Article: The affinities of Liassic and later ichthyosaurs

Ichthyosaurs from low in the Hettangian of Britain, with fore limbs showing a mixture of longipinnate and latipinnate characters, unusual cranial features, differing vertebral column functions, and differing hind-limb proportions, suggest that latipinnates evolved from Hettangian or late Triassic longipinnates and not from the Middle Triassic mixosaurs as has long been held. The latter are removed from ancestry of Liassic and later latipinnates on the grounds of their earlier and short stratigraphical range, their more advanced morphology, and because a comparison of the trends in mixosaurs and in Liassic and later latipinnates shows that these are often divergent. Ichthyosaurus intermedius and I. communis are restored as separate species, and the following new taxa are erected: Order Longipinnatoidea, Order Latipinnatoidea, Order Heteropinnatoidea, and Order Mixosauroidea. Two new families, the Protoichthyosauridae and the Leptopterygiidae, are defined and referred to the heteropinnatoids. The new genus Protoichthyosaurus and its two species Protoichthyosaurus prostaxalis sp. nov. and P. prosostealis sp. nov. are described. Leptopterygius tenuirostris presents particular taxonomic difficulties since it is an indivisible species showing every gradation of structure between longipinnatoid and heteropinnatoid individuals.