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Utah Dinos May Have Been Killed by Drought

John Roach
for National Geographic News
December 20, 2002
 
Drought—not the perils of a muddy bog—may explain why millions
of years ago hundreds of large, lumbering meat-eating cousins to
Tyrannosaurus rex perished in what is now a dusty, rocky desert
in southern Utah.

The site, named the Cleveland-Lloyd Dinosaur Quarry, is one of the world's most prolific dinosaur fossil sources. It has yielded more than 70 partial skeletons, 12,000 individual bones, and single dinosaur eggs.

Over two-thirds of the fossilized remains belong to one species, Allosaurus fragilis, a meat-eating dinosaur that preyed on herbivores such as Stegosaurus.

Paleontologists consider this to be an unusual concentration of predators and have long sought to explain how so many Allosaurus wound up dead in the same place. Predators are thought to have made up only 10 percent of the total dinosaur population.



Dinosaur Trap

The prevailing theory suggests that the predatory dinosaurs died as a result being trapped in a swampy mud bog at the edge of a lake that once covered. The dinosaurs were drawn into the bog as they chased after yelping prey already stuck, explained Terry Gates, a paleontologist at the University of Utah in Salt Lake.

"You get a few herbivores stuck and you get a whole bunch of predators stuck," he said.

Gates is a graduate student in paleontology studying the process of dinosaur death and bone accumulation in the quarry. He presented a paper on an alternative theory to the demise of dinosaurs found in the Cleveland-Lloyd Dinosaur Quarry at the annual meeting of the Society of Vertebrate Paleontology in Norman, Oklahoma, in October.

Gates' analysis of the bones and his unsuccessful search for a modern analog to the predator trap scenario lead him to believe that the dinosaurs died as a result of drought, which causes animals to congregate around watering holes and, eventually, die.

The bones, he says, do not show the expected signs of a dinosaur being stuck, helpless, in the mud: vertical articulation, or orientation. Scientists say that bones that remain together after burial are articulated. Gates found no articulated bones during his excavations.

"If you image the animals walking into a pond with a soft muddy bottom, the animals will sink straight down and their locomotor limbs will be preserved vertically," he said. "At least that is the prediction."

The prediction has been supported by ancient flightless birds called moa that were preserved in this manner at the Pyramid Valley Moa Swamp in New Zealand, said Gates. However, there are no modern analogs of a scavenger dying as a result of going after an animal that is stuck in the mud.

Another strike against the so-called predator trap theory is the high proportion of Allosaurus juvenile remains recovered from the quarry.

"Juveniles have been shown to be very susceptible to death during environmental stress," said Gates. "At Cleveland-Lloyd Dinosaur Quarry about 82 percent of the population is juvenile, compelling evidence that something went on to concentrate a bunch of juveniles together."

Gates looked at the sediments in the quarry and found several signs indicative of drought, such as preserved mud cracks and rip-up clasts, which are curled up sections of mud cracks that have been worn down and flattened by water flows.

The formation of mud cracks and clasts signal that periodic drought conditions occurred in the Cleveland-Lloyd Dinosaur Quarry, said Gates. This string of evidence has led him to theorize that the dinosaurs died a prolonged and agonizing death due to drought.

In the scenario Gates envisions, numerous herbivores accumulate around a water hole as the dry season approaches. As they eat up all the available vegetation, a few of them die, letting off a stench that attracts carnivorous Allosaurus.

"You get a whole bunch of Allosaurus to accumulate around the water hole and they begin to pick off the herbivores left and right," he said. The Allosaurus would eventually eat all the herbivores, leaving a huge concentration of like-minded predators that would scare off any would-be intruder.

Previous studies indicate that similar carnivorous dinosaurs may have practiced cannibalism out of necessity. Thus Allosaurus may have eaten their own kind, allowing them to stick remain in the area instead of moving on in search of other food sources. As a result, they died a slow death of dehydration, bacterial diseases, and sun stroke, said Gates.

Theory Questioned

Sue Ann Bilbey, a paleontologist with the Utah Field House of Natural History in Vernal and a recognized expert in the sediments of the Cleveland-Lloyd Dinosaur Quarry, believes the predator-trap scenario is still the most reasonable explanation for the dinosaur demise in the quarry.

"I think it was a predator trap associated with a spring, with wet sticky volcanic mud that captured several animals that came to drink," she said. "Then the smell attracted more carnivores."

Bilbey, who heard Gates' presentation in Oklahoma, does allow that one area of the quarry out of the 18 that she studied had a high abundance of gypsum, a mineral which can be an indication of drought. However, gypsum can also be deposited by ground water.

Bilbey's analyses show that the quarry is composed primarily of mudstone made from volcanic ash, which leads her to believe that a spring in the area gave rise to a water table that eventually covered the quarry and formed a lake.

The dinosaurs died as the spring turned the volcanic ash into a muddy, sticky bog, she said. Her theory is consistent with the predator-trap scenario.

"Sedimentologically, the lethal spring still makes the most sense to me, although a drought could have restricted the trap area," she said.

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Dinosaur Tracks Shed Light on Sauropod Evolution
New Find: Pterosaur Had Strange Crest, Fishing Style
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New Study Supports Idea That Primates, Dinosaurs Coexisted
Mass Extinction That Led to Age of Dinosaurs Was Swift, Study Shows
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Scientist's Finds Spur New Thinking on Dino Evolution
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