A Third of Dinosaur Species Never Existed?

Brian Handwerkfor National Geographic News
October 9, 2009

ON TV Dinosaurs Decoded, airs Sunday, October 11 at 9 p.m. ET/PT on the National Geographic Channel. Preview Dinosaurs Decoded >>

Many dinosaurs may be facing a new kind of extinction—a controversial theory suggests as many as a third of all known dinosaur species never existed in the first place.

That's because young dinosaurs didn't look like Mini-Me versions of their parents, according to new analyses by paleontologists Mark Goodwin, University of California, Berkeley, and Jack Horner, of Montana State University.

Instead, like birds and some other living animals, the juveniles went through dramatic physical changes during adulthood.

This means many fossils of young dinosaurs, including T. rex relatives, have been misidentified as unique species, the researchers argue.

How T. Rex Became a Terror

The lean and graceful Nanotyrannus is one strong example. Thought to be a smaller relative of T. rex, the supposed species is now considered by many experts to be based on a misidentified fossil of a juvenile T. rex.

The purported Nanotyrannus fossils have the look of a teenage T. rex, Horner said in the new documentary. That's because T. rex's skull changed dramatically as it grew, he said.

The skull morphed from an elongated shape to the more familiar, short snout and jaw, which could take in large quantities of food.

But the smoking gun, Horner said, was the discovery of a dinosaur between the size of an adult T. rex and Nanotyrannus.

Nanotyrannus—actually a young T. rex in Horner's view—had 17 lower-jaw teeth, and an adult T. rex had 12.

The midsize dinosaur had 14 lower-jaw teeth—suggesting that it was also a young T. rex, and that tyrannosaurs gradually traded their smaller, blade-like teeth for fewer bone-crushing grinders in adulthood.

Triceratops Transformation

The paleontologists also amassed a large collection of Triceratops fossils, which had died in various stages of life, from eastern Montana's Hell Creek formation from the late Cretaceous epoch (145.5 to 65.5 million years ago).

The dinosaur skulls, which ranged from dinner plate-size to human-size, came from a range of animals.

When the paleontologists studied the skulls, they found that the youngest animals' tiny, straight horns changed as they got older: Juveniles' horns actually curved backward, whereas adult horns pointed forward.

The animal's distinctive neck frill also changed—the triangular spiked bones surrounding the frill in juveniles became flattened and lengthened into a bony fan-like shield.

"In this ten-year project we were able to collect a very good growth series that no one had ever seen before, and see this transformation that occurs," Goodwin said.

"We could document the extreme changes that occur with growth, [like] the direction that the horns are pointing."

http://news.nationalgeographic.com/news/2009/10/091009-dinosaur-species-never-existed.html