Tyrannosaurus Teeth

T. rex had bigger choppers, more varied in size and shape, than those of other tyrannosaurids. There might have been fifty teeth in its mouth at any one time. Up front, it had incisor-like teeth, as all tyrannosaurids do, four on each premaxilla, the bone in the front of the upper jaw. Among meat-eating dinosaurs, only the little Troodons and tyrannosaurids had these. The shape of these teeth was fairly wide and flat, designed for nipping, according to tooth expert Phil Currie. They would have been more precise in raking meat from bone, Phil says, without biting right through the bone as T. rex's bigger teeth could.

T. rex had twelve or thirteen additional teeth on each side of the upper jaw. These cheek teeth were enormous, a foot long including the root, and as big around as a child's fist. They were some of the biggest teeth of any dinosaur (Spinosaurus, a North African dinosaur known only from bits and pieces, had bigger teeth and may have been a bigger carnivore). You'd expect T. rex to have big teeth, since it was one of the biggest carnivores ever. But as Jim Farlow, a dinosaur paleontologist at the University of Indiana, Purdue, found, even in proportion to T. rex's size advantage over other predators, its teeth were oversized.

With thick, strong teeth the size of bananas, T. rex could well have penetrated the hide (except when armored), the flesh, and the bones of another dinosaur with a single bite. We know T. rex broke off some of those teeth on or in the bones of other dinosaurs.

Fortunately for T. rex., like all dinosaurs, it was always making new teeth. Whether or not they broke off in feeding, old teeth were constandy being replaced by new teeth that pushed their way out of every socket. Within the jaw, two or three teeth for each tooth position were always developing, getting ready to emerge.

Greg Erickson, a graduate student of mine, has made a special study of our T. rex's teeth. Greg looked at what are called "von Ebner lines," incremental markers of growth in the dentin at the edge of pulp cavity of the tooth. Greg compares these lines to "reverse tree rings. They grow from outside to inside." You can see these thin lines under a microscope if you cut a thin section of a tooth and look at it at 100 x magnification. Greg figures that an adult T. rex would have laid down one brown and one black line each day.

We can't know for sure how fast T. rex made those tooth rings. But Greg found a clever way to test for an answer. He went to a crocodile farm in Louisiana and fed tetracycline to the crocodiles. The antibiotic has the neat effect of staining their day's tooth growth chartreuse green. Greg also injected Puff and Bubba, two small crocodiles we keep at the museum. None of his subjects was happy about a needle in the belly or the base of the tail, and Greg wasn't too eager to give it to them, but he had help from some experienced crocodile-handlers.

Days later, after the farm crocodiles were on their way to becoming shoes and handbags, Greg got their three-

this clawlike fossil is a tooth from "stan" the t. rex.
Rex Single Tooth
a t. rex tooth, actual size.

inch-long teeth. He sectioned them and found • that the dye was confined to a single set of growth rings. It's a reasonable guess that dinosaur teeth grew new rings daily as well. My fellow vertebrate paleontologists in North America thought well enough of Greg's study that they made him a co-winner of the 1991 student prize for the best presentation at our annual meeting.

Counting up the number of lines on T. rex teeth, Greg worked out that T. rex would have shed its teeth every two or three years. Crocodile teeth don't last much more than a year. And Greg figures a duckbill tooth would grow in within half a year.

T. rex had such huge teeth that you'd figure it would need a while to grow them. Most dinosaurs were always producing backup teeth. But T. rex's replacement teeth didn't start growing until its working teeth were 70 percent grown. Crocodiles start building replacements when their working teeth are less than half-grown.

T. rex had a never-ending supply of teeth. But if its teeth broke off often, T. rex would have had an uneven bite. Then it may have been hard for it to make a smooth, slicing cut. For a replacement tooth to fully emerge might take eight months to more than a year. That's how it is with crocodiles, which also replace their teeth continually throughout life.

T. rex's teeth were serrated, so they could saw as well as puncture and rip meat. The serrations were extremely wide, with razor-sharp edges, "beautifully adapted to saw bone and meat," according to Phil.

The back of the T. rex tooth had the biggest serrations, so perhaps T. rex bit down hard to break skin or bone, then cut backward through its prey. Serrations might have been a way to get the cutting action of a thin blade without sacrificing the strength of a stout tooth. Jim Farlow, an expert on dinosaur teeth (and footprints), thinks that's true. But Jim thinks the small serrations might have helped to cut by binding tissue more than by slicingit. Coupled with T. rex's enormous biting strength, the hlnrllne would have resulted in more teariine and a better grip on the victim.

Jim is impressed that T. rex's upper teeth combined features that made them useful for cutting into and grabbing onto prey with other features that would make them withstand the stress. Each tooth had a good point. It wasn't blunt at the dp like the teeth of animals that crunch shells or grind vegetation. It was knife tipped to pierce flesh, maybe even break through bone. Overall, the tooth was shaped somewhat like a blade, though not so flat. And the base was fairly broad, providing resistance to lateral forces that might have made a slimmer tooth break off.

Jim has studied more than five hundred meat-eating-dinosaur teeth and calculated their bending strength. He finds tyrannosaur teeth superior in strength to those of any other carnivorous dinosaur. Maybe they were thickjust because of their size, but that thickness would have helped them bite into, even break through, solid bone. It's hard to say exactly what T. rex teeth could do, as no modern carnivores I know of, except sharks and Komodo dragons, have serrated teeth. Modern mammalian hunters use their big canines to hold prey rather than to cut meat off their victims.

Jim and others have found that the teeth of large meat-

above and right: t. rex's teeth were evenly notched with good-sized serrations.

above and right: t. rex's teeth were evenly notched with good-sized serrations.

greg erickson found these lines of growth, probably laid down daily. in t. rex's teeth.

eating dinosaurs were far less specialized than those of mammals—no molars or canines as you and I have. But not all T. rex teeth were as big and thick as bananas. Those in the back of the jaw were thinner and shorter, perhaps for fine slicing-and-dicing of bones and flesh, as paleontologist Bob Bakker suggests.

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Responses

  • Tesfay Dawit
    What are trex teeth made of?
    8 years ago
  • Clizia Conti
    Why did tyrannosaurus rex have big teeth?
    8 years ago
  • frankie
    How wide were a trex's teeth?
    8 years ago
  • negisti
    Why were the teeth of carnivorous dinosaurs serrated?
    8 years ago
  • Welde
    Do t rex teeth have serrations?
    8 years ago
  • keegan
    Why is there dinasour teeth in smooth round river rocks?
    8 years ago
  • allan
    Why does the tyrannosaurus rex teeth give it an advantage?
    3 months ago

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