Limb osteology and ossification patterns inCryptoclidus(Reptilia: Plesiosauroidea) with a review of sauropterygian limbs

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Abstract
Limb osteology and ontogenetic patterns of limb ossification are described for the plesiosaur Cryptoclidus eurymerus (Upper Jurassic: Callovian), and compared to those of other sauropterygians. Major features of limb ossification in Cryptoclidus are identified: 1) delayed mesopodial ossification; 2) alterations to the ossification sequence of the radius/ulna, tibia/fibula, and some metacarpals and metatarsals; 3) the loss of perichondral bone from the margins of the radius/ulna, tibia/fibula, and some metacarpals and metatarsals; 4) altered bone morphology is correlated with loss of perichondral bone. Recognition of some of these features in basal sauropterygians, and their application to the study of limb elements in derived sauropterygians such as Cryptoclidus, alters traditional identifications of several bones. The conventional intermedium is re-identified as a centrale. The ‘true’ intermedium is found to be a small bone that is variably free, ossifies to the base of the radius forming a distinct process, or ossifies to the base of the ulna. Additional endochondral ossifications are found along the preaxial and postaxial margins of the limb. These bones have perichondral bone on some margins, articulate with distinct facets on other limb elements, and therefore cannot be dismissed as sesamoid ossifications. Bones conventionally identified as the 4th distal carpal, 4th distal tarsal, and calcaneum, are re-identified as the 4th metacarpal, and the 4th and 5th metatarsals, respectively.