The skull is the most complicated piece of any animal's anatomy, and the one that undergoes the most evolutionary change. It has many openings (fenestrae) and tubes for air passages and nerve canals. T. rex had an enormous skull, nearly as long as I am, and I'm six feet tall.
Where it is solid, the roof of T. rex's skull is three or four inches thick—"heavy duty," in the words of paleontologist Phil Currie. On some dinosaurs, skulls were light, small, and delicate and so very hard to come by as fossils. The biggest dinosaurs, such as the four-legged sauropods Diplodocus and Brachiosaurus, had tiny skulls with brains no bigger than those of house cats. We know hundreds of sauropod skeletons, but in two centuries, paleontologists have found fewer than two dozen sauropod skulls.
For horned dinosaurs, such as Triceratops, the preservation story is the opposite—we've got lots of skulls but relatively litde of the rest of the body. But you can get lucky. Out in the Hell Creek badlands in the summer of 1991, Diane Gabriel, a graduate student at our museum, found the skull and most of the skeleton of a Tricer-atops.
Our T. rex's skull is big and sturdy, and though full of holes, it seems to have held up pretty well. Phil Currie and I will look closely at the skull and at the brain cavity in particular. And thanks to the folks at General Electric in Cincinnati, we're now able to look at T. rex's skull inside and out in thousands of ways.
So much of the interesting anatomy of a skull is inside it—in the braincase, air passages, and nerve pathways. It's hard to get a good look at these features without
breaking apart the individual skull bones. Then it's the skull of t. rex is a difficult to get them back together. Breaking them apart great maze of bone and IS not a risk you d want to take with the few T. rexskulls , in existence. But with a CAT scan you can photograph thousands of microscopically thin sections of bone and then piece the images back together in a three-dimensional view from any direction. And since it's all done by X rays, you never damage the skull.
Our T. rex's skull is too big to fit in hospital CAT scanners, but at the GE labs in Cincinnati there's a CAT scanner big enough to handle a five-foot-long skull. We trucked T. rex out there in a U-Haul, along with every other good skull we had in our collections—two and a half days of nonstop driving. (As the boss, I got to take the plane.)
We took thousands of pictures of T. rex. It took us seven hours just to photograph the T. rex jaw, since we made an image every fiftieth of an inch along the skull. In the end we had a gigabyte's worth of image informa-
tion—that's a thousand megabytes. GE says they'll let us use their Cray computer, one of those super number-crunchers, to help generate and analyze our images. Sue, the Black Hills Institute T. rex,, was scheduled for a similar CAT scan treatment in spring of 1992 at the Marshall Space Flight Center in Alabama, just days before the skeleton was impounded. It was never scanned.
It's hard to know what we'll find, even what questions to ask, until we get a look inside the skull with CAT scans. We need to see how thin and how porous the bones are. We'll be looking at tooth growth, upper and lower jaw mechanics, the sinuses, and the size and shape of the braincase. CAT scans of T. rexes of different ages would be a great source of information. It would be interesting to look at changes in the brain, shown in the braincase, as the animal grows up. We can't yet do that with T. rex, since we have only adult animals.
Paleontologist Ralph Molnar has done the most de-
tailed studies of T. rex's skull. In fact, they are the only scientific studies since Osborn s early in the century. Each feature of the T. rex skull that Molnar outlines in such detail helps determine which aspects of T. rex distinguish it from other dinosaurs. For instance, Ralph notes fifteen skull features that distinguish T. rex from its closest relatives, the other tyrannosaurids. One is beady eyes—T. rex had a narrow eye socket with what San Francisco paleontologist Jacques Gauthier calls a narrow "keyhole" shape. The eye sockets of Alberto-saurus and other close relations of T. rex were oval or elliptical.
A key similarity between the skulls of birds and the late dinosaur carnivores, big and small, is the many holes in their skulls. These holes could have served any number of purposes, and in T. rex the most logical is the simple one of reducing the weight of the huge skull. Among the many holes in T. rex's head were lots of air spaces to help keep the skull light and make room for an elaborate network of muscles, blood vessels, and nerves.
There were other big holes in the T. rex skull that may put flesh over the muscle and you have a pretty lifelike t. rex head.
the nasal bone of t. rex is a plate studded with peculiar holes.
be sinuses like ours. One system of T. rex skull holes connected to the nose, another to the middle ear, and another led from the back of the skull to air sacs in the lungs. At least the tubes leading to the lungs would have been filled with air. There were also many holes for nerves in the snout of T. rex.
The nasal bone of T. rex is an especially fun bone. It's a long thin strip, like the bark off one side of a log. And it's rugose—full of bumps).
Besides its many holes, T. rex's braincase had room for a brain that was one of the largest in the history of life, bigger than a gorilla's or a cliimpanzee's, though not as big as ours.
We can think up all kinds of talents for T. rex from guesswork about its skull. Maybe it could hear and see with depth perception and feel with lips. But the holes in T. rex's skull don't prove anything about T. rex's senses.
What we can tell with more certainty from examining many of T. rex's skull features is how it ate. T. rex's wide cheeks and deepjaws supported bigger, more powerful jaw muscles than those of any other dinosaur we know. Atop its head, between the temples, was a crest of bone. Muscles along that ridge, and extra long muscles around the lower jaw, could give T. rex a powerful bite.
T. rex may have had an overbite, as skull expert Ralph Molnar contends. I don't know of any tyrannosaur jaws that shut all the way. So is the overbite we see natural, or an after-death change in the jaw alignment? According to Ralph, it's natural. With its powerful cheek muscles and overhanging top teeth, T. rex could have the nasal bone of t. rex is a plate studded with peculiar holes.
combined two different biting techniques: scissors and the nutcracker bites. Scissoring could shred meat, nutcracking could chomp into bone. But at least one study indicates T. rex's bite wasn't so efficient for clean bites and instant kills.
Reservations about T. rex's chewing style shouldn't take away too much from T. rex's impressive skull power. T. rex had neck muscles a football lineman would envy. Its skull bones had thick walls and were joined more tighdy than in other hunters. Bars across the eye and cheek sockets added more stability. Bigger teeth and fewer of them, relative to other tyrannosaurs, also contributed to T. rex's biting force.
T. rex's palate was light, supported by thin struts that would have allowed the skull to flatten out sideways with the force of a bite, and stretch a bit to help hundreds of pounds of dinosaur tissue slide down its gullet.
That doesn't mean T. rex's jaws could spread open like a snake's. The right
and left jawbones were joined at the chin like ours, but not fused together. Maybe they slid back and forth against each other slighdy, but we don't know that. The back of the lower jaw was firmly joined to the skull by a bone called the quadrate. The bottom of the quadrate pivoted on a hinge, allowing T. rex to spread itsjaws and open its maw wide. How wide we don't know. A third joint allowed flexing between the toothy front and the muscular back of each jawbone. Put it all together and you have a jaw anatomy with aspects of the strength of a crocodile and the lightness of a bird.
With this equipment, it's not likely T. rex was a finicky eater. But it could have chewed well before swallowing. Sure, it could have eaten Arnold Schwarzenegger in one bite or turned him into two hors d'oeuvres with a single snap. But there's no reason to swallow a lump that big when you could take it apart first with a spectacular set of teeth.
below: the quadrate is part of the hinge that works t. rex's jaw.
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