Dinosaur - Alioramus altai

Introduction

Pronounced AL-ee-o-RAY-mus AL-tai

Alioramus means ‘other [evolutionary] branch’ in Latin. The species name altai refers to the Altai Mountains, near the fossil site where the species was first found.

This species is one of the smallest of the subfamily Tyrannosaurinae, and was about half the size of the closely-related Tyrannosaurus rex.

Identification

Alioramus altai was a small, gracile, long-snouted carnivore that differs from other tyrannosaurids (those in the subfamily Tyrannosauridae) in its body plan and presumably its ecological habits.

Alioramus has a skull never seen in a tyrannosaur before: it had a long, comparatively slender snout, projecting horns on the midline of the skull as well as on the cheekbones, blade-like teeth, and many air-filled cavities within the skull.

Although its skeleton is anatomically similar to its larger relative Tarbosaurus, Alioramus altai is about half the size. Its weight was probably only about 370 kilograms.

Habitat

Fossils were discovered in the Late Cretaceous Nemegt Formation in the Gobi Desert of Mongolia. It consists of river channel sediments and contains a variety of fossils. When Alioramus lived, Mongolia was wet, humid and seasonally rainy, although periods of drought occasionally occurred. The region had floodplains, rivers and lakes, and forests of araucarian pines. The rich mosaic of habitats supported fish, turtles, crocodiles, mammals, birds and dinosaurs (including plant-eaters and a variety of theropods – dromaeosaurs like Velociraptor, ornithomimids, oviraptors and the tyrannosaurs Alioramus and Tarbosaurus).

Distribution

The first fossils were found in 2001, in what is now the Gobi Desert, in southern Mongolia. Alioramus altai lived 72-66 million years ago, during the Late Cretaceous Period.

Feeding and diet

Alioramus altai was a carnivore, but did not have the bone-crushing jaws of larger tyrannosaurids. Instead, it had a long delicate snout with over 75 shallow-rooted teeth - the most of any tyrannosaurid.

Alioramus altai probably relied on speed and agility to hunt, as it competed for food with Tarbosaurus, which was more than twice its size. However, differences in their skull shape, size and build probably reflect their different hunting lifestyles, which allowed the two to co-exist.

Other behaviours and adaptations

The spectacular fossils discovered in 2001 tells us that there is a lot more anatomical and ecological variety in tyrannosaurids than we previously thought - not all tyrannosaurids were mega-predators adapted for stalking and dismembering large prey. Some were small and slender.

We know Alioramus altai was about half the size of a T. rex of the same age. How? Researchers determined the age of one specimen when it died by examining thin sections of bone and counting growth rings. It was then possible to compare the size and features of this 9-year-old with specimens of similar-aged T. rexes. Growth rings are layers of bone laid down during periods of interrupted or slowed growth – often due to cold seasons – and probably reflect a yearly growth stage. Growth rings are commonly used to determine the age of dinosaurs when they died.

Fossils description

Two species of Alioramus have been discovered, both in Mongolia. The first, Alioramus remotus, was discovered by Sergei Kurzanov in 1976 and described based on a partial skull and an incomplete skeleton. The second is the more complete Alioramus altai, found in 2001 by Julia Clarke, a member of a joint team from the American Museum of Natural History and Mongolian Academy of Sciences. It was named in 2009, with a full description published in 2012.

Evolutionary relationships

Alioramus altai is the smallest of the tyrannosaurines, along with Teratophoneus, and shared the Late Cretaceous Mongolian landscape with one of the largest, Tarbosaurus.Other tyrannosaurines include Daspletosaurus, Tyrannosaurus and Zhuchengtyrannus.

Further reading

Stephen L. Brusatte, Thomas D. Carr, Gregory M. Erickson, Gabe S. Bever, and Mark A. Norell. A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. PNAS October 13, 2009 106 (41) 17261-17266