Tyrannosaurus and its close kin
At first glance you could be excused for thinking that Raptorex was the juvenile stage of one of the later tyrannosaurids. At only about 10 feet long, it had long, gracile legs, a slender-looking head, a large eye socket, and ridiculously small forelimbs which terminated in claws. While it was not quite a fully mature individual, it was not the juvenile stage of an already-known dinosaur. It was something scientists had never seen before, one that can tell us much about how its giant cousins evolved.
Up until now paleontologists have been working with the bookends to the tyrannosaur evolutionary series. There were the classic, large-bodied terrors like Tyrannosaurus and the more recently-discovered, raptor-like dinosaurs like Dilong and Guanlong from China. Raptorex fits somewhere in between, and even though it was small it possessed many of the characteristics seen in its larger relatives.
While Raptorex did not have the heavy, knobbly head of Tyrannosaurus, its head was relatively large for its body size; about 10 percent larger for its body size than the skulls of earlier relatives like Guanlong. Raptorex also had incisor-like teeth in the front of its mouth, a condition seen dinosaurs like Tyrannosaurus but not the early tyrannosauroids. Add to that the expanded areas for jaw muscle attachment and you get a "miniature" tryannosaur with a terrifying bite.
What is really interesting, though, is that Raptorex had small arms in which the humerus (upper arm bone) is longer than the lower arm. These short, robust arms were tipped in claws. While a biomechanical study of the arms of Raptorex has yet to be carried out, this arrangement suggests that short-yet-strong arms of the tyrannosaurids evolved when the lineage was still relatively small.
This discovery is very important because just why Tyrannosaurus and its close relatives had such small arms has long been an evolutionary puzzle. The arms of the relatively small Raptorex may help paleontologists understand whether arm size was an adaptation to a particular hunting style or the consequence of something else, like a change in growth rate. In addition to the other characteristics documented by the paper's authors, many of the tell-tale tyrannosaurid traits first evolved when the lineage was still small, contrary to what paleontologists had expected.
The description of Raptorex is made all the sweeter by the fact that it was almost lost to science. In an University of Chicago interview, Paul Sereno, the lead author on the new paper, explains that the skeleton of Raptorex was excavated by a local person and sold to a private owner. This private owner then approached Sereno, and the scientist was adamant that the skeleton be donated to science (and eventually returned to China). Had Raptorex stayed in private hands we would still be in the dark about this crucial point in tyrannosaur evolution.