Mixosauridae was an early group of ichthyosaurs, living between 247.2 and 235 million years ago, during the Triassic period.[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.
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.[4] The name Mixosauria has been used for a larger group contianing Mixosauridae,[5][6] but also as an equvialent term for Mixosauridae,[3] resulting in Mixosauria being regarded as a junior synonym of Mixosauridae.[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.[8] 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.[7]
There are six species of mixosaurids widely accepted as valid: Mixosaurus cornalianus, Mixosaurus kuhnschnyderi, Mixosaurus panxianensis, Phalarodon atavus, Phalardon callawayi, and Phalarodon fraasi.[9][10] Mixosaurus xindianensis is sometimes also considered valid,[11] but has also been treated as a species inquirenda.[9] Other mixosaurid species have been proposed in the past but subsequently had their validity questioned or rejected. These include Mixosaurus nordenskioeldii, of which Phalarodon fraasi was traditionally seen a junior synonym,[12] and Mixosaurus maotianensis, for which the genus Barracudasaurus was proposed, before the referred specimens were reassigned to M. panxianensis,[13] among others.[14] Grippia was once considered a junior synonym of Mixosaurus,[15] however, restudy has revealed that the two genera are significantly different,[16] and Grippia is now understood to instead be a basal ichthyopterygian, not a mixosaurid.[8][7] The very poorly-known Tholodus has also been proposed to be a mixosaurid,[17]: 69 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.[18] Additionally, a specimen potentially belonging to the toretocnemid Qianichthyosaurus was initially misidentified as a species of Mixosaurus, M. guanlingensis.[19][17]: 127
The number of mixosaurid genera is controversial.[20][21][14] Traditionally, Mixosaurus was generally regarded as the only valid genus of mixosaurids,[22][15][23] and this system of classification continued to be used into the 21st century.[21][14] However, Phalarodon was also sometimes treated as a separate genus,[5][1] a position which later became widely accepted.[14][8][10] In 1998, Maisch and Matzke named a new genus, Contectopalatus, for P. atavus, and later maintained its distinctiveness from Mixosaurus and Phalarodon.[1][24][6] M. panxianensis is also sometimes treated as a separate genus, Barracudasauroides.[6][18][25] Additionally, M. kuhnschnyderi was initially named as a separate genus, Sangiorgiosaurus, by Brinkmann in 1998,[23] who sunk it into Mixosaurus later during the same year,[26] an assignment agreed upon by other authors.[20] In 2017, Moon cautioned that the standard concepts of Mixosaurus and Phalarodon may not be monophyletic.[7] The following cladograms show two hypotheses for the evolutionary relationships between different mixosaurid species.