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Falcarius utahensis (was RE: Newfound Dinosaur a Transitional Creature)

Here's the official article...

Kirkland, J.I., Zanno, L.E., Sampson, S.D., Clark, J.M., and DeBlieux, D.D. (2005). A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature 435: 84-87.

Abstract: 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 beaks. 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 group. 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 Utah. 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 Therizinosauroidea, 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.

Theropoda Marsh 1881
Coelurosauria von Huene 1914
Therizinosauroidea Maleev 1954
_Falcarius utahensis_ gen. et sp. nov.

Etymology. From Falcarius (Latin, a sickle-maker) and utahensis (refers to Utah as its place of origin).

The material hails from the Crystal Geyser Quarry, basal Yellow Cat Member, Cedar Mountain Formation, which directly overlies the Morrison Formation. Around 90% of the skeleton is represented (but disarticulated). "_Falcarius_ is a gracile, small- to medium-sized theropod, approximately 1 m in height at hips and 4 m in length." The dentary is downturned snout and lacks a lateral shelf - two characters that are seen in _Eshanosaurus_, and cited by the authors to cast doubt on the theriziosauroid affinities of the Early Jurassic _Eshanosaurus_ (which lived much earlier).

The hands are long and gracile (as in _Beipiaosaurus_ and most oviraptorosaurs). The wrist is very interesting: "Distal carpals are preserved in two morphs: one fused into a semilunate that caps metacarpals I and II, and the other as unfused pairs of distal carpals as in other therizinosauroids." I would guess that the name "Falcarius" references the recurved manual unguals. The pelvis is not opisthopubic, with the pubis restored in a subvertical orientation. If I am interpreting the authors' description correctly, the suite of features in the pelvis suggest some accommodation for a more capacious gut via the lateral expansion of the ilium and the pubic orientation. So, _Falcarius_ had an incipient "pot belly", befitting its omnivorous habits.

The anatomy of _Falcarius_ indicates that the teeth (small, leaf-like) and pelvis were among the first things to change in the transition from carnivory to herbivory in therizinosauroid evolution. Nevertheless, "similarities between the dentition of the basal therizinosaur _Falcarius_ and the basal oviraptorosaur _Incisivosaurus_, ... raises the possibility that the common ancestor of these clades [Oviraptorosauria, Therizinosauroidea] had already undertaken the initial steps in this transition." But I wonder if this transition might have begun even earlier, based on the dentition of troodontids and the posterior shift of the pubic shaft that appears to be primitive for the Maniraptora.