Xavier A. Jenkins

@semifossorial.bsky.social

757 Followers 296 Following

Paleontologist and postdoc at the American Museum of Natural History. I study tetrapod origins, sensory evolution, and all things Permian🦎🐢🐊

Various constrain analyses showing the additional steps required to place turtle origins among other groups of fossil reptiles
A figure showing the skeleton of the millerettid Milleretta, the skeleton of a juvenile Eunotosaurus, and an adult Eunotosaurus. They shared broadened ribs that become T-shaped during ontogeny
A figure showing just some of the millerettid features present in Eunotosaurus and Milleretta. Look how similar their skulls are!
A figure showing numerous plesiomorphic (primtive) features present in Eunotosaurus and millerettids (left) and crown reptiles (right). Specifically showing the ear region, occiput, and hindlimb
Our segmentation of Eunotosaurus specimen CM-777 and reconstructions of the taxon based on all available specimens
The crushed skull of Eunotosaurus specimen CM-777 in all of its distorted glory. For decades this was the best preserved specimen of a Eunotosaurus skull
The braincase figure from earlier again (:
A red eared slider blinking and show off its nictitating membrane
A bunch of turtle skulls in lateral view as well as a single captorhinid (A). Gaffney’s 1990 monograph on turtle evolution and the Triassic turtle Proganochelys compared turtles not to living reptiles but to captorhinids, a group of Paleozoic tetrapods that may not have even been reptiles at all
Figure from Lyson et al 2016 showing Eunotosaurus as pivotal to the origins of the turtle shell. Note, however, that the next figured unambiguous stem turtles Pappochelys and Odontochelys do not have broad, overlapping ribs.
Artwork of Milleretta by Gabriel Ugueto
A subadult specimen of Eunotosaurus in dorsal (left) and ventrolateral (right) views. Note the broad, overlapping ribs.
The braincases of four early reptiles in lateral view, showing similarities between an early shelled turtle Proganochelys (C) with an early archosauromorph (B). Eunotosaurus (A) has anatomy most similar to primitive stem reptiles
A pair of Eunotosaurus by Gabriel Ugueto: a sense of scale is provided by Glossopterus leaves in the fore-and-background and by a prosbolid hemipteran insect
Line drawing of Scyllacerta in numerous anatomical views, done by paleontologist Cy Marchant
The holotype of Youngina, a little worse for wear but bearing a conspicuous emargination on the back of its overly-prepped quadrate…
Phylogeny of early reptiles showing the placement of Scyllacerta as an early Younginid and mapping the appearance of the tympanic fossa, loss of lower temporal bar, and appearance of the cephalic Condyle on the quadrate in these taxa.
A close of a monitor lizard (Komodo dragon) with a visible external ear towards the right
Zoom in showing the middle ear, tympanum, and occiput of Scyllacerta
Biostratigraphic map of the Karoo basin in South Africa, showing where in the section Scyllacerta and other younginids are from.
The holotype aggregation in multiple views, with numerous individuals visible
Skull of Scyllacerta, reconstructed based on scans of four nearly complete skulls.
A cladogram of early reptile relationships showing the origin of important anatomical features in the foot of these reptiles
Tarsus, or foot, of Galesphyrus.
The holotype of Galesphyrus, an articulated skeleton of a small stem reptile preserved as a mold. It is beautifully preserved, but decided to lose it’s skull prior to discovery
Graphical abstract and simplified phylogeny of early reptile relationships showing the gradual evolution of the reptile middle ear
Simple cladogram demonstrating the relationships of Amenoyengi (on the left) with other moradisaurine captorhinids. Skull silhouettes show the size disparity present in this group
Maxilla of Amenoyengi in ventral view showing four rows of dentition, which are numbered from 1-4. A small diastema, or gap in the tooth row, is present anteriorly
Photos of a skull of a captorhinid in standard anatomical views. The skull is preserved in a brownish red mudstone and is badly weathered, but the internal anatomy is well-preserved.
Brandon Peecook prepares to introduce Xavier as Xavier nervously drinks a coffee
A paleoartist's *beautiful* interpretation of Milleropsis pricei based on new data from our paper