, and gracile little forms like Pakasuchus which had mammal-like teeth, to pick just a few.
Stratiosuchus represents another unusual croc body plan. This thirteen-foot-long crocodyliform belonged to a subgroup of land-dwelling, specialized predators called the baurusuchids. These quadrupedal carnivores chased down prey during the Cretaceous, and their ilk persisted for millions of years afterward as mammals rose to dominance in a post-dinosaur world. Rather than being squat, semi-aquatic creatures, they carried their legs beneath their bodies and had deep skulls set with compressed, sharp teeth better suited to slashing flesh than gripping or crushing. They almost seemed to be a recapitulation of much earlier predators – such as Postosuchus and Teratosaurus – which ruled before dinosaurs gained dominance.
Just how closely crocs like Stratiosuchus approximated some aspects of theropods was appreciated early on. Baurusuchid teeth were so much like those of carnivorous dinosaurs that, after they were discovered in Cretaceous strata of South America, paleontologists started to wonder if isolated teeth assigned to theropods really belonged to the crocs instead. And perhaps dinosaurs did not have the total dominance over the world’s ecosystems as had been presumed – the baurusuchids seemed to indicate that crocs were giving carnivorous dinosaurs some formidable competition. Exactly how closely creatures like Stratiosuchus came to resemble theropod dinosaurs has previously been estimated at a relatively superficial level, however, and so Riff and Kellner investigated how deep the evolutionary convergence extended.
The skull of Stratiosuchus does not look like a typical modern croc head. Stratiosuchus had a relatively deep skull which, when viewed from the top, had the hatchet-shaped profile seen in some predatory dinosaurs in which the skull narrows from the back into a relatively thin snout. And there’s a particular feature of that thin snout the croc shared with the famous tyrannosaurs – fused nasal bones, thought to be an adaptation to crushing, high-force bites that would have considerably stressed the skull. Those features, plus the large and prominent canine-shaped teeth, hint that Stratiosuchus was a predator in the mold of some of the famous predatory theropods.
Not all the dinosaur-like features of Stratiosuchus were restricted to the skull, though. In their analysis of the post-cranial skeleton, Riff and Kellner point out that some of the muscle attachment sites around the hip – such as the area where a major retractor of the femur would have been located – were closer to the anatomy of dinosaurs than other crocs. This is consistent with the idea that Stratiosuchus held its limbs beneath its body and moved them forward-and-back in pursuit of prey. A shared hunting method may have driven the convergence.
And Stratiosuchus seems to have out-competed smaller guilds of theropod dinosaurs. Where you find baurusuchids, there is often a conspicuous lack of dinosaurian predators of the light- and middleweight classes. Baurusuchids appear to have taken a chunk of niches often occupied by carnivorous dinosaurs in other places and times.
In the well-studied Late Jurassic Morrison Formation, for example, paleontologists have found a full swath of predators, form small (Coelurus, juveniles of larger predators), to medium (Marshosaurus, Stokesosaurus, Ceratosaurus), to giant (Allosaurus, Torvosaurus) alongside a diversity of herbivorous dinosaurs (Camptosaurus, Apatosaurus, Camarasaurus, Diplodocus, Stegosaurus, etc.). But in the Adamantina Formation where Stratiosuchus is found, the small and medium classes of carnivorous dinosaurs are virtually unknown. Two large predatory dinosaurs have been found in the Adamantina Formation, but most of the rest of the dinosaurian fauna belonged to a group of long-necked sauropod dinosaurs called titanosaurs. While small- and medium-sized theropods may yet turn up, the assemblage uncovered so far is consistent with the idea that the baurusuchids were the primary mid-size carnivores on the landscape. Young sauropod dinosaurs may have been a staple of the Cretaceous hypercarnivorous croc diet.
During the reign of the dinosaurs, crocodyliforms were not just hanging out in the background, waiting to win the evolutionary lottery. They were a fantastic, varied group that sometimes pushed dinosaurs out of their vaunted, apex roles in Cretaceous ecosystems. And, like the dinosaurs, crocodyliforms belonged to that great group of “ruling reptiles” called archosaurs. Dinosaurs might have been the flashiest monarchs, but they were not the only reptilian royalty around.
RIFF, D., & KELLNER, A. (2011). Baurusuchid crocodyliforms as theropod mimics: clues from the skull and appendicular morphology of Stratiotosuchus maxhechti (Upper Cretaceous of Brazil) Zoological Journal of the Linnean Society, 163 DOI: 10.1111/j.1096-3642.2011.00713.x