Identification of growth cessation in dinosaurs based on microscopy of long bone articular surfaces: preliminary results

Abstract As applied to bone, ‘determinate growth’ identifies an upper limit to size and the point when normal endochondral ossification ceases. This contrasts with ‘indeterminate growth’, which proceeds through the entire life of the animal. In this study, a non-destructive method, epi-illumination surface microscopy of the articular surfaces of long bones, is applied for the first time in 40 taxa of non-avian dinosaurs to determine cessation of endochondral growth. Thereby, the presence or absence of articular vascular channels between the endochondral bone and the cartilage is assessed. As articular vascular channels are the major source of nutrients for continued longitudinal growth, atrophy or loss of those channels would preclude continued growth of bone. We correlated our findings with published histological data and bone length measurements. We found articular vascular channels in all assessed dinosaur groups, but some individuals showed a loss of detectable articular vascular channels – what we interpret as evidence of longitudinal skeletal growth cessation. This observation contrasts with the hypothesis of continuous indeterminate growth in dinosaurs, at least for the taxa identified here, in which channels have been documented as closed or closing over. The new method introduced here provides a phylogenetic tool for definitively distinguishing new ‘dwarf’ species from juveniles of known species. Furthermore, this study confirms the rarity of skeletally mature dinosaurs discovered to date and indicates that we have only begun to witness the full extent of dinosaur growth. Bruce M. Rothschild [spondylair@gmail.com], Carnegie Museum, 4400 Forbes Ave, Pittsburgh, PA 15213, USA; Florian Witzmann* [florian.witzmann@mfn.berlin], Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstraße 43, 10115 Berlin, Germany.