Of all the organisms in the animal kingdom, turtles do not often come to mind as among the most mysterious, but their placement in the tree of life is one of the most important debates in evolutionary biology.
A new study from researchers at Yale, the New York Institute of Technology and the Denver Museum of Nature and Science provides key insights into turtles’ origins. The research, an analysis of the skull of Eunotosaurus africanus, a reptile that lived in the Karoo Basin of South Africa 260 million years ago, has provided strong evidence that turtles evolved from the same ancestors as other reptiles and birds. The study was published Sept. 2 in the journal Nature.
“For a long time one of the most controversial topics in all of vertebrate zoology had to do with exactly what turtles are,” said Daniel Field GRD ’16, who is finishing his Ph.D. in Geology and Geophysics.
Most reptiles and birds are diapsid. This means that they are descended from ancestors with two holes on each side of their skulls. Turtles, however, are anapsid and have no holes in their skulls. Genetic evidence has long pointed to turtles originating from diapsid ancestors. Despite this, paleontologists were skeptical of the notion that turtles could have evolved to be anapsid from a diapsid ancestor. The mystery shrouding the species’ origins left a significant gap in the mapping of the tree of life.
Field explained that, although the first Eunotosaurus fossil was collected over 100 years ago, for a long time it was not thought to be closely related to any living animals, and so was not examined.
It was not until 2008, when Gabe Bever, now a professor at NYIT, was a postdoctoral researcher at Yale, that work on the Eunotosaurus began in earnest. He began a collaboration with Tyler Lyson, curator of vertebrate paleontology at the Denver Museum of Nature and Science, to study the Eunotosaurus in the context of evolutionary history after realizing that the anatomy of Eunotosaurus, specifically its rib structure, indicated that Eunotosaurus was the oldest relative of turtles.
Field joined the team last year after publishing a genetic analysis of turtles that placed them in the diapsid lineage, specifically close to crocodiles and birds.
By using high resolution CT scans that allowed them to look at the anatomy of the skull in three dimensions, the researchers discovered hidden evidence that the skull was, in fact, diapsid. There did not appear to be a hole in the skull because an expanded super temporal bone had covered it up.
“Since living turtles don’t have that opening and their earliest known relatives did, at some point that opening was lost,” Field said, adding that whether the hole continued to close up because of the super temporal bone over the course of turtle evolution is not clear.
According to Gregory Watkins-Colwell, collection manager of herpetology and ichthyology at the Peabody Museum of Natural History, who was not involved in the study, intermediate evolutionary stages for turtles were not well known prior to this study.
“We now know there were ancestors longer ago, and we know more of what they looked like,” he said.
Bever said he is not done working with the Eunotosaurus. He is still looking at the details of the skeleton, how it lived, what conditions it lived in and what those conditions mean for the origin of modern turtles.
The oldest tortoise alive is 183 years old.