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

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          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.

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          Most cited references54

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          On the Classification of the Fossil Animals Commonly Named Dinosauria

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            The paranasal air sinuses of predatory and armored dinosaurs (archosauria: theropoda and ankylosauria) and their contribution to cephalic structure.

            The paranasal air sinuses and nasal cavities were studied along with other cephalic spaces (brain cavity, paratympanic sinuses) in certain dinosaurs via CT scanning and 3D visualization to document the anatomy and examine the contribution of the sinuses to the morphological organization of the head as a whole. Two representatives each of two dinosaur clades are compared: the theropod saurischians Majungasaurus and Tyrannosaurus and the ankylosaurian ornithischians Panoplosaurus and Euoplocephalus. Their extant archosaurian outgroups, birds and crocodilians (exemplified by ostrich and alligator), display a diversity of paranasal sinuses, yet they share only a single homologous antorbital sinus, which in birds has an important subsidiary diverticulum, the suborbital sinus. Both of the theropods had a large antorbital sinus that pneumatized many of the facial and palatal bones as well as a birdlike suborbital sinus. Given that the suborbital sinus interleaves with jaw muscles, the paranasal sinuses of at least some theropods (including birds) were actively ventilated rather than being dead-air spaces. Although many ankylosaurians have been thought to have had extensive paranasal sinuses, most of the snout is instead (and surprisingly) often occupied by a highly convoluted airway. Digital segmentation, coupled with 3D visualization and analysis, allows the positions of the sinuses to be viewed in place within both the skull and the head and then measured volumetrically. These quantitative data allow the first reliable estimates of dinosaur head mass and an assessment of the potential savings in mass afforded by the sinuses.
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              Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus

              Background Extended neoteny and late stage allometric growth increase morphological disparity between growth stages in at least some dinosaurs. Coupled with relatively low dinosaur density in the Upper Cretaceous of North America, ontogenetic transformational representatives are often difficult to distinguish. For example, many hadrosaurids previously reported to represent relatively small lambeosaurine species were demonstrated to be juveniles of the larger taxa. Marginocephalians (pachycephalosaurids + ceratopsids) undergo comparable and extreme cranial morphological change during ontogeny. Methodology/Principal Findings Cranial histology, morphology and computer tomography reveal patterns of internal skull development that show the purported diagnostic characters for the pachycephalosaurids Dracorex hogwartsia and Stygimoloch spinifer are ontogenetically derived features. Coronal histological sections of the frontoparietal dome of an adult Pachycephalosaurus wyomingensis reveal a dense structure composed of metaplastic bone with a variety of extremely fibrous and acellular tissue. Coronal histological sections and computer tomography of a skull and frontoparietal dome of Stygimoloch spinifer reveal an open intrafrontal suture indicative of a subadult stage of development. These dinosaurs employed metaplasia to rapidly grow and change the size and shape of their horns, cranial ornaments and frontoparietal domes, resulting in extreme cranial alterations during late stages of growth. We propose that Dracorex hogwartsia, Stygimoloch spinifer and Pachycephalosaurus wyomingensis are the same taxon and represent an ontogenetic series united by shared morphology and increasing skull length. Conclusions/Significance Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are reinterpreted as younger growth stages of Pachycephalosaurus wyomingensis (adult). This synonymy reduces the number of pachycephalosaurid taxa from the Upper Cretaceous of North America and demonstrates the importance of cranial ontogeny in evaluating dinosaur diversity and taxonomy. These growth stages reflect a continuum rather than specific developmental steps defined by “known” terminal morphologies.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2013
                8 May 2013
                14 May 2013
                : 8
                : 5
                : e62421
                Affiliations
                [1]Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
                Raymond M. Alf Museum of Paleontology, United States of America
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: VMA. Performed the experiments: VMA. Analyzed the data: VMA. Contributed reagents/materials/analysis tools: VMA PJC. Wrote the paper: VMA PJC.

                Article
                PONE-D-13-02249
                10.1371/journal.pone.0062421
                3648582
                23690940
                0cb06929-474a-46e2-8a81-3b370c2c3a96
                Copyright @ 2013

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 11 January 2013
                : 21 March 2013
                Page count
                Pages: 39
                Funding
                This work was supported by Alberta Ingenuity Studentship ( http://www.albertainnovates.ca/), Dinosaur Research Institute ( http://www.dinosaurresearch.com/index.htm), Izaak Walton Killam Doctoral Scholarship ( http://www.killamtrusts.ca/uofAlberta.asp), Korea-Mongolia International Dinosaur Project ( http://www.digitaldreammachine.com/gobifieldwork/kid/home.html), National Science and Engineering Research Council (Canada Graduate Scholarship – Doctoral; Postgraduate Scholarship – Masters; Michael Smith Foreign Study Supplement) ( http://www.nserc-crsng.gc.ca/index_eng.asp), University of Alberta China Institute ( http://www.china.ualberta.ca/), University of Alberta Graduate Students Association ( http://www.gsa.ualberta.ca/), University of Alberta Women in Scholarship, Engineering, Science and Technology ( http://www.wisest.ualberta.ca/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology
                Anatomy and Physiology
                Comparative Anatomy
                Evolutionary Biology
                Paleontology
                Vertebrate Paleontology
                Evolutionary Systematics
                Paleontology
                Biogeography
                Vertebrate Paleontology
                Zoology
                Animal Phylogenetics
                Earth Sciences
                Paleontology
                Biogeography
                Biostratigraphy
                Vertebrate Paleontology

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