Ontogeny and the fossil record: what, if anything, is an adult dinosaur?

Abstract

Identification of the ontogenetic status of an extinct organism is complex, and yet this underpins major areas of research, from taxonomy and systematics to ecology and evolution. In the case of the non-avialan dinosaurs, at least some were reproductively mature before they were skeletally mature, and a lack of consensus on how to define an 'adult' animal causes problems for even basic scientific investigations. Here we review the current methods available to determine the age of non-avialan dinosaurs, discuss the definitions of different ontogenetic stages, and summarize the implications of these disparate definitions for dinosaur palaeontology. Most critically, a growing body of evidence suggests that many dinosaurs that would be considered 'adults' in a modern-day field study are considered 'juveniles' or 'subadults' in palaeontological contexts.

Keywords: adult; dinosauria; growth; histology; juvenile; subadult.

Figure 1.

A tableau of Tyrannosaurus rex skeletal reconstructions, on display at the Natural History Museum of Los Angeles County. The largest individual represents typical adult size for the taxon—current mainstream scientific consensus considers them all different ontogenetic stages of T. rex but the smaller specimens were originally referred to different genera. Photo: DWEH. (Online version in colour.)

Figure 2.

Various methods that may be used to determine the age/ontogenetic status of a given dinosaur specimen. Central image is a reconstruction of the skeleton of an adult ceratopsian Zuniceratops, with surrounding indications of maturity (taken from multiple sources and do not necessarily relate to this taxon). (a) Development of sociosexual signals (adult left, juvenile right—modified from [9]), (b) surface bone texture (traced from [17]), (c) large size, represented here by an ilium of the same taxon that is considerably larger than that of a known adult specimen, (d) reproductive maturity, here based on the presence of medullary bone here shown below the black arrow (traced from [18]), (e) fusion of the neurocentral arch—location of the obliterated synchondrosis indicated by black arrow (traced from [19]), (f) asymptote of growth based on multiple species indicated by black arrow (based on [20]). Central image by Julius Csotonyi, used with permission. (Online version in colour.)