Phylogeny of the ankylosaurian dinosaurs (Ornithischia: Thyreophora)

Abstract

Ankylosauria is a diverse clade of quadrupedal ornithischian dinosaurs whose remains are known from Middle Jurassic to latest Cretaceous sediments worldwide. Despite a long history of research, ankylosaur interrelationships remain poorly resolved and existing cladistic analyses suffer from limited character and taxon sampling. Here, we present the most comprehensive phylogenetic analysis of the group attempted to date. The traditional ankylosaurid–nodosaurid dichotomy is maintained. Ankylosauridae forms a well-resolved clade, which includes Zhongyuansaurus, the first ankylosaurid known to lack a tail club. Nodosauridae includes a number of taxa that were resolved either as ‘polacanthids’ or basal ankylosaurids in previous analyses. The use of a broader character sample allows analysis of the interrelationships of all valid ankylosaur species; this has revealed several previously unrecognized relationships. Stegosauria is recovered as the sister taxon to Ankylosauria, while Scelidosaurus is found to be a basal thyreophoran. Dedicated methods for coding continuous characters could be used in future to improve the resolution of ankylosaur phylogeny, particularly in order to explore the relationships within the poorly resolved nodosaurid clade.

Keywords:

• Ankylosauridae
• Nodosauridae
• polacanthid
• dinosaur
• cladistic analysis
• phylogeny
• evolution

Acknowledgements

This work was carried out as part of a Master of Research degree at Imperial College, London and the Natural History Museum by RST, supervised by PMB and SCRM. The dataset was based on PhD research carried out by JP at the University of Oxford and supervised by PMB. During the course of this work, RST was funded by the Natural Environment Research Council; JP was funded by a NERC studentship and SCRM was funded NERC grant number NE/G001898/1 to PMB. The Willi Hennig Society made TNT freely available to download at http://www.cladistics.org/tnt.html. We thank Aodhán Butler and Greg Edgecombe (Natural History Museum) for help and advice using TNT, Matt Vickaryous (University of Guelph) for clarifying some character statements from his analyses and Attila Ösi (Hungarian Natural History Museum) for providing some character scores for Hungarosaurus tormai. The following people allowed the authors access to specimens in their care: Sandra Chapman, Mike Dorling, Rod Long, Kevin Seymour, Hans-Dieter Sues, Margaret Feuerstack, Kieran Shepherd, Ken Carpenter, Logan Ivy, Don Foster, Don Burge, John Bird, Duane Taylor, Lyndon Murray, Michael Brett-Surman, Chris Collins, Halszka Osmólska, Teresa Maryańska, Tomasz Sulej, Xu Xing, Li Hong, Tatyana Tumanova, Alex Cook, and Steve Salisbury. Xabier Pereda-Suberbiola and one anonymous referee are thanked for their constructive views of this article.