Evolutionary Origin of the Turtle Skull

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Written by Tyler Lyson

Eunotosaurus

Turtles and their bizarre body plan have fascinated me ever since I had a pet box turtle as a child. Their hard shells and bony skull roof that lacks openings for the jaw musculature are unlike that in any living animal. It was not until reading through my freshman year biology textbook that I realized that turtles were a real scientific enigma. Both the origin of their body plan and whom they were related to remained elusive. Part of the problem was that at that point in 2002 there were no transitional fossils that bridged the gap between the ancestral lizard body and the highly modified body of turtles.

Recently a number of fossil animals have been put forth as early “proto” turtles, including a 260 million year old animal I have had the privilege to work on, Eunotosaurus africanus. Eunotosaurus is from South Africa and has shed considerable light on the origin of the turtle body plan.

Watch these cool animations that show how you change from an animal that looks like a lizard to one that looks like a turtle, changes that took place over the course of 50 million years: https://www.youtube.com/watch?v=qIOPDysP74g and https://www.youtube.com/watch?v=NphNApmSZ0U.

Now my colleagues and I have tackled the other major question surrounding turtles: who are turtles related to??

This has become a classic morphology versus molecular data problem. Partly based on the lack of any openings in the skull roof of living turtles, morphologists have always placed turtles outside of those groups (lizards, snakes, birds, crocodilians) that have two pairs of openings in their skull roof. On the other hand, molecular data has consistently allied turtles with birds and crocodilians.

Our new study, led by my collaborator Dr. Gabe Bever from the New York Institute of Technology, was recently published in the scientific journal Nature, and presents a detailed study of the skull of the earliest “proto” turtle Eunotosaurus. We used high-resolution computed tomography to digitally dissect each bone in the skull (see this animation) and then incorporated all of these observations into a new analysis of the reptile tree of life.

A major result of the study shows that Eunotosaurus has a pair of openings situated behind the eyes, indicating that it is closely related to other modern groups with a similar set of holes (snakes, lizards, birds, and crocodiles).

Eunotosaurus has closed the morphological gap between lizards and turtles and has now brought the morphological and molecular data closer into alignment, both major steps towards understanding the reptile tree of life. However several questions remain: what were the ecological conditions that led to the evolution of the turtle’s shell and bony skull roof? Which specific group of living animal (snakes, lizards, birds, crocodilians) is most closely related to turtles? The study of Eunotosaurus and other “proto” turtles will likely play a key role in addressing these interesting questions, so stay tuned! 

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