When we compare Kathy Wankel's T. rex to the ten others we have, we get some sense of the variation that may have existed among individual T. rexes. When you look at human skeletons, you can see a great many differences. No two human skeletons, or two dinosaur skeletons, look exactiy alike. And we know that human skeletons vary according to race and by sex as well.
As for T. rex, eleven skeletons, all of them seemingly adult, still don't make for a big enough sample to say much about the variety you might find in T. rex. But it does seem that the creatures we call T. rex fall into two different categories. Some of them, including Kathy's T. rex, were relatively slim and delicate, at least for a multiton giant. Others were more robustiy built. So some researchers have suggested that there were actually two different kinds of tyrannosaurs we've been calling T. rex—that we've gotten it wrong ever since
Osborn combined Dynamosaurus and T. rex into one in 1906.
Dale Russell, a Canadian dinosaur paleontologist, was the first modern scientist to suggest we were calling two different animals T. rex. He thought Osborn's two names should have been kept separate. Dynamosaurus imperiosus would fit the more massive specimens like the one in Los Angeles; T. rex would be just the slighter ones like the skeleton at the Carnegie Museum. Bob Bakker also thinks T. rex is two different animals, though he calls them T. rex and Tyrannosaurus "x".
Clearly, some T. rexes were more robust than others, significandy broader in the head and body. T. rex skull expert Ralph Molnar looked at all the T. rex skulls except for Kathy Wankel's and Sue. He noticed some differences between them. The one in Los Angeles, for example, had unfused bones in its nose and not much of a bump over its eyes. As I mentioned, on human skulls, and those of many other animals including dinosaurs, skull bones aren't fused in young individuals. Ralph wasn't sure whether the Los Angeles skull was from a different sex or just a younger animal than others he'd examined, even though it didn't vary considerably in size from the other specimens.
The biggest, most noticeable difference between the T. rexrskulls is the bump overthe eyes. In the Los Angeles and Canadian T. rex skulls, there is almost no bump at all. In one of the American Museum skulls, the bump is hemisphere shaped. Looking at the bump through a display case, Ralph Molnar thinks it might be a plaster invention. The biggest bump is on the first T. rex from the Museum of the Rockies. On that animal the over-the-eye bump is peaked like the eave of a house. So maybe the bump was something that grew on all T rexes as they got older. Then again, it might be a feature of just the females or males. I suspect males had bigger bumps than females, and that the same was the case for features on other dinosaurs that might have been used for display. That's the way it is with horned animals today.
Peter Dodson of the University of Pennsylvania has done a lot of work on the crests of lambeosaur duckbilled dinosaurs, showing how the crests probably varied within a species according to age and sex. You might figure that the ones with showy crests were the males, who need to display to attract a mate—that's how it is with birds, the descendants of carnivorous dinosaurs, and with rams and deer—but you really can't be sure which dinosaur sex got the headgear.
Ralph didn't find any consistent pattern of variation between the T. rexes, judging from just the skulls. But Ken Carpenter did when he looked at T. rex skulls and skeletons together. He says the namesake T. rex at the Carnegie Museum and Sue are the best examples of the robust T. rex. Their jaws are more massive, and they have bigger horns over the eyes and bigger teeth. Kathy Wankers T. rex and those in New York and Los Angeles are the more delicate form.
Ken found several ways in which the T. rex skeletons fit into two different groups, from the shape of the arm bones to the number of teeth. For instance, the skinnier, "gracile" T. rexes like Kathy's and the T. rexes in Los Angeles and New York have straight shafts on their upper arm bones. The bigger T. rexes like Sue and the namesake T. rex at the Carnegie have larger, bowed upper arm bones. (Skeletons of Tarbosaurus, T. rex's Mongolian cousin, show the same division between curved-arm and straight-arm-types.)
So maybe there are two types of skeletons of animals we're calling T. rex. But I think that before we create two different names for these animals, we have to resolve the question of whether the variation we see is just differences between males and females of one kind of animal, the one we call T. rex. That doesn't mean the big ones were males, the thin ones females. That's our prejudice because, on average, men are bigger, women smaller. A lot of mammals turn out that way. But it's not the case for many other animals—many birds, for instance.
As it happens, the female T. rex may have been the larger, stronger form. That's what Ken Carpenterthinks. He finds that the bigger T. rexes had pubic bones angled t. rex skulls also appear to sort into two categories, one considerably more stoutly built than the other. sue's skull (above)
is more robust than the american museum specimen (below).
farther away from the back vertebrae. Ken thinks this made for additional abdominal space. That room would have helped eggs to pass through.
Pete Larson of the Black Hills Institute has told me about a scientist who thinks he's got a way to figure out the sex of dinosaurs. His name is Dr. Eberhard "Dino" Frey, and he's a reptile expert at the natural history museum in Karlsruhe, Germany. Pete compares him to Bob Bakker, a brilliant guy with a wide-ranging curiosity.
From Dr. Frey's nickname you could guess dinosaurs are one of those interests. Frey has dissected a lot of crocodiles in his time. The males are always distinct from the females in the architecture of their tailbones. There is an arch with V-shaped spines on the belly side of the tail vertebrae of reptiles. In anatomy books these are called hemal arches or chevrons. On male crocodil-ians the first hemal arch, the one closest to the body, is the same size as the second arch. On females the second arch is twice as large as the first. Frey thinks that the male's arch is tight from the muscles that retract the penis and that the female's is enlarged to help it hatch eggs.
Whatever the reason, Frey finds the distinction consistent for crocodiles. And he finds it holds up for dinosaurs, too. The first few tail bones of Sue are scrambled up, but those of our T. rex at the Museum of the Rockies are in place. The first and second arches are the same size. According to Frey, that would make our T. rex a male. And since our T. rex is slimmer than Sue, it might be that the more lightly built T. rexes were males. Sue, then, would be the right name. (The bones of a subadult found along with Sue would belong to a male.) And Stan, the T. rex Pete Larson excavated in 1991, is relatively slim, like our T. rex.. So it, too, might be male. Our T. rex, if it had a name, would have to be named for Tom Wankel, not Kathy.
Yet another clue to the sex of dinosaurs could come from bone studies. Females of many animals, us included, are prone to osteoporosis after menopause. And bone changes during pregnancy can show up in bone
a big female and a smaller male t. rex? here's artist brian franczak's depiction of two sexes of t. rexes.
studies. A veterinarian who works in our histology laboratory thinks he may be able to detect these changes in dinosaur bone. If so, then we'd know we had a female on our hands.
But I don't think I'll be convinced there are two species or sexes of rexes until we find a lot more than eleven specimens. How many do we need? Maybe thirty. You could take those thirty specimens and compare them to see if they fall into two groups. If they do, and they're all from one vicinity so that the size difference isn't caused by a slightly different gene pool, then the chances are you are looking at two sexes.
Male and female T. rexes both were enormous by comparison with most animals, living or dead. Which raises another intriguing question.
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