The fearsome family of dinosaurs topped by Tyrannosaurus rex began with a miniature version of the tyrant that was only the size of a human being.

The new find from China was made public Thursday in a press conference and is already rewriting T. rex's evolutionary story. It'd long been thought that the multiton dinosaur's massive skull, dinky arms, and runner's legs evolved as a set of compromises necessitated by its increasingly massive size.

The new Raptorex kriegsteini proves that the T. rex's distinctive features predated its scaling up.

"The most interesting and important thing about this new fossil is that it is completely unexpected," said Stephen Brusatte, a paleontologist at the American Museum of Natural History, who co-authored a paper on the find in the journal Science. "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."

The tiny tyrannosaurid from 130 million years ago is not just awesome fodder for a future Jurassic Park sequel, it's also a scientifically valuable find. The scientists are sure that the new mini tyrant is not just a juvenile. Bones in animals tend to fuse in sequence, providing a key to an animal's maturity level. Raptorex's pelvic girdle was completely fused and its scapula and shoulder blade were nearly so, indicating that it was an adolescent at the end of its growth.

"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," said Thomas Holtz, a paleontologist at the University of Maryland, who was not involved with the research.

Many of the giant dinosaurs have their familial roots in smaller dinosaurs like T. rex, but usually the animals' body plans — how their knee bones connect to their hip bones, and so on — change a lot.

"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," Holtz said. "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."

That the T. rex body plan turned out to be flexible enough to just become 90 times larger without major changes is astounding.

"It is one design, almost unlike any other you can point to in dinosaurs, that can withstand two orders of magnitude enlargement," said paleontologist Paul Sereno of the University of Chicago, a co-author of the Science paper.

Raptorex also helps answer a lot of lingering questions about why T. rex looked the way it looked. Why, for example, did such a huge animal have a specialized foot with a built-in shock absorber, clearly an adaptation for running?

"For T. rex itself, it’s living in a world where the laws of physics are saying you can’t be as fast running as you look," Holtz said. "They are more speed adapted than you’d expect. It’s like having a spoiler on an SUV."

With Raptorex fully described, paleontologists can explain the speed-boosting leg structure as a leftover from T. rex's evolutionary past. The smaller animal could have really taken advantage of its fleet-footedness.

But maybe not for hunting prey.

The researchers described Raptorex kriegsteini as a predator, running down prey and "dispatching" it with its jaws. But Jack Horner, a paleontologist at the Museum of the Rockies who was not involved in the research, did not see evidence that the small dinosaur was a predator. In recent years, debate has raged over whether T. rex was the fearsome predator we'd long assumed or whether he was an enormous scavenger.

"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," Horner wrote in an email to Wired.com. "I think their evidence clearly supports an hypothesis that tyrannosaurids were small scavengers early on in their evolution."

Holtz staked out some middle ground, noting that most meat-eaters do some scavenging and do some hunting, so it's difficult to draw a clear line between the two kinds of animals.

"No carnivore passes up a free meal," he said. "[Raptorex] was a well built little animal for running fast and catching small dinosaurs."

And in a world where some dinosaurs were much larger than itself, those legs could have helped Raptorex from becoming some other animal's dinner.

Regardless of how the Raptorex primarily earned its calories, the new find will change the direction and scope of research on the T. rex family.

"It’s inspiring me to do new types of research going back and reexamining our previous ideas about dinosaurs," Holtz said.

Image: 1. Illustration/Paul Marshall 2. Photo/Mike Hettwer.*

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