[2][1][3] Fossils of mixosaurs have been found all over the world: China, Timor, Indonesia, Italy, Germany, Spitsbergen, Switzerland, Svalbard, Canada, Alaska, and Nevada.

[1][7] Motani defined the clade Mixosauria as comprising all descendants of the last common ancestor of Mixosaurus cornalianus and M. nordenskioeldii,[3] which was applied to its equivalent group Mixosauridae by Maisch and Matzke in 2000.

[1]: 95  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.

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.

[21][22][15] Traditionally, Mixosaurus was generally regarded as the only valid genus of mixosaurids,[23][16][24] and this system of classification continued to be used into the 21st century.

[6][19][26] Additionally, M. kuhnschnyderi was initially named as a separate genus, Sangiorgiosaurus, by Brinkmann in 1998,[24] who sunk it into Mixosaurus later during the same year,[27] an assignment agreed upon by other authors.

[4] In 1904, Boulenger considered Ichthyosauria as splittable into three divisions, with Mixosaurus an early member of the group leading to the wide-finned Ichthyosaurus.

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.

[29]: 87–89  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.

[30] 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.

[31] Under this classification scheme, mixosaurids were classified as early latipinnates, with von Huene believing them to be the direct ancestors of Ichthyosaurus.

Consequently, he named a new monotypic order for the mixosaurids, Mixosauroidea, and instead argued that the post-Triassic latipinnates evolved from the longipinnate line.

[23] Despite initially supporting the dichotomy, McGowan would go even further than Appleby in overturning the latipinnate-longipinnate classification, considering that the differences separating the two groups were too ambiguous to be valid.

[17] 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.

[3] The phylogenetic analysis run by Sander and Maisch and Matzke the next year instead found mixosaurids to be more basal than Cymbospondylus.

[7] Parvinatator wapitiensis Thalattoarchon saurophagis Xinminosaurus catactes Mixosauridae Pessopteryx nisseri Cymbospondylus spp.

Phantomosaurus neubigi Besanosaurus leptorhynchus Californosaurus perrini Phalarodon major Qianichthyosaurus xingyiensis Wimanius odontopalatus Shastasauridae Euichthyosauria

[18]: 47 [36][34] Mixosaurids show a variety of adaptations for life in the water, with their limbs modified into fins[22] and their deep,[18]: 47  streamlined bodies.

[22][39] The front region of the mixosaurid skull is drawn out into a long, thin snout, with lengthened premaxillae in the upper jaw.

At their hind ends, the premaxillae taper to points, and do not form much of the rims of the external nares, the openings which housed the nostrils.

[22] The upper, or dorsal, processes of the maxillae contact the prefrontals on both the inside and the outside of the skull,[1]: 95  which blocks the lacrimals from the borders of the external nares.

[24] These terraces flank a long sagittal crest that extends along the skull roof, spanning across three pairs of bones, the nasals, frontals, and parietals, from front to back.

[22][24] The parietals bear a ridge towards their posterior end, and the processes stemming from their rear fork are short, as seen in later ichthyosaurs.

[22][37] Mixosaurids have distinctively tall and thin neural spines,[24][1]: 95  whose height is much greater than that of the centra (the bodies of the vertebrae).

[33] The zygapophyses, pairs of interlocking projections, are strongly developed in the neck region, but are reduced further back in the trunk, if not totally lacking.

The underside of the trunk sported gracile gastralia, or belly ribs, each of which have a small prong extending forwards.

[1]: 42  The scapulae are broad, semicurcular bones, with their blades resembling fans or axheads in shape,[22][37] except in P. atavus, where the edge before the shoulder joint is straight.

Both the front and back sides of the radius are bowed inwards, while the ulna's rear edge is unnotched, with the exception of P.

Each pubic bone typically bears a small opening called an obturator foramen, though it is occasionally lacking in M. cornalianus.

These specimens also show that M. cornalianus had a dorsal fin, which was internally supported by collagen fibers and set far forwards on the body, where the trunk was deepest.