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.
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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