The shell and pelvic anatomy of the Late Jurassic turtle Platychelys oberndorferi based on material from Solothurn, Switzerland

Abstract

Platychelys oberndorferi is a stem pleurodire from the Late Jurassic of Europe. The majority of informative specimens originate from the Late Jurassic (late Kimmeridgian) Turtle Limestones exposed north of the city of Solothurn, Switzerland, but these findings remain poorly documented despite their global importance for understanding the evolution of pleurodire turtles. The available material of this species from this locality currently consists of five shells, one of which is associated with two cervical and one caudal vertebrae and a second with two caudal vertebrae and a phalanx, and 16 fragments mostly found in close proximity to one another but representing at least two different individuals. The Swiss material documents variation in the shape of the dorsal protuberances formed by the vertebrals, the number of supramarginals, the presence of a preneural, the orientation and lateral contacts of the neurals, fusion of neural VIII with suprapygal I, fusion of suprapygal II with the pygal, and the shape of the anal notch. All documented variation falls within the range of variation observed in material from the Late Jurassic (early Tithonian) type locality of Kelheim, Germany, and their referral to Platychelys oberndorferi is therefore confirmed. The utility of confluent plastral fontanelles in diagnosing the sexes is refuted. However, two specimens document substantial differences in anal notch morphology, which are confirmed to be consistent with sexual dimorphism, with females having a shallow but broad anal notch framed by rounded xiphiplastral processes, whereas males have a narrow anal notch that is framed by pointed xiphiplastral processes.