Spinosaurs Were Lousy Croc Mimics

I love dinosaur names. Many of them are so beautifully evocative of the animals they represent. There may be no more perfect title in the history of nomenclature than Tyrannosaurus rex, to pick just one classic. Yet dinosaur names can also be reminders of incorrect interpretations or assumptions about those animals. Stegosaurus – the “roofed lizard” – was so named because paleontologist O.C. Marsh thought the dinosaur’s characteristic plates laid flat on the herbivore’s back as bony tiling, rather than sticking straight up out of the skin as fossil finds later made clear.

Suchomimus may also be a less than apt name. This sail-backed spinosaur was given the “crocodile mimic” moniker by Paul Sereno and colleagues for the dinosaur’s long, shallow snout. The Cretaceous carnivore seemed to have more of a crocodile profile than other deep-skulled predatory dinosaurs. And if spinosaurs had croc-like snouts, perhaps they spent a great deal of time hunting fish (an idea supported by fish in the gut contents of the spinosaur Baryonyx.) Furthermore, a paper by Emily Rayfield and coauthors found that Baryonyx had a skull suited to resisting the kind of forces expected to be created by struggling fish. But now, along with coauthor Andrew Cuff, Rayfield has gone back to the spinosaurs and found that the connection between long, slender jaws and a diet of freshly-caught fish isn’t as clear as previously thought.

Of the spinosaurs known so far, Cuff and Rayfield investigated the snouts of Baryonyx – a roughly 125 million year old genus found in the strata of England – and the famous Spinosaurus from the 97 million year old rock of Egypt and Morocco. Using CT scans to restore the spinosaur snouts, the researchers simulated how the jaws of these dinosaurs would react to the stresses caused by biting, and compared these results to the virtual bites of an African slender-snouted crocodile, an Indian gharial, and an American alligator.

After running the virtual snouts through their biting paces, Cuff and Rayfield found that the upper jaws of the spinosaurs “absolutely outperform all crocodilian taxa” and showed similar resistance to bending and twisting to each other. As far as the front part of the upper jaw is concerned, at least, the spinosaurs were not just like big gharials. The spinosaur jaws were better at dealing with up-and-down stresses than side-to-side stress, as well, which is consistent with their relatively narrow jaws. And the apparent superiority of the spinosaurs seems to have been a result of their size. Rather than being a case of convergent evolution with slender-snouted crocs because of a fish-based diet, spinosaurs might have been able to get away with having relatively narrow jaws thanks to mechanical advantages conferred by size. They weren’t bonecrushers like Tyrannosaurus – not even close – but the spinosaurs had jaws that were capable of doing more than holding onto slippery fish.

So maybe spinosaurs weren’t dedicated fish-snatchers. The fact that dinosaur remains have been found in the gut contents of Baryonyx and a spinosaur tooth was discovered embedded in a pterosaur bone indicate that these carnivores were capable of consuming a wider variety of flesh, if not actually catching such terrestrial or airborne meals.

Spinosaurs through time were capable of tossing a variety of savory bites down their throats, but the real question is how they did so and whether there was a particular sort of prey they preferred. On that score, we know relatively little, but lucky finds of gut contents, geochemical clues locked in their skeletons, and future attempts to make these dinosaurs bite again might start to show us how these utterly bizarre predators made a living on Cretaceous floodplains.

[Top image from Flickr user theMatthewBlack.]

Reference:

Cuff, A., Rayfield, E. 2013. Feeding Mechanics in Spinosaurid Theropods and Extant Crocodilians. PLoS ONE. 8, 5: e65295. doi:10.1371/journal.pone.0065295