How turtles evolved their skulls

Eunotosaurus

Eunotosaurus africanus, the earliest-known turtle, even though it had no shell. It lived in southern Africa 260 million years ago.

Turtles are famous for the their shells, but it was their skulls that led to the identification of the earliest known turtle – a 260 million-year-old, shell-less reptile known as Eunotosaurus africanus. Detailed in a recent study in Nature, co-authored by Bhart-Anjan Bhullar, PhD, while he was a postdoctoral fellow in the lab of Zhe-Xi Luo, PhD, professor of organismal biology and anatomy, the findings shed light on an important transition in the evolutionary history of the turtle. Bhullar, now an assistant professor at Yale University, describes the discovery in a guest article for ScienceLife.

By Bhart-Anjan Bhullar

Anjan

Bhart-Anjan Bhullar

As a postdoctoral scholar at the University of Chicago, I was able to develop my interests in great transformations and transitions in the history of vertebrates among perhaps the most vibrant community in the world for addressing these questions. I worked primarily with Professor of Organismal Biology and Anatomy Zhe-Xi Luo on the major transformations leading to the mammalian skull.  However, although the mammal story was my major focus, I also collaborated on work aimed toward addressing cranial evolution in other major vertebrate groups. The latest study, led by Professor Gabe S. Bever from the New York Institute of Technology, is significant because it explains the evolution of the reptile skull during the most ancient events in reptilian evolution, when the modern groups that compose today’s reptilian biodiversity originated.

We studied Eunotosaurus, a 260 million-year-old reptile species found in South Africa that appears to be a very early ancestor of turtles. Using microCT scanning to create advanced 3D images, we digitally dissected the Eunotosaurus skull to learn about its internal anatomy, nearly 300 million years after the animal walked the earth. One of the things we learned was that there are a number of cryptic, or hidden, features that resemble modern turtles, including a unique bone in the front of the braincase sometimes called the laterosphenoid (I published a short paper with Prof. Bever on this feature of turtles a number of years ago).

To me, though, the most important aspect of this new study is that we were able to show some correspondence to genetic investigations of where turtles fit in the tree of life. This work suggests that they are part of a group of reptiles that have two openings in the back of their skull, known as “diapsids.” If turtles are diapsids, then all living reptiles would belong to this group. Modern turtles’ skulls, however, are closed at the back, to at least superficially resemble more primitive fossil forms. If they truly descended from diapsids, then that means their closed skulls represent reversals. Another recent paper on an early turtle relative, Pappochelys or “grandfather turtle,” also indicates that the earliest members of the turtles lineage had diapsid skulls.

Crucially, one way we supported this hypothesis was by looking at how the Eunotosaurus skull changes during the transition from hatchling to adult—in other words, its ontogeny. The skulls of many organisms change drastically with age; think about how different an infant human or puppy skull is from that of an adult person or a dog (unless it’s a Chihuahua, of course).

Much of my work, and a large part of the work I was doing with Luo at UChicago, focused precisely on this issue—changes in skull shape during ontogeny and how those relate to evolutionary history, and even evolutionary innovation. In the case of Eunotosaurus, we were lucky enough to gain access to a very small juvenile specimen. Unlike the more closed skull of the adult, this juvenile had two perfect openings in the skull, appearing for all the world like any diapsid. Even in the adult specimens, we found a ‘hidden’ opening atop the skull that had been covered over by the overgrowth of another bone. This indicates that a more ancestral condition was retained at younger stages in Eunotosaurus, and evolution had operated on the later parts of the animal’s life. Thus we were able to explain with a bit more detail how the transition from an ancestral diapsid skull to the closed modern turtle skull occurred.

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