There’s more than one way to supersize a dinosaur. Scientists studying the ancient bones of sauropod relatives that walked the Earth more than 200 million years ago have found that they grew to multiton masses 30 million years before the appearance of their cousins, the titanosaurs.
The findings described in the journal Nature Ecology & Evolution could fill in a more complex portrait of the ecology of early dinosaurs and the evolution of sauropods and their relatives.
“This discovery helps shed light on how the different traits that we think led to the extreme gigantism in sauropods first evolved,” said Kristina Curry Rogers, a vertebrate paleontologist at Macalester College in St. Paul, Minn., who was not involved in the study. “The discovery is important because it helps connect some dots between earlier bipedal dinosaurs and the giants that ruled the Mesozoic later on.”
Think of a sauropod, and a species like Brachiosaurus may come to mind: long-necked, long-tailed, straight-legged with giant bodies and small heads. They arose from the sauropodomorphs, whose members included “the largest animals recorded in the history of life,” the study authors wrote.
Brachiosaurus, which plodded across the terrain roughly 150 million years ago, could outweigh several elephants; some scientists estimate that the largest of the sauropod species could hit 70 tons or more. But the ancestors of these titans were very different: They were small, nimble and walked on two legs instead of four.
In order to grow into these massive eusauropoda, or “true sauropods,” these animals had to undergo some major modifications to their body plan, scientists say. Among the changes, they had to develop straighter, tree-trunk legs to support their weight; they also had to grow quickly and consistently. But there isn’t a whole lot of evidence to show what that transition looked like.
“It was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda,” the study authors wrote. (A tonne is a metric ton, or roughly 1.1 U.S. tons.) “Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change.”
Two strange-looking sauropodomorphs offer a surprising twist to the story of gigantism in these iconic dinosaurs. Cecilia Apaldetti of the National University of San Juan in Argentina and her colleagues examined a new species, Ingentia prima, as well as a previously known species called Lessemsaurus sauropoides.
These so-called lessemsaurids lived roughly 237 million to 201 million years ago in present-day Argentina, roughly 47 million years before Brachiosaurus did, and yet they had already managed to grow to gigantic sizes — that is, roughly 8 to 11 tons, about the size of a large elephant. They also had long necks and tails, though not as long as the true sauropods.
These animals didn’t just grow to massive sizes roughly 30 million years before true sauropods did — they also managed this feat with unexpected adaptations. Their legs were more bent rather than trunk-like, in spite of this great mass. Like their giant sauropod cousins, they had air sacs in their vertebrae, though not quite as many, Curry Rogers said; these may have helped lighten the animals’ mass. And instead of growing continuously, they grew in short, intense bursts until reaching their final size.
“That’s one freaky-looking animal,” said Matthew Lamanna, a dinosaur paleontologist at the Carnegie Museum of Natural History in Pittsburgh who was not involved in the study. “That’s what struck me, more than anything else.”
The idea that such large dinosaurs developed so soon after the emergence of dinosaurs as a group roughly 245 million years ago was something of a surprise, he added.
“That’s pretty exciting,” he said. “It shows that dinosaurs, even in their early evolutionary history, were more diverse than we give them credit for.”
Still, the new analysis offers a piece of the puzzle rather than a full picture of the sauropodomorph family tree, Curry Rogers said.
“I'd argue that we still have a lot to learn,” she said. “We actually don't yet have a great understanding of how this transition occurred, because the rocks that are the critical age are pretty rare, and dinosaurs in those rocks are even more hard to come by.”
The discovery does show that there are a lot more clues to be found in specimens like these that could help scientists fill in this stage of dinosaur evolution, she added, “especially in terms of how all these organisms are related to each other, and in terms the pattern of how their different characteristics evolved.”