Description, phylogenetic analysis and taphonomy of a new Thalattosaur from the Brisbois member of the Vester Formation (Carnian/Norian) of central Oregon

Abstract

Thalattosaurs are a poorly understood group of exclusively Triassic, secondarily aquatic tetrapods. Despite being first described over a century ago, their morphology and evolutionary history has been clouded by a spotty fossil record and poor specimen preservation. Here, a new thalattosaur genus is established on the basis of newly-discovered three dimensional cranial and postcranial elements from multiple individuals of different ontogenetic stages. The specimens were found within a single calcareous conglomerate nodule from the Brisbois Member of the Vester Formation in central Oregon, USA. The Brisbois Member thalattosaur is a relatively large taxon with an estimated total length of 4 - 5 m. Numerous cranial autapomorphies help diagnose the new taxon, including a ventrally deflected rostrum bearing a rugose ornamentation. The first three dimensionally preserved thalattosaur braincase anatomy is also described along with elements from nearly every region of the postcranium. Using high resolution surface laser scans, the first three-dimensional digital reconstruction of any thalattosaur skull is presented. Phylogenetic analysis indicates the Brisbois Member thalattosaur is a basal member of Thalattosauroidea, a clade that includes other taxa with highly modified rostra from Europe and China.