Revision of the Early Jurassic ichthyosaurian Temnodontosaurus zetlandicus

Parvipelvia is a major clade of ichthyosaurians that appeared during the Late Triassic and rapidly diversified into a disparate assemblage of morphologies during the Early Jurassic. However, the interrelationships of early parvipelvians are unclear and many genera are loosely diagnosed, such as Temnodontosaurus, an ecologically important genus from the Early Jurassic of Western Europe. One taxon concentrates many taxonomic issues: Temnodontosaurus acutirostris, to which ‘Ichthyosaurus’ zetlandicus is currently assigned. We redescribe the holotype of ‘Ichthyosaurus’ zetlandicus (CAMSM J35176) as well as a new specimen attributable to this taxon (MNHNL TU885), from the Toarcian of Luxembourg. We find that Temnodontosaurus zetlandicus is a valid species that can be referred to Temnodontosaurus, sharing a number of morphological traits with Temnodontosaurus nuertinguensis and Temnodontosaurus trigonodon, despite having a distinct cranial architecture. Our phylogenetic analyses under both implied weighting maximum parsimony and Bayesian inference support the integration of T. zetlandicus in Temnodontosaurus. More broadly, temnodontosaurids are relatively well clustered among Early Jurassic parvipelvians. However, the genus still appears polyphyletic, mainly due to the stronger affinity of Temnodontosaurus azerguensis with leptonectids. This highly attests the need of a more-thorough reinvestigation of the Temnodontosaurus monophyly and early parvipelvians relationships.