(Dave Thomas has another take on this discovery at the Panda's Thumb.)

It's another transitional fossil! This one won't cause the creationists much discomfort, though, since it is transitional between two extinct groups and they can therefore ignore it, just as they do 99.99% of all science. Falcarius utahensis is a member of a strange little side branch within the theropods that had begun to switch over from eating meat to living on plants—they were a bunch of Cretaceous vegans. The cool thing about this particular specimen is that it was caught in the early stages of this shift, and it exhibits a mix of carnivorous and herbivorous traits, and therefore lets us see what kind of adaptations were involved.

Here's a reconstruction of the animal, and you can see it's overall raptor-like form, including the nasty great claws. This skeleton is built up from several individuals, and you can also see that one thing missing is a good skull.

f, reconstructed right pectoral girdle UMNH VP 12279, 12281 and forelimb UMNH VP 12284, 12287, 12289, 12291, 12294−12296, 12298, 12300, 12302, 12314, 12304, 12306, 12308, 12310, 12312, 12316, 12320 in lateral view; g, Falcarius skeletal reconstruction; h, reconstructed left foot UMNH-VP 12330−12342, 12352−12356 in anteriodorsal view; i, medial caudal vertebrae UMNH VP 12405, 12407, 12409 with chevron UMNH VP12391 in lateral view.

The cladogram shows where Falcarius is in the grand scheme of the theropods.

Despite the lack of a complete skull, the investigators do have jaws and maxillae and teeth—and there's where the difference between this lineage and other theropods is apparent. Those are leaf-shaped, iguana-like teeth; this guy wasn't ripping into flesh, he was chomping on leaves. He also had a toothless beak.

a, Partial left maxilla UMNH VP 14526 in lateral view; b, c, close-up of maxillary teeth in lingual (b) and labial (c) views; d, detail of maxillary tooth denticles in labial view; e, dentary teeth UMNH VP 14527 in lingual view; f, detail of dentary tooth denticles in labial view; g, same dentary teeth in labial view; h, composite reconstruction of the dentary of Falcarius based on UMNH VP 14527, UMNH VP 14528 and UMNH VP 14529. nf, nutritive foramina.

The authors compare this skeleton to its nearest relatives:

Therizinosaurs are here proposed as shifting their dietary habit from predation to herbivory on the basis of the development of a number of features that seem convergent with clades of other herbivorous dinosaurs. The most significant of these features include small, leaf-shaped teeth, an edentulous beak, posterior displacement of the pubis and lateral expansion of the pelvis associated with greatly increased intestinal volume, and shortening of the tibia relative to the femur and an increased number of weight-supporting pedal digits—the latter two being specific reversals of the cursorial condition. Falcarius demonstrates the mosaic nature of this evolutionary transition, indicating that the dentition and pelvis were among the first hard-tissue structures to undergo modification. These changes probably coincided with modifications in food acquisition and digestion during the early stages of therizinosauroid evolution. Moreover, similarities between the dentition of the basal therizinosaur Falcarius and the basal oviraptorosaur Incisivosaurus, in combination with their proposed sister relationship, raises the possibility that the common ancestor of these clades had already undertaken the initial steps in this transition.

Another small puzzle piece falls into place, and it fits neatly into the framework provided by other fossil specimens and evolutionary theory.

Kirkland JI, Zanno LE, Sampson SD, Clark JM, DeBlieux DD (2005) A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature 435:84-87.