This page serves supplemental imagery for a paper entitled Evolutionary Origin of the Turtle Skull, by G.S. Bever, T.R. Lyson, D.J. Field, and B-A.S. Bhullar (2015, Nature, 525, 239-242). The abstract is as follows:
Transitional fossils informing the origin of turtles are among the most sought-after discoveries in palaeontology. Despite strong genomic evidence indicating that turtles evolved from within the diapsid radiation (which includes all other living reptiles), evidence of the inferred transformation between an ancestral turtle with an open, diapsid skull to the closed, anapsid condition of modern turtles remains elusive. Here we use high-resolution computed tomography and a novel character/taxon matrix to study the skull of Eunotosaurus africanus, a 260-million-year-old fossil reptile from the Karoo Basin of South Africa, whose distinctive postcranial skeleton shares many unique features with the shelled body plan of turtles. Scepticism regarding the status of Eunotosaurus as the earliest stem turtle arises from the possibility that these shell-related features are the products of evolutionary convergence. Our phylogenetic analyses indicate strong cranial support for Eunotosaurus as a critical transitional form in turtle evolution, thus fortifying a 40-million-year extension to the turtle stem and moving the ecological context of its origin back onto land. Furthermore, we find unexpected evidence that Eunotosaurus is a diapsid reptile in the process of becoming secondarily anapsid. This is important because categorizing the skull based on the number of openings in the complex of dermal bone covering the adductor chamber has long held sway in amniote systematics, and still represents a common organizational scheme for teaching the evolutionary history of the group. These discoveries allow us to articulate a detailed and testable hypothesis of fenestral closure along the turtle stem. Our results suggest that Eunotosaurus represents a crucially important link in a chain that will eventually lead to consilience in reptile systematics, paving the way for synthetic studies of amniote evolution and development.
See 'Additional Imagery' for animations of the segmented skull.
About the Species
This specimen was collected from the Karoo Basin of South Africa. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Gabe Bever of the New York Institute of Technology. Funding for scanning was provided by Dr. Bever. Funding for additional image processing was provided by UTCT.
About this Specimen
This specimen was scanned by Matthew Colbert on 5 August 2010 along the coronal axis for a total of 1764 slices. Each 1024 x 1024 pixel slice is 0.06618 mm thick, with an interslice spacing of 0.06618 mm and a field of reconstruction of 62.1 mm.
Bever, G.S., Lyson, T.R., Field, D.J., and B-A.S. Bhullar. In press. Evolutionary origin of the turtle skull. Nature.
Eunotosaurus page on Wikipedia