Hadrosaurs. David A. Eberth
on the position of the tooth within the tooth row: more mesially and distally positioned teeth in hadrosaurids often have lower crown height-to-width ratios than those in the middle of the tooth row (Gallimore and Evans, 2009). As the exact position of the holotype tooth within the tooth row cannot be established, we adopt a conservative approach and refrain from proposing a new generic name for this very limited material and regard the taxon as a nomen dubium (see also Horner et al., 2004). Nevertheless, it is likely that this single tooth, and possibly other isolated indeterminate material from the Cambridge Greensand Member (such as the isolated maxillary fragment described above), represents an otherwise unknown basal hadrosauroid taxon, given its geographic and stratigraphic provenance and its anatomical distinctiveness from other European hadrosauroids and more basally positioned iguanodontians.
6.3. Morphometric analysis of hadrosauroid tooth shape. (A) Principal component space defined by components 1 and 2; (B) principal component space defined by components 1 and 3; (C) UPGMA cluster analysis of hadrosauroid shape distances for the first four eigenvalue scores. Gray areas indicate shape spaces defined by Hadrosauridae.
‘Iguanodon’ hillii can be distinguished from “Trachodon cantabrigiensis” on the basis of its denticle morphology, suggesting that it, too, represents an otherwise unknown basal hadrosauroid taxon from the “middle” Cretaceous of England. However, the tooth possesses no autapomorphic features and as far as can be determined is similar in morphology to teeth of Telmatosaurus. We also regard this taxon as a nomen dubium (see also Horner et al., 2004).
CONCLUSIONS
Morphometric analysis and consideration of dental character distributions indicate that neither “Trachodon cantabrigiensis” nor ‘Iguanodon’ hillii can be referred to Hadrosauridae, contrary to previous accounts (e.g., Horner et al., 2004). Nevertheless, these specimens represent the most derived non-hadrosaurid hadrosauroids to have been recovered from western Europe and are taxa that, on the basis of the limited available evidence, are potentially very close to the origin of Hadrosauridae. All other basal hadrosauroids are known from the “middle” to early Late Cretaceous of eastern Asia (e.g., Bactrosaurus, Equijubus, Levnesovia, Probactrosaurus) and North America (Eolambia, Protohadros), along with the relictual taxon Telmatosaurus from the latest Cretaceous of Romania (Norman, 2004; Sues and Averianov, 2009). The “middle” Cretaceous English taxa therefore extend significantly the already broad Laurasian geographic distribution of basal hadrosauroids.
Frustratingly, the apparent phylogenetic proximity of “T. cantabrigiensis” and ‘I.’ hillii to Bactrosaurus, Protohadros, Telmatosaurus, and hadrosaurids, along with the western European provenance of the former taxa, clouds further the paleobiogeography of hadrosaurid origins, by offering another geographic region that yields plausible hadrosaurid sister taxa (see Horner et al. [2004] and Prieto-Márquez [2010b] for recent discussions on this issue). However, the early (late Albian) occurrence of “T. cantabrigiensis” is noteworthy. Although it is not the earliest-known hadrosauroid (contra Sues and Averianov [2009]; various Asian taxa are potentially of Aptian age [e.g., Norman, 1998; You et al., 2003]), the dental morphology of “T. cantabrigiensis” exhibits a set of derived character states relative to those present in other late Early Cretaceous hadrosauroids (e.g., Altirhinus, Equijubus, Probactrosaurus) that place it much closer to hadrosaurids than any of these near-contemporaries. In addition, it occurs earlier than all of the other most-proximate hadrosaurid outgroups, which are Cenomanian in age or younger (Head, 1998; Sues and Averianov, 2009; Prieto-Márquez, 2010b). If our inferred phylogenetic position for “T. cantabrigiensis” is correct, this could imply that hadrosaurid origin occurred close to the Early–Late Cretaceous boundary, rather than in the Santonian as previously suggested (see Sues and Averianov, 2009; Prieto-Márquez, 2010a, b).
ACKNOWLEDGMENTS
This chapter is dedicated to David Weishampel, the doyen of hadrosaurid workers. He was one of PMB’s Ph.D. examiners and was influential in JJH’s graduate training. We thank M. Riley (CAMSM) and P. Shepherd (GSM) for arranging the loan of material and P. Hurst (NHMUK, Photographic Unit) for his excellent photographs of the holotype teeth. A. McDonald and J. Kirkland provided photographs of the teeth of Equijubus and Eolambia, respectively; D. Norman is thanked for numerous interesting discussions on the Cambridge Greensand dinosaur fauna; and D. Eberth and Kh. Tsogtbaatar provided helpful reviews of the manuscript.
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