Victoria M. Arbour

A redescription of the ankylosaurid dinosaur Dyoplosaurus acutosquameus Parks, 1924 (Ornithischia: Ankylosauria) and a revision of the genus

ABSTRACT The holotype of Dyoplosaurus acutosquameus Parks, 1924 (=Euoplocephalus tutus Lambe, 1910), represents one of the most complete ankylosaurid specimens collected to date. It consists of a partial skull roof, four mandible fragments with two teeth in situ, portions of postcranial armor (some in situ), skin impressions, articulated post-thoracic vertebrae, ten partial thoracic ribs, a partial left ilium, both ischia, and tail club, and associated right radius, left metacarpal IV, femur, tibia, fibula, and pes. We provide a detailed redescription of this material as well as comparisons with other ankylosaurid specimens referred to E. tutus. The synonymy of the genus with Euoplocephalus is not supported, and Dyoplosaurus is a valid taxon. It is possible that the fragmentary nature of the holotype of Euoplocephalus leads to the inference that the diversity of Late Cretaceous North American ankylosaurids is lower than it actually is. As such, it might be necessary to look beyond traditional cranial characters in order to accurately appraise the number and nature of various ankylosaurid taxa.

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

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.

The internal cranial morphology of an armoured dinosaur Euoplocephalus corroborated by X-ray computed tomographic reconstruction

Internal cranial anatomy is a challenging area to study in fossilized skulls because of small sample sizes and varied post‐mortem preservational alterations. This difficulty has led to the lack of correspondence between results obtained from direct osteological observation and from more indirect reconstruction methods. This paper presents corroborating evidence from direct osteological observation and from reconstruction based on computed X‐ray tomography (CT) on the internal cranial anatomy of the ankylosaurid dinosaur Euoplocephalus tutus. A remarkable specimen of Euoplocephalus preserves rarely observed internal cranial structures such as vascular impressions in the nasal cavity, olfactory turbinates and possible impressions of conchae. Comparison with fossils and CT models of other taxa and other Euoplocephalus specimens adds osteological evidence for the previously reconstructed nasal cavity in this dinosaur and revises the previously described braincase morphology. A new interpretation of the ethmoidal homology identifies a mesethmoid, sphenethmoid and ectethmoid. These ethmoidal ossifications are continuous with the mineralized walls of the nasal cavity. The location of the olfactory fenestra provides further evidence that the olfactory regions of the nasal cavity are pushed to the sides of the main airway. This implies that the function of the vascular impressions in the nasal cavity and the looping of the cavity are not related to olfaction. A byproduct of the elongate, looping airway is a dramatic increase in surface area of the nasal respiratory mucosa, which in extant species has been linked to heat and water balance. A role in vocalization as a resonating chamber is another possible function of the looping and elongation of the nasal cavity. Olfaction remains as a possible function for the enlarged olfactory region, suggesting that multiple functions account for different parts of the ankylosaurid nasal cavity that underwent substantial modification. Cranial endocasts show negligible variation within Euoplocephalus, which lends some confidence to interspecific comparisons of endocranial morphology.

Analyzing Taphonomic Deformation of Ankylosaur Skulls Using Retrodeformation and Finite Element Analysis

Taphonomic deformation can make the interpretation of vertebrate fossil morphology difficult. The effects of taphonomic deformation are investigated in two ankylosaurid dinosaur taxa, Euoplocephalus tutus (to investigate effects on our understanding of intraspecific variation) and Minotaurasaurus ramachandrani (to investigate the validity of this genus). The ratio of orbit maximum rostrocaudal length to perpendicular height is used as a strain ellipse, which can be used to determine if ankylosaur skull fossils have been dorsoventrally compacted during fossilization and diagenesis. The software program Geomagic is used to retrodeform three-dimensional (3D) digital models of the ankylosaur skulls. The effects of sediment compaction are modeled using finite element analysis, and the resulting strain distributions are compared with the retrodeformed models as a test of the retrodeformation method. Taphonomic deformation can account for a large amount of intraspecific variation in Euoplocephalus, but finite element analysis and retrodeformation of Minotaurasaurus shows that many of its diagnostic features are unlikely to result from deformation.

An ankylosaurid dinosaur from Mongolia with in situ armour and keratinous scale impressions

A Mongolian ankylosaurid specimen identified as Tarchia gigantea is an articulated skeleton including dorsal ribs, the sacrum, a nearly complete caudal series, and in situ osteoderms. The tail is the longest complete tail of any known ankylosaurid. Remarkably, the specimen is also the first Mongolian ankylosaurid that preserves impressions of the keratinous scales overlying the bony osteoderms. This specimen provides new information on the shape, texture, and arrangement of osteoderms. Large flat, keeled osteoderms are found over the pelvis, and osteoderms along the tail include large keeled osteoderms, elongate osteoderms lacking distinct apices, and medium-sized, oval osteoderms. The specimen differs in some respects from other Tarchia gigantea specimens, including the morphology of the neural spines of the tail club handle and several of the largest osteoderms.

Locomotion in ornithischian dinosaurs: an assessment using three‐dimensional computational modelling

Ornithischian dinosaurs were primitively bipedal with forelimbs modified for grasping, but quadrupedalism evolved in the clade on at least three occasions independently. Outside of Ornithischia, quadrupedality from bipedal ancestors has only evolved on two other occasions, making this one of the rarest locomotory transitions in tetrapod evolutionary history. The osteological and myological changes associated with these transitions have only recently been documented, and the biomechanical consequences of these changes remain to be examined. Here, we review previous approaches to understanding locomotion in extinct animals, which can be broadly split into form–function approaches using analogy based on extant animals, limb‐bone scaling, and computational approaches. We then carry out the first systematic attempt to quantify changes in locomotor muscle function in bipedal and quadrupedal ornithischian dinosaurs. Using three‐dimensional computational modelling of the major pelvic locomotor muscle moment arms, we examine similarities and differences among individual taxa, between quadrupedal and bipedal taxa, and among taxa representing the three major ornithischian lineages (Thyreophora, Ornithopoda, Marginocephalia). Our results suggest that the ceratopsid Chasmosaurus and the ornithopod Hypsilophodon have relatively low moment arms for most muscles and most functions, perhaps suggesting poor locomotor performance in these taxa. Quadrupeds have higher abductor moment arms than bipeds, which we suggest is due to the overall wider bodies of the quadrupeds modelled. A peak in extensor moment arms at more extended hip angles and lower medial rotator moment arms in quadrupeds than in bipeds may be due to a more columnar hindlimb and loss of medial rotation as a form of lateral limb support in quadrupeds. We are not able to identify trends in moment arm evolution across Ornithischia as a whole, suggesting that the bipedal ancestry of ornithischians did not constrain the development of quadrupedal locomotion via a limited number of functional pathways. Functional anatomy appears to have had a greater effect on moment arms than phylogeny, and the differences identified between individual taxa and individual clades may relate to differences in locomotor performance required for living in different environments or for clade‐specific behaviours.

Finite Element Analyses of Ankylosaurid Dinosaur Tail Club Impacts

Ankylosaurid dinosaurs have modified distal caudal vertebrae (the handle) and large terminal caudal osteoderms (the knob) that together form a tail club. Three‐dimensional digital models of four tail clubs referred to Euoplocephalus tutus were created from computed tomography scans of fossil specimens. We propose to use finite element modeling to examine the distribution of stress in simulated tail club impacts in order to determine the biological feasibility of hypothesized tail clubbing behavior. Results show that peak stresses were artificially high at the rigid constraint. The data suggest that tail clubs with small and average‐sized knobs were unlikely to fail during forceful impacts, but large clubs may have been at risk of fracture cranial to the knob. The modified handle vertebrae were capable of supporting the weight of even very large knobs. Long prezygapophyses and neural spines in the handle vertebrae helped distribute stress evenly along the handle. We conclude that tail swinging‐behavior may have been possible in Euoplocephalus, but more sophisticated models incorporating flexible constraints are needed to support this hypothesis. Anat Rec, 292:1412–1426, 2009.

A Review of Pelvic Shield Morphology in Ankylosaurs (Dinosauria: Ornithischia)

Abstract The pelvic shield of ankylosaurian dinosaurs refers to an area of osteoderms lacking differentiated transverse bands over the pelvic region and it is used as a diagnostic character for various ankylosaur groups. The pelvic shield character varies across ankylosaur taxa but is typically coded as a binary character or is excluded from phylogenetic analyses, which obscures evolutionary trends and relationships. This study investigates for the first time pelvic shield morphology in a stratigraphic and geographic context. This paper comprehensively reviews pelvic shield morphology with firsthand observations of specimens, and proposes three categories of pelvic shield morphology. Category 1 pelvic shields have un-fused but tightly interlocking osteoderms. Category 2 pelvic shields have fused osteoderms forming rosettes and are restricted to the Late Jurassic to mid Cretaceous of North America and Europe. Category 3 pelvic shields have fused polygonal osteoderms of similar size, and are found in the mid- to Late Cretaceous of North America. Although the pelvic shield is used to characterize the Polacanthidae, an interpretation supported by this review, the validity of such a clade is dependent upon a global parsimony analysis incorporating this character. Future analyses of the Ankylosauria should incorporate a more detailed treatment of the pelvic shield to determine its diagnostic value within the group.

Epidermal and dermal integumentary structures of ankylosaurian dinosaurs

Ankylosaurian dinosaurs are most notable for their abundant and morphologically diverse osteoderms, which would have given them a spiky appearance in life. Isolated osteoderms are relatively common and provide important information about the structure of the ankylosaur dermis, but fossilized impressions of the soft‐tissue epidermis of ankylosaurs are rare. Nevertheless, well‐preserved integument exists on several ankylosaur fossils that shows osteoderms were covered by a single epidermal scale, but one or many millimeter‐sized ossicles may be present under polygonal, basement epidermal scales. Evidence for the taxonomic utility of ankylosaurid epidermal scale architecture is presented for the first time. This study builds on previous osteological work that argues for a greater diversity of ankylosaurids in the Dinosaur Park Formation of Alberta than has been traditionally recognized and adds to the hypothesis that epidermal skin impressions are taxonomically relevant across diverse dinosaur clades. J. Morphol. 275:39–50, 2014.

A new ankylosaurid dinosaur from the Upper Cretaceous (Kirtlandian) of New Mexico with implications for ankylosaurid diversity in the Upper Cretaceous of western North America.

A new ankylosaurid (Ankylosauria: Dinosauria), Ziapelta sanjuanensis, gen. et sp. nov., is based on a complete skull, an incomplete first cervical half ring, a possible fragment of the second cervical half ring, and additional fragmentary osteoderms. The holotype specimen is from the Upper Cretaceous (Upper Campanian, Kirtlandian Land-Vertebrate Age) Kirtland Formation (De-na-zin Member) at Hunter Wash, San Juan Basin, in northwestern New Mexico, USA. Diagnostic characters of Ziapelta include: a large, prominent triangular median nasal caputegulum; a mixture of flat and bulbous frontonasal caputegulae; ventrolaterally oriented squamosal horns with a sharp, prominent dorsal keel; and the ventral surface of basicranium with three prominent anteroposteriorly oriented fossae. A phylogenetic analysis suggests that Ziapelta is not closely related to the other ankylosaurid from the De-na-zin Member, Nodocephalosaurus, but allies it to the northern North American ankylosaurids Ankylosaurus, Anodontosaurus, Euoplocephalus, Dyoplosaurus, and Scolosaurus.