A newly described Cretaceous crocodile is helping scientists determine how (and where) modern crocodiles got their start.
Modern crocodiles, alligators, caimans, and gavials are united in the order Crocodylia, which includes the last common ancestor of these groups and all its descendants. But the Crocodylia is only one branch on the great family tree of the Crocodyliformes. Today it is the only living branch, but millions of years ago the diversity of the Crocodyliformes was much greater. During the Age of Dinosaurs crocodyliforms evolved into flippered sea monsters, wolf-like hunters, duck-billed filter-feeders, and even pug-nosed vegetarians.
The details of how modern crocodiles evolved within this diverse group have long been lacking. Fossils show that today’s croc families have been around for quite a while—the oldest alligatoroids, for example, are known from 80,000,000-year-old remains. But the closest-known relatives of these early forms were a good 40 million years older and possessed very few of the anatomical features that define modern crocs.
Until now.
An international team of paleontologists has just described a new crocodyliform from Australia. Isisfordia duncani (named after the town of Isisford and its discoverer, Ian Duncan) is known from nearly complete skeletons. It looked very much like a very small modern crocodile, measuring just over 1 meter in length and probably weighing about 3 kilograms (7 pounds).
Isisfordia was found in 95,000,000-year-old rocks, helping to fill the time-gap between the first modern crocs and their next-nearest relatives. Details of its anatomy show that it was an intermediate between older crocodyliformes and modern crocodylians, as well.
Older crocodyliforms had rows of armor plates along their backs tightly arranged to form a sort of shield, while today’s crocodylians have broken up these plates into rows of single scutes. The armor Isisfordia is sagitally segmented—more tightly packed than in modern crocodiles, but in more individual pieces than in earlier forms. The vertebrae of modern crocs are procoelous (forwardly hollow), that is, a rounded “ball” at the back of each vertebra fits into a hollow cup in the front of the next. Earlier crocodyliforms have amphicoelous vertebrae—cupped at both ends. The tail vertebrae of Isifordia are “incipiently procoelous”: a small “ball” at the back fits into the hollow front of the next vertebra.
Taken together, these features show how crocodiles gained a degree of flexibility. Breaking up their armor shield gave their backs a greater range of movement, but less support. The tighter fit of their procoelous vertebrae increased their back support without compromising their flexibility. This unique combination of support and flexibility may be the key behind the evolutionary success of modern crocs.
Further reading
- Salisbury, S. W., Molnar, R. E., Frey, E. and Willis, P. M. A. 2006. The origin of modern crocodyliformes: new evidence from the Cretaceous of Australia. Proceedings of the Royal Society B. doi: 10.1098/rspb.2006.3613 (published online). When it first came out, the Royal Society made the paper freely available. They’ve taken down the links, but the URL still works. Supplemental info.
- Press release from the University of Queensland, and a whole slew of images.
- Palaeoblog and Afarensis have the story, as well.
—Matt Celeskey.
File under: Cretaceous, Recent Discoveries, Reptiles.
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