Shonisaurus is a genus of very largeichthyosaurs. At least 37 incomplete fossil specimens of thetype species,Shonisaurus popularis, have been found in theLuning Formation ofNevada, USA. This formation dates to the lateCarnian-earlyNorian age of theLate Triassic, around 227 million years ago.[3] Other possible species ofShonisaurus have been discovered from the middleNorian deposits ofCanada andAlaska.[2][4]
Size ofS. popularis (green) andShastasaurus sikanniensis (red) compared with a human (blue)Shonisaurus skeleton, Nevada State Museum
Shonisaurus lived during the lateCarnian toNorian stages of theLate Triassic. With a large skull about 2.75 m (9.0 ft) long,S. popularis measured around 13.5–15 metres (44–49 ft) in length and 21.6–29.7 metric tons (23.8–32.7 short tons) in body mass.[5][6]S. sikanniensis was one of the largest marine reptiles of all time, measuring 21 metres (69 ft) long and weighing 81.5 metric tons (89.8 short tons).[5]
Shonisaurus had a long snout, and its flippers were much longer and narrower than in other ichthyosaurs. WhileShonisaurus was initially reported to have had socketed teeth (rather than teeth set in a groove as in more advanced forms), these were present only at the jaw tips, and only in the very smallest, juvenile specimens. All of these features suggest thatShonisaurus may be a relatively specialised offshoot of the main ichthyosaur evolutionary line.[7] More recent finds however indicate thatShonisaurus possessed teeth in all ontogenetic stages.[8] Robust sectorial teeth and gut contents indicate thatShonisaurus was a macrophagous raptorialpredator which fed on vertebrates and shelled mollusks like cephalopods, possibly even large-bodied prey.[9][10] Additionally,Shonisaurus was historically depicted with a rather rotund body, but studies of its body shape since the early 1990s have shown that the body was much more slender than traditionally thought,[11] and had a relatively deep body compared with related marine reptiles.[2]
Fossils ofShonisaurus were first found in a large deposit inNevada in 1920. Thirty years later, they were excavated, uncovering the remains of 37 very large ichthyosaurs. These were namedShonisaurus, which means "lizard from theShoshone Mountains", after the formation where the fossils were found.
A second species from thePardonet Formation ofBritish Columbia was namedShonisaurus sikanniensis in 2004.[2] However, aphylogenetic study by Sander and colleagues in 2011 later showedS. sikanniensis to be a species ofShastasaurus rather thanShonisaurus.[13]A subsequent study by Ji and colleagues published in 2013 reasserted the original classification, finding it more closely related toShonisaurus than toShastasaurus.[14] Support for both hypotheses has been found in later studies, with some authors classifying the species inShonisaurus and others inShastasaurus.[15][16]
Specimens belonging toS. sikanniensis have been found in thePardonet FormationBritish Columbia, dating to the middleNorian age.[2] An isolated humerus from a smaller individual (TMP 94.381.4) and a postorbital region (TMP 98.75.9) from a juvenile were also reported from the same formation and were referred to asShonisaurus sp.[2] Other fossils from this formation include the ichthyosaursMacgowania andCallawayia, coelacanthsWhiteia banffensis and possiblyGarnbergia, and various genera of molluscs including ammonites and bivalves.[2][17] Large ichthyosaur remains found in Alaska have also been identified asShonisaurus sp.[4]
^abcdefgNicholls, Elizabeth L.; Manabe, Makoto (2004). "Giant Ichthyosaurs of the Triassic—A New Species ofShonisaurus from the Pardonet Formation (Norian: Late Triassic) of British Columbia".Journal of Vertebrate Paleontology.24 (4):838–849.doi:10.1671/0272-4634(2004)024[0838:GIOTTN]2.0.CO;2.ISSN0272-4634.
^Balini, M.; Jenks, James F.; Martin, R.; McRoberts, Christopher A.; Orchard, Michael J.; Silberling, Norman J. (2015). "The Carnian/Norian boundary succession at Berlin-Ichthyosaur State Park (Upper Triassic, central Nevada, USA)".Paläontologische Zeitschrift.89 (3):399–433.Bibcode:2015PalZ...89..399B.doi:10.1007/s12542-014-0244-2.hdl:2434/259929.
^abAdams, Thomas L. (2009). "Deposition and taphonomy of the Hound Island Late Triassic vertebrate fauna: Fossil preservation within subaqueous gravity flows".PALAIOS.24 (9):603–615.Bibcode:2009Palai..24..603A.doi:10.2110/palo.2009.p09-010r.
^Palmer, D., ed. (1999).The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. pp. 78–79.ISBN1-84028-152-9.
^Ji, C.; Jiang, D. Y.; Motani, R.; Hao, W. C.; Sun, Z. Y.; Cai, T. (2013). "A new juvenile specimen ofGuanlingsaurus (Ichthyosauria, Shastasauridae) from the Upper Triassic of southwestern China".Journal of Vertebrate Paleontology.33 (2): 340.Bibcode:2013JVPal..33..340J.doi:10.1080/02724634.2013.723082.S2CID83784699.
