Ontogenetic stages in the long bone histology of sauropod dinosaurs

Abstract

Long bones (femora, humeri) are the most abundant remains of sauropod dinosaurs. Their length is a good proxy for body length and body mass, and their histology is informative about ontogenetic age. Here we provide a comparative assessment of histologic changes in growth series of several sauropod taxa, including diplodocids (Apatosaurus, Diplodocus, indeterminate Diplodocinae from the Tendaguru Beds and from the Morrison Formation), basal macronarians (Camarasaurus, Brachiosaurus, Europasaurus), and titanosaurs (Phuwiangosaurus, Ampelosaurus). A total of 167 long bones, mainly humeri and femora, and 18 limb girdle bones were sampled. Sampling was performed by core drilling at prescribed locations at midshaft, and 13 histologic ontogenetic stages (HOS stages) were recognized. Because growth of all sauropod long bones is quite uniform, with laminar fibrolamellar bone being the dominant tissue, HOS stages could be recognized across taxa, although with minor differences. Histologic ontogenetic stages generally correlate closely with body size and thus provide a means to resolve important issue like the ontogenetic status of questionable specimens. We hypothesize that sexual maturity was attained at HOS-8, well before maximum size was attained, but we did not find sexually differentiated growth trajectories subsequent to HOS-8. On the basis of HOS stages, we detected two morphotypes in the Camarasaurus sample, a small one (type 1) and a larger one (type 2), presumably representing different species or sexual dimorphism.

Small body size and extreme cortical bone remodeling indicate phyletic dwarfism inMagyarosaurus dacus(Sauropoda: Titanosauria)

“…dacus Long Bone Histology. Like those of other sauropods (34,35,38,46), M. dacus long bones are characterized by a small medullary cavity and relatively thick cortex ( Fig. 2A).…”

“…2 E and F). However, the Magyarosaurus dacus sample is still amenable to relative age determination of individuals using histologic ontogentic stages (HOS) (35). The smallest individual, represented by specimen MAFI Ob.3092, records HOS 12 (Fig.…”

“…In a number of femora and humeri (FGGUB R.1046, FGGUB R.1992, MAFI Ob.3089, MAFI Ob.3128, FGGUB R.1048), at least one additional generation of secondary osteons, crosscutting secondary osteons of the first or subsequent generations are present in the outer cortex. The microstructure of these specimens corresponds to tissue type H, and is thus assigned to HOS 14 (35) (Fig. 3, Materials and Methods, and SI Text).…”

“…Sauropods as well as theropods [and ornithischian dinosaurs, where sample size is sufficient (29)(30)(31)] follow a narrow growth trajectory, i.e., they lack developmental plasticity. In sauropods, this is documented by a close correlation between histologic ontogenetic stage (HOS) and body size (32)(33)(34)(35)(36)(37)(38).…”

Does morphology reflect osteohistology-based ontogeny? A case study of Late Cretaceous pterosaur jaw symphyses from Hungary reveals hidden taxonomic diversity

“…On one hand, more complete 2006; Klein and Sander 2008;Horner and Goodwin 2009;Scannella and Horner 2010;Knoll et al 2010;Padian 2013;Shelton et al 2013), because absolute size may not be a true measure of relative ontogeny (e.g., Johnson 1977;Andrews 1982;Galton 1982;Gibson and Hamilton 1984;Brinkman 1988;Bennett 1993;Sander and Klein 2005;). …”