Dinosaur paleontologists don’t look for fossils simply because dinosaurs are cool. They want to solve evolutionary mysteries. Like all living things, dinosaurs form groups of species. You’ve got your long-necked sauropods, your head-shield-sporting ceratopsians, and so on. The distinctiveness of a group can make it difficult to determine how it evolved from an ancestor. Whales may be mammals (they nurse their young, for example), but they’re all fish-shaped.
Some of the best clues to the origins of these groups come from transitional fossils, which are formed by species that evolved some, but not all, the traits that set a group of species off as a group. Transitional fossils can sometimes be very weird. My personal favorites are the early tetrapods (basically fish with fingers) and the early whales (whales with feet). But that’s probably because they were the subject of my first book. Each year brings new transitional fossils to choose from. And now comes the latest addition: Guanlong wucaii, a forerunner of Tyrannosaurus rex.
T rex and its kin–known as tyrannosaurids–were a particularly distinctive bunch. They had big heads with fearsome teeth, little arms ending in two-fingered hands, long legs, and a massive overall body. That distinctiveness made it hard for paleontologists to determine exactly where in the dinosaur family tree they fit. Thus Guanlong comes as such a delight. In a paper published today in Nature, a team of paleontologists from China and the United States describe two skeletons of Guanlong, discovered in western China. It lived about
120 160 million years ago, and measured up to nine feet long. It sported a number of traits that are found only in tyrannosaurids. Some are pretty easy to recognize, like the fusion of the nasal bones in the skull into a single unit. Others are a bit more esoteric, like the “centropostzygapophyseal lamina on cervicodorsal vertebrae.” (And breathe….)
But Guanlong lacked some of the traits found in better known tyrannosaurids. For example, it had three fingers instead of two. Only after Guanlong branched off that other tyrannosaurids lost one of their fingers. This and several other key differences set off Guanlong as a very primitive tyrannosaurid.
The scientists combined the information about Guanlong with information about other dinosaurs to calculate their most likely evolutionary relationships. Tom Holtz of the University of Maryland, who has contributed some excellent comments here at the Loom, wrote an accompanying commentary on Guanlong. It includes a nice illustration of that tree, showing the ages of the different lineages based on their fossils. Guanlong appears at the base of the tyrannosaurid branch. The similarities Guanlong has to certain other dinosaurs helps reveal the closest relatives of tyrannosaurids. On a superficial level, tyrannosaurids look a lot like an older group of dinosaurs called carnosaurs. But that resemblance is just a result of convergent evolution. In fact, the scientists conclude, tyrannosaurids evolved from a small two-legged ancestor that also gave rise to several lineages including one alive today: birds. Here’s the tree:
Guanlong’s place on this tree led the scientists to reconstruct it with the primitive feathers shown here. The branches closest to Guanlong on the dinosaur tree have yielded fossils with feather-like impressions (that includes Dilong, itself a tyrannosaurid). So it’s reasonable to infer that Guanlong had them too–not full-blown flight feathers, but simpler ones that appear to have been the evolutionary precursors of bird plumage.
I certainly wouldn’t want to bump into Guanlong in a dark alley, but as tyrannosaurids go, it wasn’t very big. As Holtz points out, it would have been dwarfed by big carnosaurs. It would take over fifty million years for tyrannosaurids to get to the monstrous sizes of T rex and its closest relatives.
As this evolutionary tree makes clear, Guanlong was an ancient relative of T rex, not a direct ancestor. After its lineage branched off from the other tyrannosaurids, some unique traits emerged. One of them is quite obvious in this picture: its enormous crest. This thin wedge of bone wouldn’t have helped the dinosaur in a fight, which leads the paleontologists to propose that it was a sexual display. Crests appeared on some other dinosaurs, not to mention their close flying relatives, the pterosaurs. Here’s a picture of Tapejara, a wonderfully bizarre species discovered in Brazil.
And just a few thousand years ago, another extravagant piece of headgear could be found on the Irish elk, shown here.
There’s a remarkable disconnect between these sexual displays and the rest of the bodies to which they’re attached. Guanlong looks very much like its closest relatives except for its crest. Tapejara has a standard pterosaur body. Take away the Irish elk’s antlers, and the rest of the skeleton remains very much like those of living elk and deer. Sexual displays show signs of evolving very quickly, thanks to the big difference they can make to a male’s reproductive success. But other parts of the body take much longer to evolve and tend to stick around once they’ve appeared.