Textbooks aren’t known for their originality. They build on the basics, and often include the same standard examples from one generation of students to the next. (I haven’t checked, but I wouldn’t be surprised if the fox terrier clone is still creeping somewhere.) That’s why ichthyosaurs are a textbook staple.
Mesozoic “fish lizards”, ichthyosaurs were marine reptiles that independently became adapted to a life at sea around 200 million years before dolphins. Despite their distance from the oceanic mammals in both time and evolutionary history, though, ichthyosaurs look enough like dolphins for the two to be practically inseparable in textbooks. They’re a striking example of convergent evolution – two lineages independently evolving extremely similar anatomy from different starting points.
But how similar are the two, exactly? An Opthalmosaurus looks kind of like a bottlenose dolphin, sure, but stopping at superficial similarities isn’t very scientific. Here’s where a new Biology Letters study by National Museum of Natural History paleontologist Neil Kelley and U.C. Davis’ Ryosuke Motani offers an opportunity to see whether such resemblances are only skin deep.
Kelley and Motani focused on skulls of marine tetrapods – descendants of the four-legged vertebrates that crawled out of the swamps over 360 million years ago. They’re an ideal group for such comparisons because all of them – from seals to turtles to whales – had terrestrial ancestors that eventually took on life in the seas. By combining skull, jaw, and tooth measurements from 69 living species with data on what they actually eat, Kelley and Motani were able to pick out how form relates to feeding.
As it turns out, skull anatomy is a fairly good predictor of feeding style regardless of ancestry. For example, herbivores like the marine iguana, green sea turtle have short skulls with larger areas of attachment for powerful jaws muscles to crop and crush vegetation. Species that snatch up fish and squid, on the other hand, tend to have longer, toothier snouts better-suited to “snap feeding” and swallowing prey whole. Apex predators such as the saltwater crocodile, leopard seal, and orca fell in-between, characterized by elongated jaws and relatively deep skulls that give them the power to tear apart larger prey.
Despite diverging far back in the prehistoric past, creatures as distantly-related as iguanas and dugongs have evolved similar skull shapes to cope with similar diets. And with this proof-of-concept in place, the same technique can be applied to the fossil record. Paleontologists will able to investigate the similarities, and differences, between ichthyosaurs and dolphins, and perhaps even gauge how marine reptiles like different species of mosasaur may have been able to coexist by picking different items off the marine menu. There are plenty of prehistoric secrets embodied by evolution’s greatest hits.
Kelley, N., Motani, R. 2015. Trophic convergence drives morphological convergence in marine tetrapods. Biology Letters. doi: 10.1098/rsbl.2014.0709