Do you wanna evolve a sauropod? The Triassic’s the perfect time. Down on four legs instead of two, I’m telling you, these were dinos in their prime.

*ahem*

Sorry. Had to get that out of my system.

But the Triassic really was the time when those famous long-necked dinosaurs – the sauropods – initially evolved. Scrappy fossil finds have allowed paleontologists to draw back the origin of these prodigious plant-eaters to about 208 million years ago, a scant 8 million years before a mass extinction cleared the ecological decks for them to proliferate and grow into the famous Jurassic giants we adore.

Sauropods didn’t pop out of thin air, of course. They evolved from ancestors called sauropodomorphs – gawky dinosaurs than tromped about on two legs, holding their impressive hand claws in the air, while already using their elongated necks to reach vegetation high and low. Once called “prosauropods”, these dinosaurs form a diverse, widespread, and successful group that set the stage of the origin of their heftier cousins. The challenge facing paleontologists has been to figure out how the awkward-looking sauropodomorphs evolved into classic sauropods of a shape more similar to the later Apatosaurus and Brachiosaurus.

A little-known dinosaur helps outline that change. In a comprehensive new paper in the Zoological Journal of the Linnean Society, paleontologists Blair McPhee, Adam Yates, Jonah Choiniere, and Fernando Abdala make the case that a dinosaur named Antetonitrus ingenipes has a peculiar set of traits that helps flesh out the transition between the sauropodomorphs and sauropods.

Found in the roughly 208 million year old strata of South Africa, the Antetonitrus fossils found so far represent an animal that was quite sizable for a Triassic dinosaur – in the neighborhood of 30 feet long. The timing, size, and anatomy of the dinosaur made it seem like a great candidate for one of the first true sauropods, hence the genus name meaning “before thunder.” McPhee and coauthors have now revised that view. Rather than being a dedicated four-on-the-floor sauropod, Antetonitrus was a transitional sauropodomorph that was probably able to switch between walking on two and four legs.

While there’s plenty of skeletal nitty-gritty that places Antetonitrus as almost, but not quite, a sauropod, the dinosaur’s limbs are among the most striking points of transformation. Antetonitrus didn’t have column-like limbs that held up the bulk of sauropods such as Diplodocus. Instead, the sauropodomorph had a long humerus (upper arm bone) and a shortened hand. The dinosaur still had a grasping hand for raking or pulling down vegetation, but one that could also support more of the dinosaur’s weight. This arrangement, McPhee and colleagues point out, signals a sauropodomorph switch to walking on all-fours more often.

View Images A reconstruction of Lessemsaurus, a close sauropodiform relative of Antetonitrus. Photo by Katharina Surhoff, distributed under a Attribution-NonCommercial-NonDerivative 3.0 (US) license.

Not that Antetonitrus was a definite ancestor of later Jurassic giants. In the new analysis, McPhee and colleagues found that Antetonitrus was one of several sauropodomorphs that come out in the evolutionary gray area between sauropodomorph and sauropod. They’ve named these dinosaurs sauropodiformes – an array of animals that exhibit features related to the origin of the first sauropods but that aren’t necessarily direct ancestors of Apatosaurus et al.

Such revisions are wonderful illustrations of evolution. As paleontologists discover more dinosaurs at critical points of transformation, they find tangled, diverse branches rather than straight-line pathways of progress. Each new find adds just that much more to the big picture of transcendent change until stark and distant waypoints become blurred into the rapid-fire sequence of dramatic evolutionary change.