Naming and definitions Mixosauridae was named by
Georg Baur in 1887 as a
family-level group to contain the new genus,
Mixosaurus that he named in the same publication. The name Mixosauria has been used for a larger group containing Mixosauridae, which was applied to its equivalent group Mixosauridae by Maisch and Matzke in 2000. This definition was emended by Ji and colleagues in 2016 by replacing
Mixosaurus nordenskioeldii with
Phalarodon fraasi, as the former had since been determined to not be diagnostic. The definition was changed again in 2017, this time by Moon. As the evolutionary relationships his analyses found would have resulted in many traditional mixosaurids falling outside of the group, he redefined it as being all ichthyosaurs more closely related to
Mixosaurus cornalianus than
Ichthyosaurus communis for consistency.
Composition There are six species of mixosaurids widely accepted as valid:
Mixosaurus cornalianus,
Mixosaurus kuhnschnyderi,
Mixosaurus panxianensis,
Phalarodon atavus,
Phalardon callawayi, and
Phalarodon fraasi. An additional species,
Mixosaurus luxiensis, was named in 2024.
Mixosaurus xindianensis is sometimes also considered valid, but has also been treated as a
species inquirenda. and
Mixosaurus maotianensis, for which the genus
Barracudasaurus was proposed, before the referred specimens were reassigned to
M. panxianensis, among others.
Grippia was once considered a junior synonym of
Mixosaurus, however, the very fragmentary nature of its remains make its relationships unclear, and it has also been proposed to be related to various other ichthyopterygians. and this system of classification continued to be used into the 21st century. Additionally,
M. kuhnschnyderi was initially named as a separate genus,
Sangiorgiosaurus, by Brinkmann in 1998, an assignment agreed upon by other authors. In 1908, Merriam remarked that it was difficult to reconstruct the interrelationships of ichthyosaurs with confidence. However, he considered that all well-known Triassic ichthyosaurs at the time were too specialized to have been ancestral to the later species, pointing to the anatomy of the ribs in particular. Therefore, he proposed an early split between the Triassic ichthyosaurs and the post-Triassic ichthyosaurs. In the 1920s, von Huene proposed a classification scheme where ichthyosaurs were divided into two different groups, the latipinnates and longipinnates, which split from each other in the Triassic and both persisted into the Cretaceous. These divisions were based primarily on the structure of the forelimb, though McGowan argued in 1972 that the two groups could be differentiated by skull proportions as well. Under this classification scheme, mixosaurids were classified as early latipinnates, with von Huene believing them to be the direct ancestors of
Ichthyosaurus. However, in 1979, Appleby reassessed mixosaurid anatomy, and found it to be very specialized. These specializations did not suggest to him that mixosaurids were ancestral to the later latipinnates; often their features differed markedly or the mixosaurids were more specialized than their supposed descendants. Consequently, he named a new monotypic order for the mixosaurids, Mixosauroidea, and instead argued that the post-Triassic latipinnates evolved from the longipinnate line. The first cladogram of Ichthyopterygia was published by Mazin in 1981, Nicholls and colleagues in 1999 placed mixosaurids in Ichthyosauria, arguing based on tooth and shoulder girdle anatomy that they were the sister taxon of a group composed of
Utatsusaurus,
Grippia, and
Omphalosaurus. These two groups were placed in a suborder that was named Mixosauria. In 1999 and 2000, multiple major phylogenetic analyses of Ichthyopterygia were published. Thus, mixosaurids were found to be members of Ichthyosauria. Maisch did not follow this hypothesis in his 2010 review, though he used the name Mixosauria for a group containing mixosaurids and
Wimanius. Cladogram following Ji and colleagues, 2016. }} Cladogram following the preferred tree of Moon, 2017. }} ==Description==