Stubby Dino Find Blurs Image of Long-Neck Lumberers

John Roach
National Geographic News

June 1, 2005

Paleontologists today unveiled a sauropod dinosaur with a stubby neck. The discovery smudges the common picture of sauropods as unspecialized, lumbering dinosaurs that used very long necks to munch away at any greenery in sight, including treetops.

Sauropods were the largest animals ever to walk on land. They are characterized by their small heads, elephant-like limbs, and long tails and necks. Some sauropods' necks were four times as long as their backs.

The new dinosaur, Brachytrachelopan mesai, measured less than 33 feet (10 meters) long, making it unusually small for a sauropod. Its neck was shorter than its backbone.

"It is by far the shortest neck reported in any sauropod," Oliver Rauhut, a paleontologist with the Bavarian State Collection for Paleontology and Geology in Munich, Germany, said in an e-mail to National Geographic News.

Rauhut and German and Argentinean colleagues report the discovery in tomorrow's issue of the journal Nature.

The dinosaur was discovered in Argentina's Patagonia region by Daniel Mesa, a local farmer. Mesa was looking for stray sheep when he came across the dinosaur fossil. Brachytrachelopan mesai translates to "Mesa's short-neck shepherd god." Rauhut and colleagues date the new dinosaur to the late Jurassic period, about 150 million years ago.

Short Neck

The Brachytrachelopan fossils include most of the dinosaur's neck and back vertebrae, all the ribs, portions of the pelvis, and fragments of the hind legs. The rest of the fossils were destroyed by erosion in the last thousand years or so, Rauhut said.

Brachytrachelopan's neck vertebrae are remarkably short for a sauropod: Each individual vertebra is as long as or shorter than it is tall. In other sauropods, the neck vertebrae are at least twice as long as they are tall, according to Rauhut.

The short neck, Rauhut said, was likely an adaptation to eating an abundant or nutritious low-growing plant—a niche that may have limited Brachytrachelopan's size.

In addition, an analysis of the vertebrae fossils indicate Brachytrachelopan was unable to lifts its neck above the horizontal, meaning it was unable to crane its neck up into the treetops.

Adam Yates is a paleontologist at the University of the Witwatersrand in Johannesburg, South Africa. He commented in an e-mail that the discovery of the short-neck sauropod is exciting and confirms an unusual sauropod body plan that earlier discoveries had suggested.

Brachytrachelopan belongs to the family of sauropods known as dicraeosaurids. Prior to the new discovery, dicraeosaurids with relatively short necks had been found and showed adaptations to eating plants close to the ground, Yates said.

"So Brachytrachelopan fits in nicely as an extension of this adaptive plan and is not totally unexpected," he said.

Yates agreed that the short-neck adaptation was likely to fill the niche of browsing on low-growing plants, suggesting that sauropods were not locked into the big, lumbering, long-neck body plan.

Rapid Radiation?

According to Rauhut, the discovery of Brachytrachelopan in modern-day South America indicates that the dicraeosaurids evolved and quickly spread out after the supercontinent of Pangea separated into Northern and Southern Hemisphere continents in the Middle Jurassic, about 200 million years ago.

Dicraeosaurids are unknown in the fossil record prior to the late Jurassic, Rauhut said. When they appear, they are specialized as niche feeders and found dispersed throughout lands that were once part the Southern Hemisphere landmass known as Gondwanaland.

"This is best explained if their major radiation took place after the separation of the continents of the Southern and Northern Hemispheres, which happened in the latest middle Jurassic," Rauhut said.

Yates, the University of the Witwatersrand paleontologist, expressed caution over this interpretation. He said the fossil record of sauropods from the middle Jurassic to early late Jurassic is insufficient to determine if dicraeosaurids existed in the middle Jurassic.

A sister group to the dicraeosaurids, the diplodocids, were found in both hemispheres in the late Jurassic, suggesting that "dicraeosaurids were also around before the [continental] split became uncrossable," Yates said.

According to Yates, the lack of any dicraeosaurid fossils yet discovered in the northern continents may indicate that the shorter-neck sauropods were outcompeted by iguanodontids (large, two-footed herbivores)—or "there may be a northern dicraeosaurid waiting to be discovered."

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