The Loom

Irish elk of the Jurassic

Guanlong%20280px.jpgDinosaur 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%20tree%20500.jpg

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.

Tapejara%20250.jpg

And just a few thousand years ago, another extravagant piece of headgear could be found on the Irish elk, shown here.

Irish%20elk%20200.jpgThere’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.

Comments

  1. #1 Dan S.
    February 8, 2006

    “Dinosaur paleontologists don’t look for fossils simply because dinosaurs are cool”

    Yeah, but look at the reconstruction. That’s cool.

    re: Sexual displays and sexual selection – is this a widespread thing? I’m trying to think . . . most of them do seem to have that odd take a generic so-and-so and stick-something-on look, but I don’t know enough about any of the species involved to judge.
    And behaviorally, I wonder?

    Now for people – I know human males are rather well-endowed compared to our relatives* – might this be another example, or are their more plausible/supported explanations . . .

    * and they don’t even get showered in spam about it. Go figure?

  2. #2 steve s
    February 8, 2006

    It must be a pretty sucky time to be a creationist.

  3. #3 Jaime A. Headden
    February 9, 2006

    I think Creationists will use the crest to indicate how DIFFERENT this animal is and thus capsize on the ignorance involved similarities versus differences when dealing with biology and phylogeny. As it is, arguments of lumping by “kind” are done so not based on similarities, but differences, since if it doesn’t look like it’s “folks”, then it’s not the same thing. This is in ignorance of the details of anatomy.

  4. #4 Michael
    February 9, 2006

    Does this mean that T-Rex had feathers?

  5. #5 Jim A.
    February 10, 2006

    Carl, did you mean to write a different date in your third paragraph? At National Geographic’s website they estimated the fossils to be 160 rather than 120 million years old.

  6. #6 Dave Hone
    February 10, 2006

    To Michael:

    T.rex *may* have had feathers, early tyrannosaurus like Dilong seem to have had feather filaments so it is certainly a posiibility. However, an adult T. rex would be looking at losing heat, not insulating as it was so big (Surface area to volume ratios) so probably did not. However, it is possible, even probable that baby rexes had downy coats to keep them warm.

    As for crests in general Tarpejara is goos, but check out Nyctosaurus! It had a crest approachin 1.8m even though its wingspan was only about 4m. Yes. Really. Most reconstructions give it a ‘sail’ of skin, but this probably did not exist as it would make it too unsatble to fy well.

    As for theropods, see also Dilophosaurus, Oviraptor and Cryolophosaurus for some cool headgear.

  7. #7 Carl Zimmer
    February 10, 2006

    Jim, thanks on catching the mistake on the date. Now fixed.

  8. #8 Hank Roberts
    February 11, 2006

    > 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.

    I flew hang gliders starting in 1978 — when the wing was entirely in a plane. They had stability issues. The first change was a ‘keel pocket’ to add a vertical panel, one that could shift from side to side. It helped.

    I wonder what aerodynamics engineers the paleontologists consulted and what steps they took to rule out the possibility that the crest aided flight control, before deciding they were sexual displays?

  9. #9 Richard Hamilton
    December 20, 2006

    So does that mean that some birds have ancestors from Maniraptora and some have ancestors from Tyrannisauriodea?
    I thought that all birds evolved from Maniraptora.

    And as far as headgear in dinosaurs go, excluding Ceratopsians, Stygmoloch takes the cake without a doubt.

  10. #10 Sordes
    October 26, 2007

    What I find interesting enough is the fact that Guanlong had a prominent crest, no matter what actual purpose it had. Among modern predators neither mammals nor birds evovled similar structures. Some carnivores like lions and hyeneas evolved more or less big manes, but they are in general only hair which is longer on some bodyparts. But the complex bony structures at the heads of different theropods are actually weird.
    The crest and comparably weak jaws of Dilophosaurus were (and are) often used as an argument for a scavenging behavior. As I am a strict opponent of such ideas about specialications towards scavenging in big carnivores, I took a closer look at Dilophosaurus´ skull. The shape of the teeth and especially the dent in the upper jaws resembled a bit those of crocodiles, spinosaurids and some other piscivorous reptiles. The whole body with the long neck and the comparably gracile body would also fit well the idea that Dilophosaurus was some kind of super-heron. It was really interesting that a short time ago evidence was discovered that a relative of Dilophosaurus did wade into the water to catch fish. So what does this mean´ Perhaps the crests on the heads do indicate also for Guanlong an unusual way of life, which was not typical for a theropod of its size. I?m not saying it was a fish-eater, but perhaps there was another kind of special ecological niche which was occupied by this dinosaur.

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