Euoplocephalus tutus and the Diversity of Ankylosaurid Dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA

Abstract

Few ankylosaurs are known from more than a single specimen, but the ankylosaurid Euoplocephalus tutus (from the Late Cretaceous of Alberta, Canada and Montana, USA) is represented by dozens of skulls and partial skeletons, and is therefore an important taxon for understanding intraspecific variation in ankylosaurs. Euoplocephalus is unusual compared to other dinosaurs from the Late Cretaceous of Alberta because it is recognized from the Dinosaur Park, Horseshoe Canyon, and Two Medicine formations. A comprehensive review of material attributed to Euoplocephalus finds support for the resurrection of its purported synonyms Anodontosaurus lambei and Scolosaurus cutleri, and the previously resurrected Dyoplosaurus acutosquameus. Anodontosaurus is found primarily in the Horseshoe Canyon Formation of Alberta and is characterized by ornamentation posterior to the orbits and on the first cervical half ring, and wide, triangular knob osteoderms. Euoplocephalus is primarily found in Megaherbivore Assemblage Zone 1 in the Dinosaur Park Formation of Alberta and is characterized by the absence of ornamentation posterior to the orbits and on the first cervical half ring, and keeled medial osteoderms on the first cervical half ring. Scolosaurus is found primarily in the Two Medicine Formation of Montana (although the holotype is from Dinosaur Provincial Park), and is characterized by long, back-swept squamosal horns, ornamentation posterior to the orbit, and low medial osteoderms on the first cervical half ring; Oohkotokia horneri is morphologically indistinguishable from Scolosaurus cutleri. Dyoplosaurus was previously differentiated from Euoplocephalus sensu lato by the morphology of the pelvis and pes, and these features also differentiate Dyoplosaurus from Anodontosaurus and Scolosaurus; a narrow tail club knob is probably also characteristic for Dyoplosaurus.

Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs

“…MPCA-SM 1 is similar in size to a juvenile Anodontosaurus (AMNH 5266), which is 25.5 cm long (Arbour & Currie 2013a). The femur has a distinct, hemispherical head, similar to the condition in Dongyangopelta (Chen et al 2013), Hoplitosaurus (USNM 4752) and Nodosaurus (Lull 1921).…”

“…The femur (MPCA-SM 1) is 25.24 cm long, compared to 53.5 cm (AMNH 5404) and 51.5 cm (UALVP 31) in Euoplocephalus (Arbour & Currie 2013a) and 67 cm in Ankylosaurus (Carpenter 2004). MPCA-SM 1 is similar in size to a juvenile Anodontosaurus (AMNH 5266), which is 25.5 cm long (Arbour & Currie 2013a).…”

“…Gargoyleosaurus and Gastonia were recovered as basal nodosaurids, and Cedarpelta and Liaoningosaurus were recovered as basal ankylosaurids. Arbour & Currie (2013a) modified many of the character codings from Thompson et al (2012), and Arbour et al (2014a) revised the character list substantially, removing numerous characters and adding in new characters. The analysis presented in this paper further modifies the character matrix presented in Arbour et al (2014c …”

External and internal structure of ankylosaur (Dinosauria, Ornithischia) osteoderms and their systematic relevance

“…This is likely attributable to various sources, but primarily includes taxonomic differences (Carpenter, 1982;Penkalski, 2001, in press;Arbour et al, 2009;Penkalski and Blows, 2012;Arbour and Currie, 2013). Most of the information on the arrangement of osteoderms across the body in Euoplocephalus is actually from the holotypes of Dyoplosaurus acutosquameus Parks, 1924 (ROM 784), and Scolosaurus cutleri Nopsca, 1928 (NHMUK R5161).…”

“…UALVP 31 includes a skull and has been assigned to Euoplocephalus (Vickaryous and Russell, 2001;Arbour and Currie, 2013). Although its body placement is not known, the osteoderm studied (Fig.…”