| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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horizons The Better to Eat You WithA century of debate has surrounded the question of whether Tyrannosaurus earned its keep through hunting or scavenging. Those who recall the lawyer in "Jurassic Park" who made an ill-timed trip to the outhouse know how the movie sided in this debate, despite the fact that its scientific advisor, paleontologist Jack Horner, favors the scavenging view. Most paleontologists, though, would probably agree with the comic-strip character Calvin, who delivered this report to his class: "I say tyrannosaurs were predators, because it would be so bogus if they just ate things that were already dead. The end." One would think that a creature as dentally well endowed as Tyrannosaurus rex--sporting the largest teeth of any dinosaur--makes a pretty strong case for being a predator of reckoning, even if it does have puny forearms. Yet a minority of paleontologists maintained that T. rex teeth and jaws couldn't stand up to the wear and tear of being an aggressive predator. The case to the contrary just got literally and figuratively stronger. In the first experimental attempt to measure the jaw- clamping capacity of any dinosaur, a Berkeley biologist and a team of Stanford biomechanical engineers have determined that T. rex wielded a bite force exceeding that of any living animal. Its teeth could exert a crushing force of more than 3,000 pounds. According to biologist and lead author Gregory Erickson, a former student of Horner's, "This is like the weight of a pickup truck behind each tooth." The new evidence comes from a 70-million-year-old fossilized Triceratops pelvis found five years ago in Montana's fossil-rich Hell Creek Formation by amateur dinosaur hunter Kenneth Olson. The four-and-a-half-foot-long bone contains 58 definite bite marks and another two dozen possible dental impressions from teeth that could only have belonged to a tyrannosaur. Many of the marks display distinctive furrows as though the biter had struggled to deflesh its prey. Repeated biting removed a sizable chunk from the front of the largest pelvic bone. For his study that appeared in the August 22 issue of Nature, Erickson, a graduate student in the Department of Integrative Biology at the University of California at Berkeley, collaborated with Stanford University engineer Dennis Carter, an expert on the mechanics of bone, graduate students Samuel Van Kirk and Jinntung Su, and two other Stanford colleagues to calculate how much force the T. rex teeth endured while biting Triceratops. They installed a cast of a real T. rex tooth made of aluminum and bronze (which mimics the rigidity of enamel) into a hydraulic mechanical loading machine--a substitute for jaws that looks and acts a bit like a guillotine. A piston-powered bar holding the tooth made punctures in a cow pelvis that mirrored those in the Triceratops bone, and the team measured the amount of force needed to recreate those wounds. Erickson chose cow bone for the surrogate victim because it has a microscopic structure similar to bone from Triceratops, also a large herbivore. One of the two tooth casts got dented when the scientists underestimated their machine's power and the tooth penetrated the bone completely, striking the steel table beneath it. Real-life rexs, Erickson says, would have broken teeth occasionally after violently impacting bone, and they regularly replaced each tooth every few years. To leave a half-inch deep mark in a bone, a T. rex canine would have absorbed 1,440 pounds of force. By being closer to the jaw joint, the rear teeth were even more powerful, and the team estimated a force there of 3,011 pounds. (For the record, the most force that the rear teeth of a human can generate is 175 pounds--suitable for cracking Corn Nuts, but little use on pelvic blades.) The results suggest that T. rex's dental arsenal is consistent with the idea that they hunted live prey. A scavenger wouldn't need as much bite to deflesh an animal that couldn't escape, and the strong teeth of tyrannosaurs could presumably handle the torquing and compressing that would be part of a day's work capturing and subduing gargantuan prey. The teeth of T. rex also closely resemble those of two renowned modern-day hunters: American alligators, the dinosaur's closest living relatives, and great white sharks. Like the alligator, T. rex has stout, rounded canine-like teeth embedded and cemented in sockets which can withstand large forces. Skulls of both species often display bite marks from teeth used in intraspecific sparring. Alligators can exert a bite force of just under 3,000 pounds when rapidly snapping their jaws, but T. rex bites exceed this force with minimal effort. Like the white shark, T. rex teeth exceed those of all their relatives in size and bear serrated edges that can cut through bone. Serrations run along the front and back of tyrannosaur teeth, as on a steak knife, which suggests to Erickson that the dinosaur used "puncture and pull" biting to inflict big cuts on the head, neck, or spine and then perhaps let their prey bleed to death. Komodo dragons, though they have relatively weak teeth, employ a similar biting and slicing strategy on prey. But, says Erickson, "There is no great analogy [among living animals] for T. rex. If there were we'd have a scary world." Erickson is quick to point out that his study does not settle the issue of how T. rex got its meals. He hopes that others will make similar studies of more dinosaurs, such as the fleet-footed predator Deinonychus, or begin searching for additional bite-marked bones. As for T. rex, finding fossil skulls or vertebrae from prey dinosaur species that bear evidence of lethal bone-crunching bites--rather than the more shallow feeding bites analyzed in this study--could bolster the view that this carnivore killed for food, thus vindicating Steven Spielberg's depiction. "If the T. rex in 'Jurassic Park' had waited around for everything to die," says Erickson, "it would have been a pretty boring movie."
Interplanetary EIRs In the meantime, NASA has been taking steps to stem an invasion from Mars--of the microscopic variety--or at least the public perception of one. The concept is called "planetary protection," which boils down to preventing biological contamination of planets during space missions. No transfer of Earth life that might be misinterpreted as native to Mars, and bringing nothing back alive without taking care to keep it contained. To do that, three things need to happen, says Donald DeVincenzi, NASA's former planetary protection officer. The craft must be clean--if not sterile--when it leaves Earth; it must depart Mars without any hitchhikers inside or out; and a portion of the returned samples must pass a biological quarantine to test for potential hazards to Earthlings. "In the absence of firm data about life on Mars, you have to be prudent," says DeVincenzi, who considers these to be minimal precautions. The specifics, and the cost, of each step have yet to be worked out, but the quarantine will likely take advantage of new molecular techniques and cell cultures to avoid using live plants and animals. Because of rocket size constraints, the first samples from Mars will be about the size of a coffee cup--about a pound or so (unlike the 835 pounds of Moon rocks). And for the foreseeable future, no humans will join the payload. Still, the price tag could be in the millions, and scientists have questioned whether it's all worthwhile. Some say the vast distance between Mars and Earth ensures that life on one planet would have diverged genetically to the point that it could not infect anything on the other. Others assume just the opposite, viewing planetary protection as too little, too late. Earth life may have already been transported elsewhere by meteorites, or Martians may have already arrived here aboard incoming rocks. Says geophysicist Norman Sleep of Stanford University, "We basically don't have to quarantine because we've already been contaminated." We have received organic matter from Mars, but no proof exists that meteorites have already cross-contaminated the planets with living creatures. Meteorites have been randomly circulating in a hostile environment for millennia, whereas NASA plans to keep samples in Mars-like conditions and ship them straight back. "Now we're talking about bringing back a sample 'fresh' and optimized to preserve the very things that we're afraid of," says DeVincenzi. "The concern is really over a replicating organism," says ecologist Margaret Race, principal investigator with the SETI Institute. Having studied alien species in San Francisco Bay, she now ponders the legal, political, and social issues surrounding the introduction of hypothetical aliens from Mars. This convergence of science and public concern reveals how much things have changed since the sixties and planetary protection for Apollo. Then the top priority was to get men safely to the Moon and back. No one protested when NASA released its lunar quarantine regulations the same day that it launched Apollo 11. Mars will be another story. Now we have the Environmental Protection Agency, the National Environmental Policy Act, and a host of international treaties which have something to say about how a sample return mission should proceed. Environmental impact assessments and extensive public hearings will occur, and the threat of lawsuits may prove much greater than that of biological contamination. NASA has been taking all of this to heart, if only to avoid the embarrassing prospect of going all the way to Mars to procure rocks--at a cost of perhaps $500 million--only to discover organisms that its rocket brought with it from Earth. Blake Edgar is Associate Editor of California Wild. |
Winter 1997
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