The sauropod dinosaurs were the largest animals to have ever walked on the earth. They were so incredibly huge, in fact, that they had to move about on four legs—but since the earliest dinosaurs were bipedal, paleontologists have long known that the ancestors of giants like Brachiosaurus and Apatosaurus actually trotted about on two legs. A dinosaur just described in the Proceedings of the Royal Society B sat close to this major transition in sauropod evolution.
Recovered from Early Jurassic (about 183 - 200 million year old) rock in South Africa, Aardonyx celestae was an approximately 20-foot-long dinosaur that combined elements that are both strange and familiar. It had a small head, a long neck, a large body, and a long tail, but it still had relatively short forelimbs compared to its hind legs. While it could occasionally walk on four legs, its limbs indicate that it primarily walked around on two , and an evolutionary analysis that was part of the new study placed it relatively close to the earliest sauropod dinosaurs (thus fitting Aardonyx within the larger category of dinosaurs called sauropodomorphs).
Aardonyx was not actually ancestral to the larger, four-feet-on-the-floor sauropods—it lived during a time when such dinosaurs already existed—but it preserves some of the transitional features that we would expect to find in the actual ancestor. (Contrary to a headline published by the BBC, it is not a "missing link" and the entire concept of "missing links" is a hopelessly out-of-date idea that was discarded by scientists long ago. The phrase goes back to a time when life was viewed as proceeding from "lower" forms to "higher" ones in a straight line, and scientists have rightly rejected it in favor of a branching bush of evolutionary diversity.)
While not a direct ancestor of dinosaurs like Diplodocus, this new dinosaur will help us better understand how sauropod dinosaurs evolved. If you would like to know more about it check out the blog of the lead author of the new description, Adam Yates, where he summarizes the important details about Aardonyx. It is good to see working paleontologists take a more active role in communicating their discoveries to the public, and I hope that other dinosaur specialists will follow the example made by Yates and others.