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A Re-evaluation of Purported Hadrosaurid Dinosaur Specimens from the “Middle” Cretaceous of England
Paul M. Barrett, David C. Evans, and Jason J. Head
ABSTRACT
“Trachodon cantabrigiensis” and ‘Iguanodon’ hillii are poorly known ornithopod taxa from the “middle” Cretaceous of England that were both named on the basis of a single isolated tooth. The holotype of “T.” cantabrigiensis was collected from the Cambridge Greensand Member of the Melbury Marly Chalk Formation (Cenomanian) and was reworked from late Albian deposits, while that of ‘I.’ hillii is from the slightly younger Totternhoe Stone Member of the Zig Zag Chalk Formation (middle Cenomanian). Both taxa have been suggested to represent hadrosaurids, and if correctly identified would form the earliest-known records of this predominantly latest Cretaceous dinosaur clade. Here we provide redescriptions of these teeth, and evaluate their morphology with reference to the phylogenetic distributions of hadrosauroid dental characters. In addition, we incorporate “T. cantabrigiensis” into a morphometric analysis of iguanodontian tooth crown shape. Results indicate that although both taxa share many features in common with hadrosaurids, neither can be referred to this clade. Nevertheless, both taxa appear to nest in a grade of hadrosauroid taxa, including Bactrosaurus and Telmatosaurus, which are among the closest relatives of hadrosaurids. Consequently, these isolated European records need to be considered when discussing the paleobiogeography of hadrosaurid origins.
INTRODUCTION
The Lower Cretaceous portion of the Purbeck Limestone Formation (Tithonian–Berriasian) and the Wealden Group (upper Berriasian–lowermost Aptian) of central and southern England have yielded historically important dinosaur faunas that contain diverse ornithopod dinosaurs (see recent reviews in Naish and Martill [2008], Galton [2009], and Norman [2011a]). These faunas include basal ornithopods (Hypsilophodon [Galton, 1974, 2009]) and numerous iguanodontians, including dryosaurids (Valdosaurus and other indeterminate material [Galton, 2009; Barrett et al., 2011]) and many ankylopollexians, such as Barilium, Hypselospinus, Iguanodon, Kukufeldia, Mantellisaurus, and Owenodon (Paul, 2007, 2008; Galton, 2009; Carpenter and Ishida, 2010; McDonald, Barrett, and Chapman, 2010; Norman, 2010, 2011b; McDonald, 2012). By contrast only a small amount of ornithopod material has been recovered from post-Wealden deposits, namely the Lower Greensand Group (Aptian–lower Albian), which has yielded a partial skeleton of Mantellisaurus (the historically important “Mantel-piece”: Norman, 1993) and other fragmentary material (e.g., Keeping, 1883), the Cambridge Greensand Member of the West Melbury Marly Chalk Formation (Seeley, 1879; Lydekker, 1888a; see below), and the Totternhoe Stone Member of the Zig Zag Chalk Formation (Newton, 1892; see below). Although material from these units is rare it is potentially important paleobiogeographically and phylogenetically, as specimens from contemporary units in North America and eastern Asia indicate that this interval witnessed the beginning of the hadrosauroid radiation (e.g., Head, 1998; Kirkland, 1998; Norman, 1998, 2002; You et al., 2003; Wang et al., 2011). Few other European localities yield material of this age (Weishampel et al., 2004), so specimens from these formations have the potential to provide some insight into the distribution and species richness of “middle” Cretaceous hadrosauroids.
Dating the Cambridge Greensand Member has been contentious, but there is now a general consensus that it is of early Cenomanian age, although many of its macrofossils were probably reworked from the upper part of the underlying late Albian Gault Formation (see Pereda-Suberbiola and Barrett [1999] and Unwin [2001] for reviews of the sedimentology, vertebrate paleontology, and dating of this unit). The Cambridge Greensand vertebrate fauna includes numerous chelonians, fish, ichthyosaurs, dinosaurs, plesiosaurs, and pterosaurs, and rare birds, crocodilians, and squamates. Ankylosaur specimens comprise the majority of the dinosaur remains recovered, but sauropod and ornithopod specimens also occur (see brief summary in Pereda-Suberbiola and Barrett, 1999). Several putative ornithopod taxa have been named on the basis of these finds, including “Anoplosaurus major” (partim), “Eucercosaurus tanyspondylus,” “Syngonosaurus macrocercus,” and “Trachodon cantabrigiensis” (Seeley, 1869, 1879; Lydekker, 1888a), all of which are currently considered to be nomina dubia and either iguanodontian ornithopods, or chimeras of iguanodontian and ankylosaur material (Pereda-Suberbiola and Barrett, 1999; Horner et al., 2004; Norman, 2004; Vickaryous et al., 2004). Of these, “Trachodon cantabrigiensis” Lydekker, 1888a, which is represented by the isolated holotype tooth (NHMUK R496; Fig. 6.1) and a set of referred material (Lydekker, 1888b; Nopcsa, 1923), is of special interest as, historically, it has been regarded as an early member of the Hadrosauridae (or a “trachodontid” in earlier works).
6.1. Holotype left dentary tooth of “Trachodon cantabrigiensis” (NHMUK R496). (A) lingual view; (B) mesial view; (C) labial view; (D) distal view; (E) close-up on medial margin showing denticles with a single row of mammillae. Scale bars equal 10 mm (A–D) and 3 mm (E).
Owen (1861:30, pl. 7, figs. 15–16) was the first person to describe NHMUK R496 – as a “young Iguanodon” – but this identification was questioned by Leidy (1865:86–87) who noted its resemblance to teeth of Hadrosaurus and “Trachodon.” The “trachodontid” affinity of the tooth was upheld by Lydekker (1888a), who erected the name “Trachodon cantabrigiensis” for its reception, and referred additional unassociated specimens (all pedal elements) from the Cambridge Greensand to the species (Lydekker, 1888b). Nopcsa (1923) also referred more specimens to the taxon (an isolated maxilla and pedal phalanges) and commented upon the similarities between all of these elements and those of hadrosaurids. The hadrosaurid identity of the holotype tooth was subsequently accepted by many authors, usually in the context of systematic lists of taxa or paleobiogeographic discussions (e.g., Steel, 1969; Kirkland, 1998; Horner et al., 2004; Dalla Vecchia, 2006; Benton et al., 2010). Head (1998) was the first to cast doubt on this identification, although the specimen was not formally removed from Hadrosauridae. Rather, a combination of reasons, including the absence of other definitive hadrosaurid material from the English dinosaur record (and the European pre-Campanian record more generally), the geographic isolation of England from the Aptian onward, and the possibility of homoplasy in the distribution of various iguanodontian dental characters were cited as confounding factors in accepting the hadrosaurid identification of the tooth (Head, 1998:733). Sues and Averianov (2009) also questioned the possible hadrosaurid affinities of “T. cantabrigiensis,” regarding it as a more basal hadrosauroid, although they provided no evidence in support of this conclusion.
A second putative hadrosaurid, ‘Iguanodon’ hillii Newton, 1892, was recovered from the Totternhoe Stone Member of the Zig Zag Chalk Formation (middle Cenomanian). It is known from a single partial tooth crown (GSM 1966; Fig. 6.2) and no other ornithopod material has been recovered from any other Late Cretaceous British fossil reptile locality (Benton and Spencer, 1995; Weishampel et al., 2004). ‘Iguanodon’ hillii has received much less attention in the literature than “Trachodon cantabrigiensis,” but it is has also been regarded as an indeterminate hadrosaurid (Horner et al., 2004; Dalla Vecchia, 2006) or an indeterminate iguanodontian (Head, 1998), in both cases without discussion. Naish and Martill (2008:620) questioned Head’s (1998) conclusion stating that “the tooth is highly derived,” but offered no alternative identification or other remarks on this specimen.
Here, we