Osteology and Taxonomy of British Wealden Supergroup (Berriasian–aptian) Ankylosaurs (Ornithischia, Ankylosauria)

ABSTRACT

Ankylosaurs, dinosaurs possessing extensive body armor, were significant components of terrestrial ecosystems from the Middle Jurassic–latest Cretaceous. They diversified during the Early Cretaceous, becoming globally widespread. The Lower Cretaceous Wealden Supergroup (Berriasian–Aptian) of Britain has produced abundant ankylosaur material, with three currently recognized taxa: Hylaeosaurus armatus (Grinstead Clay Formation, West Sussex); Polacanthus foxii (Wessex Formation, Isle of Wight); and Horshamosaurus rudgwickensis (Weald Clay Group, West Sussex). However, these taxa are poorly understood; the initial descriptions of Hylaeosaurus and Polacanthus date from the 1800s and subsequent referrals of specimens have been based largely on provenance rather than morphological comparisons. This has led to uncertainty over the definitions of these taxa and the compositions of their hypodigms. Here, we redescribe the holotypes of Hylaeosaurus and Polacanthus, provide comparisons between these taxa, and use this information to assess the taxonomy of all ankylosaur specimens from the British Wealden Supergroup. We conclude that Hylaeosaurus and Polacanthus are valid, distinct taxa, which can be diagnosed by a combination of autapomorphies and a unique combination of characters. However, in both cases, we restrict their hypodigms to the holotypes. ‘Horshamosaurus rudgwickensis’ is a nomen dubium (an indeterminate nodosaurid dinosaur) and the majority of ankylosaur specimens from the Wealden Supergroup are taxonomically indeterminate. Hylaeosaurus and Polacanthus are separated stratigraphically, with Hylaeosaurus from the Valanginian of the Weald Sub-basin and Polacanthus from the Barremian of the Wessex Sub-basin. This separation supports the hypothesis of distinct lower and upper dinosaur faunas in the Wealden Supergroup of Britain.

ACKNOWLEDGMENTS

T.J.R. was funded by a University of Brighton Science Scholarship, the SYNTHESYS Project Research Infrastructure Action under the FP7 “Capacities” Program, the Palaeontological Association Whittington Award, the Natural History Museum Earth Sciences Departmental Investment Fund, the M. A. Fritz Travel Grant for the Royal Ontario Museum, the Society of Systematic Biologists Graduate Research Award, the Jurassic Foundation Grant, the Universities’ China Committee in London Grant, the Geological Society Daniel Pidgeon Fund, the Geologists’ Association New Researcher’s Award, and the Palaeontographical Society Richard Owen Award. We thank C. Mehling (AMNH), J. Porter (Bexhill Museum), D. Hutchinson (Bristol Museum), J. Cooper and L. Ismail (Booth Museum), M. Riley (Sedgwick Museum), K. Corneli (CEUM), M. Currie, K. Shepherd, and J. Mallon (CMN), M. Munt and A. Peaker (Dinosaur Isle), K. MacKenzie and J. Sertich (DMNH), B. Simpson, P. Makovicky, P. Viglietti, and E. Gorscak (Field Museum), J. Knight (Horsham Museum), Z. Szentesi, M. Gasparik, D. Bernadett, and A. Ösi (Hungarian Natural History Museum), J. Jobylinska (Institute of Paleobiology Warsaw), X. Xing and Z. Fang (IVPP), K. Purevorj and K. Tsogtbaatar (Mongolian Paleontological Center), J. Gillette and D. Gillette (Museum of Northern Arizona), R. Irmis and C. Levitt-Bussian (Natural History Museum of Utah), T. Tumanova, V. Alifanov, and A. Sennikov (Palaeontological Institute Moscow), K. Seymour and D. Evans (Royal Ontario Museum), A. Millhouse, M. Carrano, and A. Jukar (Smithsonian Institution), P. Sereno (University of Chicago), J. Kriwet and S. Stumpf (University of Vienna), and D. Brinkman (Yale Peabody Museum) for access to specimens and help during museum trips. D. Lomax, J. Kirkland, and J. Russo are thanked for specimen photos, as is V. Arbour for discussion about all things ankylosaur. K. Webb (NHMUK Image Resources) took photographs of the specimens used herein. M. Graham (NHMUK Conservation Centre) provided information on the preparation history of Hylaeosaurus and, together with D. Gray and S. Moore-Fay, carried out some of the preparation. V. Fernandez (NHMUK Imaging and Analyses Centre) provided support with the X-ray CT. This work has benefitted from the authors’ involvement in the London Palaeobiology Research Group and the NHM Fossil Reptile Group, and we thank D. Button in particular for discussion. We thank the reviewers V. Arbour and R. Hill and the editor L. Zanno for their constructive critiques, which greatly improved the manuscript.