Just yesterday, a new primitive dromaeosauroid was announced in that finest of scientific journals, Science. This tiny little terror, Mahakala omnogovae, is a Mongolian addition to the Dromaeosauridae, as well as its basalmost member. It seems to have just as many anatomical features in common with primitive birds and primitive troodontids as dromaeosaurs. It's position among the fearsome raptor dinosaurs is solidified only thanks to its foot morphology, in which the distal (far) end of metatarsal II is composed of an asymmetrical ginglymoid articular surface, and phalanx II-2 (the raptorial toe) has a well-developed proximal (near) heel and hypertrophied ginglymoid trochlea (for movin' that big ol' claw). Had a partial foot not been unearthed, we would know Mahakala omnogovae only as a primitive paravian.
In fact, Mahakala mixes features found in primitive troodontids like Mei and Sinovenator, primitive birds like Yixianornis, Archaeopteryx, and Jeholornis, and both groups of dromaeosauroids, that is, the Eurasian velociraptorines and the Gondwanna unenlagiians. Such a mix of characters speaks to the fact that, not only is Mahakala a step or two away from the original paravian, but also that the three groups (avialans, troodontids, dromaeosaurs) are very closely related. Many of Mahakala's anatomical features, such as lack of antiliac shelf, presence of a large semilunate carpal, and concave calcaneum, are plesiomorphic for Paraves.
The most interesting aspect of Mahakala's anatomy, however, is its miniscule size. At an estimated 70 cm long, the little raptor is about the same size as Mei long and the largest Archaeopteryx specimens. It's smaller even than Microraptor. This means that small body size is ancestral to Paraves. This renders moot any questions of how these big mean theropod dinosaurs shrunk down to reach a size that would facilitate flight. It's actually the other way around: how or why did such small animals (the authors estimate that the "original" members of each family would have been 64-70 cm long and weighed between 600 and 700 grams) reach such large sizes not once but four times?
Four times? Well, it happened once in the Troodontidae. All of the later members of the family, including Troodon, Byronosaurus, and Saurornithoides were huge by comparison to their theoretical ancestor. Among the dromaeosaurs, a significant size increase occurred three more times. Among the unenlagiians, it occurred once, with Unenlagia. Among the velociraptorines, it occurred once, with Deinonychus. Among the dromaeosaurines, it occurred again with Utahraptor and Achillobator. But even those genera are among the giants for their larger families. Even Velociraptor, at a whopping four feet long, could snatch up Mahakala for lunch. Cope's Rule was alive and well even among the paravians.
The authors of Mahakala's description, at the end of the paper, imply that the ancestor of Paraves must have, itself, shrunk from some previous larger ancestor among the maniraptorans. I disagree on this point, and believe that perhaps the Maniraptora is far older than we commonly think. It is entirely possible that the Maniraptora is nearly as old as the Eutheropoda itself, and that theropods diversified incredibly quickly during the Late Triassic and Early Jurassic. Why would a large lineage shrink at all, only to get big again? In my mind, it would make more sense to say that the ancestral theropod was quite small, and that small theropods were diverse throughout the Mesozoic. Now, there is, of course, a strong preservation bias against small animals, especially theropods (what with their excavated bones). But a preservation bias does not mean that these animals did not exist. Just like there are small mammals today, one would think there were small dinosaurs, too.
Turner, A. H., Pol, D., Clarke, J. A., Erickson, G. M & Norell, M. A. (2007). A Basal Dromaeosaurid and Size Evolution Preceding Avian Flight. Science (317): 1378-1381.
P.S. I drew that picture. The short arms are apparently diagnostic of the taxon. Being a 3D fossil, feathers are not know, but the fact that avialans had 'em (Archaeopteryx), troodontids had 'em (Jinfengopteryx), and, of course, other dromaeosaurs had 'em (Microraptor) means that Makahala would have been similarly adorned. The skull is virtually unknown save the braincase, and frontals, which show evidence of large eyes. The tibia/fibula is probably too long but hey--I whipped this drawing out in like 45 minutes just for this blog. It's all you get! :-)