The history of ichthyosaurs is briefly traced from the Middle Triassic to the Upper Cretaceous.Platypterygius Von Huene, the only Cretaceous representative, is compared with other longipinnates, including Cymbospondylus petrinus Leidy, Middle Triassic; Proteosaurus platyodon (Conybeare), Lower Liassic; and Stenopterygius quadriscissus (Quenstedt), Upper Liassic.Several evolutionary trends occur in the skull and fore fin of ichthyosaurs (McGowan, 1972) and whereas some of these trends are well advanced in Platypterygius, others are retarded.The diameters of the orbit and internal sclerotic ring are small in Platypterygius, as in Cymbospondylus Leidy, whereas the orbit lies more posteriorly, and is in this respect comparable with Stenopterygius Jaekel.Maxillary reduction in Platypterygius is intermediate between Cymbospondylus and Stenopterygius.The fore-fin trend towards an increase in the.total number of digits is more advanced in Platypterygiusthan in any other longipinnate ichthyosaur, and the fin is unique for the presence of three pre-axial accessory digits.Platypterygius also differs from other longipinnates in its body proportions, the skull is relatively larger, the tail fin much smaller.An attempt to reconstruct the locomotor habits of Platypterygius indicate that the fore fins were used for propulsion, unlike most ichthyosaurs, and it is suggested that speed and manoeuverability may have been inferior to Jurassic longipinnates.Longipinnate evolution is discussed, and an attempt is made to quantify the rates of cranial evolution.Rates of change were highest during the Liassic and lowest during the long, post-Liassic interval.The remarkable homogeneity of ichthyosaurian body form during the Cretaceous, exemplified by the single genus Platypterygius, supports the conclusion that the rate of evolutionary change was at its lowest level.Loss of adaptive plasticity at a time when a new competitor, the mosasaur, was rapidly evolving is suggested as a possible cause of extinction.
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