Figure 1. Ichthyosaur skulls in phylogenetic order (top to bottom). Those below the red line have not been ordered yet. All of those below the red line have a naris/lacrimal contact and many do not have a naris/maxilla contact. They are mostly Jurassic and Cretaceous taxa. Boxed specimens are not yet tested. Many illustrations from Maisch and Matzke 2000. Click to enlarge. Not to scale.
Ichthyosaur phylogeny has been examined by Motani (1999), Maisch and Matzke (2000) and Maisch (2010). The large reptile tree (Fig. 2) offers yet another solution and finally have the correct outgroup taxa included. All four of these studies are broadly similar, but do differ from each other in detail.
Figure 2. Ichthyosaur subset of the large reptile tree. This tree follows the broad topology of prior trees, but all trees differ in detail.
Note (Fig. 2) the two Shonisaurus specimens do not nest together. Neither do the two Cymbospondylus specimens. Earlier we talked about all the specimens attributed to Shastasaurus.
This is a continuing study.
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
Maisch MW and Matzke AT 2000. “The Ichthyosauria”. Stuttgarter Beiträge zur Naturkunde: Serie B 298: 159.
Motani R 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology 19(3):473-496.’