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A Subadult Frontal of Daspletosaurus Torosus

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A Subadult Frontal of Daspletosaurus Torosus Yun, DASPLETOSAURUS TOROSUS frontal PJVP, 17(2) (2020) A SUBADULT FRONTAL OF DASPLETOSAURUS TOROSUS (THEROPODA: TYRANNOSAURIDAE) FROM THE LATE CRETACEOUS OF ALBERTA, CANADA WITH IMPLICATIONS FOR TYRANNOSAURID ONTOGENY AND TAXONOMY Chan-gyu Yun* *Vertebrate Paleontological Institute of Incheon, Incheon 21974, Republic of Korea. Biological Sciences, Inha University, Incheon 22212, Republic of Korea [email protected] Yun, Chan-gyu. A Subadult Frontal of Daspletosaurus torosus (Theropoda: Tyranno- sauridae) from the Late Cretaceous of Alberta, Canada with Implications for Tyranno- saurid Ontogeny and Taxonomy. – Palarch’s Journal of Vertebrate Palaeontology 17(2) (2020), 1-13. ISSN 1567-2158. 13 pages + 6 figures. Keywords: Daspletosaurus, Dinosaur Park Formation, Frontal, Theropoda, Tyranno- sauridae ABSTRACT An isolated frontal bone of Daspletosaurus torosus (Theropoda: Tyrannosauridae) is desecribed which was probably found in the Dinosaur Park Formation (Upper Cretaceous) of Dinosaur Provincial Park, Alberta (Canada). It is important in terms of the first detailed osteological description of the frontal ofDaspletosaurus torosus. The size and anatomical details of the specimen indicates the frontal belongs to a large subadult indi- vidual. This subadult frontal suggests that although ontogeny of Daspletosaurus torosus was generally similar to that of Tyrannosaurus rex, there were some distinct differences. Finally, certain features of this frontal bone indicate that some autapomorphies that have recently suggested for some tyrannosaurid taxa are inadequate due to their broad distribution within a clade. PalArch Foundation 1 Yun, DASPLETOSAURUS TOROSUS frontal PJVP, 17(2) (2020) INTRODUCTION Tyrannosaurid theropods, which dominated terrestrial ecosystems of western North America and Asia during the Late Cretaceous as apex predators (Holtz, 2004; Brusatte et al., 2010), represent the most iconic non-avian dinosaurs of all time. The members of this group are characterized by large, deep skulls with robust teeth, short forelimbs with two functional digits, and elongate hindlimbs with adaptations for cursorial behavior (Brochu, 2003; Holtz, 2004). Tyranno- sauridae is highly diverse in cranial and skeletal morphology, however, as it in- cludes relatively small-sized taxa with longirostrine skulls (Brusatte et al., 2012; Lü et al., 2014), medium-sized taxa with short snouts (Carr et al., 2011; Loewen et al., 2013) and very large taxa with robust skulls (Brochu, 2003; Holtz, 2004; Hone et al., 2011; Yun, 2017; Persons et al., 2019). As a result, tyrannosaurids have been included in many paleobiological studies related to systematics (e.g., Loewen et al., 2013; Carr et al., 2017), ecological behavior (e.g., Holtz, 2008), as well as paleodiversity and anatomy (e.g., Carr, 1999; Currie, 2003; Carr & Williamson, 2004). Indeed, tyrannosaurid theropods are particularly important in understanding dinosaur paleobiology and Mesozoic terrestrial ecosystems as they represent advanced, bird-like coelurosaurian theropods, yet morphologically divergent from closely related lineages (Brusatte et al., 2010; Yun, 2016; Carr et al., 2017; Persons et al., 2019). Daspletosaurus is a genus of tyrannosaurid theropod erected by Russell (1970), which include the type species Daspletosaurus torosus, from the Oldman and Di- nosaur Park Formations of Alberta and the referred species Daspletosaurus horneri from the Two Medicine Formation of Montana (Currie, 2003; Carr et al., 2017). Unfortunately, despite the large numbers of individuals that have been men- tioned (Currie, 2003) most of the referred specimens of Daspletosaurus torosus have not been properly described. This is especially true of juveniles and subadult individuals, as the majority of currently known Daspletosaurus torosus individuals are adults (Delcourt, 2017; Voris et al., 2019). Currie (2003) only briefly de- scribed several Daspletosaurus specimens in his review on tyrannosaurid cranial anatomy, and the frontal element of juvenile Daspletosaurus torosus he had figured (TMP 1994.143.1) is now known as the misidentified Gorgosaurus libratus (Voris et al., 2019). In summary, little is known about the morphological changes that occurred during the ontogeny of this species, and many questions remain unre- solved. Thus, the present paper describes an isolated tyrannosaurid frontal bone (SDNHM 32701) from the Dinosaur Park Formation of southern Alberta that is assignable to Daspletosaurus torosus in an attempt to contribute to clarifications of these questions and problems. Although, admittedly, the specimen is fragmen- tary, it provides several informative anatomical features that are hard to observe in articulated specimens nonetheless. Furthermore, the discovery of this specimen is historically significant, as SDNHM 32701 predates the discovery of holotype in 1921 and naming of the Daspletosaurus torosus (Russell, 1970). INSTITUTIONAL ABBREVIATIONS CMN: Canadian Museum of Nature, Ottawa, Canada LACM: Los Angeles County Museum, California, U.S.A. SDNHM: San Diego Natural History Museum, San Diego, U.S.A. TMP: Royal Tyrrell Museum of Palaeontology, Drumheller, Canada UMNH: Utah Museum of Natural History, Utah, U.S.A. PalArch Foundation 2 Yun, DASPLETOSAURUS TOROSUS frontal PJVP, 17(2) (2020) GEOLOGICAL SETTING The specimen was purchased by San Diego Natural History Museum from Charles H. Sternberg in 1921. It was collected below Steveville, Red Deer River of Alberta which is within the boundaries of what is now Dinosaur Provincial Park. Unfortunately, no additional information was provided. In the Park area, both Oldman Formation and Dinosaur Park Formation are exposed (Eberth & Evans, 2011). However, it is probable that this was collected from the Dinosaur Park Formation as Sternberg collected dinosaurs exclusively from this formation during the 1910s (Currie, 2005). Of note, Currie (2003) listed SDNHM 32701 as coming from the Dinosaur Park Formation as well, but he gave no reason for it. It is possible that the specimen was collected in 1917, as there are records that imply that Sternberg sold fossils that he collected in that year to San Diego Natural History Museum (Currie, 2005). If this assumption is correct, it is likely that it was collected from the south side of the Red Deer River as Sternberg in- vestigated this area in 1917 (Currie, 2005). Considering that Sternberg found dinosaur taxa (e.g., Centrosaurus, Corythosaurus) that are exclusive in lower or middle part of the formation in this area (e.g., Arbour et al., 2009; Eberth & Ev- ans, 2011; Fowler, 2017), it is probable that the specimen that is described in this paper came from either one of these stratigraphic positions instead. This is further supported by the taxonomic identity of the specimen, Daspletosaurus torosus, and certain materials of this taxon have been only recovered in the lower two-thirds of the Dinosaur Park Formation (Carr et al., 2017). The Dinosaur Park Formation is approximately 75 m thick. The sediments composing it were derived from the erosion of western mountains and deposited during the initial stages of the last major transgression of the Western Interior Seaway (Eberth, 2005; Fowler, 2017). 40Ar / 39Ar radiometric dating constrains the age of the Dinosaur Park Formation to between 77.3 Ma (the uppermost part of the Oldman Formation) and 75.46 Ma (the base of Bearpaw Formation) (Fowler, 2017). Given that the sedimentation rates for the upper 22 meters of the Dinosaur Park Formation and Bearpaw Formation transition were slow (Carr et al., 2017), the top of the Dinosaur Park Formation is likely slightly older than 75.46 Ma. The formation is famous for the rich abundance of vertebrate fos- sils including fishes, amphibians, turtles, squamates, crocodiles, pterosaurs, di- nosaurs and mammals (e.g., Currie & Koppelhus, 2005; Longrich, 2008). The Dinosaur Park Formation is partly equivalent in age and partly correlates with the Campanian deposits in Montana such as Judith River Formation (Eberth, 1997). MATERIAL AND METHODS Due to the lack of funding at the beginning of this project, this anatomical study was conducted using high-quality casts of the original specimen housed at the San Diego Natural History Museum (SDNHM 32701) made through 3D scanning. Anatomical comparisons were made with other North American tyrannosauroids such as Bistahieversor and tyrannosaurids. The comparisons were made through on an extensive review on the literature and direct observations on several casts. The anatomical nomenclature used in this study follows Currie (2003), Carr et al. (2017) and McDonald et al. (2018). SYSTEMATIC PALEONTOLOGY Tyrannosauroidea Osborn, 1906 Tyrannosauridae Osborn, 1906 Tyrannosaurinae Osborn, 1906 Daspletosaurini Voris et al., 2020 [in press] PalArch Foundation 3 Yun, DASPLETOSAURUS TOROSUS frontal PJVP, 17(2) (2020) Daspletosaurus Russell, 1970 Daspletosaurus torosus Russell, 1970 MATERIAL SDNHM 32701. An incomplete right frontal bone, lacking the rostral end of the nasal process, most of the sagittal crest and caudolateral parts of the bone. LOCALITY AND AGE Dinosaur Provincial Park, Alberta, Canada. Upper Cretaceous (Campanian) Di- nosaur Park Formation. ASSIGNMENT Currie (2003) listed SDNHM 32701 as Daspletosaurus sp., but did not give a reason for this referral. The specimen can be unambiguously referred asDaspleto - saurus based on the presence of one autapomorphy of the genus: the presence of rostrocaudal ridge that is present along the nasal process of the frontal (Carr et al., 2017;
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