Ontogenetic evolution of bone structurein Late Cretaceous Plesiosauria from New Zealand

Abstract

Histological observations of homologous bones (vertebrae, ribs, humerus, phalanges) among conspecific juvenileand adult Upper Cretaceous plesiosaurs from New Zealand (elasmosaurs and pliosaurs) demonstrates a unique “ontogenetic trajectory” of skeletal histogenesis in these animals. 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 specializations was known among aquatic tetrapods, adapted along contrasting pathways 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 these constrasting conditions during ontogenesis of a single organism had never been reported, as far as we know, but could well be an ontogenetic characteristic of Plesiosaurs sensu lato. The significance of these findings are discussed in various phylogenetical, functional and paleoecological contexts. The ontogenetic trajectory of the plesiosaur skeleton is interpreted within the general framework of developmental heterochrony. Specifically, it suggests that juvenile plesiosaurs kept a conservative (plesiomorphic) ecology for sauropterygians, as poorly mobile, lagoon or shore dwellers while, in contrast, the adults would shift towards much more active locomotory behaviors in the open sea.