A total evidence analysis of the evolutionary history of the thunnosaur ichthyosaur

Ichthyosaurs first appear in the Early Triassic with an elongate, lizard-shaped anatomy. The most derived ichthyosaurs, including the Late Triassic to Early Jurassic Eurhinosauria and the Late Triassic to Mid Cretaceous Thunnosauria, evolved more streamlined fish-shaped bodies. A species-level cladistic analysis of this derived group of ichthyosaurs, using 60 characters for 17 outgroup taxa and one ingroup taxon, was conducted using PAUP* (Swofford, 1998). The new analysis is compared to that of Motani's (1999b) genus-level analysis of ichthyosaurs by looking at the clade-defining character state changes in each. I use a total evidence approach, meaning a detailed phylogenetic analysis plus data on stratigraphic and geographic occurrences, to answer two questions. First, is it more plausible to place Stenopterygius as a sister to the Ophthalmosauria or to permit the longer ghost lineage for Ophthalmosauria that would be required if Ichthyosaurus is its sister taxon, a question suggested by Motani (1999b)? A phylogenetic tree incorporating known stratigraphic ranges was constructed, based on the species-level cladistic analysis, to help quantify the stratigraphic debt using the relative completeness index (RCI), stratigraphic congruency index (SCI) and the gap excess ratio (GER). When a species-level analysis was performed, four previously defined clades fell apart, including Leptonectes, Stenopterygius, Eurhinosauria and Ophthalmosauria. Hence, Motani's (1999b) suggestion that Stenopterygius and Ophthalmosauria be placed as sister taxa is not supported by this analysis; nor is placing Ichthyosaurus as the sister group supported. Rather, the ophthalmosaurian taxa were pulled down the tree in the species-level analysis, creating even longer ghost lineages than were previously present. These longer ghost lineages are the primary cause of the observed stratigraphic debt, along with Eurhinosaurus longirostris, Leptonectes solei, and Excalibosaurus costini. Despite these increases, overall the species-level phylogeny is more stratigraphically parsimonious than Motani's (1999b) genus-level one. The second question this study addressed was to determine whether the radiation of the Ophthalmosauria was influenced by the opening of new marine habitats from the widening of the Atlantic and/or the appearance of morphological novelties. The radiation of the Ophthalmosauria into North and South America coincides with the widening of the North and South Atlantic during the Late Jurassic, as well as nine synapomorphies involving changes to the ear and lower jaw region, numerous changes in the forefins (including extra digits) and pelvis. These characters may be associated with adaptations to more efficient swimming in open ocean habitats, helping the groups' biogeographic dispersion. Hence, both tectonic events and the evolution of morphological innovations likely influenced the radiation of the Ophthalmosauria.