Читать книгу Hadrosaurs - David A. Eberth - Страница 23

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.