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 Kh ural) has been shown to have been established on holotype material tha t is non-diagnostic, but appears to be closely similar osteologically to the Western European species I. bernissartensis. Additional materia l collected from the locality known as Khuren Dukh (Dornogov', Mongoli a), which had previously been referred to I. orientalis has been re-ex amined and shown to represent a new genus and species (Altirhinus kurz anovi gen. et sp. nov) of ornithopod dinosaur from the Early Cretaceou s (Late Aptian/Early Albian). The anatomy of this new ornithopod is de scribed it shows a number of similarities to the known species of Igua nodon, but also demonstrates features which are in some instances uniq ue to this taxon, while others seem to be either convergent upon or tr ansformational with respect to, the more derived hadrosaurid ornithopo ds of the Late Cretaceous. Palaeogeographic evidence is corroborative in that it suggests contemporaneous, albeit episodic, links between th e Northern Hemisphere landmasses during the Barremian-Albian interval; these explain the appearance of very 'European' large ornithopods (Ig uanodan) in Asia in Barremian/Aptian times as a consequence of land-ba sed 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 a n Asian centre of origin for the family Hadrosauridae in middle Cretac eous 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, latera l expansion and lowering of the cropping beak relative to the jaw line , increase in the number of replacement teeth (but no significant mini aturization of the crowns) and the trend toward formation of a more in tegrated battery of cheek teeth, seen to var)ring degrees in several m id-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 th;se change s 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 an d abrasive (xeric adapted) foliage; others, notably the modifications to the nasal cavity (perhaps associated with providing space for a cou ntercurrent moisture conserving turbinal system), are suggestive of a biological response to increasingly seasonal/xeric conditions in the m iddle of the Cretaceous Period or changes in the floral composition of these times. (C) 1998 The Linnean Society of London.