His findings also add evidence that dinosaur populations were stable, and not on the decline, in the time shortly before their extinction 65 million years ago.

Dodson proposes that 1,850 genera (the plural of genus, an organizational group comprised of one or more separate species) will eventually be discovered, in total. Since the dinosaur research began in earnest in the 19th century, only 527 genera have so far been found, although that number is currently changing at the rate of 10 to 20 per year.

"It's a safe bet that a child born today could expect a very fruitful career in dinosaur paleontology," said Dodson, professor of anatomy in Penn's School of Veterinary Medicine and professor in Penn's Department of Earth and Environmental Sciences. "Unfortunately, there is a finite limit to what can be discovered, so our estimates show that the child's grandchildren won't be so fortunate as new discoveries will likely decline sharply in the early 22nd century."

Dodson and co-author Steve Wang, a statistician at Swarthmore College, estimate that 71% of all dinosaur genera that could be found are still awaiting discovery. The researchers predict that 75% of discoverable genera will be found within 60-100 years and 90% within the next 140 years.

In 1990, Dodson first census paper coincided with his publication of The Dinosauria," the first book to comprehensively depict known dinosaurs. In preparing for the second edition, published last year, Dodson revisited his projections to account for the sudden increase of dinosaur fossil discoveries during the 1990s.

"The 1990s saw an 85% increase in the number of new fossil discoveries," Dodson said. "The dinosaur field used to be the pursuit of white Anglo-Saxons, but, with recent explosion in of dinosaur paleontology in places like China, Mongolia and South America, that is clearly no longer the case. "

Historically, Dodson contends, dinosaur discovery was largely in the hands of British, Canadian and American researchers, with few exceptions in other countries. In recent decades, however, the discovery of new fossil beds, especially in China and Mongolia, has resulted in a greater diversity among dinosaur researchers.

Ultimately, however, there are only so many dinosaurs that can be found. Fossilization itself is a rare event, and there are very few places on Earth, as a whole, where the rocks are of the right age to contain fossils dating back to the dinosaur era. At some point, paleontologists will find all the fossils that could possibly be found.

Dodson and Wang's analysis also offers evidence that dinosaur populations were stable before the extinction event that ended their reign of global dominance. Dodson, however, warns that the picture of the fossil record at the time of extinction is not resolved enough to say definitively.

"We have enough information to say for certain that, within six million years of the meteorite's arrival, dinosaur populations were stable," Dodson said. "But we don't know for certain if there was a decline within that six-million-year slice of time before the extinction event."

Their estimates for total dinosaur diversity take into account the number of dinosaurs already found, the rate of discovery and potential richness of the fossil locations that can be reasonably explored.

They hold that it is still an open question as to whether their estimates of discoverable genera mirror the actual diversity of dinosaur genera that walked the earth. Their findings, combined with previous studies suggest that nearly half of all dinosaur genera that existed did not leave behind fossils that could be found.

"I would never suggest that this prediction, however statistically sound, is the final word on dinosaur diversity," Dodson said. "My intention is to fuel the discussion using the facts at hand, and this is the best estimate we can make with the data available."

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