Mixosauridae
Mixosauridae was an early group of ichthyosaurs, living between 247.2 and 235 million years ago, during the Triassic period. Fossils of mixosaurs have been found across the Northern Hemisphere.
History and classification
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, but also as an equivalent term for Mixosauridae, resulting in Mixosauria being regarded as a junior synonym of Mixosauridae. Motani defined the clade Mixosauria as comprising all descendants of the last common ancestor of Mixosaurus cornalianus and M. nordenskioeldii, 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. 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, 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, restudy has revealed that the two genera are significantly different, and Grippia is now understood to instead be a basal ichthyopterygian, not a mixosaurid. The very poorly-known Tholodus has also been proposed to be a mixosaurid, 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. Additionally, a specimen potentially belonging to the toretocnemid Qianichthyosaurus was initially misidentified as a species of Mixosaurus, M. guanlingensis.The number of mixosaurid genera is controversial. Traditionally, Mixosaurus was generally regarded as the only valid genus of mixosaurids, and this system of classification continued to be used into the 21st century. However, Phalarodon was also sometimes treated as a separate genus, a position which later became widely accepted. In 1998, Maisch and Matzke named a new genus, Contectopalatus, for P. atavus, and later maintained its distinctiveness from Mixosaurus and Phalarodon. M. panxianensis is also sometimes treated as a separate genus, Barracudasauroides. Additionally, M. kuhnschnyderi was initially named as a separate genus, Sangiorgiosaurus, by Brinkmann in 1998, who sunk it into Mixosaurus later during the same year, an assignment agreed upon by other authors. While most researchers accept only Mixosaurus and Phalarodon as valid, in 2017, Moon cautioned that the standard concepts of Mixosaurus and Phalarodon may not be monophyletic. The following cladograms show two hypotheses for the evolutionary relationships between different mixosaurid species.
Cladogram following the 50% majority rule consensus tree of Fang and colleagues, 2024.
Cladogram following the preferred tree of Moon, 2017.
Higher-level classification
In his 1887 description, Baur recognized mixosaurids as a "primitive" group of ichthyosaurs. In 1904, Boulenger considered Ichthyosauria as splittable into three divisions, with Mixosaurus an early member of the group leading to the wide-finned Ichthyosaurus. 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.
The classification of ichthyosaurs into latipinnates and longipinnates persisted for many decades.
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. 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.
The first cladogram of Ichthyopterygia was published by Mazin in 1981, in which mixosaurids were found to fall outside Ichthyosauria, though in a more derived position than Grippia. Based on these results, Mazin argued that heterodonty was the ancestral conidition in ichthyopterygians. 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. These studies all agreed upon a general framework, with three nested groups: the early "basal grade", followed by an intermediate grade, followed in turn by the derived, "fish-shaped" ichthyosaurs. Utatsusaurus and Grippia were found to belong to the basal grade, but mixosaurids were instead recovered in the intermediate grade, together with the shastasaurids and Cymbospondylus. Thus, mixosaurids were found to be members of Ichthyosauria. The first of these analyses was done by Motani in 1999, who found mixosaurids to be more derived than Cymbospondylus but less so than the true shastasaurs. The phylogenetic analysis run by Sander and Maisch and Matzke the next year instead found mixosaurids to be more basal than Cymbospondylus. Additionally, Maisch and Matzke argued that the poorly-known Wimanius was the sister taxon of Mixosauridae. Due to its fragmentary nature, however, both Motani and Sander considered the relationships of this genus provisional, with Sander instead considering it a shastasaurid.
In 2008, Maisch and colleagues noted that toretocnemids shared multiple features with mixosaurids, and suggested that the two groups might be closely related, rather than the toretocnemids having branched off later. As they did not run an analysis to test this hypothesis, however, they considered it provisional. Maisch did not follow this hypothesis in his 2010 review, though he used the name Mixosauria for a group containing mixosaurids and Wimanius. Further phylogenetic analyses were conducted since, often drawing from the analysis of Motani and that of Maisch and Matzke. In 2016, Ji and colleagues found mixosaurids to be more derived than cymbospondylids based on their analysis. However, in 2017, Moon recovered the mixosaurids as more basal. Additionally, Wimanius was found to be a more derived member of Ichthyosauria than the mixosaurids. Moon also noted that the "intermediate grade" ichthyosaurs could be divided into two subgroups, with mixosaurids and Cymbospondylus part of the early grade, paraphyletic to the later grade which gave rise to the later ichthyosaurs. The relative positions of mixosaurids and Cymbospondylus remain unresolved.
Cladogram following Ji and colleagues, 2016.
Cladogram following the preferred tree of Moon, 2017.