Iguanodon

Mcdonald, Andrew T., Barrett, Paul M. & Chapman, Sandra D., 2010, A new basal iguanodont (Dinosauria: Ornithischia) from the Wealden (Lower Cretaceous) of England, Zootaxa 2569, pp. 1-43 : 2-4

Note on Iguanodon Taxonomy

The taxonomy of Iguanodon has been problematic ever since Mantell (1825) erected the genus without a type species. Holl (1829) provided the species name I. anglicum (emended to anglicus: Norman 1986 ) shortly thereafter based upon the teeth described by Mantell, including the lectotype NHMUK R2392 ( Norman 1986). However, these teeth possess no features that would definitively demarcate them from the teeth of other, better known iguanodontian species ( Norman 1986; Charig & Chapman 1998). Thus, I. anglicus was declared a nomen dubium and I. bernissartensis , by virtue of its complete lectotype skeleton from Belgium, was made the type species of Iguanodon ( Charig & Chapman 1998) . Furthermore, some species and specimens formerly regarded as pertaining to Iguanodon have been recently removed ( Norman & Barrett 2002; Paul 2007, 2008; Galton 2009; Norman 2010); the name Mantellisaurus Paul, 2007 is used herein for “ I.” atherfieldensis, Owenodon Galton, 2009 for “ I.” hoggii, and Barilium Norman, 2010 and Hypselospinus Norman, 2010 respectively for “ I.” dawsoni and “ I.” fittoni.

Norman (2010) discussed NHMUK 28660 and illustrated it in medial view ( Norman 2010: fig. 10C), suggesting that it might be referable to Barilium, which occurs in the middle Valanginian Wadhurst Clay Formation with Hypselospinus. However, there are several reasons for not referring NHMUK 28660 to either of the Wadhurst Clay taxa. The unit in which NHMUK 28660 was found, the Grinstead Clay Member of the Tunbridge Wells Sand Formation, is stratigraphically above and slightly younger than the Wadhurst Clay Formation ( Allen & Wimbledon 1991; Radley 2004; Rawson 2006). More importantly, NHMUK 28660 differs from the two basal iguanodont dentaries from the Wadhurst Clay: NHMUK R1834 and NHMUK R1831. NHMUK R1834 is a partial associated skeleton that includes a partial left dentary, numerous vertebral centra, a fragmented right scapula, a well preserved right ilium, and several pedal elements; the specimen is herein considered referable to Barilium due to the similar morphologies (e.g., smoothly convex dorsal margin) shared by its ilium and NHMUK R802, the holotype ilium of Barilium (contra Norman 2010, who referred NHMUK R1834 to Hypselospinus). The ventrally inflected rostral ramus of the dentary of NHMUK R1834 differs from the straight rostral ramus of NHMUK 28660 ( Figs 1 A, 2A), suggesting that they do not represent the same taxon. Dentaries of basal iguanodonts known from multiple specimens from the same locality and horizon (e.g., Camptosaurus dispar [USNM 4281, 5818, 5961; YPM 1886, 1888] from Quarry 13 in the Morrison Formation, Iguanodon bernissartensis [IRSNB 1536, 1561, 1639] from Bernissart, and Eolambia caroljonesa [CEUM 34357, 34447, 35482, 35714, 52139, 52995] from Eolambia #2 Quarry in the Mussentuchit Member, Cedar Mountain Formation) indicate that there is no intraspecific variation in the morphology of the rostral ramus, i.e. straight versus ventrally inflected, and that this character is probably taxonomically informative.

NHMUK R1831, a right dentary, is part of an associated skeleton with elements catalogued under numbers NHMUK R 1832–33 and NHMUK R 1835–36. The taxonomic identification of this skeleton is undecided, with Paul (2008) regarding it as a potential new taxon and Norman (2010) referring it to Hypselospinus; however, the specimen includes only a shard of an ilium and a very poorly preserved femur, and thus possesses no overlapping features that would support referral to Hypselospinus (based on the holotype NHMUK R1635 and referred material comprising the holotype of the subjective junior synonym “ Iguanodon hollingtoniensis ”; Norman 2010). NHMUK R1831 is similar to NHMUK 28660 in its straight rostral ramus ( Fig. 1 B), but differs in having a longer diastema (though not as long as suggested by Paul 2008; ATM pers. obs.; Norman 2010) and teeth with more prominent accessory ridges arising from the marginal denticles directly mesial to the primary ridge. The coronoid process of NHMUK R1831 is broken both at its base and apex, thus rendering its inclination and expansion, or lack thereof, difficult to evaluate. Given that there are two named taxa (Barilium and Hypselospinus) and two different dentary morphologies (NHMUK R1834, referred herein to Barilium, and NHMUK R1831) from the Wadhurst Clay, it would be convenient to refer NHMUK R1831 and associated postcranial specimens to Hypselospinus; however, this is not supportable in the absence of shared morphologies and overlapping material. This situation is akin to that of the derived titanosaurs Nemegtosaurus (known from a skull) and Opisthocoelicaudia (known from a partial postcranium) from the Nemegt Formation of Mongolia ( Wilson 2005), i.e. the possibilities that the specimens represent the same taxon and that they represent different diagnostic taxa must be considered equally likely given the known material.

Both NHMUK R1834 and NHMUK R1831 lack the autapomorphic row of foramina observed in NHMUK 28660 (see below). These differences indicate that NHMUK 28660 should not be referred to the same taxon as either of the Wadhurst Clay dentaries. Because the distinctive dentary NHMUK 28660 was found in the same unit (Grinstead Clay Member of the Tunbridge Wells Sand Formation) and vicinity as the non-diagnostic teeth of Iguanodon anglicus , it would be tempting to refer NHMUK 28660 to I. anglicus and thus render that taxon diagnostic. However, there are two reasons for not doing so. Firstly, the isolated dentary teeth of I. anglicus do not share any synapomorphies with the in situ tooth of NHMUK 28660 to the exclusion of other iguanodontian specimens, such as NHMUK R1831, Owenodon hoggii (NHMUK R2998), Iguanodon bernissartensis (MIWG 1997.55), and Mantellisaurus atherfieldensis (NHMUK R5764). Secondly, although NHMUK 28660 and the teeth of I. anglicus both came from quarries near Cuckfield, it is evident that more than one lithostratigraphic horizon was exposed in those quarries ( Gallois & Worssam 1993). Among the numerous isolated and disarticulated iguanodontian bones found near Cuckfield, some are embedded in a tan, well sorted, fine-grained sandstone matrix, whilst others are in dark brown, poorly sorted, coarser sandstone. The disarticulated nature of the bones and presence of more than one bone-bearing layer make it impossible to refer disparate elements to the same taxon with any certainty. Considering these factors, it is best to continue to regard “ Iguanodon anglicus ” as a nomen dubium and to create the novel genus and species Kukufeldia tilgatensis for NHMUK 28660.

Institutional abbreviations

AMNH American Museum of Natural History, New York, NY, USA ANSP Academy of Natural Sciences, Philadelphia, PA, USA

CEUM College of Eastern Utah Prehistoric Museum, Price, UT, USA CM Carnegie Museum of Natural History, Pittsburgh, PA, USA DMNH Denver Museum of Nature and Science, Denver, CO, USA HERM Hull and East Riding Museum, Hull, UK

IRSNB Institut royal des Sciences naturelles de Belgique, Brussels, Belgium IVPP Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China MIWG Museum of Isle of Wight Geology (Dinosaur Isle Museum), Sandown, UK MNHN Muséum national d’Histoire naturelle, Paris, France

MSM Mesa Southwest Museum (now Arizona Museum of Natural History), Mesa, AZ, USA NHMUK Natural History Museum (formerly BMNH, British Museum of Natural History), London, UK OXFUM Oxford University Museum of Natural History, Oxford, UK

PIN Palaeontological Institute, Moscow, Russia

QM Queensland Museum, South Brisbane, Australia

SDSM South Dakota School of Mines and Technology, Rapid City, SD, USA SMU Southern Methodist University Shuler Museum of Paleontology, Dallas, TX, USA UMNH Utah Museum of Natural History, Salt Lake City, UT, USA

USNM National Museum of Natural History, Washington, DC, USA

YPM Yale University Peabody Museum of Natural History, New Haven, CT, USA

Figures

FIGURE 1. Dentaries of basal iguanodonts from the Wadhurst Clay Formation. Left dentary of NHMUK R 1834, referred herein to Barilium dawsoni, in lateral (A) view; right dentary NHMUK R 1831 in lateral (B) view. Scale bars equal 10 cm.