Shastasaurus

''Shastasaurus'' ("Mount Shasta lizard") is an extinct genus of ichthyosaur from the Late Triassic.Hilton, Richard P., ''Dinosaurs and Other Mesozoic Animals of California'', University of California Press, Berkeley 2003 , at pages 90-91. Specimens have been found in the United States, Canada, and China.

Description

left, Size of '' Shonisaurus popularis'' (green) and ''S. sikanniensis'' (red), a possible species of ''Shastasaurus'', compared with a human
''Shastasaurus'' lived during the late Triassic period. The type species, ''S. pacificus'', is known from California, with the name ''Shastasaurus'' directly referencing Shasta County, Northern California, where the type specimen was found. ''S. pacificus'' was a medium-sized ichthyosaur, measuring over in length. A second possible species of ''Shastasaurus'', ''S. sikanniensis'', is known from the Pardonet Formation British Columbia, dating to the middle Norian age (about 210 million years ago). By comparison, ''S.sikanniensis'' was one of the largest known ichthyosaurs, similar in size to modern-day cetaceans, measuring up to in length and weighing .

''Shastasaurus'' was highly specialized, and differed considerably from other ichthyosaurs. It was very slender in profile. ''S. sikanniensis'' had a ribcage slightly less than deep despite a distance of over between its flippers. With its unusually short, toothless snout (compared to the longer, toothed, dolphin-like snouts of most ichthyosaurs), it had been proposed that ''Shastasaurus'' was a suction feeder (or filled a similar ecological niche as extant beaked whales), preying primarily on soft-bodied cephalopods. However, 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.

It is unknown whether ''Shastasaurus'' had a dorsal fin; however, the smaller, more basal ichthyosaur '' Mixosaurus'' had one. The upper fluke of the tail was probably much less-developed than the shark-like tails found in later species.Wallace, D.R. (2008). ''Neptune's Ark: From Ichthyosaurs to Orcas''. University of California Press, 282pp.

Species and synonyms

The type species of ''Shastasaurus'' is ''S. pacificus'', from the late Carnian of northern California. It is known only from fragmentary remains, which have led to the assumption that it was a 'normal' ichthyosaur in terms of proportions, especially skull proportions. Several species of long-snouted ichthyosaur were referred to ''Shastasaurus'' based on this misinterpretation, but are now placed in other genera (including '' Callawayia'' and '' Guizhouichthyosaurus''). Elizabeth Nicholls and Makoto Manabe considered this species as a ''nomen dubium'' in 2000.

''Shastasaurus'' may include a second species, ''Shastasaurus liangae''. It is known from several good specimens, and was originally placed in the separate genus '' Guanlingsaurus''. Complete skulls show that it had an unusual short and toothless snout. ''S. pacificus'' probably also had a short snout, although its skull is incompletely known. However, a new juvenile specimen discovered in 2013 shows that the hyoid bone of ''Guanlingsaurus'' is much shorter, and considered it as a distinct genus based on phylogenetic analysis.

''S. sikanniensis'' was originally described in 2004 as a large species of '' Shonisaurus''. However, this classification was not based on any phylogenetic analysis, and the authors also noted similarities with ''Shastasaurus''. The first study testing its relationships, in 2011, supported the hypothesis that it was indeed more closely related to ''Shastasaurus'' than to ''Shonisaurus'', and it was reclassified as ''Shastasaurus sikanniensis''. However, a 2013 analysis supported the original classification, finding it more closely related to ''Shonisaurus'' than to ''Shastasaurus''. In the 2019 study, ''S. sikanniensis'' was pertained within the genus ''Shastasaurus''. In the 2021 analysis, ''S. sikanniensis'' forms a clade with ''Shonisaurus'', indicating that it is closer to ''Shonisaurus'' than to ''Shastasaurus''. Specimens belonging to ''S. sikanniensis'' have been found in the Pardonet Formation British Columbia, dating to the middle Norian age.

In 2009, Shang & Li reclassified the species ''Guizhouichthyosaurus tangae'' as ''Shastasaurus tangae''. However, later analysis showed that ''Guizhouichthyosaurus'' was in fact closer to more advanced ichthyosaurs, and so cannot be considered a species of ''Shastasaurus''.

Dubious species that were referred to this genus include ''S. carinthiacus'' (Huene, 1925) from the Austrian Alps and ''S. neubigi'' (Sander, 1997) from the German Muschelkalk. ''S. neubigi'', however, was re-described and reassigned to its own genus, '' Phantomosaurus''.

Synonyms of ''S.'' / ''G. liangae'':

''Guanlingichthyosaurus liangae'' Wang ''et al.'', 2008 ('' lapsus calami'')

Synonyms of ''S. pacificus'':

''Shastasaurus alexandrae'' Merriam, 1902

''Shastasaurus osmonti'' Merriam, 1902

See also

* List of ichthyosaurs
* Timeline of ichthyosaur research
* '' Shonisaurus''

References

{{Taxonbar, from=Q2700110

Late Triassic ichthyosaurs of North America
Late Triassic reptiles of Asia
Triassic California
Ichthyosauromorph genera
Fossils of Canada
Triassic Canada
Carnian genus first appearances
Norian genus extinctions
Fossil taxa described in 1895