Rare juvenile material constrains estimation of skeletal allometry in Gryposaurus notabilis (Dinosauria: Hadrosauridae)

Abstract

In studying the skeletal allometry of any vertebrate, it is important to sample the ontogenetic extremes to ensure the accuracy of parameter estimation; this is particularly true for fossil taxa, where sampling of ontogenetic series is incomplete and sporadic. Previous studies have examined allometry in the skull of the duck-billed dinosaur Gryposaurus notabilis, but these did not include individuals smaller than $65% the maximum known size (based on linear dimensions). Here, we report on the two smallest known examples of this species (a mostly complete skeleton and a partial skull), which are $37% the known maximal size of G. notabilis. Osteohistology indicates that these represent individuals $2 years of age. Allometric analysis demonstrates that most aspects of the skull of G. notabilis grew isometrically, although the height of the nasal arch grew with positive allometry. Early in the ontogeny of G. notabilis, the dentary teeth possessed secondary ridges, which were lost later in life. This finding has important bearing on hadrosaurid tooth taxonomy.The limb proportions of G. notabilis largely grew isometrically (or with weak negative allometry, at most), like some other hadrosaurids, suggesting that the species did not undergo a gait shift with increasing age (unless it occurred very early in ontogeny). We argue that the lack of significant locomotory performance compensation exhibited by young hadrosaurids helps to explain why they apparently formed small, mutualistic aggregations, presumably for protection from large predators.

Calvarial suture interdigitation in hadrosaurids (Ornithischia: Ornithopoda): Perspectives through ontogeny and evolution

“…Although a two‐dimensional view of the suture surface does not capture complete information on the three‐dimensional suture face, isolated calvarial bones in iguanodontians show that they are fairly consistent in suture interdigitation along these sutural contacts, and studying the suture surface is, therefore, a good approximation of suture shape. Gryposaurus specimens were identified as species following Lowi‐Merri and Evans (2020) and Mallon et al (2022). Isolated lambeosaurine braincases from Dinosaur Park were tentatively identified by us as Lambeosaurus if the frontal‐nasal suture formed a raised ridge, and as Corythosaurus if this ridge was absent.…”

The dentary of hadrosauroid dinosaurs: evolution through heterochrony

“…The same is true for Saurolophinae, for example, in Gryposaurus notabilis (juvenile ROM 1939 with 25 tooth families (Mallon et al . 2022); large individuals with up to 40 (Lull & Wright 1942)) and Edmontosaurus annectens (juvenile UCMP 235860 with 9 tooth families, and subadult ROM 73859 with 44 (Wosik et al . 2019); adult specimens with up to 55 (Prieto‐Márquez 2008, p. 263)).…”

Introduction to Dinosaurs: New Ideas from Old Bones papers honoring Peter Dodson

“…Their study showed that the ceratobranchial, basihyal, cricoid, and paraglossal are likely keys in archosaur hyolaryngeal evolution (Yoshida et al, 2023). Mallon et al (2023) examined the smallest known examples of Gryposaurus notabilis that either reinforce or shed new light on developmental growth and biomechanics within the hadrosaurs. Previous studies did not include individuals smaller than 65% of the maximum known adult size.…”

“…Mallon et al (2023) examined the smallest known examples of Gryposaurus notabilis that either reinforce or shed new light on developmental growth and biomechanics within the hadrosaurs. Previous studies did not include individuals smaller than ~65% of the maximum known adult size.…”

“…Previous studies did not include individuals smaller than ~65% of the maximum known adult size. Their study included specimens that were only about ~40% the maximal size of G. notabilis and therefore extended the sampled ontogenetic sequence of G. notabilis (Mallon et al, 2023). They conclude that in the development of the postcranium, this taxon was like other hadrosaurines in that the postcranium grew isometrically or with positive allometry, suggesting that there was no strong selection pressure for more efficient locomotor performance among young hadrosaurids (Mallon et al, 2023).…”