Reassessment of a juvenile Daspletosaurus from the Late Cretaceous of Alberta, Canada with implications for the identification of immature tyrannosaurids

Abstract

Daspletosaurus is a large tyrannosaurine found in upper Campanian deposits of Alberta and Montana. Although several large subadult and adult individuals of this taxon are known, only one juvenile individual, TMP 1994.143.1, has been identified. This specimen has played a key role in the idea that juvenile tyrannosaurid individuals are difficult to differentiate among species. Here the taxonomic affinity of TMP 1994.143.1 is reassessed in light of a juvenile tyrannosaurine postorbital recently discovered in the Dinosaur Park Formation of Alberta. Anatomical comparisons and phylogenetic analyses reveal that TMP 1994.143.1 is referable to the albertosaurine Gorgosaurus libratus, whereas the new postorbital belongs to a small juvenile Daspletosaurus. This taxonomic reassignment of TMP 1994.143.1 results in the juvenile ontogenetic stage of Daspletosaurus being known only from two isolated cranial elements. The new postorbital provides insights into early Daspletosaurus ontogeny, revealing that the cornual process developed earlier or faster than in other tyrannosaurids. Although some ontogenetic changes in the postorbital are found to be unique to Daspletosaurus, overall changes are most consistent with those of other large tyrannosaurines. Our results also show that diagnostic features develop early in ontogeny, such that juveniles of different tyrannosaurid species are easier to differentiate than previously thought.

Intraspecific facial bite marks in tyrannosaurids provide insight into sexual maturity and evolution of bird-like intersexual display

“…Data for these latter taxa are excluded from the systematic analysis, although the raw data are reported. Following Currie (2003b) and Voris and colleagues (2019, 2020), taxonomic identity could be confidently determined for many articulated/associated skulls and bone bed material, but not for all isolated specimens. Isolated specimens from the Horseshoe Canyon Formation were attributed to A. sarcophagus ; however, isolated specimens from formations containing two tyrannosaurid taxa (Dinosaur Park Formation: Daspletosaurus sp., G. libratus ) could occasionally not be identified beyond Tyrannosauridae indet.…”

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

“…Osteohistological data have enabled detailed analyses of tyrannosaurid growth rate and life history (Erickson et al 2004, Horner and Padian 2004, Woodward et al 2020, showing that tyrannosaurids grew at high but inconsistent rates in the later stages of their lives. Recently discovered small tyrannosauroid taxa from the Early Cretaceous and early Late Cretaceous show that many of the characters once considered distinctive of larger tyrannosaurids evolved at smaller body sizes (Brusatte et al 2016a, Nesbitt et al 2019, Voris et al 2019, Zanno et al 2019. These may have enabled mid-sized tyrannosauroids to flourish in the Late Cretaceous after the extinction of allosauroids in North America Makovicky 2011, 2013).…”

“…In contrast, little is known of the earliest stages of tyrannosaurid development. Eggs and embryos remain elusive, only a handful of perinatal teeth have been described (Carpenter 1982), and juvenile specimens-although known-are rare (Carr 1999, Tsuihiji et al 2011, Voris et al 2019, Woodward et al 2020. These immature specimens are essential because it is now recognized that the tyrannosaurid skeleton undergoes dramatic changes throughout ontogeny (Carr 1999, Currie 2003b, Carr 2020.…”

A Reassessment of the Taxonomic Validity of Dynamoterror dynastes (Theropoda, Tyrannosauridae)

“…8, b). Similar variations can be observed between multiple Gorgosaurus libratus Lambe, 1914 specimens as well (Currie, 1987(Currie, , 2003Voris et al, 2019). In summary, the close position of these two processes and the presence of a shallow notch between them cannot be an autapomorphy of Dynamoterror as nearly identical condition occurs in Daspletosaurus torosus, and such configuration might represent individual or ontogenetic variation rather than phylogenetic.…”