The status of Dollodon and other basal iguanodonts (Dinosauria: Ornithischia) from the Lower Cretaceous of Europe
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.
Section snippets
Materials and methods
In addition to NHMUK R5764 (holotype of M. atherfieldensis) and IRSNB 1551 (holotype of D. bampingi), I examined two other nearly complete associated skulls and postcrania, both from the Wessex Formation of the Isle of Wight: NHMUK R11521 and MIWG 6344. NHMUK R11521 was referred to as Mantellisaurus sp. and MIWG 6344 as Dollodon sp. by Paul (2008). The postcranial material of NHMUK R5764 is suspended several meters above the floor of the NHMUK dinosaur hall, and was thus difficult to view and
Original diagnostic characters
The cranial and postcranial skeletons of NHMUK R5764, IRSNB 1551, NHMUK R11521, and MIWG 6344 are virtually indistinguishable from each other. Nevertheless, Paul (2008) proposed that NHMUK R5764 and IRSNB 1551 are sufficiently different to warrant placement in the distinct taxa M. atherfieldensis and D. bampingi, respectively. To assess this hypothesis, it is necessary to evaluate the characters by which the diagnoses of the two taxa differ. In the following list, “M” stands for Mantellisaurus
Systematic paleontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Ornithopoda Marsh, 1881
Iguanodontia Dollo, 1888
Ankylopollexia Sereno, 1986
Styracosterna Sereno, 1986
Hadrosauriformes Sereno, 1997 (sensu Sereno, 1998)
Mantellisaurus atherfieldensis (Hooley, 1925) Paul, 2006
1925 Iguanodon atherfieldensis Hooley, p. 3
1986 Iguanodon atherfieldensis Norman, p. 283
2006 Mantellisaurus atherfieldensis Paul, p. 74
2008 Dollodon bampingi Paul, p. 200
Holotype. NHMUK R5764, nearly complete skull and postcranium.
Referred
Discussion and conclusions
In addition to M. atherfieldensis, D. bampingi, and I. bernissartensis, three other basal iguanodonts have been recognized from Lower Cretaceous rocks on the Isle of Wight: Sphenospondylus gracilis (Lydekker, 1888, Seeley, 1883), Vectisaurus valdensis (Hulke, 1879), and Proplanicoxa galtoni (Carpenter and Ishida, 2010). Sphenospondylus is based upon a series of six incomplete dorsal vertebrae (NHMUK R142) that do not present any features that would allow them to be associated with any other
Acknowledgements
I am very grateful to my advisor, Peter Dodson, for his tireless encouragement and for reading a rough draft of this paper. I am also grateful to Steve Hutt for discussions of MIWG specimens and Paul Barrett for measurements and photographs of the dentary of NHMUK R11521, and to both of them for the opportunity to work on specimens under their care. I thank the following people for access to and assistance with specimens under their care: J. Bartlett and J. Bird (CEUM); A. Folie and H. du
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