Stubby Dino Find Blurs Image of Long-Neck Lumberers

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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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