Convergence and functional evolution of longirostry in crocodylomorphs

Mar 2019 Palaeontology

Ballell, A., Moon, B.C., Porro, L.B., Benton, M.J. & Rayfield, E.J. 2019 Convergence and functional evolution of longirostry in crocodylomorphs. Palaeontology 62: 867–887 doi:10.1111/pala.12432

This project was completed by Antonio while he was an MSc student at the University of Bristol. We used CT scan data of two crocodiles: Pelagosaurus, from 180 million years ago, and a modern gharial (Gavialis) to recreate and compare their musculature and jaw mechanics – the way that they bite.

While these two crocodiles look superficially similar, both having long snouts, they are different in the amount of muscle that the skulls can hold and thus the resulting forces created when biting. The gharial has a larger muscle chamber and somewhat more robust skull that can handle the forces more easily. However, despite differences, it is still likely that both these crocodiles had a similar diet and feeding method: catching fast-moving, slippery prey, possibly by ambush.

Plots of the stress distribution from biting in Pelagosaurus (left) and Gavialis (right).

We also looked at how these feeding-related features of the skull changed across the whole tree of crocodiles. In particular, there are rapid rates of change in the clade Thalattosuchia. These are marine crocodiles that first evolved in the Early Jurassic and have characteristically elongated snouts, including Pelagosaurus. The shift in skull morphology gave these crocodiles ‘low mechanical advantage’, resulting a less powerful but faster bite. This appears to be linked to moving into water and changing to eating more fish, and fast-moving prey.

Evolution of mechanical advantage in crocodylians. The red colours indicate rapid change in that part of the tree. Here Thalattosuchians (Teoleosauridae and Metriorhynchidae) rapid evolve a faster but weaker bite.

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