Modified perichondral ossification and the evolution of paddle-like limbs in ichthyosaurs and plesiosaurs

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Abstract
Evolution of paddle-like limbs in ichthyosaurs and plesiosaurs is correlated with loss of perichondral bone from the shafts of long bones. Among ichthyosaurs, loss of perichondral bone is first observed on the shafts of digit bones of Early Triassic taxa. Late Triassic ichthyosaurs show perichondral bone loss on the postaxial margins of the ulna and fibula. Among plesiosaurs, loss of perichondral bone is first observed on the postaxial margins of the ulna and fibula of Lower Jurassic taxa. In geologically later species of both groups, perichondral bone is progressively lost on all margins of the ulna and radius, and fibula and tibia. Late Triassic and Jurassic ichthyosaurs show an absence of perichondral ossifications on all limb bones distal to the humerus and femur. Delayed ossification of the mesopodium is not observed in ichthyosaurs. Evolutionary changes to the ossification of perichondral tissues appear to affect the sequence of limb ossification as long bones lose perichondral bone. Limb bones of ichthyosaurs and plesiosaurs resemble mesopodial elements through loss of perichondral bone. The proportion of endochondral bone found on these transformed bones is increased, resulting in an increased articular surface area and increased articular complexity.