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.
Highlights
The Triassic witnessed an unprecedented radiation of marine reptiles, possibly triggered by the Permian/Triassic extinction event which left the marine environment largely devoid of metazoans [1,2] or by the dramatic decline of atmospheric oxygen level [3,4] during this period
Based on new ichthyosaur finds from China, and the reexamination and reinterpretation of material from the western USA, we suggest that in the Late Triassic there was a previously unrecognized global diversification of large suction-feeding ichthyosaurs that probably were the ecological equivalent to the extant suction-feeding odontocetes
Toothlessness in ichthyosaurs Previously, complete toothlessness had only been described for the adults of one other Triassic ichthyosaur species, i.e., the giant Shastasaurus sikanniensis comb. nov
Summary
The Triassic witnessed an unprecedented radiation of marine reptiles, possibly triggered by the Permian/Triassic extinction event which left the marine environment largely devoid of metazoans [1,2] or by the dramatic decline of atmospheric oxygen level [3,4] during this period. Ichthyosaurs were very well adapted to the marine environment as evidenced by their worldwide record in open marine sediments. As attested to by fossil stomach contents, the elongated and toothed ichthyosaur rostrum was used to capture fish and squid, the diet of most extant odontocetes. Some of these odontocetes, such as the beaked whales (Ziphiidae), some delphinids, pygmy and dwarf sperm whales (Kogia sima and K. breviceps), and even the sperm whales (Physeter), use suction feeding instead of an elongate tooth-bearing rostrum to capture their prey [10,11,12]. 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
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