Bactrosaurus

Abstract: MSM P4166, a specimen from the Moreno Hill Formation (middle Turonian) of New Mexico, is described as the holotype of a new genus and species of hadrosauroid dinosaur. Jeyawati rugoculus, gen. et sp. nov., is diagnosed by a rugose texture that covers the entire lateral surface of the postorbital and the presence of a large neurovascular foramen at the base of the jugal process of the postorbital, as well as a unique combination of characters. A preliminary phylogenetic analysis reveals that Jeyawati is a basal hadrosauroid more derived than Probactrosaurus, Eolambia, and Protohadros, but more basal than Shuangmiaosaurus, Bactrosaurus, and Telmatosaurus. Assessment of ontogenetic criteria indicates that MSM P4166 represents a subadult or adult individual. Even with the recognition of Jeyawati, Late Cretaceous hadrosauroid biogeography remains somewhat ambiguous because of the lack of material from the late Turonian—early Santonian in western North America.

Abstract: The juvenile anatomy of various cranial and appendicular elements of the hadrosauroid dinosaur Bactro- saurus johnsoni is described in detail. Growth changes are documented from juvenile to adult stages for each skeletal element available. In the studied skull, ontogenetic trends consist of: development of features on the ventral surface of the frontal; reduction in the slope of the posteromedial pro- cess of the premaxilla; a posterior shift of the dorsal process of the maxilla; development of concavities on the medial surface of the prefrontal; increased robustness and develop- ment of the ventral flange of the jugal; decreased curvature of the long axis of the quadrate; increased ventral deflection of the dentary; and changes in the length ⁄ width proportions and depth of the anterior surface of the predentary. In the appendicular skeleton, the majority of ontogenetic variation from juvenile to adult occurs in the limb bones, including increased robustness of the deltopectoral crest of the humerus; relative shortening of the ulna; increased develop- ment of the fourth trochanter and mediolateral widening of the distal end of the femur; increased expansion of the cnemial crest of the tibia; and the increased prominence of articular protuberances and flanges of the metatarsals. A survey of the phylogenetically informative characters present in B. johnsoni indicates that several characters concerning the frontal, maxilla, jugal, quadrate, predentary, dentary, scapula, humerus and ilium are affected by ontogeny. Nevertheless, the majority of phylogenetic characters are not ontogenetically variable, suggesting that a substantial amount of the information provided by juvenile and suba- dult specimens for phylogenetic inference is reliable in basal hadrosauroids.

Abstract: A new genus and species of non-hadrosaurid hadrosauroid, Gobihadros mongoliensis, is described from a virtually complete and undeformed skull and postcranial skeleton, as well as extensive referred material, collected from the Baynshire Formation (Cenomanian-Santonian) of the central and eastern Gobi Desert, Mongolia. Gobihadros mongoliensis is the first non-hadrosaurid hadrosauroid from the Late Cretaceous of central Asia known from a complete, articulated skull and skeleton. The material reveals the skeletal anatomy of a proximate sister taxon to Hadrosauridae in remarkable detail. Gobihadros is similar to Bactrosaurus johnsoni and Gilmoreosaurus mongoliensis, but can be distinguished from them in several autapomorphic traits, including the maximum number (three) of functional dentary teeth per tooth position, a premaxillary oral margin with a ‘double-layer morphology’, and a sigmoidal dorsal outline of the ilium with a well-developed, fan-shaped posterior process. All of these characters in Gobihadros are inferred to be convergent in Hadrosauridae. Phylogenetic analysis positions Gobihadros mongoliensis as a Bactrosaurus-grade hadrosauromorph hadrosauroid. Its relationship with Maastrichtian hadrosaurids from Asia (e.g., Saurolophus angustirostris, Kerberosaurus manakini, Wulagasaurus dongi, Kundurosaurus nagornyi) are sufficiently distant to indicate that these latter taxa owe their distribution to migration from North America across Beringia, rather than having a common Asian origin with Go. mongoliensis.

Abstract: 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. johnso...