A new species-level phylogeny of the Ichthyopterygia and analysis of macroevolutionary trends

Two centuries of ichthyosaur research have revealed a great deal of information on the anatomy of ichthyosaurs. Recent research has put this in a systematic context using numerical phylogenetic methods, but many of these have only considered the group down to genus level, and many include a limited taxon sample. Here, I present new phylogenies for the Ichthyopterygia, coded to species level, and including every reasonably complete taxon. The data matrix comprises over 280 characters, built using a critical supermatrix methodology, coded for almost 100 taxa. Characters from these analyses were evaluated based on similarity, informativeness and exclusivity; reductive coding and non-additive binary coding methods were preferred. Character codings were derived mostly from published descriptions, focusing on type specimens where possible. Analysis was completed using several parsimony- and likelihood-based methods. These include the heuristic search, ratchet and drift functions in PAUP* and TNT, and Mk model Bayesian analysis in Mr Bayes. The phylogenies produced similar topologies from all methods, and in comparison to to previous studies, but several genera were found to be paraphyletic, such as Ichthyosaurus and Stenopterygius. Other subclades within the Ichthyopterygia were found to be well supported, and these mark transitions in several character states, for example Mixosauridae and Ophthalmosauridae. Bayesian analysis yielded poor resolution, except for clades supported by several characters. This suggests that the characters used are not all truly informative in a likelihood context: the higher resolution found in parsimony analyses may therefore be misleading. These new results show the need to reappraise the taxonomy of several ichthyosaurian taxa, particularly within the Mixosauridae and several Liassic taxa, for example Ichthyosaurus and Temnodontosaurus. Despite this, these data allow preliminary analysis of large-scale trends within the evolution of the Ichthyopterygia. The group generally shows gradual acquisition of characters throughout its evolution, punctuated by times of rapid diversification, as marked by the best-supported clades. The combination of the new, comprehensive phylogeny and comparative analysis sheds new light on the key macroevolutionary models and phases in ichthyosaurian evolution.