Читать книгу Hadrosaurs - David A. Eberth - Страница 24

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. Derived from Batten (2011). The vertical bars on the right-hand side indicate the approximate stratigraphic distribution of the four principal Wealden neoiguanodontian taxa.

This contribution concerns itself with updating our current understanding of the anatomy, taxonomy and systematics of an Early Cretaceous group of neoiguanodontians from the Wealden of northwest Europe (Fig. 2.1). Given their older chronostratigraphic occurence relative to hadrosaurids, these taxa contribute to an analysis of taxa that are considered topologically basal to hadrosaurids. This review probes our understanding of an important phase in ornithopod evolution, and highlights areas where more research is needed.

Institutional abbreviations MIWG, Museum of the Isle of Wight Geology, Sandown, Isle of Wight, U.K.; NHMUK, Natural History Museum, London, U.K.; RBINS [formerly IRSNB], Royal Belgian Institute of Natural Sciences, Brussels, Belgium.