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Osteological Correlates for Quadrupedality in Ornithischian Dinosaurs

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Osteological Correlates for Quadrupedality in Ornithischian Dinosaurs Osteological correlates for quadrupedality in ornithischian dinosaurs SUSANNAH C.R. MAIDMENT and PAUL M. BARRETT Maidment, S.C.R. and Barrett, P.M. 2014. Osteological correlates for quadrupedality in ornithischian dinosaurs. Acta Palaeontologica Polonica 59 (1): 53–70. The evolution of quadrupedality from bipedal ancestors is an exceptionally rare transition in tetrapod evolution, but it has occurred several times within the herbivorous dinosaur clade Ornithischia. Stegosauria, Ankylosauria, and Ceratopsidae are all uncontroversially quadrupedal, while basal ornithischians and basal ornithopods are uncontroversially bipedal. However, stance in iguanodontian ornithopods, including the hadrosaurs, and in non-ceratopsid ceratopsians is debated because robust osteological correlates of quadrupedality have not been identified. We examine a suite of characteristics that have been previously proposed as osteological correlates for bipedality or quadrupedality in dinosaurs. These in- clude both discrete anatomical features, which we assess as correlates for quadrupedality using character optimization onto a composite cladogram, and proportional ratios, which we assess as correlates by reconstructing nodal ancestral states using squared-change parsimony, followed by optimization. We also examine the correlation of these features with body size. An anterolateral process on the proximal ulna, hoof-shaped manual unguals, a transversely broadened ilium, a reduced fourth trochanter and a femur longer than the tibia are found to be robust correlates of quadrupedality in ornithischian dinosaurs. Along the ceratopsid “stem” lineage, quadrupedal characters were acquired in a stepwise fashion, with forelimb characters developing prior to changes in the hind limb. In contrast, iguanodontid ornithopods display a mosaic of character states, indicating varying degrees of facultative quadrupedality that probably arose for a variety of different reasons. Hadrosaurs are found to possess all character states associated with quadrupedality and were probably predominantly quadrupedal. In general, quadrupedal ornithischians do not appear to have been constrained by their bipedal ancestry to a particular order of character acquisition. Key words: Dinosauria, Ornithischia, osteological correlate, quadrupedality. Susannah C.R. Maidment [[email protected]], Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom; current address: Department of Earth Science and Engineering, Imperial College, South Kensington Campus, London SW7 2AZ, UK; Paul M. Barrett [[email protected]], Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. Received 20 June 2012, accepted 14 September 2012, available online 21 September 2012. Copyright © 2014 S.C.R. Maidment and P.M. Barrett. 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. 2004; Dodson et al. 2004) that evolved quadrupedality inde- Introduction pendently of each other (Figs. 1, 2). However, debate continues over the stance of iguanodon- During the entire history of tetrapod evolution, a reversion tian ornithopods, which has been controversial since their to quadrupedality from primitively bipedal ancestors has initial discovery nearly 200 years ago (e.g., review in Nor- occurred only among the ornithodiran archosaurs (Fig. 1), man 1980). Although early workers debated the quadrupedal making it one of the rarest locomotory transitions. Within the or bipedal nature of iguanodontians (e.g., Leidy 1858; Cope herbivorous dinosaur clade Ornithischia, reversions to qua- 1869; Huxley 1870; Dollo 1905), the mounting of Iguanodon drupedality have occurred on several occasions. While basal bernissartensis in the tripodal “kangaroo stance” in the Royal ornithischians and the basal-most members of all ornithis- Institute of Natural Sciences, Belgium (Dollo 1883) and the chian lineages are considered to have been unequivocally discovery of bipedal dinosaur trackways (e.g., Beckles 1862) bipedal (Galton 1971; Sereno 1991; Senter 2007), eurypodan led to an early consensus that these dinosaurs were biped- thyreophorans (stegosaurs and ankylosaurs) and ceratopsids al. Subsequently, Gilmore (1909) and Norman (1980, 1986) were clearly obligate quadrupeds (e.g., Galton and Upchurch both suggested that iguanodontids adopted both bipedal and Acta Palaeontol. Pol. 59 (1): 53–70, 2014 http://dx.doi.org/10.4202/app.2012.0065 54 ACTA PALAEONTOLOGICA POLONICA 59 (1), 2014 Fig. 1. Simplified relationships of ornithodiran archosaurs showing major clades discussed in the text. Circles on branches indicate clades where second- ary quadrupedality has evolved. quadrupedal stances (facultative quadrupedality), but Galton pods (“hypsilophodontids”) were bipedal. Finally, we use the (1970) argued that hadrosaurs were bipedal. Lockley and resulting character distributions to examine stance in basal Wright (2001) provided evidence for quadrupedality in some thyreophorans, basal ceratopsians and iguanodontians. Cretaceous ornithopods from numerous quadrupedal orni- thopod trackways, and Dilkes (2001) suggested that juvenile Institutional abbreviations.—AMNH, American Museum of hadrosaurs were bipedal but that they adopted a quadrupedal Natural History, New York, USA; CMN, Canadian Museum mode of life in adulthood. Carpenter and Wilson (2008) have of Nature, Ottawa, Canada; NHMUK, Natural History Mu- recently argued that the basal iguanodontian Uteodon aph- seum, London, UK; ROM, Royal Ontario Museum, Toronto, anoecetes was quadrupedal, while Maidment et al. (2012) Canada; UALVP, University of Alberta, Calgary, Canada; provided osteological evidence suggesting that hadrosaurs USNM, National Museum of Natural History, Smithsonian predominantly used their forelimbs for locomotion. Institution, Washington DC, USA. One reason that the determination of stance in iguano- dontians is so controversial is that there are no universally agreed osteological correlates of quadrupedality in ornithis- Previously-proposed osteological chians, even though previous workers (Colbert 1964; Galton 1970; Norman 1980; Bonnan 2003; Yates and Kitching 2003; correlates for quadrupedality Chinnery 2004; Yates et al. 2010; Maidment et al. 2012) Most earlier work has focused on the transition from biped- have proposed various characteristics that they considered to ality in basal saurischians and basal sauropodomorphs to the be indicative of either bipedality or quadrupedality, but the obligate quadrupedality of eusauropods, via a number of taxa utility of these features as robust osteological correlates of that show the gradual acquisition of “quadrupedal” features quadrupedality has never been tested. (e.g., Bonnan 2003; Yates and Kitching 2003; Yates et al. Here, we assess the validity of the osteological correlates 2010). Table 1 shows the previously proposed osteological that have been proposed as indicative of quadrupedality us- correlates for quadrupedality and the character states used to ing phylogenetic optimization of discrete osteological char- assess their robustness herein. acters and ancestral state reconstruction and optimization of continuous characters, including characters dealing with Fig. 2. Composite cladogram of ornithischians sampled in this study. → single elements and those comprising ratios between multi- Relationships based on Butler et al. (2008), Brown et al. (2011), Maid- ple elements. Following the existing consensus, we assume ment (2010), Farke et al. (2011), McDonald et al. (2010), Prieto-Mar- that ceratopsids and eurypodan thyreophorans were obligate quez (2010), Sampson et al. (2010), Sereno (2010), Xu et al. (2010), and quadrupeds, and that basal ornithischians and basal ornitho- Thompson et al. (2012). MAIDMENT AND BARRETT—QUADRUPEDALITY IN ORNITHISCHIAN DINOSAURS 55 Lesothosaurus diagnosticus HETERODONTOSAURIDAE Abrictosaurus consors cf. Heterodontosauridae Scutellosaurus lawleri THYREOPHORA Scelidosaurus harrisonii ORNITISHIA Huayangosaurus taibaii Kentrosaurus aethiopicus Loricatosaurus priscus STEGOSAURIA Dacentrurus armatus Stegosaurus mjosi Stegosaurus armatus EURYPODA Ankylosaurus magniventris Dyoplosaurus acutosquameus ANKYLOSAURIDAE Euoplocephalus tutus Polacanthus foxii ANKYLOSAURIA Hoplitosaurus marshi GENASAURIA Sauropelta edwardsi Panoplosaurus mirus NOSDOSAURIDAE Struthiosaurus languedocensis Edmontonia rugosidens Edmontonia longiceps Stormbergia dangershoeki Othneilosaurus consors Jeholosaurus shangyuanensis ORNITHOPODA Parksosaurus warreni Thescelosaurus neglectus Hypsiliphodon foxii NEORNITHISCHIA Zalmoxes robustus Zalmoxes shqiperorum IGUANODONTIA Tenontosaurus tilletti Dysalotosaurus lettowvorbecki Valdosaurus canaliculatus DRYOMORPHA Camptosaurus dispar Uteodon aphanoecetes ANKYLOPOLLEXIA Cumnoria prestwichii Hypselospinus fittoni Osmakasaurus depressus Barilium dawsoni Mantellisaurus atherfieldensis Equijubus normani HADROSAURIFORMES Bactrosaurus johnsoni Gilmoreosaurus mongoliensis HADROSAUROIDEA Telmatosaurus transsylvanicus Gryposaurus notabilis Edmontosaurus annectens Edmontosaurus regalis SAUROLOPINAE Brachylophosaurus canadensis HADROSAURIDAE Parasaurolophus walkeri Hypacrosaurus altispinus taxa considered LAMBEOSAURINAE bipedal a priori Lambeosaurus lambei Lambeosaurus magnicristatus taxa considered quadrupedal a priori Corythosaurus casuarius
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