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Friday, July 23, 2010

Far fewer dinosaur species than previously suspected

Since their earliest discovery, dinosaurs have been divided into different species based on various morphological differences. Evidence is now mounting that many of these differences represent different life cycle stages within one species, rather than different species. That idea was first presented by Jack Horner of Montana State University’s Museum of the Rockies.

Horner previously established this concept with respect to the dinosaur Pachycephalosaurus. Now, he and his student John Scannella have extended that idea to Triceratops and Torosaurus. Previously, these were thought to be distinct species. However, Horner and Scannella have shown that Torosaurus is in fact the most mature stage of the Triceratops life cycle.

The paleontologists spent the last decade collecting and examining hundreds of fossils at the Hell Creek Formation in eastern Montana. Among the finds were Triceratops skulls of various sizes. The few Torosaurus skulls, in contrast, were all of adults. Upon closer inspection, the Triceratops fossils showed signs of growth and change consistent with juvenile life stages. Other so-called Ceratopsid species may also turn out to be life cycle stages of the same species.

Horner believes there may actually have been one third fewer dinosaur species than previously thought, especially toward the end of their reign. This may have been one reason the dinosaurs did not survive the impact of the meteor that ended the Cretaceous period; as a group they were not genetically diverse enough to withstand the decimation.