Shonisauridae

Shastasauridae is an extinct family of ichthyosaurs from the Late Triassic with a possible Early Jurassic record. The family contains the largest known species of ichthyosaurs, which include some of and possibly the largest known marine reptiles.

Taxonomy

Shastasauridae was named by American paleontologist John Campbell Merriam in 1895 along with the newly described genus ''Shastasaurus''. In 1999, Ryosuke Motani erected the clade Shastasauria to include '' Shastasaurus'', '' Shonisaurus'', and several other traditional shastasaurids, defining it as a stem-based taxon including "all merriamosaurians more closely related to '' Shastasaurus pacificus'' than to '' Ichthyosaurus communis''." He also redefined Shastasauridae as a node-based taxon including "the last common ancestor of ''Shastasaurus pacificus'' and '' Besanosaurus leptorhynchus'', and all its descendants" and Shastasaurinae, which Merriam named in 1908, as a stem taxon including "the last common ancestor of ''Shastasaurus'' and ''Shonisaurus'', and all its descendants." In an alternative classification scheme, paleontologist Michael Maisch restricted Shastasauridae to the genus ''Shastasaurus'' and placed ''Shonisaurus'' and ''Besanosaurus'' in separate families, Shonisauridae and Besanosauridae, respectively. In various studies, the grouping of Shastasauridae has been variously found to be either monophyletic or paraphyletic. Studies that have recovered the group as monophyletic generally include ''Shastasaurus'', ''Besanosaurus'', '' Guanlingsaurus'', '' Guizhouichthyosaurus'', ''Shonisaurus'' and ''‘Callawayia''’ ''wolonggangense'' within the group.

Description

Shastasaurids as typically defined have elongate bodies, with over 55 presacral vertebrae. They were the largest ichthyosaurs, with even some of the smaller species like '' Guanlingsaurus'' measuring over in length. One of the largest specimens was discovered in England in May 2016, when researcher and fossil collector Paul de la Salle discovered a partial jawbone measuring long which was catalogued as BRSMG Cg2488, also referred to as the Lilstock specimen. In 2018, Dean Lomax, de la Salle, Judy Massare, and Ramues Gallois identified the Lilstock specimen as a shastasaurid. While its incompleteness made the size of the animal difficult to suggest, it clearly was very large. Using ''Shonisaurus sikanniensis'' as a model, the researchers estimated the ichthyosaur to have been long, nearly the size of a blue whale. Scaling based on ''Besanosaurus'', however, found a shorter length estimate of . In 2024, the Lilstock specimen was referred to the newly described species '' Ichthyotitan'', with a length estimate of up to .

Feeding habits

Unlike other Triassic ichthyosaurs, which fed almost exclusively on cephalopods, shastasaurians fed on a variety of prey. Evidence for this prey diversity includes gut contents from '' Guizhouichthyosarus tangae,'' '' Shonisaurus popularis,'' and an unnamed specimen from the Brooks Range of Alaska.

Although older studies have suggested that shastasaurids were suction-feeders, current research indicates that the jaws of shastasaurid ichthyosaurs do not fit the suction-feeding profile, since their short and narrow hyoid bones are unsuitable to withstand impact forces for such kind of feeding, and since some species like '' Shonisaurus'' had robust sectorial teeth with gut contents of mollusk shells and vertebrates.

References

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Ichthyosauria
Triassic ichthyosaurs
Triassic reptiles of North America
Triassic California
Fossils of the United States
Paleontology in California
Prehistoric reptile families