Phylogeny, systematics, and origin of the Ichthyosauria - the state of the art

A historical overview and a synopsis of the current knowledge on the phylogeny, systematics, and phylogenetic position of the Ichthyosauria are presented. All known ichthyosaur taxa are listed, new taxa are discussed and diagnosed. The following new taxa are erected: Thaisauridae n. fam., Wimaniidae n. fam., Barracudasauroides n. gen. with Mixosaurus panxianensis JIANG et al., 2006 as type species, as well as the species Omphalosaurus merriami n. sp. Merriamosaurus MAISCH & MATZKE, 2003 is a junior synonym of Pessopteryx WIMAN, 1910. “Ichthyosaurus” acutirostris OWEN, 1840 probably represents a genus of its own. Based on this summary of our current understanding of the group as a whole, ichthyosaurs are included in two of the largest and more widely used phylogenetic analyses of the Amniota. Character codings and their justifi cations are discussed. No clear signal on the phylogenetic position of the Ichthyosauria results from these analyses. Instead, they are in one case nested within the Diapsida, but with the anapsid Mesosauria as their sister group. In the other case they are nested within Parareptilia, with Procolophonia (Testudines, Pareiasauria) as sister group. In contrast to previous assumptions, the inclusion of ichthyosaurs in these large-scale analyses always changes the original topology of the resulting cladograms so that turtles and pareiasaurs become sister groups. This underlines the importance of the taxon Ichthyosauria for any future large-scale phylogenetic analyses of amniotes. At the present state of knowledege, a defi nite decision on the origin and phylogenetic position of ichthyosaurs is still impossible, but descent from primarily anapsid ancestors and an origin from among the Parareptilia can not be excluded as an alternative to a diapsid origin of the group.