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.)
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 ), (b) surface bone texture (traced from ), (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 ), (e) fusion of the neurocentral arch—location of the obliterated synchondrosis indicated by black arrow (traced from ), (f) asymptote of growth based on multiple species indicated by black arrow (based on ). Central image by Julius Csotonyi, used with permission. (Online version in colour.)
This is not an exhaustive list of terms used or definitions given. Age classes are given as used in the original sources and the definitions or reasoning for the assignment to this age class are direct quotes from the text. Additional details are often provided in the respective sources for assigning age classes, but these quotes are intended to be representative and not overarching. EFS, external fundamental system.
these occur both in situ and inside fragments of eggs exposed on erosional surfaces
we use the term ‘perinate’ (‘around birth’)
the bone tissue that forms the shafts of the longer limb bones is … composed of vascular canals surrounded by an undifferentiated mineralized bone matrix
in cross section, the shafts of the long bones generally have a cortex that is well differentiated from the marrow cavity
anumerous differences in cranial and postcranial morphology given between ‘young’ and ‘adult’ Protoceratops
a bone that is less than one-half the size of that of a typical adult specimen
all vertebrae in embryonic and neonate ornithischian material, unsurprisingly, preserve open neurocentral sutures
histological section of the tibia shows well-vascularized, woven and parallel-fibred primary cortical bone typical of juvenile ornithopods
the frontoparietal fontanelle is open in late-stage subadults
individuals of adult or virtually adult size, with additional characters indicating pre-adult status … but individuals lack several adult characters
a bone between one-half and two-thirds the size of that of a typical adult specimen
the individuals in this stage have both ‘young’ and ‘adult’ characters
subadult or young adult
aneurocentral sutures have closed, partial fusion of scapula and coracoid and of the ilium and ischium, fusion of some cranial elements
fully grown individuals with full expression of adult characters, often including fusion of skull elements
a bone that is approximately the size of that of a typical adult specimen
this histology is typical of an EFS and indicates that the individual was fully grown
nearly all of the cranial sutures are obliterated by co-ossification
aThe quotations have been compressed for brevity.
The timing of macroscopic changes in sauropod skeletons over ontogeny is not consistent among taxa. ‘yes’ indicates fusion to the adjacent respective spines or centra, ‘no’ indicates lack of fusion, and a blank indicates that the relevant material is not preserved. Modified from Wedel & Taylor [23, table 1].