Observations on Triassic ichthyosaurs.: Part X:: The Lower Triassic Merriamosaurus from Spitzbergen -: additional data on its anatomy and phylogenetic position

The material of the Lower Triassic ichthyosaur Merriamosaurus hulkei from the Sticky Keep Formation (Spathian) of Spitzbergen housed in the Paris collection is described in detail. Important new data on all parts of the skeleton, particularly the skull, caudal vertebrae, limb girdles and hindlimb of this little known taxon are provided. Phylogenetically, Merriamosaurus is the most highly derived Lower Triassic ichthyosaur, being the most basal member of the Merriamosauria. This indicates that the early radiation of ichthyosaurs in the Lower Triassic was much more extensive than previously assumed as considerable ghost lineages have to be postulated for several Triassic ichthyosaur clades, including, apart from the Merriamosauria, the families Cymbospondylidae, Toretocnemidae and Mixosauridae which must have all formed part of this radiation.