The study appears in the September 12 issue of the journal Proceedings of the National Academy of Sciences.
In their study Dodson and Wang also analyzed dinosaur diversity in the ten-million-year period that immediately preceded the dinosaurs' mass extinction, which occurred about 65 million years ago.
The researchers found no evidence to suggest dinosaur diversity declined near the end of the dinosaur age.
"It's a very surprising finding, on the one hand, that about 40 percent of all dinosaurs that we know occurred within about 12 million years of the end of dinosaur times," Dodson, a National Geographic Society research-grant recipient, said. (National Geographic News is part of the National Geographic Society.)
"But it appears that in the last six million years [of their reign], there was no strong trend to a decrease in diversity," he added.
The scientists say their findings support the theory that an asteroid impact with Earth wiped out the dinosaurs. (Related: "Asteroid Rained Glass Over Entire Earth, Scientists Say" [April 2005].)
A less spectacular demise of the giant lizards would likely have been marked by steady extinctions, causing a gradual decline in the number of dinosaur species.
But Dodson cautions that, without more detailed fossil data, it's difficult to say precisely how the makeup of dinosaur species or genera changed in the final six million years of dinosaur history.
"Our study suggests that a very high diversity was maintained into the last stage of the Maastrichtian," he said, referring to the last stage of the Cretaceous period, about 71 to 65 million years ago.
"But we can't say for sure whether the extinction occurred over six million years or between a Tuesday night and a Wednesday morning."
Gregory Erickson, an evolutionary biologist and paleontologist at Florida State University in Tallahassee, says the study is insightful.
"I think that our field needs a lot more statistical rigor, and this is a very nice example of neat things you can find when you throw some stats at dinosaur paleontology," he said.
Spotlighting the fact that dinosaur diversity did not decline prior to their mass extinction was "a really neat find," as were the study's other factoids, he adds.
For example, 59 percent of all known dinosaur genera are identified by just a single set of remains, which is often incomplete.
Dale A. Russell, a paleobiologist at North Carolina State University in Raleigh, also says the study is important, in part because it tries to answer what we currently don't know about dinosaurs.
Using a different approach, based on the sizes of ancient continents, Russell has estimated dinosaur diversity at 3,400 species.
But unlike Dodson and Wang's estimate, Russell's applies to all dinosaursnot just those likely to be found as fossils.
That's why Russell sees his and Dodson's estimates as complementary, helping to define the low and high end of true dinosaur diversity.
As Russell noted, "This is a case of violent agreement."
Editor's Note: Peter Dodson has received several grants for his research from the National Geographic Society's Committee for Research and Exploration.
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