Every dinosaur enthusiast knows about sauropods, if not in name then at least in appearance. Members of this large group of quadrupedal herbivores are easily recognizable among museum fossils and artistic reconstructions thanks to their hefty size, stout elephant-like legs, massive straight tails, and – most famously – long necks. Brontosaurus, Brachiosaurus, and Diplodocus are three genera you may be familiar with thanks to pop culture, and the recently described Patagotitan mayorum currently takes the title of the largest land-dwelling animal in history.
Palaeontologists have determined that the majestic sauropods arrived on the scene during the late Triassic (when Pangea was beginning to split into the Laurasia and Gondwana supercontinents), after evolving from bipedal ancestors, and thrived in diverse ecosystems across all areas except what is now Antarctica.
Unfortunately, due to the inherent limitations of studying long-dead organisms through incomplete fossilized remains, little is known about the sauropods’ close relatives and possible forebearers, the prosauropods. This diverse group dates back to the mid-Triassic and walked on two legs with or without assistance from their front limbs, which were about one-half the length.
But now, a partial fossil unearthed in southern Africa sheds fresh light on the mysterious origins of the sauropods by introducing a charmingly awkward-looking new species of sauropodomorph. World, meet Ledumahadi mafube – named after the Sesotho words for “a giant thunderclap at dawn”.
According to lead researcher Jonah Choiniere and his colleagues, the specimen likely represents a fully grown adult, about 14 years old, that weighed 12,000 kilograms (26,455 pounds). The small assortment of vertebrae, leg, and toe bones are roughly 200 to 190 million years old based on their location within the upper Elliot Formation, a chunk of mudstone that formed in the early Jurassic. Blair Mcphee, the first author of a Current Biology paper describing the new species discovered the skeleton fragments literally sticking out of the side of a hill within the rock mass, which is now located at the border of South Africa and Lesotho.
"[This] shows us that even as far back as 200 million years ago, these animals had already become the largest vertebrates to ever walk the Earth," explained Choiniere.
An analysis of L. mafube’s limb bones revealed that the massive dinosaur mainly walked on four legs but still had flexed leg bones rather than the column-like ones of later sauropods. The team conclude that the dino still had maintained the ability to stand on its rear legs as desired, supporting the recent theory that prosauropods “experimented” with different styles of locomotion before some lineages went full quadruped – a switch that allowed them to grow significantly more massive than their bipedal kin.
"The evolution of sauropods isn't quite as straightforward as we once thought," Choiniere added. "In fact, it appears that sauropodomorphs evolved four-legged postures at least twice before they gained the ability to walk with upright limbs, which undoubtedly helped make them so successful in an evolutionary sense."
Choiniere and Mcphee’s group note that L. mafube’s appearance way back in the early Jurassic is also remarkable because it implies that some ecosystems recovered immediately after or simply persisted through the end-Triassic extinction event, which wiped out 76 percent of all marine and terrestrial life. After all, there has to be a whole lot of vegetation for an herbivore to reach 12 tonnes (13 tons).