OSSIFICATION SEQUENCES AND ASSOCIATED ONTOGENETIC CHANGES IN THE BONE HISTOLOGY OF PACHYPLEUROSAURIDS FROM MONTE SAN GIORGIO (SWITZERLAND/ITALY)

Evolutionary changes in lifestyle (e.g., terrestrial vs. aquatic habits) influence tetrapod limb morphology. Similar evolutionary trends in osteogenetic sequences (order of bone ossification) and bone microstructure are often shared in extant tetrapods with similar habit preferences: such data are sometimes available from extinct taxa. The pachypleurosaurids from Monte San Giorgio (Switzerland, Italy) are Triassic marine reptiles with high quality, complete ontogenetic series. We studied osteogenesis and bone histology in the four species from this locality, comparing these with data from Recent terrestrial lizards and secondarily aquatic reptiles in order to determine if the osteogenetic sequences of pachypleurosaurids were similar to either the hypothesized plesiomorphic condition for terrestrial eureptilians or those of Recent aquatic reptiles. Pachypleurosaurian limb osteogenesis occurs in two steps: (1) developmental sequences of ossification during embryology and/or in early neonates, (2a) additional primary periosteal compaction processes, and (2b) additional primary and secondary endosteal compaction processes during neonate ontogeny. Taphonomic patterns reveal information on the order of the initiation and termination of these steps, which are (A) onset of ossification, (B) onset of additional compaction processes (early phase), and (C) termination of additional compaction processes (final phase). An event pairing analysis found that ossification of forelimb elements in pachypleurosaurids precedes that of the hind limb elements in all osteogenetic stages except for initiation of ossification (A). The order of their early phase compaction processes is similar to the hypothesized plesiomorphic eureptilian condition, whereas their final phase compaction processes varies among pachypleurosaurids, with S. mirigiolensis showing minor heterochronic shifts and N. edwardsii showing many heterochronic shifts relative to the hypothesized ancestral condition. Pachypleurosaurids from Monte San Giorgio increase the number of heterochronic shifts with decreasing strati-graphic age, showing a transition from more 'terrestrial' to more 'aquatic' osteogenetic sequences in comparison to data on ossification sequences of Recent aquatic reptiles.