Ichthyosaur expert, Dr. Dean Lomax, brings up the question
in this video, “What is an ichthyosaur?”
The problem is,
Dr. Lomax tells us only what ichthyosaurs are not. He has fun reporting that ichthyosaurs are not swimming dinosaurs.
in the large reptile tree (LRT, 1326), which tests all candidate taxa, ichthyosaurs nest with thalattosaurs + mesosaurs, derived from basal sauropterygians. All are derived from marine younginiforms in the Permian. These are derived from Late Carboniferous diapsids arising from the pro-diapsid clade within the new Archosauromorpha.
Three years later,
Chen et al.. 2014 suggested Wumengosaurus might be related to basal ichthyosaurs and hupehsuchids. So, once again, you heard it here first… but I’m not giving Chen et al. much credit because their cladogram excludes so many taxa that, as a result, it recovers a bogus mix of archosauromorph and lepidosauromorph taxa (Fig. 4).
Dr. Lomax also asks,
“What is Ichthyosaurus (and the various species within this genus)?” In this portion of the video, Dr. Lomax is extremely informative, showing distinctions made with skeletons—not with teeth, which can vary within one set of jaws.
Chen X-H, Motani R, Long C, Jiang D-Y and Rieppel O 2014. “The enigmatic marine reptile Nanchangosaurus from the Lower Triassic of Hubei, China and the phylogenetic affinities of Hupehsuchia”. PLoS ONE. 9 (7): e102361. online here