Temnodontosaurus risoris a juvenile ofT. platyodon(Reptilia: Ichthyosauria)

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Abstract
Most ichthyosaurs from the English Lower Jurassic are dolphin-sized, with skull lengths of about 500 mm and body lengths of up to 2,500 mm. Very large ichthyosaurs are rare, and the largest and best known of these is Temnodontosaurus platyodon, with skull lengths ranging from 1,200 to 2,000 mm and body lengths of up to about 9,000 mm. Two other species are referrred to the genus Temnodontosaurus, T. eurycephalus and T. risor. The former, known from a single 950 mm long skull, is characterized by its much shortened snout. T. risor, erected upon three isolated skulls ranging in length from 655 to 800 mm, is like T. platyodon but differs in having a slightly larger orbit, smaller maxilla, and a curved snout. An almost complete, three-dimensionally preserved skeleton was recently found which can be identified as T. risor but which has a slightly longer skull than the other referred specimens. The skull is remarkably large relative to the post-cranial skeleton, suggestive of immaturity, and this is supported by size-independent osteological features. T. risor therefore represents an immature stage, but of which taxon? The most likely candidate is T. platyodon, and when comparisons are made between the new material, and specimens formerly referred to T. risor and to T. platyodon, a case can be made that they form part of a growth series. T. risor is therefore recognized as a junior synonym for T. platyodon. The new material provides valuable additonal information for T. platyodon, a rather inadequately known species, for which an emended diagnosis is given.