Shastasauridae

Shastasauridae Temporal range: Triassic
Skeleton of Shastasaurus sikanniensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Ichthyosauria
Node:	†Merriamosauria Motani, 1999
Family:	†Shastasauridae Merriam, 1895
Genera
†Besanosaurus? †Himalayasaurus †Ichthyotitan? †Guanlingsaurus †Guizhouichthyosaurus? †Shonisaurus †Shastasaurus

Shastasauridae is an extinct family of Triassic ichthyosaurs that includes the genera Shastasaurus, Shonisaurus and Himalayasaurus.^[1] Many other Triassic ichthyosaurs have been assigned to Shastasauridae in the past, but recent phylogenetic analyses suggest that these species form an evolutionary grade of early ichthyosaurs rather than a true clade or evolutionary grouping that can be called Shastasauridae.

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."^[2] 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.^[3][4]

Shonisauridae

Shonisauridae (Camp, 1980) was a prosed group of ichthyosaurs, living in the late Triassic period,^[5][4] and now within Shastasauridae.

Description

In May 2016,^[6] researcher and fossil collector Paul de la Salle discovered a partial jawbone measuring 96 centimeters (3.15 ft) 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 26 meters (85 ft) long, nearly the size of a blue whale. Scaling based on Besanosaurus, however, found a shorter length estimate of 22 meters (72 ft).^[7] In 2024, the Lilstock specimen was referred to the newly described species Ichthyotitan, with a length estimate of up to 25 meters (82 ft).^[8]

Feeding Habits

Unlike other Triassic ichthyosaurs, which fed almost exclusively on cephalopods,^[9] shastasaurians fed on a variety of prey.^[10] Evidence for this prey diversity includes gut contents from Guizhouichthyosarus tangae, Shonisaurus popularis, and an unnamed specimen from the Brooks Range of Alaska.^[10]

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,^[11] and since some species like Shonisaurus had robust sectorial teeth with gut contents of mollusk shells and vertebrates.^[12][10]

References

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