The holotype of Stenopelix valdensis is the most completely known dinosaur specimen from the “Wealden” (Lower Cretaceous) of northwestern Germany, but its phylogenetic position has remained highly controversial. Most recent authors have suggested affinities with the ornithischian clade Marginocephalia, and most commonly to the marginocephalian subclade Pachycephalosauria. A pachycephalosaurian identity would make Stenopelix the only confirmed pre−Late Cretaceous member of this clade, breaking up an extensive ghost lineage which extends to the inferred origin of Pachycephalosauria in the Middle–Late Jurassic. Based upon re−examination of the holotype we here review the characters that have previously been used to assign Stenopelix to either Pachycephalosauria or Ceratopsia. All of these characters are problematic, being based upon inaccurate anatomical interpretations, or having more widespread distributions within Ornithischia than previously realised. We conclude that although the overall anatomy of Stenopelix is consistent with marginocephalian affinities, there is insufficient evidence to support referral to either Pachycephalosauria or Ceratopsia; we consider Stenopelix ?Marginocephalia. A brief review indicates that there is no compelling fossil evidence for pachycephalosaurs prior to the Late Cretaceous.

Key words: Ornithischia, Pachycephalosauria, Marginocephalia, Stenopelix, Cretaceous, Germany.

Richard J. Butler [R.Butler@nhm.ac.uk], Department of Palaeontology, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK; and Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK (corresponding author); Robert M. Sullivan [rsullivan@state.pa.us], Section of Paleontology and Geology, The State Museum of Pennsylvania, 300 North Street, Harrisburg, PA 17120-0024, USA