page_title: Stegoceras
section_title: Ontogenetic changes
hierarchical_section_title: Stegoceras / Palaeobiology / Ontogenetic changes
caption_attribution_description: High-resolution CT images of TMP 84.5.1. A, dorsal view (left) and frontal section (right); B, lateral view (left), median sagittal section (middle), and lateral sagittal section (right); C, anterior view (left), anterior transverse section through the frontal (middle), and posterior transverse section through the parietal (right). The histological zones of Goodwin and Horner [37] are denoted. The arrow identifies an artifact likely produced by mineralized collagen fibres present across and along the interfrontal and frontoparietal sutures. Abbreviations: f, frontal; f-f, inter-frontal suture; f-p, frontoparietal suture; p, parietal; po, postorbital; pso, posterior supraorbital; sq, squamosal; Z-1 to Z-3, histological Zones I to III.
context_page_description: Stegoceras is a genus of pachycephalosaurid dinosaur that lived in what is now North America during the Late Cretaceous period, about 77.5 to 74 million years ago. The first specimens from Alberta, Canada, were described in 1902, and the type species Stegoceras validum was based on these remains. The generic name means "horn roof", and the specific name means "strong". Several other species have been placed in the genus over the years, but these have since been moved to other genera or deemed junior synonyms. Currently only S. validum and S. novomexicanum, named in 2011 from fossils found in New Mexico, remain. The validity of the latter species has also been debated.
  Stegoceras was a small, bipedal dinosaur about 2 to 2.5 metres long, and weighed around 10 to 40 kilograms. It had a rigid vertebral column, and a stiffened tail. The pelvic region was broad, perhaps due to an extended gut. The skull was roughly triangular with a short snout, and had a thick, broad, and relatively smooth dome on the top. The back of the skull had a thick "shelf" over the occiput, and it had a thick ridge over the eyes.
context_section_description: Several explanations have historically been proposed for the variation seen in the skulls of Stegoceras and other pachycephalosaurs. Brown and Schlaikjer suggested that there was sexual dimorphism in the degree of doming, and hypothesized that flat-headed specimens such as AMNH 5450 (Ornatotholus) represented the female morph of Stegoceras. This idea was supported by a 1981 morphometric study by Champan and colleagues, which found that males had larger and thicker domes. After other flat-headed pachycephalosaurs were discovered, the degree of doming was proposed to be a feature with taxonomic importance, and AMNH 5450 was therefore considered a distinct taxon from 1979 onwards. In 1998, Goodwin and colleagues instead proposed that the inflation of the dome was an ontogenetic feature that changed with age, based on a histological study of an S. validum skull that showed the dome consisted of vascular, fast-growing bone, consistent with an increase in doming through age. These authors found that the supposedly distinct features of Ornatotholus could easily be the results of ontogeny.
  In 2003, Williamson and Carr published a hypothetical growth series of S. validum, showing Ornatotholus as the juvenile stage. They suggested that juveniles were characterized by a flat, thickened frontoparietal roof, with larger supratemporal fenestrae, and studded with closely spaced tubercles and nodes. The parietosquamosal shelf was not reduced in size, and the frontoparietal suture was open. Sub-adults had mound-like domes, with the back part of the parietal and skull-roof being flat. The supratemporal fenestrae showed asymmetry in size, and the closure of the frontoparietal suture was variable. The nodes were stretched or almost obliterated as the dome expanded during growth, with a tesserated surface remaining. The pattern was often obliterated at the highest point (apex) of the dome, the area where maximum expansion occurred. The tubercles on the skull were stretched in different directions, and those at the margin of the parietosquamosal shelf may have been hypertrophied (enlarged) tubercles. The back and sides of sub-adult and adult skulls were ornamented by less modified tubercles. Before being incorporated into the enlarging dome, the skull bones expanded, resulting in junctions between these bones. The adult dome was broad and convex, and incorporated most of the shelf, which was reduced in size and overhung the occiput as a thick "lip". The supratempooral fenestrae were closed, but the suture between the frontoparietal and connected skull bones was not always closed in adults and subadults.
  In 2011, Ryan Schott and colleagues made a more comprehensive analysis of cranial dome ontogeny in S. validum. The study found that the parietosquamosal shelf conserved the arrangement of ornamentation throughout growth, and that vascularity of the frontoparietal domes decreased with size. It also found that dome shape and size was strongly correlated with growth, and that growth was allometric (in contrast to isometric) from flat to domed, supporting Ornatotholus as a juvenile Stegoceras. They also hypothesized that this model of dome growth, with dramatic changes from juvenile to adult, was the common developmental trajectory of pachycephalosaurs. These researchers noted that though Williamson and Carr's observation that the supratemporal fenestrae closed with age was generally correct, there was still a high degree of individual variation in the size of these fenestrae, regardless of the size of the frontoparietal, and this feature may therefore have been independent of ontogeny.
  A 2012 study by Schott and David C. Evans found that the number and shape of the individual nodes on the squamosal shelf of the examined S. validum skulls varied considerably, and that this variability does not seem to correlate with ontogenic changes, but was due to individual variation. These researchers found no correlation between the width of supratemporal fenestrae and the size of the squamosal.
caption: CT images of TMP 84.5.1, a juvenile or subadult S. validum, with sections
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