Shastasauridae Explained

Shastasauridae is an extinct family of Triassic ichthyosaurs.[1] 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."[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] 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.[5]

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 8m (26feet) in length.[6] One of the largest specimens was discovered in England in May 2016,[7] when researcher and fossil collector Paul de la Salle discovered a partial jawbone measuring 96cm (38inches) 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 26m (85feet) long, nearly the size of a blue whale. Scaling based on Besanosaurus, however, found a shorter length estimate of 22m (72feet).[8] In 2024, the Lilstock specimen was referred to the newly described species Ichthyotitan, with a length estimate of up to 25m (82feet).[9]

Feeding habits

Unlike other Triassic ichthyosaurs, which fed almost exclusively on cephalopods,[10] shastasaurians fed on a variety of prey.[11] 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,[12] and since some species like Shonisaurus had robust sectorial teeth with gut contents of mollusk shells and vertebrates.[13]

Notes and References

  1. N. B. . Fröbisch . J. R. . Fröbisch . P. M. . Sander . L. . Schmitz . O. . Rieppel . 2013 . Macropredatory ichthyosaur from the Middle Triassic and the origin of modern trophic networks . Proceedings of the National Academy of Sciences . 10.1073/pnas.1216750110 . 110 . 4 . 1393–1397 . 23297200 . 3557033. 2013PNAS..110.1393F . free .
  2. Motani . R. . Phylogeny of the Ichthyopterygia . 10.1080/02724634.1999.10011160 . Journal of Vertebrate Paleontology . 19 . 3 . 473–496 . 1999 . 1999JVPal..19..473M .
  3. Michael W. Maisch . 2010 . Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art . Palaeodiversity . 3 . 151–214 .
  4. Sander . P. M. . Chen . X. . Cheng . L. . Wang . X. . Claessens . Leon . Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaurs . 10.1371/journal.pone.0019480 . PLOS ONE . 6 . 5 . e19480 . 2011 . 21625429. 3100301 . 2011PLoSO...619480S . free .
  5. Bindellini . Gabriele . Wolniewicz . Andrzej S. . Miedema . Feiko . Scheyer . Torsten M. . Sasso . Cristiano Dal . 2021-05-06 . Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications . PeerJ . en . 9 . e11179 . 10.7717/peerj.11179 . free . 33996277 . 8106916 . 2167-8359. 2434/924636 . free .
  6. Sander . P.M. . Chen . X. . Cheng . L. . Wang . X. . vanc . 2011 . Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaurs . PLOS ONE . 6 . 5. e19480 . 10.1371/journal.pone.0019480 . Claessens . Leon . 21625429 . 3100301. 2011PLoSO...619480S . free .
  7. News: Geggel . Laura . 2018 . Prehistoric Sea Monster Was Nearly the Size of a Blue Whale .
  8. De la Salle . Paul . R. Lomax . Dean . A. Massare . Judy . Gallois . Ramues . vanc . 2018 . A giant Late Triassic ichthyosaur from the UK and a reinterpretation of the Aust Cliff 'dinosaurian' bones . PLOS ONE . 10.6084/m9.figshare.5975440 . 13 . 4 .
  9. Lomax. Dean R.. De la Salle. Paul. Massare. Judy A.. Gallois. Ramues. 2018-04-09. Wong. William Oki. A giant Late Triassic ichthyosaur from the UK and a reinterpretation of the Aust Cliff 'dinosaurian' bones. PLOS ONE. en. 13. 4. e0194742. 10.1371/journal.pone.0194742. 1932-6203. 5890986. 29630618. 2018PLoSO..1394742L. free.
  10. Rieber. Hans. May 1970. Phragmoteuthis? ticinensis n. sp., ein Coleoidea-Rest aus der Grenzbitumenzone (Mittlere Trias) des Monte San Giorgio (Kt. Tessin, Schweiz). Paläontologische Zeitschrift. 44. 1–2. 32–40. 10.1007/bf02989793. 1970PalZ...44...32R . 129041989. 0031-0220.
  11. Druckenmiller. Patrick S.. Kelley. Neil. Whalen. Michael T.. Mcroberts. Christopher. Carter. Joseph G.. 2014-09-19. An Upper Triassic (Norian) ichthyosaur (Reptilia, Ichthyopterygia) from northern Alaska and dietary insight based on gut contents. Journal of Vertebrate Paleontology. 34. 6. 1460–1465. 10.1080/02724634.2014.866573. 2014JVPal..34.1460D . 129908740. 0272-4634.
  12. Motani . R. . Tomita . T. . Maxwell . E. . Jiang . D. . Sander . P. . vanc . 2013 . Absence of Suction Feeding Ichthyosaurs and Its Implications for Triassic Mesopelagic Paleoecology . PLOS ONE . 10.1371/journal.pone.0066075 . 8 . 12 . e66075 . 24348983 . 3859474. 2013PLoSO...866075M . free .
  13. Kelley . Neil P. . Irmis . Randall B. . dePolo . Paige E. . Noble . Paula J. . Montague-Judd . Danielle . Little . Holly . Blundell . Jon . Rasmussen . Cornelia . Percival . Lawrence M.E. . Mather . Tamsin A. . Pyenson . Nicholas D. . December 2022 . Grouping behavior in a Triassic marine apex predator . Current Biology . 32 . 24 . 5398–5405.e3 . 10.1016/j.cub.2022.11.005 . 36538877 . 254874088 . 0960-9822. free . 2022CBio...32E5398K .

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