New Iguanodonts from Central Asia

Ornithischian dinosaur remains from the Maortu site in Alashan' occur in three bone bearing horizons; those in the lower two are of advanced iguanodont affinity and those in the upper are very primitive hadrosaurs. These are the first beds in which iguanodonts have been found so closely associated with hadrosaurs. The uppermost bed is well developed at Iren-Nor, not far from Maortu, from which the primitive hadrosaur Bactrosaurus johnsoni was described. Other remains from Asia described as iguanodonts, upon re-diagnosis turn out to be sauropods. The iguanodont remains from Maortu are assigned to a new genus Probactrosaurus, on the basis of hadrosaur-like characters of dentition and limbs and other distinctive characters that show no particular trend. Two species are described, P. gobiensis from the lowermost horizon, and P. alashanicus from the next superjacent horizon. P. alashanicus is more nearly like the hadrosaur B. johnsoni in most respects, and the sequence P. gobiensis - P. alashanicus - B. johnsoni may indicate stages in a rapid transition from an iguanodont to a hadrosaurian level of organization. On the basis of the morphological affinities of the dinosaur remains, the uppermost bed at Maortu is assigned a Cenomanian (earliest Upper Cretaceous) age, while the two lower beds may be dated to the Aptian-Albian (end of the Lower Cretaceous). — N.H. Hotton III.