Shastasaurus sikanniensis

''Shastasaurus'' ("Mount Shasta lizard") is a very large extinct genus of ichthyosaur from the middle and late Triassic, and is the largest known marine reptile.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 ''Shastasaurus pacificus'' is known from California. ''S. pacificus'' was a medium-sized ichthyosaur, measuring over in length and weighing . 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). If ''S.sikanniensis'' belongs to ''Shastasaurus'', it would be the larger species, 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. Due to its unusually short, toothless snout (compared to the long, toothed, dolphin-like snouts of most ichthyosaurs) it was proposed that ''Shastasaurus'' was a suction feeder, feeding primarily on soft-bodied cephalopods, although current research indicates ichthyosaur jaws do not fit the suction-feeding profile.

It is unknown whether ''Shastasaurus'' had a dorsal fin, however, the more basal ichthyosaur ''Mixosaurus'' had dorsal fin. 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'').

''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. ''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 (about 210 million years ago).

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

Middle Triassic ichthyosaurs
Late Triassic ichthyosaurs of North America
Middle Triassic reptiles of Asia
Middle Triassic reptiles of North America
Late Triassic reptiles of Asia
Triassic California
Ichthyosauromorph genera
Fossils of Canada
Triassic Canada