Study Suggests Chelonia Evolved from Diapsids

Turtles and their Kin Diapsids Not Anapsids

A study into an ancestral form of the Chelonia (turtles, tortoises and terrapins) has revealed that the skull had a pair of holes in it just behind the eye.  So what, you might say, but this is a big deal, a very big deal, as it means that the history of the reptiles and any subsequent cladograms (family trees), constructed will have to be fundamentally changed.  This new research using the skulls of 260 million-year-old South African Permian reptiles suggests that turtles and their kin are not anapsid reptiles but that this group evolved from diapsids.

Chelonia – Very Ancient Reptiles

Unravelling the history of the Reptilia is a very complicated process.  However, a tenet in helping to establish evolutionary relationships between different types of reptile is the anatomy of their skulls.  It is differences in the number of holes found in the skull behind the eye socket that has helped palaeontologists to understand more about how the first reptiles evolved from amphibians.  Skull anatomy has been used to map the radiation of different forms of reptile, those that led to the birds, crocodiles and the dinosaurs and those who took a different evolutionary line eventually leading to mammals.  Some of this may have to be re-examined in the light of new research conducted by Dr. Gaberiel Bever (Honorary Research Associate at Witwatersrand University and scientist at the New York Institute of Technology) with co-author of the scientific paper, just published in “Nature”, Dr Tyler Lyson of the Denver Museum of Nature and Science.  Three-dimensional models of the skull of the ancestral turtle, a primitive reptile named Eunotosaurus have revealed that this reptile was a diapsid, suggesting that the tortoises and turtles evolved from diapsids and not from what are thought to be more primitive reptiles with anapsid skulls.

Four Basic Types of Reptile Skull

anapsid, synapsid, euryapsid and diapsid

anapsid, synapsid, euryapsid and diapsid

Picture Credit: Before the Ark/BBC

A traditional method of attempting to unravel the phylogeny of early reptiles is to examine the layout of the bones in the skull.  There are four basic skull patterns for reptiles (living and extinct forms):

  • Anapsid – the most simple skull type with the skull bones only having holes for the eyes and the nostrils. Up until this research was published, turtles, tortoises and terrapins were thought to belong to this group.
  • Synapsid – has one hole behind the eye socket on either side of the skull between the squamosal and the postorbital skull bones (sq and po), the lower opening when compared to diapsids.
  • Euryapsid – has one hole behind the eye socket above the squamosal and the postorbital skull bones (sq and po), the upper opening when compared to diapsids.
  • Diapsid – has two holes behind the eye socket on either side of the skull (lower and upper openings in the skull between the squamosal and postorbital bones.

Helping in the research effort involved using CT scans and computer modelling techniques to produce three-dimensional images of the fossil skulls, were Yale University’s Daniel Field and Bhart-Anjan Bhullar (Assistant Professor, Department of Geology & Geophysics).

The fossilised skulls come from the famous Karoo Basin and are assigned to an ancient ancestor of modern turtles called Eunotosaurus africanus, Everything Dinosaur reported on the significance of these fossils in terms of tracing the evolutionary history of the turtle family back in 2013.

To read more about E. africanusHow the Turtle Got Its Shell

Commenting on their research Dr. Gaberiel Bever stated:

“Eunotosaurus is a critical link connecting modern turtles to their evolutionary past.”

Dr. Bever and his co-workers examined high-resolution images created by computer tomography (CT scans).  Their research revealed the complex anatomy of the Permian skulls and supports the theory that animals like Eunotosaurus are indeed the ancestors of today’s tortoises, terrapins and turtles.  Once the skull anatomy had been mapped, the team re-drew the cladogram of the Reptilia based on their findings.

Dr. Bever explained:

“Using imaging technology gave us the opportunity to take the first look inside the skull of Eunotosaurus.  What we found not only illuminates the close relationship of Eunotosaurus to turtles, but also how turtles are related to other modern reptiles.”

A key finding of the study was that Eunotosaurus had a pair of openings set behind the eyes. Eunotosaurus was a diapsid.

The Chelonia Are More Closely Related to Other Reptiles Than Previously Thought

Eunotosaurus a diapsid reptile.

Eunotosaurus a diapsid reptile.

Picture Credit: Witwatersrand University with additional annotation by Everything Dinosaur

This suggests that the ancestors of the turtle lineage were not very primitive anapsids, with skulls similar to those found in amphibians, they were diapsids and therefore the turtle family is more closely related to crocodiles, lizards, snakes, birds and dinosaurs than previously thought.

The holes behind the eye socket served to lighten the skull to help make it more manoeuvrable.  The jaw muscles were able to lengthen and flex to a greater degree, this would have given Eunotosaurus a more powerful and quicker bite than modern tortoises and their kin.

Explaining the significance of this study Dr. Bever stated:

“We can now draw the well-supported and satisfying conclusion that Eunotosaurus is the diapsid turtle.”

In linking turtles to their diapsid ancestry, the skull of Eunotosaurus also reveals how the evidence of that ancestry became obscured during later stages of turtle evolution.

“The skull of Eunotosaurus grows in such a way that its diapsid nature is obvious in juveniles but almost completely obscured in adults.  If that same growth trajectory was accelerated in subsequent generations, then the original diapsid skull of the turtle ancestor would eventually be replaced by an anapsid skull, which is what we find in modern turtles.”

This new study means that the reptile cladogram will have to be re-drawn.  This research helps to cement the phylogenetic relationships between different reptile groups, but the researchers admit that this study is not the end of the matter, merely a beginning.

Witwatersrand University Professor Bruce Rubidge, who has been leading the collaborative effort to explore the rich vertebrate fossil assemblage of the Karoo Basin, explained that although the focus of the research had been on piecing together evidence to outline the diverse fauna of the Middle and Late Permian, their work had implications for studying modern reptiles too.

He stated:

“This is a major step towards understanding the interrelationships of reptiles.  Also of great significance is that Eunotosaurus, which is known only from South Africa, is a critical transitional form in the origin of tortoises and this finding indicates that the tortoise lineage had its origins in Gondwana.”

Dr. Bever summarised the way forward for the research team:

“There is still much we don’t know about the origin of turtles or which of the other diapsid groups form their closest cousin?  What were the ecological conditions that led to the evolution of the turtle’s shell and the anapsid skull?  How much of the deep history of turtle evolution can be discovered by studying the genes and developmental pathway of modern turtles.”

One thing that is for sure, most of the published works on the phylogenetic relationships between the Reptilia (alive and extinct) have just become in need of an update.

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