Crocodilians.  Big, meaty, toothy, armored death machines.  For nearly 100 million years, they have lurked in the shallow, muddy waters of wetlands around the world, preying on anything small enough to be swallowed.  Crocs are famous for remaining seemingly resistant to the forces of evolution, simply refusing to make any major changes to their basic body plan.  For reference, the earliest hominids, which bridged the gap between the rest of the great apes and humans, first appeared only about six million years ago.  The effectiveness of the crocodilian design is evident not only in their continued existence to the present day, but also in the multitude of cases of convergent evolution. 

The phenomenon of lineages evolving into the basic form and lifestyle we associate with today’s crocodilians has occurred on numerous occasions throughout Earth’s history.  Perhaps no group of animals better reflects this than the phytosaurs, which are superficially nearly identical to crocodilians.  Bearing remarkably similar scales, teeth, In fact, one of the few ways to distinguish phytosaurs from crocs is by their nose.  Whereas crocodilians have nostrils at the end of their snouts, phytosaurs keep their noses near their eyes.  Although the nostrils are in different locations on phytosaurs and crocodilians, in both cases, the dorsal placement of the nose allows the animal to remain almost entirely beneath the surface of the water while stalking terrestrial prey. 

Fig. 1. Kobierska, Joanna. Parasuchus sp.  

The striking similarities between phytosaurs and crocodilians are remarkable in their own right, but perhaps even more peculiar are the parallels between crocodilians and animals that do not even qualify as reptiles.  When one contemplates amphibians, scarcely do images of giant, thirty-foot-long aquatic predators with gaping, toothy maws appear in their brains, yet that is exactly what Prionosuchus was.  The largest amphibian to have ever lived, Prionosuchus patrolled the shallows of Permian South America some 270 million years ago.  This oversized salamander weighed in at over two tons, doubling the mass of even the largest modern-day crocodiles. 

Fig. 2. Sutton, Thomas. Prionosuchus plummeri

A looser, yet still applicable instance of convergence with crocodilians can be found in the evolution of what we know as whales today.  At one point a terrestrial lineage, whales gradually returned to the water starting about 50 million years ago.  During their transition back to the sea, the ancestors of whales lived semiaquatic, predatory lives, much like crocodiles do today.  A notable example of this can be seen in Ambulocetus, which resembles an otter distorted into more crocodilian-like proportions. 

Fig. 3. Tamura, Nobu. Ambulocetus

The consistency with which the crocodilian form appears throughout Earth’s timeline is truly a testament not only to its versatility, but also to its robustness.  Even in spite of all the challenges modern wildlife faces, I am confident we can expect crocodiles to survive for millennia to come.