A revised cranial description of Massospondylus carinatus Owen (Dinosauria: Sauropodomorpha) based on computed tomographic scans and a review of cranial characters for basal Sauropodomorpha

Massospondylus carinatus is a basal sauropodomorph dinosaur from the early Jurassic Elliot Formation of South Africa. It is one of the best-represented fossil dinosaur taxa, known from hundreds of specimens including at least 13 complete or nearly complete skulls. Surprisingly, the internal cranial anatomy of M. carinatus has never been described using computed tomography (CT) methods. Using CT scans and 3D digital representations, we digitally reconstruct the bones of the facial skeleton, braincase, and palate of a complete, undistorted cranium of M. carinatus (BP/1/5241). We describe the anatomical features of the cranial bones, and compare them to other closely related sauropodomorph taxa such as Plateosaurus erlenbergiensis, Lufengosaurus huenei, Sarahsaurus aurifontanalis and Efraasia minor. We identify a suite of character states of the skull and braincase for M. carinatus that sets it apart from other taxa, but these remain tentative due to the lack of comparative sauropodomorph braincase descriptions in the literature. Furthermore, we hypothesize 27 new cranial characters useful for determining relationships in non-sauropodan Sauropodomorpha, delete five pre-existing characters and revise the scores of several existing cranial characters to make more explicit homology statements. All the characters that we hypothesized or revised are illustrated. Using parsimony as an optimality criterion, we then test the relationships of M. carinatus (using BP/1/5241 as a specimen-level exemplar) in our revised phylogenetic data matrix

A chronostratigraphic framework for the upper Stormberg Group: Implications for the Triassic-Jurassic boundary in southern Africa

The upper Stormberg Group (Elliot and Clarens formations) of the main Karoo Basin is well-known for its fossil vertebrate fauna, comprising early branching members of lineages including mammals, dinosaurs, and turtles. Despite 150 years of scientific study, the upper Stormberg Group lacks radioisotopic age constraints and remains coarsely dated via imprecise faunal correlations. Here we synthesise previous litho- and magnetostratigraphic studies, and present a comprehensive biostratigraphic review of the upper Stormberg fauna. We also present the results of the first geochronological assessment of the unit across the basin, using U-Pb dates derived from detrital zircons obtained from tuffaceous sandstones and siltstones, the youngest of which are considered maximum depositional ages. Our results confirm that the Elliot Formation contains the Triassic–Jurassic boundary, making it one of the few fossiliferous continental units that records the effects of the end-Triassic Mass Extinction event. Our work suggests a mid-Norian–Rhaetian age for the lower Elliot Formation and a Hettangian–Sinemurian age for the upper Elliot Formation, although the precise stratigraphic position of the Triassic/Jurassic (Rhaetian/Hettangian) boundary remains somewhat uncertain. A mainly Pliensbachian age is obtained for the Clarens Formation. The new dates allow direct comparison with better-calibrated Triassic-Jurassic faunas of the Western Hemisphere (e.g., Chinle and Los Colorados formations). We show that sauropodomorph, but not ornithischian or theropod, dinosaurs were well-established in the main Karoo Basin ~220 million years ago, and that typical Norian faunas (e.g., aetosaurs, phytosaurs) are either rare or absent in the lower Elliot Formation, which is paucispecific compared to the upper Elliot. While this is unlikely the result of geographic sampling biases, it could be due to historical sampling intensity differences