It isn’t a brachiosaur — the type of giant, plant-eating dinosaur that had a titanic body and a peculiarly tiny head.

Nor is it quite like a diplodicid, the sauropod known best for its fantastically long, whip-like tail.

And its leg and foot bones are all wrong for it to have been an earth-shaking titanosaur.

Instead, the dinosaur that’s been identified from just a few fossil bones from the Black Hills of South Dakota seems to represent a huge new species to science.

What’s more, it lived some 139 million years ago, but it closely resembles a type of dinosaur that disappeared from North America’s fossil record at least 5 million years earlier. [Read about another recent South Dakota dinosaur discovery: “Giant Crested Bird-Like Dinosaur Discovered in the Dakotas“]

The new species, whose remains are scant enough that it hasn’t yet been given a name, fills an important gap in the history of American dinosaurs, and suggests that its ancestors survived a key transition in America’s climatic past.

“The importance lies in the fact that no sauropods, and few dinosaurs in general, are known from the period between about 148 million to 125 million years ago,” said Dr. Michael D’Emic, a paleontologist with Stony Brook University who reported the find.

“These bones are from about 139 million years ago, partially filling that gap.”

That gap in the fossil record occurs just after a change took place in North America’s climate, about 145 million years ago, when a brief span of cooler, wetter conditions marked the end of the Jurassic period and the beginning of the Cretaceous.

The newfound dinosaur is the earliest, oldest known sauropod from the Cretaceous period in North America, D’Emic noted.

Plant-eaters from this period were thought almost exclusively to be a type known as titanosauriforms, which includes the brachiosaurids, the diplodicids, and the titanosaurs.

But the foot and leg bones found in the South Dakota specimen show that it doesn’t belong to this group, D’Emic said.

Instead, it closely resembles Camarasaurus, a genus of giant herbivore that roamed North America in great numbers during the Jurassic, until it disappeared about 145 million years ago.

While Camarasaurus may not have survived the transition from the Jurassic to the Cretaceous, these bones suggest that some of its non-titanosauriform relatives did.

“[These bones] look like those of Jurassic sauropods, from around 148 million years ago, rather than of later Cretaceous ones, from about 125 million to 95 million years ago, which suggests that some of the older sauropods didn’t go extinct until after 139 million years ago,” he said.

A clearer understanding of which dinosaurs survived into the Cretaceous will take much more data, D’Emic said — a challenge, considering that the fossils he studied were originally excavated in the 1960s and “languished, miscatalogued” in Chicago’s Field Museum until he discovered them. [See another dinosaur discovery made nearby: “T. Rex Tooth Found in Dinosaur’s Tail Proves Tyrannosaurus Was a Predator, Study Says“]

With no other material to work with, it will take more exploring and excavating in western South Dakota to round out this gap in the fossil record, he added.

“It does show that more digging is needed in that part of the U.S.,” D’Emic said.

D’Emic and his colleague J.R. Foster report the find in the journal Historical Biology.

D’Emic, M., & Foster, J. (2014). The oldest Cretaceous North American sauropod dinosaur Historical Biology, 1-9 DOI: 10.1080/08912963.2014.976817