Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaurs

Abstract

BackgroundIchthyosaurs were an important group of Mesozoic marine reptiles and existed from the Early Triassic to the early Late Cretaceous. Despite a great diversity in body shapes and feeding adaptations, all share greatly enlarged eyes, an elongated rostrum with numerous conical teeth, and a streamlined body.Methodology/Principal FindingsBased on new material from China and the restudy of Shastasaurus pacificus, we here reinterpret the classical large-bodied Late Triassic ichthyosaur genus Shastasaurus to differ greatly from the standard ichthyosaurian body plan, indicating much greater morphological diversity and range of feeding adaptations in ichthyosaurs than previously recognized. Phylogenetic analysis indicates a monophyletic clade consisting of the giant Shonisaurus sikanniensis, Guanlingsaurus liangae, and Shastasaurus pacificus to which the genus name Shastasaurus is applied. Shastasaurus liangae comb. nov. is from the Late Triassic (Carnian) Xiaowa Formation of Guizhou Province, southwestern China. The species combines a diminutive head with an entirely toothless and greatly reduced snout. The species also has by far the highest vertebral count among ichthyosaurs (86 presacral vertebrae and >110 caudal vertebrae), a count that is also very high for tetrapods in general. A reduced toothless snout and a diminutive head is also apparently present in the giant S. sikanniensis and presumably in S. pacificus.Conclusions/SignificanceIn analogy to many modern odontocetes, Shastasaurus is interpreted as a specialized suction feeder on unshelled cephalopods and fish, suggesting a unique but widespread Late Triassic diversification of toothless, suction-feeding ichthyosaurs. Suction feeding has not been hypothesized for any of the other diverse marine reptiles of the Mesozoic before, but in Shastasaurus may be linked to the Late Triassic minimum in atmospheric oxygen.

A Giant Chelonioid Turtle from the Late Cretaceous of Morocco with a Suction Feeding Apparatus Unique among Tetrapods

“…However, despite this considerable diversity in the Mesozoic marine reptile modes of predation [40], [41], as well as a broad overlap of their tooth morphospaces with those of Cenozoic marine mammals, indicating multiple feeding behavior convergences [42], suction feeding, though being a common feeding strategy in aquatic vertebrates [34] has been extremely rarely reported among Mesozoic marine reptiles. It has been hypothesized (but without any concrete argument) for Hupesuchus , a small marine reptile from the Middle Triassic of China [43] and recently postulated for Shonisaurus [44], [45] and Shastasaurus [46], both large toothless Triassic ichthyosaurs, interpreted as suction feeders also comparable to many extant odontocetes.…”

Macropredatory ichthyosaur from the Middle Triassic and the origin of modern trophic networks

“…The first of these secondarily aquatic groups are three major lineages of reptiles: Sauropterygia, Thalattosauria, and Ichthyosauria. They suddenly appear in the marine fossil record by the late Spathian (Early Triassic), ∼4 My after the P/T crisis (8). Intriguingly, the first taxonomic diversity peak of marine tetrapods is already reached in the Anisian (Middle Triassic) (21,22) and coincides with great variation in dentitions, body shape, and body size.…”

“…The orbits are elongate, the left measuring 29 cm in length, with a well-preserved scleral ring. The upper temporal openings are large and oval, reminiscent of those of Shastasaurus (8). The postorbital region is long, and the lower temporal embayment is very shallow.…”

Mesozoic marine reptile palaeobiogeography in response to drifting plates

“…By the Middle Triassic, ichthyosaurs dispersed within the northern hemisphere, reaching Europe, Nevada, and California (Fröbisch et al, 2006;Jiang et al, 2007;Sander, 1989). During the Late Triassic, the whale-sized shastasaurids colonized the Tethys and Pantalassa oceans, with remains found in California, British Columbia, China and Tibet (Ji et al, 2013;Sander et al, 2011). At the same time (Carnian), the parvipelvians, derived ichthyosaurs with a peculiar thunniform body shape, appeared on the northwestern coast of the Pangaea (British Columbia) (McGowan, 1995(McGowan, , 1997.…”

“…At the same time (Carnian), the parvipelvians, derived ichthyosaurs with a peculiar thunniform body shape, appeared on the northwestern coast of the Pangaea (British Columbia) (McGowan, 1995(McGowan, , 1997. The Late Triassic is often regarded as the 'golden age' of ichthyosaurs, with the presence of numerous taxa exhibiting a wide variety of body plans and feeding ecologies (Sander et al, 2011;Thorne et al, 2011), as well as extreme disparities in adult body sizes, from the <1 m long Hudsonelpidia brevirostris (McGowan, 1995) to the >20 m long Shonisaurus sikkaniensis (Nicholls and Manabe, 2004).…”