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Background  Ichthyosaurs 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 Findings  Based 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/Significance  In 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.

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