The anatomy, taxonomy and systematics of Middle Triassic-Early Jurassic ichthyosaurs (Reptilia: Ichthyopterygia) and the phylogeny of Ichthyopterygia

Ichthyosaurs were a successful group of Mesozoic marine reptiles spanning a time interval of nearly 160 million years, from the Early Triassic to the early Late Cretaceous. Although ichthyosaurs have a long history of research, dating back to the early 18th century, significant controversies still surround their anatomy, taxonomy and evolution. The revision of the cranial anatomy of the basal ichthyosaur Cymbospondylus from the Middle Triassic (Anisian) resolves a 100-year controversy surrounding the arrangement its skull bones. Comparisons with the recently discovered colossal, predatory ichthyosaur Thalattoarchon result in the identification of shared synapomorphies, which provide evidence for a close relationship between the two taxa. Together with other, recent discoveries from China, this demonstrates the rapid ecomorphological diversification of ichthyosaurs in the early Middle Triassic. The incompletely sampled Late Triassic marine fossil record hinders our understanding of ichthyosaur evolution across the Triassic-Jurassic boundary. A new genus and species of large-bodied, early parvipelvian (fish-shaped) ichthyosaur from the Late Triassic (upper Norian) of British Columbia, Canada, is described. The new taxon is characterized by the possession of labiolingually flattened, bicarinate tooth crowns. Its co-occurrence with other small- to medium-sized parvipelvians, representing different ecomorphologies, demonstrates that parvipelvian ichthyosaurs were already ecomorphologically diverse at the beginning of their evolutionary history and maintained high ecological diversity throughout the Late Triassic. The ichthyosaur diversity of the Lower Jurassic (Hettangian–Pliensbachian) of the United Kingdom is still poorly understood, due to the lack of detailed studies of ichthyosaur anatomy. Here, I describe a new genus and species of ichthyosaur, defined by the possession of five autapomorphies and a unique combination of both plesiomorphic and derived characters. Phylogenetic analysis recovers the new taxon as an early-diverging parvipelvian, providing evidence of a temporally staggered, rather than catastrophic, ichthyosaur turnover at the Triassic-Jurassic boundary.