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A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah

2005; Nature Portfolio; Volume: 435; Issue: 7038 Linguagem: Inglês

10.1038/nature03468

1476-4687

James I. Kirkland, Lindsay E. Zanno, Scott D. Sampson, James M. Clark, Donald D. DeBlieux,

Evolution and Paleontology Studies

The discovery of a well preserved fossil of an early therizinosauroid dinosaur in a Utah quarry marks the first North American record of what is largely an Asian group, predating the formation of the Alaskan land bridge. This raises new questions about the origins of these dinosaurs as well as indicating a faunal exchange between Asia and North America via Europe in the Early Cretaceous. In addition various primitive features of this fossil suggest a pattern by which herbivory could have developed from a carnivorous ancestor. Therizinosauroids are an enigmatic group of dinosaurs known mostly from the Cretaceous period of Asia, whose derived members are characterized by elongate necks, laterally expanded pelves, small, leaf-shaped teeth, edentulous rostra and mandibular symphyses that probably bore keratinized beaks1,2. Although more than a dozen therizinosauroid taxa are known, their relationships within Dinosauria have remained controversial because of fragmentary remains and an unusual suite of characters. The recently discovered 'feathered' therizinosauroid Beipiaosaurus from the Early Cretaceous of China helped to clarify the theropod affinities of the group3. However, Beipiaosaurus is also poorly represented. Here we describe a new, primitive therizinosauroid from an extensive paucispecific bonebed at the base of the Cedar Mountain Formation (Early Cretaceous) of east-central Utah4,5. This new taxon represents the most complete and most basal therizinosauroid yet discovered. Phylogenetic analysis of coelurosaurian theropods incorporating this taxon places it at the base of the clade Therizinosauroiden, indicating that this species documents the earliest known stage in the poorly understood transition from carnivory to herbivory within Therizinosauroidea. The taxon provides the first documentation, to our knowledge, of therizinosauroids in North America during the Early Cretaceous.