Looming larger than a double-decker bus and baring teeth that have been likened to serrated bananas, Tyrannosaurus rex has long been considered one of the most fearsome creatures ever to have walked the earth. Other familiar tyrannosaurs, such as Albertosaurus and Tarbosaurus, were likewise terrifying in their size and bite—despite those absurd-looking but characteristic arms. But it turns out that not all tyrannosaurs have these hallmark features.

This past fall paleontologists unveiled two tyrannosaurs new to science that are shaking up long-standing ideas about everyone’s favorite mega-predator. The finds are forcing researchers to reevaluate the origin of the tyrannosaur body plan and reconsider what they thought they knew about the diversity of this well-studied group. “Our view of tyrannosaur evolution has changed dramatically,” says doctoral student Stephen L. Brusatte of the American Museum of Natural History in New York City.

Over the past decade researchers have established that behemoth tyrannosaurs such as T. rex evolved from smaller ancestors. And they thought that the signature features of T. rex—including a huge skull built for tearing into flesh and bone, puny arms, and running legs and feet—were inextricably linked to the evolution of large body size. But in the October 16 Science, Paul C. Sereno of the University of Chicago and his colleagues, including Brusatte, described a new tyrannosaur, Raptorex kriegsteini, that upends this idea.

Unearthed in Inner Mongolia, Raptorex lived 125 million years ago—60 million years before T. rex terrorized North America. The fossil shows that, in fact, the T. rex body design debuted in a dainty dino, one that weighed little more than a human and was about 1/100th the size of T. rex. As such, Raptorex clears up some puzzling aspects of T. rex anatomy, observes Thomas R. Holtz, Jr., of the University of Maryland, a tyrannosaur authority who was not involved in the research. For example, paleontologists have long wondered why T. rex had lower limbs engineered for speed when the animal’s sheer heft would have precluded swift locomotion. But the presence of fleet legs and feet in the much older Raptorex indicates that T. rex’s lower limb architecture is just an evolutionary holdover from a smaller, faster ancestor.

The fact that the Raptorex body plan was simply scaled up in later tyrannosaurs such as T. rex attests to the adaptive value of those trademark traits. But not all members of this group went that evolutionary route, as Brusatte and his colleagues revealed in a paper published in the October 13 Proceedings of the National Academy of Sciences USA that describes the second new tyrannosaur.

Discovered in 2001 on an expedition in Mongolia’s Gobi Desert, the 65-million-year-old specimen, named Alioramus altai, has a suite of features that deviate sharply from the tyrannosaur norm. Tipping the scales at an estimated 350 kilograms, this dinosaur—believed to have been about nine years old when it died—is larger than Raptorex but still only half the size of a nine-year-old T. rex (which reached full size in 18 years). Furthermore, it has a “totally weird skull shape,” Brusatte asserts. Among other bizarre traits, the skull is long and slender, somewhat like a crocodile’s, and it lacks the banana-shaped teeth and enlarged browridges that enabled T. rex and other tyrannosaurs to bite with bone-crushing force.

The skull of A. altai also exhibits eight small horns, including one on each cheek that stuck out to the side. Though modest compared with the horns of dinosaurs like Triceratops, the horns of Alioramus are quite extravagant for a tyrannosaur. They would not have done much good in combat, so Brusatte surmises that they served to attract mates once the animal reached sexual maturity.

A. altai might seem wimpy, but its more delicate features may have been the key to surviving alongside its bigger, badder brother, Tarbosaurus. “Different skull shapes and different bodies probably allowed Alioramus and Tarbosaurus to coexist” much as lions and cheetahs share the African grasslands today, Brusatte comments. Whereas Tarbosaurus probably pursued large animals using brute force, Alioramus could have snagged smaller animals using the speed and stealth that its smaller proportions allowed. In the lush forests of Late Cretaceous Mongolia, Alioramus would have had plenty of manageable prey to choose from, including numerous species of small dinosaurs.

The new A. altai fossil resolves an enduring mystery about a genus that scientists previously knew from a single fragmentary and poorly described specimen, named Alioramus remotus. Based on the earlier find, researchers had debated whether Alioramus was a primitive tyrannosaur ancestor or a more derived creature possibly related to T. rex. Some investigators even wondered whether it was a distinct genus at all, suggesting that the creature could just be a juvenile Tarbosaurus. But analysis of the new specimen, which is much more complete, reveals a creature that is very closely related to T. rex, despite lacking the hallmark tyrannosaur traits.

Questions remain about Alioramus, however. Holtz notes that researchers will need to find an adult Alioramus to assess to what extent the animal’s unusual features are the result of having a slower growth rate than larger tyrannosaurs versus simply stopping growth earlier.

Note: This article was originally printed with the title, "Tinier Tyrannosaurs."