Baby tyrannosaurid bones and teeth from the Late Cretaceous of western North America¹

Abstract

Tyrannosaurids were the apex predators of Late Cretaceous Laurasia and their status as dominant carnivores has garnered considerable interest since their discovery, both in the popular and scientific realms. As a result, they are well studied and much is known of their anatomy, diversity, growth, and evolution. In contrast, little is known of the earliest stages of tyrannosaurid development. Tyrannosaurid eggs and embryos remain elusive, and juvenile specimens — although known — are rare. Perinatal tyrannosaurid bones and teeth from the Campanian–Maastrichtian of western North America provide the first window into this critical period of the life of a tyrannosaurid. An embryonic dentary (cf. Daspletosaurus) from the Two Medicine Formation of Montana, measuring just 3 cm long, already exhibits distinctive tyrannosaurine characters like a “chin” and a deep Meckelian groove, and reveals the earliest stages of tooth development. When considered together with a remarkably large embryonic ungual from the Horseshoe Canyon Formation of Alberta, minimum hatchling size of tyrannosaurids can be roughly estimated. A perinatal premaxillary tooth from the Horseshoe Canyon Formation likely pertains to Albertosaurus sarcophagus and it shows small denticles on the carinae. This tooth shows that the hallmark characters that distinguish tyrannosaurids from other theropods were present early in life and raises questions about the ontogenetic variability of serrations in premaxillary teeth. Sedimentary and taphonomic similarities in the sites that produced the embryonic bones provide clues to the nesting habits of tyrannosaurids and may help to refine the prospecting search image in the continued quest to discover baby tyrannosaurids.

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

“…to JCM, 2021)—the secondary ridges are lost. This is the first such documented instance of secondary ridge loss in hadrosaurid ontogeny, and evokes analogous discrete dental character changes in other dinosaurian taxa, such as the acquisition of serrations in tyrannosaurine premaxillary tooth ontogeny (Carr & Williamson, 2004; Currie, 2003), although this has recently been suggested to be more complex (Funston et al, 2021).…”

Nesting at extreme polar latitudes by non-avian dinosaurs

“…In total, perinatal material was recovered from seven dinosaur families, as well as birds, representing 70% of those known to occur in the PCF (Figure 4). The only PCF families not currently recognized by perinates are Pachycephalosauridae, the neonates of which are unknown globally, and Ornithomimidae, which lack teeth and make their discovery less Data sources, by formation: Prince Creek (this study); Wapiti; 34 Horseshoe Canyon; 35,36 Dinosaur Park and Oldman; [36][37][38] Judith River; 38 Two Medicine; 36,[38][39][40][41] Hell Creek. 38,42 likely.…”

“…23 We also compared UAMES 17610 to TMP 1996.005.011, a very small premaxillary tooth (crown height = 8.5 mm) referred to Albertosaurus sarcophagus from the Horseshoe Canyon Formation of Alberta, Canada. 36 TMP 1996.005.011 has nearly the exact same crown height as UAMES 17610 and its developmental status is similarly interpreted as being likely a nestling or young juvenile.…”

Celebrating dinosaurs: their behaviour, evolution, growth, and physiology

“…These studies rose to prominence in the early years of the Dinosaur Renaissance when they were used to study tooth replacement, growth rates, and even physiology of dinosaurs (de Ricqlés 1974;Reid 1997). Although histological studies in their simplest form require relatively unsophisticated equipment, they have been improved recently by micro-computerized tomography (CT) scans, synchrotrons (Funston et al 2021), and even XRF. The number of papers that include histological studies have increased dramatically over the last 20 years, and it has reached the point where some histological work needs to be done for each paper submitted, if only to determine how old the dinosaur was when it died.…”