Chilesaurus

Well, sic transit gloria mundi to Chilesaurus, I guess. A couple of days on top of the world, and then hustled off the stage for a tiny-maniraptoran-slash-Batman-cosplayer. Could be worse; at least it's Batman. 'Round here, though, I've never really had much interest in the origins of flight, early bird evolution, or so on, so nuts to Yi. Let's bring Chilesaurus back on the stage for a few more minutes.

Genus and species: Chilesaurus diegosuarezi, in reference to being a dinosaur from Chile (precisely zero points for guessing), with the species name honoring Diego Suárez, who discovered the first bones at the age of 7.
Citation: Novas, F. E., L. Salgado, M. Suárez, F. L. Agnolín, M. D. Ezcurra, N. R. Chimento, R. de la Cruz, M. P. Isasi, A. O. Vargas, and D. Rubilar-Rogers. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature (advance online publication) doi:10.1038/nature14307.
Stratigraphy and geography: Toqui Formation, Tithonian Stage (Upper Jurassic), Aysén, Chile
Holotype: Servicio Nacional de Geología y Minería 1935, a nearly complete and articulated skeleton of a partially grown individual lacking most of the tail and part of the skull and lower jaw. Partial skeletons of four other individuals are known, although initially they were thought to represent multiple taxa because of the unusual combination of features.

The body plan of the little guy is a bit like what you'd get if you merged the long trunk and long skinny neck of an Elaphrosaurus with the shoulder and robust arms and legs of a basal tetanuran like Piatnitzkysaurus, then threw in the pelvis of an alvarezsaur and made the head and hands a custom job. Unlike most small theropods, the femur and tibia are close to the same length, and the metatarsals are short, a combination which does not suggest great speed. In addition, the first toe, instead of being a simple dewclaw, is relatively long. The pelvis is opisthopubic, meaning the pubic bones are not pointing toward the front of the animal, like in your average good honest theropod, but point back toward the tail, in the fashion of dromaeosaurids, therizinosaurians, alvarezsaurids, and birds. In this case, the immediate hypothesis that comes to mind is that the pubis shifted back to give the guts more room. The five described individuals represent different growth stages, which is always convenient, and are estimated to have ranged from 1.2 to 3.2 m long, or about 4 ft to 10 ft. With its robust limb bones, one gets the impression that it was halfway to being Thescelosaurus before Thescelosaurus.

The most distinctive parts are the skull and hands. The skull is quite deep and blunt, albeit incompletely known. (Typical; you never get the whole skull when you want it. So many recent lambeosaurines have suffered the same fate. Give us crests!) It would be interesting to know if the jaw joint is lower than the tooth row, as is seen in other herbivorous dinosaurs. The teeth are described as leaf-like, although from the photos they look a bit more like chisels, with serrations small and limited to the tips. The business end of the upper jaw is interpreted as possibly hosting a beak-like structure. Each hand is equipped with a total of two useful fingers, although given how stocky they were, chilesaurs were not getting by in any trades requiring a high degree of manual dexterity. Power is a more likely option; although short, the fingers are stout, and the humerus has a large deltopectoral crest.

Although theropods get a lot of press, especially because you can always work in a reference to Tyrannosaurus, there are whole swaths of Theropoda that get forgotten, being anatomically unremarkable small or large theropods. Chilesaurus does the great mass of basal tetanurans a favor by showing that they had more tricks up their short sleeves than "be bitey".

The other really interesting bit is we're now accumulating unusual Jurassic herbivorous theropods. Aside from Chilesaurus and Limusaurus (which also had dinky hands, but was more optimized for speed), we've also got Eshanosaurus, and Elaphrosaurus has sometimes been mooted as an herbivore despite the notable disability of lacking a skull. Eshanosaurus, of course, was described controversially as the oldest known therizinosaur. If anything, the identification has gotten more controversial over the years because we've now got several basal therizinosaurs from the Early Cretaceous, and they appear to be much more basal than Eshanosaurus from the Early Jurassic. But what if Eshanosaurus was actually a member of an early herbivorous branch that resembled therizinosaurs, but was not related? And could this possibly do any favors for my old discarded personal crackpot hypothesis of Protognathosaurus as a theropod?

References:

Novas, F. E., L. Salgado, M. Suárez, F. L. Agnolín, M. D. Ezcurra, N. R. Chimento, R. de la Cruz, M. P. Isasi, A. O. Vargas, and D. Rubilar-Rogers. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature (advance online publication) doi:10.1038/nature14307.