Hadrosaurs. David A. Eberth

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      Iguanodonts from the Wealden of England: Do They Contribute to the Discussion Concerning Hadrosaur Origins?

       David B. Norman

       ABSTRACT

      The earliest known hadrosaur-like ornithopod is represented by a tooth from the early Cenomanian (Cambridge Greensand) of England. The Wealden outcrops (late Berriasian–early Aptian) of England include a range and variety of iguanodonts that, in many anatomical respects, presage the structures seen in a succession of Albian–Maastrichtian, hadrosaur-like neoiguanodontians, hadrosauromorphans, and euhadrosaurians. The anatomy and taxonomic assignments applicable to known Wealden iguanodonts are reviewed, albeit briefly, and the recently published proposition that Wealden taxonomic diversity was far higher than previously supposed is regarded as unfounded. A new systematic analysis has generated a consistent topological framework that provides the basis for a consideration of the general pattern of assembly of anatomical features, within the neoiguanodontian lineage, that culminated in the appearance of true hadrosaurs (euhadrosaurians) during the Late Cretaceous. The general topology generated by the present analysis largely conforms to previous analyses. However, the primary region of inconsistency is located across a range of taxa that appear to form a plexus of late Early and early Late Cretaceous age; they are widely distributed geographically, vary in their degrees of preservation, and have been described mostly in the last two decades. A revised classification is proposed, based upon the new topology, and generalized phylogenetic inferences have also been drawn from the successional pattern and its associated character distributions. The systematic pattern and therefore the phylogenetic (evolutionary) origin of euhadrosaurians from within the plexus of derived neoiguanodontians is potentially tractable. However, questions focused upon the geographic (area of) origin of hadrosaurs are unlikely to be resolved satisfactorily because of definitional instability (an inherent problem of fossil-based systematic analyses), compounded by the more or less constant flow of new discoveries.

       INTRODUCTION

      The zenith of ornithopod evolution is represented by the Late Cretaceous duck-billed, or hadrosaurian dinosaurs (e.g., Lull and Wright, 1942; Ostrom, 1961; Horner et al., 2004; Prieto-Márquez, 2010), which were highly speciose, geographically widespread, and anatomically (and probably behaviorally) complex herbivores. However, the details governing the evolutionary transition from derived (neoiguanodontian) to the definitive hadrosaurian state, although understood in general terms, have proved elusive. Initially (encompassing the time between the 1870s and 1970s) the fossil record was comparatively mute on the subject: “middle” Cretaceous (Albian–Cenomanian) ornithopods were extremely rare and poorly described, as well as unreliably dated. As a consequence, evolutionary hypotheses were necessarily speculative (e.g., Gilmore, 1933; Ostrom, 1961; Rozhdestvensky, 1966; Taquet, 1975). The closing decades of the twentieth century and the opening decade of the twenty-first century mark a turning point during which a considerable number of new ornithopod taxa have been identified from both the older and established fossil hunting grounds as well as many new geographic locations. However, it seems that the abundant new data has increased ambiguity, rather than creating the expected resolution or increasing levels of consensus concerning the ancestry of the clade referred to herein as Euhadrosauria (Weishampel, Norman, and Grigorescu, 1993 [ = Hadrosauridae sensu lato, e.g., Lull and Wright, 1942; Horner et al., 2004; Prieto-Márquez, 2010]).

      Hadrosaur origins can be explored through a number of independent, yet correlated, lines of investigation: the chrono-geographical evidence suggestive of their first appearance in the fossil record; the study of ornithopod taxa that are positioned adjacent to the clade Euhadrosauria; the construction of parsimony-based and Bayesian likelihood trees (Evans, 2010; Prieto-Márquez, 2010); and the evaluation of the anatomical transformations (and phylogenetics) implied by the topology of such trees. In combination, these approaches should be able to reveal when, where, and how hadrosaurian anatomy was assembled (and, by implication, utilized by these animals in a biological sense) in the lineage(s) ancestral to the first diagnosable members of the clade Euhadrosauria.

      2.1. Stratigraphy of the Wealden of southern England. Abbreviations: Fm, Formation; GC Fm, Grinstead Clay Formation; L.T.W. Sand Fm, Lower Tunbridge Wells Sand Formation; U.T.W. Sand Fm, Upper Tunbridge Wells Sand Formation; Lower Grnsd, Lower Greensand.