The taphonomy and phylogenetic relationships of the Talkeetna mountains hadrosaur

Abstract

The fossil record of hadrosauroids (Ornithopoda, Hadrosauroidea) from the Albian to Santonian is very sparse, with few described North American and Asian taxa compared to the diverse record of Campanian to Maastrichtian hadrosaurids. In 1994, the partial postcranial remains of a hadrosauriform dinosaur were found in the Matanuska Formation of southern Alaska. The Matanuska Formation is a thick succession of Albian¬to Maastrichtian-aged, dominantly marine, sediments deposited in a forearc basin along the actively accreting western North American margin. The Alaskan specimen is assigned a Turonian age based on molluscan biostratigraphy. The skeleton consists of postcranial elements including cervical, dorsal and caudal vertebrae, a partial pectoral girdle, proximal elements of the forelimbs, a partial pelvic girdle, and representative portions of the hindlimbs. This fossil represents the most complete, single skeleton of a dinosaur known from Alaska, and one of the few skeletal remains recovered outside of the North Slope. It is only the second North American Turonian hadrosauroid described, the other being jeyawati rugoculus from New Mexico. This specimen also represents a new taxon of basal hadrosauroid that can be diagnosed by its unique combination of humeral, filial, and femoral characters. A phylogenetic analysis recovers the new taxon nested within a paraphyletic assemblage of non-hadrosaurid hadrosauroids, being more derived than the North American Cenomanian taxa Eolambia and Protohadros but more basal than stratigraphically younger hadrosauroids from Asia, including Tanius, Bactrosaurus, and Gilmoreosaurus. The temporal and geographic occurrence of the Alaskan taxon provides an important new data point for hypotheses of hadrosauroid biogeography in the Late Cretaceous.