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."