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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
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.