Читать книгу Hadrosaurs - David A. Eberth - Страница 108

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).