On Asian ornithopods (Dinosauria: Ornithischia). 3. A new species of iguanodontid dinosaur



The holotype of the Mongolian species of ornithopod dinosaur Iguanodon orientalis from the Early Cretaceous (Barremian-Aptian of Khamarin Khural) has been shown to have been established on holotype material that is non-diagnostic, but appears to be closely similar osteologically to the Western European species I. bemissartensis. Additional material collected from the locality known as Khuren Dukh (Dornogov́, Mongolia), which had previously been referred to I. orientalis has been re-examined and shown, to represent a new genus and species (Altirhinus kurzanovi gen. et sp. nov) of ornithopod dinosaur from the Early Cretaceous (Late Aptian/Early Albian). The anatomy of this new ornithopod is described; it shows a number of similarities to the known species of Iguanodon, but also demonstrates features which are in some instances unique to this taxon, while others seem to be either convergent upon, or transformational with respect to, the more derived hadrosaurid ornithopods of the Late Cretaceous. Palaeogeographic evidence is corroborative in that it suggests contemporaneous, albeit episodic, links between the Northern Hemisphere landmasses during the Barremian-Albian interval; these explain the appearance of very ‘European’ large ornithopods (Iguanodon) in Asia in Barremian/Aptian times as a consequence of land-based dispersal. Subsequent isolation of Asia from the European ‘domain’ during the late Early Cretaceous (Albian) may be responsible for the appearance of derived forms such as Altirhinus, and is suggestive of an Asian centre of origin for the family Hadrosauridae in middle Cretaceous times; this contradicts an earlier vicariance-biogeographic model of ornithopod evolution. The cranial anatomical modifications seen in this new taxon: vertical expansion of the dorsal nasal cavity, lateral expansion and lowering of the cropping beak relative to the jaw line, increase in the number of replacement teeth (but no significant miniaturization of the crowns) and the trend toward formation of a more integrated battery of cheek teeth, seen to varying degrees in several mid-Cretaceous ornithopods are commented upon; they can be interpreted within the context of an evolutionary trend culminating in the cranial complexity seen in the terminal lineage of ornithopods represented by the Late Cretaceous Hadrosauridae. Functionally, some of these changes can be correlated with what can be interpreted as ‘improvements’ to the efficiency of food gathering and processing which might represent increased niche partitioning and/or responses to increasingly tough and abrasive (xeric adapted) foliage; others, notably the modifications to the nasal cavity (perhaps associated with providing space for a counter-current moisture conserving turbinal system), are suggestive of a biological response to increasingly seasonal/xeric conditions in the middle of the Cretaceous Period or changes in the floral composition of these times.