Developmental constraints and limb evolution in Permian and extant lepidosauromorph diapsids

Abstract

Evidence from studies of chondrogenic and osteogenic patterns in extant lepidosaurs is used to analyze skeletal pattern formation in ontogenetic series of three primitive Permian diapsids: Hovasaurus boulei: Thadeosaurus colcanapi; and Claudiosaurus germaini. The ossification of proximal and postaxial carpals proceeds according to the sequence: 1) ulnare; 2) distal carpal four; 3) intermedium; 4) lateral centrale; 5) distal carpal three or one. The sequence of ossification of the remaining carpals is highly variable. Tarsal ossification proceeds according to the sequences: 1) astragalus/calcaneum; 2) distal tarsal four; 3) distal tarsal three; 4) centrale; 5) distal tarsals two, one, or five. C. germaini shows great intraspecific variation in the sequence of distal tarsal ossification after ossification of the centrale. Limb ossification is highly constrained along the primary limb axis and at the beginning of the digital arch. Variation is observed preaxially along the digital arch and with respect to the development of the fifth digit. The conserved sequences are the same as those seen in living lepidosaurs; sequence variation in living lepidosaurs is noted in the preaxial region of the digital arch. The evolution of the terrestrial limb in lepidosauromorphs is interpreted as having been constrained by the processes of limb development. The fossil diapsids illustrate a primitive condition for lepidosauromorphs and provide a basis for comparison of change among other diapsids.