Pelvis of Gargoyleosaurus (Dinosauria: Ankylosauria) and the Origin and Evolution of the Ankylosaur Pelvis Kenneth Carpenter1,2*, Tony DiCroce3, Billy Kinneer3, Robert Simon4 1 Prehistoric Museum, Utah State University – Eastern, Price, Utah, United States of America, 2 Geology Section, University of Colorado Museum, Boulder, Colorado, United States of America, 3 Denver Museum of Nature and Science, Denver, Colorado, United States of America, 4 Dinosaur Safaris Inc., Ashland, Virginia, United States of America Abstract Discovery of a pelvis attributed to the Late Jurassic armor-plated dinosaur Gargoyleosaurus sheds new light on the origin of the peculiar non-vertical, broad, flaring pelvis of ankylosaurs. It further substantiates separation of the two ankylosaurs from the Morrison Formation of the western United States, Gargoyleosaurus and Mymoorapelta. Although horizontally oriented and lacking the medial curve of the preacetabular process seen in Mymoorapelta, the new ilium shows little of the lateral flaring seen in the pelvis of Cretaceous ankylosaurs. Comparison with the basal thyreophoran Scelidosaurus demonstrates that the ilium in ankylosaurs did not develop entirely by lateral rotation as is commonly believed. Rather, the preacetabular process rotated medially and ventrally and the postacetabular process rotated in opposition, i.e., lateral and ventrally. Thus, the dorsal surfaces of the preacetabular and postacetabular processes are not homologous. In contrast, a series of juvenile Stegosaurus ilia show that the postacetabular process rotated dorsally ontogenetically. Thus, the pelvis of the two major types of Thyreophora most likely developed independently. Examination of other ornithischians show that a non-vertical ilium had developed independently in several different lineages, including ceratopsids, pachycephalosaurs, and iguanodonts. Therefore, a separate origin for the non-vertical ilium in stegosaurs and ankylosaurs does have precedent. Citation: Carpenter K, DiCroce T, Kinneer B, Simon R (2013) Pelvis of Gargoyleosaurus (Dinosauria: Ankylosauria) and the Origin and Evolution of the Ankylosaur Pelvis. PLoS ONE 8(11): e79887. doi:10.1371/journal.pone.0079887 Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America Received June 14, 2013; Accepted September 25, 2013; Published November 14, 2013 Copyright: ß 2013 Carpenter et al. 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. Funding: The authors have no support or funding to report. Competing Interests: Robert Simon is affiliated with the business Dinosaur Safaris Inc., Kenneth Carpenter gets a salary from Utah State University, and Billy Kinneer and Tony DiCroce have retirement pensions. There are no patents, products in development or marketed products to declare because this is a manuscript on vertebrate fossils. This does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors. * E-mail: [email protected] Introduction Materials Armor-plated ankylosaurs are a minor component of the Late Institutional Abbreviations: AMNH – American Museum of Cretaceous landscape, except in the arid paleoenvironments of central Natural History, New York, USA); ANSP – Academy of Natural Asia. They are characterized by a broad, flaring pelvis that can be Sciences (Philadelphia, PA); BHI – Black Hills Institute of wider than long, formed by nearly horizontal ilia. The origin of this Geological Research (Hill City, SD). BMAG – Bristol Museum odd pelvis has been variously attributed to the protection of internal and Art Gallery (Bristol, UK); BMNH – Natural History Museum organs [1] or support of heavy dermal armor [2]. These hypotheses of London (London, UK); CMNH – Carnegie Museum of Natural are examined in light of the discovery of the pelvis attributed to the History (Pittsburgh, PA); DINO – Dinosaur National Monument primitive ankylosaur Gargoyleosaurus parkpinorum from the Upper (Jensen, UT); DMNH – Denver Museum of Science and Nature Jurassic Morrison Formation in the Big Horn Basin, Wyoming. (Denver, Colorado, USA); HMNH – Hayashibara Museum of Taken in context with the older Scelidosaurus from the Lower Jurassic of Natural History (Okayama, Japan); IGM - Institute of Geology, England, and the younger Euoplocephalus from the Cretaceous of North Mongolian Academy of Sciences (Ulaan Baatar, Mongolia). MNA America, the pelvis of Gargoyleosaurus is intermediate in its morphology. – Museum of Northern Arizona (Flagstaff, AZ); MWC – Museum Traditionally, the non-vertical ilium of ankylosaurs was of Western Colorado (Grand Junction, Colorado, USA); NMC – interpreted as forming from lateral folding of the ilium so that National Museums of Canada (Ottawa, ON); SAM, South African the lateral surface faced ventrally [1,2], although no evidence was Museum, Cape Town, South Africa; SGDS – Saint George presented. This ‘‘self-evident’’ interpretation is widely accepted Dinosaur Center (Saint George, Utah, USA). today [3,4]. Our study below of ankylosaur pelvic origins and Specimens And Their Localities: Gargoyleosaurus parkpinorum: evolution casts doubt on this interpretation, and has major partial pelvis (DMNS 58831) comprised of a complete synsacrum, ramifications on the phylogenetics of the Thyreophora. with damaged neural spines, articulated with a complete right ilium, from the Morrison Formation, Simon Quarry, Big Horn Methods County, Wyoming, USA. Nearly complete right pubis with Specimens were studied and photographed in their respective associated partial skeleton (DMNH 27726) from the Morrison institutions. There were no field studies; therefore, no permits were Formation, Bone Cabin Quarry West, Albany County, Wyoming required. USA. Pelvis parts of other specimens examined include: PLOS ONE | www.plosone.org 1 November 2013 | Volume 8 | Issue 11 | e79887 Ankylosaur Pelves Camptosaurus amphanoecetes pelvis from a nearly complete skeleton the bone band. The sutural zones are marked by small ossicles that (CM 11337), Morrison Formation, Dinosaur National Monument, are coossified to the larger osteoderms. These ossicles are more Utah, USA. Chasmosaurus belli pelvis (ANSP 15764, cast of NMC extensively developed along the posterior side of the referred 2245), Dinosaur Park Formation, Dinosaur Provincial Park, quarter-ring, which because of its larger size as compared to the Alberta, Canada. Dryosaurus altus pelvis (CM 3392), Morrison holotype quarter-ring, is probably ontogenetic (i.e., increased Formation, Dinosaur National Monument, Utah, USA. Edmontonia ossification with age). The other ankylosaur from the Morrison rugosidens ischia and pubis associated with holotype partial skeleton Formation, Mymoorapelta [12] shares with Gargoyleosaurus the (USNM 11868), Two Medicine Formation, Glacier County, triangular, dorsoventrally compressed lateral plates. However, Montana, USA; nearly complete pelvis in dorsal view (AMNH although Mymoorapelta apparently has rows of cervical osteoderms, FR5381), Dinosaur Park Formation, Dinosaur Provincial Park, these are not coossified into a quarter-ring or half-ring despite Alberta, Canada. Edmontonia longiceps pelvis in ventral view (NMC having been reconstructed as such on the skeletal mount (Museum 8531), Horseshoe Canyon Formation, Alberta, Canada. Edmontonia of Western Colorado; the skull on the mount is modeled on schlessmani (BHI 127327), Lance Formation, Niobrara County, Gargoyleosaurus as well). Wyoming, USA. Edmontonosaurus annectens pelvis (BHI 126,414), The synsacrum of DMNS 58831is composed of four sacral Hell Creek Formation, Ziebach County, South Dakota, USA. vertebrae (defined below), one caudal vertebra and three presacral Euoplocephalus tutus complete pelvis of a partial skeleton (AMNH vertebrae (Fig. 1B). The term ‘‘presacral’’ is used here as originally FR5337), Dinosaur Park Formation, Dinosaur Provincial Park, defined by Osborn ([13]:193): ‘‘It is best to enumerate the dorsals Alberta, Canada. Homalocephale calathoceros ilium of a partial also from the sacrum forwards, namely as presacrals 1, 2, 3, etc.’’ skeleton (IGM 100/51), Nemegt Formation, Nemegt Basin, This numerical system was introduced because the number of Mongolia. Lesothosaurus diagnosticus right ilium, pubis and ischium dorsal vertebrae was unknown for the specimen (Diplodocus) that he (BMNH RU 17), Elliot Formation, Likhoele, Lesotho. Mymoor- was describing. Osborn pointedly did not include the cervicals. apelta maysii complete left ilium (MWC 1815) and left ischium Nevertheless, in recent years the term ‘‘presacral vertebrae’’ has (MWC 4027), Morrison Formation, Mygatt-Moore Quarry, Mesa been misused, with presacral vertebra 1 being equal to either the County, Colorado, USA; natural mold of the sacral vertebrae, first cervical or first dorsal vertebrae. As so used, the term is proximal end of the ischium and partial pubis (MWC 2610), redundant with cervical 1 or dorsal 1. The presacral centra in Morrison Formation, Hups Quarry, Mesa County, Colorado, Gargoyleosaurus are spool-shaped and slightly compressed laterally USA. Planicoxa depressus ilium (USNM 4759), Lakota Sandstone, so as to be taller than wide. The centra of the first presacral, the Calico Canyon, South Dakota, USA. Saichania chulsanensis pelvis of four sacrals and first caudal of Gargoyleosaurus