First record of lithophagy in Sulcusuchus erraini (Plesiosauria; Polycotylidae) with comments on the taphonomic and collect bias in gastroliths

Gastroliths have long been reported in marine reptiles and are particularly well documented in elasmosaurid plesiosaurs, while their occurrence in short-necked polycotylids is notably rare. Here, we report the first documented occurrence of lithophagy in the polycotylid plesiosaur Sulcusuchus erraini Gasparini & Spalletti, based on 95 gastroliths recovered from the Maastrichtian levels of La Colonia Formation, Chubut, Patagonia Argentina. The MPEF-PV 11698 (MPEF, Museo Paleontológico Egidio Feruglio) represents one of the few records of lithophagy in polycotylids and is the first for the Maastrichtian stage. The observed shapes are mainly classified as discoidal or spherical. Morphometric analysis using the Maximum Projection Sphericity index (Ψ; mean = 0.7, standard deviation = 0.1) and the Oblate – Prolate index (OP; mean = 0.08, standard deviation = 4.71) suggests a probable fluvial origin. To assess the potential taphonomic bias on Ψ and OP values, two analyses were performed: one using correlation and a second one considering resampling analysis. The results of both indicated that the mean index values remain robust even with up to 50% loss of the gastrolith cluster. The maximum transverse section observed (12.24 cm2) within the gastrolith cluster suggests that S. erraini possessed a relatively large oesophagus compared to other plesiosaurs analyzed.