More recently Prieto-Márquez (2011) has contradicted previous work by proposing that Hadrosaurus foulkii is indeed a valid taxon, which he assigns (on its own) to the subfamily Hadrosaurinae (as in Prieto-Márquez, 2010). In addition he now defines the node-based clade Hadrosauridae as “the clade stemming from the the most recent common ancestor of Hadrosaurus foulkii and Parasaurolophus walkeri” (Prieto-Márquez, 2011:67). His clade Saurolophidae (= Euhadrosauria in this account) is given a node-based definition: “the last common ancestor of Saurolophus osborni, Lamebosaurus lambei, and all its descendants” (Prieto-Márquez, 2011:67; it is not made explicit why he has abandoned the use of Parasaurolophus in favour of Lambeosaurus as one specifier in this instance) and he concludes with the proposition that this latter clade (Saurolophidae = Euhadrosauria in this account) includes “the two major hadrosaurid clades: Saurolophinae and Lambeosaurinae” (Prieto-Márquez, 2011:67). Phylogenetic definition of the taxonomic scheme proposed by Weishampel et al. (1993) with respect to Hadrosauridae (the least inclusive, node-based clade containing Telmatosaurus transsylvanicus and Parasaurolophus walkeri) and Euhadrosauria (see above) solves a number of problems put forward by Prieto-Márquez (2011), and necessitates only adopting Saurolophinae for the clade traditionally recognized as Hadrosaurinae. However, if future phylogenetic analyses recover Hadrosaurus foulkii as a member of this clade, Hadrosaurinae would have precedence.
A DISCUSSION: THE ORIGIN OF HADROSAURS
The topology generated by this analysis provides an interpretative framework for exploring the morphological transition from derived neoiguanodontian ornithopod to euhadrosaurian and can be compared directly to those that have been produced more recently.
Topological Variation
Prieto-Márquez (2010) offered a detailed analysis of the systematics of hadrosaurians and included a consideration of a few of the more basal forms considered herein (Fig. 2.27). The stem-based clade Hadrosauroidea was redefined as comprising Hadrosaurus foulkii and all taxa more closely related to it than to Iguanodon bernissartensis, including Hadrosaurinae (represented solely by H. foulkii) and Saurolophidae (Prieto-Márquez, 2011:67). Equijubus and Probactrosaurus occupy significantly more basal positions in this topology than in the resolved tree generated by the latest analyses (compare Figs. 2.26 and 2.30). However, the relative positions of Eolambia, Protohadros, and Bactrosaurus are topologically more consistent.
McDonald (2012b) produced an alternative tree depicting ornithopod relationships (Fig. 2.28), which incorporated a greater proportion of the taxa considered herein and is therefore more truly comparable to the one generated in this account (Figs. 2.26, 2.30). The topology shows substantial similarity to that which has been proposed in this account. Basal taxa appear with the same topology as seen in Figure 2.30, and all the topological variation is to be found within a range of what might be termed “intermediate” taxa leading to Tethyshadros (in this particular instance Altirhinus, Equijubus, Xuwulong, Eolambia, Probactrosaurus, and Protohadros). Above Tethyshadros, the topology of the two trees is again identical. The node-group names follow conventional usage and amplify those outlined by Prieto-Márquez (2010, 2011). It should also be noted that Hadrosauroidea has a different composition to that suggested by Prieto-Márquez (compare Fig. 2.27).
Wu and Godefroit (2012) provided another analysis that is pertinent to this review (Fig. 2.29). Basal relationships are consistent with those shown in Figure 2.26. Iguanodon and Mantellisaurus plus Ouranosaurus are recognized as a clade (Iguanodontidae), although this node is weakly supported. More derived taxa show differences in relative position, the most notable of which is the comparatively basal positions of Equijubus and Probactrosaurus. Clade names were placed at nodes, and it is notable that Hadrosauridae incorporates Bactrosaurus as well as Tethyshadros, Telmatosaurus, and Euhadrosauria, and that the latter conforms to the concept of this clade created by Weishampel et al. (1993).
The latest contribution to this on-going debate focused on hadrosaur origins is presented in summary form in Figure 2.30. Although this topology was resolved from three MPTs, the topology is not entirely unambiguous. The clade Clypeodonta is recognized in order to differentiate those ornithopods that specialize their teeth in order to create an integrated dentition of distinctively shield-shaped teeth, from those that retain a simpler set of non-imbricating, straight, leaf-shaped teeth that do not develop interdentally continuous wear surfaces. Basal clypeodonts (hypsilophodonts, rhabdodonts, and tenontosaurs) have a distinct set of tooth morphologies that may eventually prove to unite them into a sister clade to more derived dryomorphans, although that is not recovered here. The node-based clade Neoiguanodontia recognizes a range of taxa that demonstrate the acquisition of the definitive Iguanodon-like tooth morphology (notably mammilate denticles along mesial and distal crown margins; dentary crowns with a shouldered crown edge, inrolled enameled ledges, and crown subdivision by primary and secondary ridges) as well as a subsidiary set of postcranial features. Five taxa (Bolong, Jinzhousaurus, Barilium, Iguanodon, and Mantellisaurus) form a relatively poorly supported clade (= Iguanodontidae), but this is not formally defined at this stage. Succeeding taxa demonstrate the gradual acquisition of hadrosaur-like anatomical features, and this inconsistency reflects the wide geographic spread of these taxa and the difficulties inherent in trying to resolve phylogenetic relationships among groups of taxa that were evidently evolving relatively rapidly during the early Late Cretaceous. This evolutionary “plexus” (see Fig. 2.24) is succeeded by more derived taxa such as Probactrosaurus that exhibit a number of hadrosaur-like characteristics. Here, the term Hadrosauromorpha encompasses taxa that are more derived than the pollex-spike-bearing Probactrosaurus. Hadrosauromorpha comprise Tethyshadros, Bactrosaurus, and Telmatosaurus, as well as the potential non-euhadrosaurians (non-saurolophids sensu Prieto-Márquez, 2010, 2011) such as Hadrosaurus foulkii, Claosaurus agilis, and Lophorhothon atopus.
2.27. Cladogram generated by the analysis of Prieto-Márquez (2010, 2011). Modified in order to present the topology that reflects (with a few exceptions) the taxa considered in this account. Clade names and positions indicated as in the original version.
2.28. Cladogram generated by the analysis of McDonald (2012b). Modified in order to present the topology that reflects the taxa considered in this account. Clade names and positions indicated as in the original version.