The status of Dollodon and other basal iguanodonts (Dinosauria: Ornithischia) from the Lower Cretaceous of Europe

Abstract

Dollodon bampingi was recently named based upon a specimen from the Bernissart Quarry that had previously been referred to Mantellisaurus atherfieldensis. The initial diagnosis of Dollodon did not adequately distinguish it from Mantellisaurus or from other basal iguanodonts, necessitating a reassessment of the material. Firsthand examination of the holotypes of the two taxa and numerous other basal iguanodont specimens, as well as a principal components analysis of basal iguanodont dentaries, did not find any morphological features to justify the distinction of Dollodon from Mantellisaurus. D. bampingi is thus best considered a junior synonym of M. atherfieldensis. Furthermore, the recent referral of the species Iguanodon seelyi to the genus Dollodon is not supported; I. seelyi is indistinguishable from Iguanodon bernissartensis, and is considered a junior synonym of that species. Finally, the recently named taxon Proplanicoxa galtoni, also based upon a specimen formerly attributed to M. atherfieldensis, is considered a nomen dubium and probable junior synonym of M. atherfieldensis. Thus, only two species of large-bodied basal iguanodont should be recognized from the Barremian-Aptian of England and Belgium: M. atherfieldensis and I. bernissartensis.

Introduction

Europe boasts one of the richest records of Early Cretaceous basal iguanodonts in the world, and yet 186 years after Mantell’s (1825) initial description of Iguanodon, the taxonomy of Europe’s basal iguanodonts is still in need of resolution. Following the designation of Iguanodon bernissartensis as the type species of the genus (Charig and Chapman, 1998), several authors have recently reassigned various species and specimens long called Iguanodon to new genera (Carpenter and Ishida, 2010, Galton, 2009, McDonald et al., 2010, Norman, 2010, Paul, 2006, Paul, 2008).

One such species is Iguanodon atherfieldensis, named by Hooley (1925) based upon a nearly complete skull and postcranium (NHMUK R5764) from the Vectis Formation of the Isle of Wight (Naish and Martill, 2008, Norman, 1986). Norman (1986) referred a singular gracile skeleton (IRSNB 1551) from the Bernissart Quarry in Belgium to I. atherfieldensis. Paul (2006) judged I. atherfieldensis to be sufficiently disparate from I. bernissartensis to warrant its removal to the new genus Mantellisaurus. Finally, Paul (2008) removed IRSNB 1551 from Mantellisaurus atherfieldensis and made it the type specimen of the novel genus and species Dollodon bampingi.

Unfortunately, Paul’s (2008) diagnoses of M. atherfieldensis and D. bampingi are inadequate to distinguish the two taxa from each other and from other basal iguanodonts, consisting mostly of subjective qualitative statements of quantitative characteristics (e.g., “posterior portion of jugal short”, Paul, 2008: 200). In an effort to evaluate the validity of D. bampingi and enhance the diagnosis of M. atherfieldensis, I examined firsthand NHMUK R5764, IRSNB 1551, and other basal iguanodont specimens in the NHMUK, IRSNB, and MIWG collections.

Carpenter and Ishida (2010) contended that the ilia of IRSNB 1551, the holotype of D. bampingi, are indistinguishable from NHMUK R2502, the ilium of the syntype of Iguanodon seelyi (Hulke, 1882); they therefore made I. seelyi the type species of Dollodon, rendering D. bampingi a junior synonym. However, this taxonomic decision is unjustified because NHMUK R2502 bears a much closer resemblance to the ilia of IRSNB 1534, the lectotype of I. bernissartensis; the two specimens are alike in their gently convex dorsal margins and tapering postacetabular processes that lack a break in slope along their dorsal margins (Fig. 1). These morphologies differ from the ilia of IRSNB 1551, as well as those of NHMUK R5764 (holotype of Mantellisaurus atherfieldensis), MIWG 6344, and NHMUK R11521, all of which have a straight dorsal margin and a sharp break in slope along the dorsal margin of the postacetabular process (Fig. 2). The other elements preserved in the syntype partial skeleton of I. seelyi (NHMUK R2502-R2509) are also virtually indistinguishable from the corresponding elements of I. bernissartensis specimens from the Bernissart Quarry. Therefore, I. seelyi is best considered a junior synonym of I. bernissartensis, as originally proposed by Norman (1986). This means that the proper type species of Dollodon is D. bampingi, and it is treated as such in this paper.

Institutional acronyms: CEUM, College of Eastern Utah Prehistoric Museum, Price, UT, USA; IRSNB, Institut royal des Sciences naturelles de Belgique, Brussels, Belgium; MIWG, Museum of Isle of Wight Geology (Dinosaur Isle Museum), Sandown, UK; NHMUK, Natural History Museum (formerly BMNH, British Museum of Natural History), London, UK; USNM, National Museum of Natural History, Washington, DC, USA; YPM, Yale Peabody Museum of Natural History, New Haven, CT, USA.