Novel insights into the morphology of Plesiochelys bigleri from the early Kimmeridgian of Northwestern Switzerland

Plesiochelyidae were relatively large coastal marine turtles, which inhabited the epicontinental seas of Western Europe during the Late Jurassic. Their fossil record can be tracked in Germany, Switzerland, the United Kingdom, France, Spain and Portugal. The Jura Mountains, in northwestern Switzerland, have been the main source for the study of this group, mostly thanks to the rich and famous historical locality of Solothurn. In the last two decades, numerous plesiochelyid remains have been collected from Kimmeridgian deposits (Lower Virgula Marls and Banné Marls) in the area of Porrentruy (Canton of Jura, Switzerland). This material was revealed by construction works of the A16 Transjurane highway between 2000 and 2011, and led to the recent description of the new species Plesiochelys bigleri. In the years 2014 and 2016, new fragmentary turtle material was collected from the Banné Marls (Reuchenette Formation, lower Kimmeridgian) near the village of Glovelier, Canton of Jura, Switzerland. The new material consists of a complete shell, additional shell elements, a few bones from the appendicular and vertebral skeleton, and a fragmentary basicranium. This material can be confidently assigned to the species P. bigleri. It supports the presence of this species in the Banné Marls, slightly extends its spatial distribution and confirms the differences with the closely related species P. etalloni. The new material reveals that the split between the cerebral and palatine branches of the internal carotid artery occurs in a vertical plane in P. bigleri. This condition could not be observed in the type material due to poor preservation. This new character clearly distinguishes P. bigleri from P. etalloni and seems to be unique among thalassochelydians.