A Problematic Tyrannosaurid (Dinosauria: Theropoda) Skeleton and Its Implications for Tyrannosaurid Diversity in the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta

Abstract

Several published censuses have noted the presence of two tyrannosaurids, Daspletosaurus sp. and Albertosaurus sarcophagus, within the Upper Cretaceous Horseshoe Canyon Formation of Alberta. Although A. sarcophagus is known from more than a dozen major discoveries in these strata, Daspletosaurus sp. is known from just a single problematic skeleton (lacking most of the skull) of a young individual. Here we describe and figure this skeleton, and marshal a variety of osteohistologic, morphometric, and phylogenetic methods to accurately determine its taxonomic status. Although none of these methods individually provides convincing evidence regarding the affinities of the specimen, together (and including other historical and biostratigraphic considerations) they strongly imply that the skeleton instead pertains to a young A. sarcophagus. In this way, we show that only a single species of tyrannosaurid is definitively present in the Horseshoe Canyon Formation, greatly simplifying interpretations of tyrannosaurid evolution and ecology in this setting. Anat Rec, 303:673–690, 2020. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Corrections and comments on the taxonomic value of anatomical features of tyrannosaurid theropods

“…They concluded that although there is no clear evidence for the identity of the specimen, combined results strongly imply the taxonomic identification as young Albertosaurus sarcophagus . Although I agree with the presented results in Mallon et al (2020), minor parts of comparative descriptions provided by Mallon et al (2020) to support the affinity of the specimen as Albertosaurus require comments and corrections. The purpose of this submission is to discussing whether some of the characters that were used for the identification are taxonomically meaningful or not.…”

“…I read with interest the recent study by Mallon et al (2020) that re‐examined the articulated tyrannosaurid skeleton (CMN 11315) with controversial identity from Upper Cretaceous Horseshoe Canyon Formation of Alberta, which assigned to Albertosaurus sarcophagus by some authors (e.g., Holtz Jr., 2004), while classified as Daspletosaurus sp. by others (e.g., Russell, 1970).…”

“…In the skull description paragraph of Mallon et al (2020), they noted that the rostral margin of dentary in Daspletosaurus differs from those of Albertosaurus and Tyrannosaurus in having rounded form, while the latter taxa they are straight. However, the form of rostral margin of dentary is highly variable in tyrannosaurids.…”

“…In albertosaurines Gorgosaurus and Albertosaurus , the transitional point is either positioned below third or fourth alveolus (Brusatte et al, 2012). Lastly, Mallon et al (2020) considered “ Raptorex kreigsteini ” lacks this feature, but this transitional point can be observed below the second alveolus in the holotype (Brusatte et al, 2012). Of note, though Brusatte et al (2012)) considered the position of transitional point in dentary has a significant metric in phylogeny, the highly variable nature of this character makes it suspicious whether this character bears any significant implications on taxonomy (Table 2).…”

“…In the description of the forelimb preserved in CMN 11315, Mallon et al (2020) assert Tarbosaurus bears the humerus length that is more than 33% of the length of the femur. However, in reality the humerus is greatly reduced as well as other forelimb elements in this taxon, as it has the humerus length that is less than 28% of the femur (Holtz Jr., 2004).…”