A perennial criticism of evolutionary theory is so-called "missing links" lack of ancestor species, or older species recently diverged from an ancestor.  Researchers have been able to eliminate some of that skepticism with the discovery of a human-sized miniature tyrannosaurid.

The beast was excavated in an illegal dig in Mongolia and sold to a private collector.  That collector had paleontologist Paul Sereno evaluate the remains and then agreed to donate the fossil to science, with the intent that it eventually be returned to China.  Professor Sereno has published a paper on the creature in the journal Science.

The dinosaur brings significant surprises.  Scientists long thought that the T. Rex's giant proportions were the result of a sharp divergence and were not present in earlier tyrannosaurid.  However, the new species, Raptorex kriegsteini shared the same bizarre proportions its descendant, albeit in a much smaller body.  This finding shakes of the traditional assumptions about the T. Rex and its evolution, and show just how much remains to be discovered in the field of paleontology.

Stephen Brusatte, a paleontologist at the American Museum of Natural History, who co-authored a paper states, "The most interesting and important thing about this new fossil is that it is completely unexpected.  As we learn more and more about dinosaurs and the evolution of life over time, it’s harder and harder to find fossils like this that throw us for a curve."

Scientists determined that the dinosaur was an adolescent by looking at its level of bone fusion.  Certain bones  tended to fuse or merge over the life span of dinosaurs, providing an important indicator of age. Raptorex’s pelvic girdle was completely fused and its scapula and shoulder blade were nearly so, showing that it is a full grown adult.

Thomas Holtz, a paleontologist at the University of Maryland, not involved with the research, lauds the discovery, stating, "To most people in dinosaur paleontology, we’d regard it as significant because it helps us more completely understand the origin of the classic tyrant dinosaurs and how they became what they were.  When Raptorex was around, the ancestors of Triceratops are similarly very small, but the ancestors of Triceratops don’t have their horns or frills yet. They are not down on all fours and so forth.  It’s almost like finding a tiny Triceratops at this stage that is four-on-the-floor with the big horns and the big frills."

The T. Rex is 90 times larger than its recently discovered predecessor in body mass.  The fact that its body design was able to scale so well is incredible and rather unique states Professor Sereno.

While some may hope to see the new beast in a future Jurassic Park sequel, for paleontologists, it's reviving a perennial debate over the T. Rex.  The dinosaur had speed features, which now appear to be vestigial remnants of those of its smaller ancestor.  Were those features meant for hunting?  Some don't believe that.  Jack Horner, a paleontologist at the Museum of the Rockie argues that both beasts were scavengers, stating in an email to Wired.com,  "It is interesting that the authors imply that it was a predator on account of its small size, as though scavenging was only possible for large tyrannosaurids.  I think their evidence clearly supports an hypothesis that tyrannosaurids were small scavengers early on in their evolution."

Professor Holtz disagrees, stating, "No carnivore passes up a free meal, [but Raptorex] was a well built little animal for running fast and catching small dinosaurs."