Limb ossification patterns of the ichthyosaur Stenopterygius, and a discussion of the proximal tarsal row of ichthyosaurs and other neodiapsid reptiles

Limb ossification patterns for the Lower Jurassic (Toarcian) ichthyosaur, Stenopterygius , are described. It is found that limb ossification follows a continuous proximal to distal sequence from the propodial elements through to the terminal elements of 1st to 4th digit in the manus and the 1st to 3rd digit in the pes. The 5th manal and 4th pedal digit begin ossification later than more preaxial digits and also show evidence of proximal addition of elements near the distal mesopodial row in a manner consistent with delayed ossification of the 5th distal mesopodial in other diapsids. Ossification of manal elements in the Supernumerary 3–4 (S3-4) digit and the 5th digit appear interdependent; if one or the other is highly ossified, ossification of the other is retarded. The 1st pedal digit is considered to be lost in Stenopterygius and the 4th pedal digit is identified as the 5th digit. Delayed ossification of the mesopodium is not observed. The most preaxial proximal tarsal is identified as the centralc; the remaining proximal tarsals are the astragalus and calcaneum, and it is inferred that the astragalus and calcaneum ossified from within a single proximal cartilage.