ONTOGENIC EVOLUTION OF BONE-STRUCTURE IN LATE CRETACEOUS PLESIOSAURIAFROM NEW-ZEALAND

Histological observations of homologous bones (vertebrae, ribs, humeru s, phalanges) among conspecific juvenile and adult Upper Cretaceous pl esiosaurs from New Zealand (elasmosaurs and pliosaurs) demonstrates a unique ''ontogenetic trajectory'' of skeletal histogenesis in these an imals. While juveniles demonstrate a ''pachyosteosclerotic'' condition of the skeleton, adults have a very light ''osteoporotic-like'' bone structure. Until now, one or the other of these histological specializ ations was known among aquatic tetrapods, adapted along contrasting pa thways to this environment, either by ballasting (pachyosteosclerosis : e.g. sirenians) or by lightening (osteoporotic-like adaptation : e.g . modern cetaceans) of the skeleton. The successive occurrence of thes e constrasting conditions during ontogenesis of a single organism had never been reported, as far as we know, but could well be an ontogenet ic characteristic of Plesiosaurs sensu lato. The significance of these findings are discussed in various phylogenetical, functional and pale oecological contexts. The ontogenetic trajectory of the plesiosaur ske leton is interpreted within the general framework of developmental het erochrony. Specifically, it suggests that juvenile plesiosaurs kept a conservative (plesiomorphic) ecology for sauropterygians, as poorly mo bile, lagoon or shore dwellers while, in contrast, the adults would sh ift towards much more active locomotory behaviours in the open sea.