Tyrannosaur 'Missing Link' Among New Dinosaurs From China

The new species are described in the Proceedings of the Royal Society B. The papers will appear in print later this year in a special volume entitled "Recent advances in Chinese palaeontology."

An early precursor to T. rex

One of the new animals is an early relative of T. rex, and is named Xiongguanlong baimoensis (shong-GWAN-long by-mo-EN-sis). The generic name derives from the ancient Chinese name Xiong Guan ("Grand Pass") for Jiayuguan and long for dragon. The specific name baimoensis comes from "bai-mo," for "white ghost," in reference to a dramatic landform in the field area known as the "white ghost castle."

Xiongguanlong would have stood about five feet tall at the hip and weighed close to 600 pounds. It had a skull over a foot and half in length and armed with over 70 teeth. "Although impressive by today's standards, Xiongguanlong was still a fly weight predator compared to its heavy-weight relatives such as T. rex," says Peter Makovicky, PhD, Curator of Dinosaurs at Chicago's Field Museum, and corresponding author on the study of this animal. The world's largest known T. rex specimen, housed at The Field Museum and popularly known as SUE, was nearly 14 feet tall at the hips and is estimated to have weighed between six and seven tons.

Xiongguanlong represents a "missing link" in the fossil record of tyrannosaur dinosaurs. Large tyrannosaurs that lived near the end of the age of dinosaurs like T. rex and Albertosaurus have been known to science for over a hundred years, and the last decade has witnessed the discovery of some of the earliest tyrannosaurs from China and England. However, until recently there has been a huge gap between these early and late chapters of tyrannosaur evolutionary history.

According to Mark Norell of the American Museum of Natural History in New York, another member of the team that worked on the two new dinosaurs, "Xiongguanlong sheds light on the missing 40 to 50 million years of tyrannosaur evolution." Xiongguanlong is unusual among tyrannosaurs in having a very long and narrow snout, rather than a wide, massive skull optimized for powerful biting as is seen in T. rex. Despite this difference, Xiongguanlong does mark the earliest appearance of several hallmark traits of larger, geologically younger tyrannosaurs, including a short, broad braincase, broad struts of bone near the temples, expanded areas for jaw muscle attachment on the skull roof, modified "nipping" teeth at the front of the mouth, and expanded vertebral structures to support a large head. Many of these features represent structural modifications for increasing bite forces and presage the specialized skulls of larger tyrannosaurs.

Norell notes that "Xiongguanlong underscores that tyrannosaurs started as small to mid-sized predators, but a number of the traits related to the enormous bite forces of T. rex were already evident at this relatively early stage of tyrannosaur evolution."

Adds Makovicky, "The proportions of Xiongguanlong's skull are similar to those of juveniles of large tyrannosaurs, confirming that massive skulls of T. rex and its closest relatives evolved from animlas with long slender snouts like Xiongguanlong."

Second find: A giant "ostrich-mimic"

The team also discovered three specimens of a remarkable, second theropod from the Yujingzi basin during the 2006 and 2007 field seasons. Beishanlong grandis (bay-SHAN-long gran-DIS) is a new species of ornithomimosaur, or ostrich-mimic dinosaur. Ornithomimosaurs are a lineage of theropods that evolved a toothless beak and were likely omnivorous or herbivorous, superficially resembling present day ostriches.

With an estimated body mass of almost 1400 lbs. (626 kg), Beishanlong is one of the largest ornithomimosaurs yet described, rivaling the Late Cretaceous ornithomimosaur Gallimimus in size. Bone microstructure analysis on a cross section of one of the lower leg bones of the principal specimen by co-author Gregory Erickson of Florida State University reveals that the holotype individual was not yet fully grown when it died. According to Dr. Erickson, "Growth line counts revealed that the animal perished during its 14th year of life. Although it is hard to fathom, this giant was still actively growing when it died. Growth line spacing in the bones of the teen-giant show only moderate decreases in width towards the periphery. Somewhere out there are even larger specimens awaiting discovery."

Beishanlong was equipped with hand claws up to six inches in length and relatively powerful forelimbs compared to most other ornithomimosaurs. "We know the forelimbs could not be brought far forward or elevated too much" says Makovicky, who is the lead author on the study, "but their range of motion and the shape of the claws suggest they may have been used for digging or raking the ground."

Profound implications for Asian dinosaur faunas

Other dinosaurs found together with Beishanlong and Xiongguanlong include the beaked and probably herbivorous therizinosauroid theropod Suzhousaurus, primitive relatives of duck-billed dinosaurs, the small horned dinosaur Auroraceratops, and both small and large sickle-clawed theropods. Most of the famous Cretaceous dinosaur faunas of China and Mongolia, like the ones that include Velociraptor and Protoceratops, comprise members of the same dinosaur lineages as those found in Gansu, but very few others. Interestingly, horned dinosaurs and sickle-clawed dinosaurs tend to dominate red rocks deposited under dry conditions across almost all Cretaceous localities in the Gobi Desert, whereas tyrannosaurs, ornithomimosaurs, and duck-billed dinosaurs tend to occur in rocks deposited in wetter environments. The fauna collected by the Chinese-American team in Gansu shows evidence of such environmental separations between different kinds.

The fauna collected by the Chinese-American team in Gansu shows evidence of such environmental separations between different kinds of dinosaurs, and represents one of the earliest instances of both the environmental sorting of animals and the very stable, but depauperate, lineage composition of Cretaceous Central Asian dinosaur faunas that reined for the next 50 million years. Professor Gao Ke-Qin of Peking University, who also took part in this research, notes, "The abundance of new dinosaurs from Chinese localities like the Yujingzi Basin, allows us to study the long history of dinosaur evolution in light of both geographic and environmental parameters in a way that is impossible elsewhere in the world."

The researchers were funded by the US National Science Foundation, the National Science Foundation of China, and a Grainger Foundation grant from The Field Museum, and were joined in the field by students and post-docs from their institutions.