Small ancestor of giant sauropods unearthed

Fossils of a newly discovered ancestor of gargantuan sauropod dinosaurs, a group that includes the tree-browsing brachiosaurs of Jurassic Park, show hints of several anatomical traits that allowed subsequent kin to become so huge.

The fossil record suggests that sauropods, some of the largest creatures ever to live on land (SN: 6/23/01, p. 397), evolved from a small bipedal dinosaur, says Matthew Bonnan, a vertebrate paleontologist at Western Illinois University in Macomb. Now, Bonnan and his colleagues describe a small sauropod ancestor with features foreshadowing those of more recent relatives. The work appears online November 10 in Proceedings of the Royal Society B.

The new species, dubbed Aardonyx celestae, lived about 195 million years ago in what is now South Africa. Paleontologists unearthed a large number of fossilized bones — most of them single, isolated bones or fragments — that probably represent at least two individuals, says Bonnan. Detailed analyses of the bones suggest that these sauropodomorphs, or sauropod-shaped creatures, were adolescents that hadn’t yet stopped growing. Nevertheless, he notes, at the time of its death, the largest of the dinosaurs unearthed by the team probably would have measured about 7 meters long from the tip of its snout to the end of its tail. 

Rib length in the new species, when compared with the size of the dino’s leg bones, suggests that Aardonyx was rather top-heavy, says Bonnan. Plus, the slightly interlocking shape of bones in the creature’s forelimb, especially those analogous to the two bones in the human forearm, indicates that those limbs could bear considerable weight. So Aardonyx, which had a moderately long neck, occasionally dropped to all fours when foraging, he suggests. True sauropods, many of which lived millions of years later, were so large that they rarely if ever left a four-legged stance.

The large number of small pits along the creature’s jawbone suggests that Aardonyx lacked a fleshy cheek that would have constrained how far the creature could open its mouth. Its wide gape, Bonnan and his colleagues suggest, enabled the dinosaur to grab large mouthfuls of foliage with each bite. Also, a strip of bone that ran along the base of the peglike teeth braced them against side-to-side forces produced when the dinosaur stripped foliage from trees. “These features are the first steps on the road to the specialized, bulk-browsing herbivory seen in sauropods,” says Adam Yates, a paleontologist at University of the Witwatersrand in Johannesburg and lead author of the new study. 

The size and shape of muscle attachments on this dinosaur’s femur indicates that the leg muscles were powerful but that the creature was, for the most part, rather slow-moving. “This dinosaur wasn’t running a lot,” says Bonnan. The overall shape of Aardonyx and particular aspects of its limbs, teeth and jawbone are “anatomical hints of things to come in later sauropods,” he notes.

“This is a very interesting animal,” says Paul Upchurch, a vertebrate paleontologist at University College London. The large number of bones recovered from various parts of the skeleton provides a good idea of what the creature might have looked like, he notes. 

Many of the features expected in a sauropod ancestor are present in Aardonyx, but they’re combined in an unusual mosaic, say Upchurch. For instance, although the creature had a wide gape that would have allowed it to grab large mouthfuls of vegetation, it had a narrow snout, unlike the broad U-shaped snout seen in true sauropods. So, he notes, “this animal is expected, but in an unexpected sort of way.”