A description of the palate and mandible of Youngina capensis (Sauropsida, Diapsida) based on synchrotron tomography, and the phylogenetic implications

Abstract

The late Permian reptile Youngina capensis (c. 254 Ma) is a non‐saurian neodiapsid whose anatomy has been used to represent the reptilian condition prior to the divergence of Sauria (crown‐group reptiles). However, despite being first described over 100 years ago, the anatomy of Youngina remains incompletely documented. Here we use synchrotron x‐ray micro‐computed tomography to document new features of the palate, braincase and mandible of Youngina. New observations include an anteriorly bifurcating vomer, dentition on the ventral surface of the parasphenoid body and cultriform process, and a strongly convex coronoid eminence. Our anatomical observations suggest that Youngina may represent a more stemward lineage among non‐saurian neodiapsids and this is supported by our phylogenetic analysis, which places Youngina as an early diverging neodiapsid. Our research will benefit future studies on saurian origins by providing improved constraints on neodiapsid anatomy prior to the divergence of the reptilian crown group.

The origin of Amniota in phylogenetic context

“… Anthracodromeus, a Paleothyris-grade animal (Carroll and Baird, 1972;Reisz and Baird, 1983) with arboreal adaptations (Mann et al, 2021a) consistently found even closer to Diapsida by Müller and Reisz (2006) but almost lacking known skull material;  Youngina, the classic example of a Permian diapsid (Gow, 1975;Currie, 1981;Gardner et al, 2010;Hunt et al, 2023);  Orovenator, found as, effectively, the link between diapsids and varanopids by Benson (2018, 2019) and Brocklehurst et al (2022);  Carbonodraco, described as the oldest parareptile (Mann et al, 2019b), remedying the previously complete lack of parareptiles in the matrix;  Delorhynchus, a parareptile with two toothed coronoid bones in each lower jaw-a set of very rare plesiomorphies among sauropsids Haridy et al, 2016Haridy et al, , 2017Rowe et al, 2021Rowe et al, , 2023;  Australothyris, a phylogenetically isolated parareptile (Modesto et al, 2009);  Erpetonyx, the second-oldest parareptile, phylogenetically distant from Carbonodraco and Delorhynchus (Modesto et al, 2015);  Eudibamus, a cursorial animal thought to be closely related to Erpetonyx (Berman et al, 2021);  Mesosaurus (Modesto, 2006(Modesto, , 2010Piñeiro et al, 2012Piñeiro et al, , 2021, whose phylogenetic positionoften found among the parareptiles-has been controversial since the first two analyses (Gauthier et al, 1988;Laurin and Reisz, 1995) and whose highly derived anatomy has been controversial for close to a century Piñeiro, 2017, 2018;MacDougall et al, 2018;Ford and Benson, 2019; and references in all four);  Aletrimyti and Dvellecanus, the two taxa named in the revision of the microsaur Rhynchonkos (Szostakiwskyj et al, 2015)-note that the referred specimen of Aletrimyti contains a partial postcranium that was not mentioned by Szostakiwskyj et al (2015) but was ...…”

The topological organization of the turtle cranium is constrained and conserved over long evolutionary timescales

“…In non-testudinatan reptiles, the jugal consists only of a vertical plate, and a connection with the palate is established by the medially adjacent ectopterygoid (Romer, 1956;Schoch & Sues, 2018). This is documented not only in stem turtles such as Pappochelys rosinae (Schoch & Sues, 2018) or the possible stem turtle Eunotosaurus africanus (Bever et al, 2015), but also other fossil taxa that form reasonable outgroup comparisons for ancestral turtle anatomy, such as the non-saurian neodiapsid Youngina capensis (Hunt et al, 2023), the early archosauriform Prolacerta broomi (Modesto & Sues, 2004), the protorosaurian archosauromorph Tanystropheus hydroides (Spiekman et al, 2020), the early lepidosauromorph Marmoretta oxoniensis (Griffiths et al, 2021), or the early rhynchocephalian Clevosaurus hudsoni (O'Brien et al, 2018). Thus, the general shape of the testudinatan jugal consists of two functional parts (in the lateral cheek side, and in a process connecting this with the palate) that in other reptiles is often formed in two bones (i.e., jugal, ectopterygoid).…”