for National Geographic News
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."
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