Osteoderm microstructure indicates ontogenetic shifts in the growth pattern of some Cretaceous notosuchians (Crocodylomorpha)

Abstract

A characteristic common to almost all crocodylomorphs, whether living or extinct, is the presence of a dermal skeleton. This covering is composed of bones known as osteoderms or dermal plates/scutes, which are interconnected by fibrous tissues. Osteoderms play essential roles in the biology of crocodylomorphs, and vary in size, shape, ornamentation pattern, and functions according to the species. This study analyzed the osteoderms of fossil species of four fossil crocodylomorphs taxa from the Upper Cretaceous of the Bauru Group and compared their morphological characteristics with other living and extinct lineages, including Candidodontidae and fossil Caimaninae. Sampling included osteoderms from different regions of the body and individuals of different ontogenetic stages. The osteoderms analyzed originate from distinct clades of crocodylomorphs (Peirosauridae, Baurusuchidae, Sphagesauridae, and the spaghesaurian Mariliasuchus), presenting distinct external morphologies at the macroscopic level. The histological analysis confirmed the similarities seen in distinct clades of crocodylomorphs (Peirosauridae, Baurusuchidae, Sphagesauridae, and Mariliasuchus), despite highly divergent macroscopic morphology. In all cases, the bony matrix is characterized by parallel and interwoven fibers, with secondary osteons much larger than the primary ones, indicating a process of bone resorption. Growth lines also provide information on the minimum age of the individuals at the time of death. Histological differences in the osteoderms of baurusuchid of different ontogenetic stages indicate a back-to-front progression in the reabsorption process, akin to the progressive ontogenetic fusion of neurocentral sutures seen in the vertebral spine of crocodylomorphs. A shift in the position of the parasagittal crest through ontogeny indicates that the growth process of the osteoderm is not symmetric at least in Baurusuchidae, and that individuals of different ages are expected to present meaningful macroscopic shifts in the morphology of their osteoderms. Peirosauridae ornamentation pattern also varies during ontogeny since pits present in early ontogenetic states are covered/obliterated by subsequent layers of bone that assume a more regular aspect.