Jonah Choiniere, a paleobiologist at the University of the Witwatersrand, Johannesburg, was boating around the world’s largest artificial lake in 2018 when he and his team found the first known fossils of Musankwa sanyatiensis — an ancestor of the sauropod dinosaurs from around 210 million years ago.
Part of a flood of similar discoveries from the past decade or so, the fossils hint at how the four-legged sauropods developed from the two-legged sauropodomorphs that preceded them.
“We knew right away this is not something we’d ever seen before,” Choiniere says. In fact, the fossils fill a gap in the early evolution of the sauropods — the long-legged, long-necked herbivores that would eventually become the world’s largest dinosaurs.
Read More: The Time of Giants: How Did Dinosaurs Get So Big?
Musankwa Discovery
Choiniere was searching for fossils on the shores of Lake Kariba when he and his team found the first signs of Musankwa sanyatiensi.
Situated in southern Africa, Lake Kariba is an artificial reservoir on the Zambezi River that straddles the borders of Zimbabwe and Zambia. The area contains many fossils dating back to the Triassic and Jurassic periods, but the majority of these fossils were submerged when the reservoir was filled in the 1950s and 1960s. In recent years, however, low water levels due to drought have provided a rare opportunity for fossil hunters, including Choiniere and his colleagues.
Renting a houseboat, which served as an on-site laboratory, and setting out in 2018, Choiniere and his team found a strange assemblage of fossils sticking out of the ground on Zimbabwe’s Spurwing Island, on Lake Kariba’s northeastern side.
Once they found the fossils, they quickly went to work extracting them with jackhammers and rock saws. (The image of paleontologists wiping delicately at fossils with toothbrushes isn’t exactly accurate in this area, Choiniere says.) They then covered the fossils with burlap and wrapped them in the kind of plaster used to mend broken arms to transport them back to their floating lab.
“It’s cool adventurous stuff,” Choiniere says. “There were crocs everywhere and hippos. We had to have an armed guard with us.”
Read More: How Sauropods Evolved to Their Enormous Size
What Makes Musankwa Unique?
The fossils, including fragments from the thigh, shin, and ankle of a single dinosaur, were the only remains of Musankwa sanyatiensis that Choiniere and his team found. But from this, they deduced that the dinosaur was a sauropodomorph, as described in a recently paper published in Acta Palaeontologica Polonica.
What tipped them off, Choiniere says, was the “funny morphology” of the bones, which revealed that the dinosaur was probably bipedal. In fact, while the sauropods that most people know, like Brontosaurus and Brachiosaurus, walked solidly on four legs, the sauropodomorphs that preceded them — Musankwa included — walked on only two.
“They looked a lot more like a theropod,” like Tyrannosaurus or Velociraptor, Choiniere says, hinting that the ancestors of the sauropods and the ancestors of the theropods split from the same ancient lineage. “The best evidence we have right now is that sauropods and theropods shared a common ancestor.”
Sauropodomorphs like Musankwa weren’t as big as sauropods, either. While the average Brontosaurus and Brachiosaurus were around 34 tons, sauropodomorphs were about the size of a dog or horse. Musankwa likely weighed about 850 pounds, Choiniere adds, or roughly the size of a small horse.
The newly described dinosaur was named partly after the houseboat, also dubbed “Musankwa,” that Choiniere and his team used to explore the lake. “We loved the boat so much that we felt like we had to name something after it one day,” Choiniere says. Meanwhile, “sanyatiensis” was derived from the Sanyati River, which flows into Lake Karabi just south of Spurwing Island.
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Musankwa and Sauropod Evolution
By dating the rock that surrounded the fossils, Choiniere and his team found that they were around 210 million years old. That put Musankwa in the Late Triassic, in the early period of sauropod development when sauropodomorphs were bipedeal but beginning to transition to using all four legs.
“Musankwa is one of the earliest sauropodomorphs that’s in that zone of experimentation,” Choiniere says.
Few dinosaurs have been discovered in Zimbabwe in the past few decades, but those that have help illustrate this transition. Described in 2022, the fossils of Mbiresaurus raathi may be the oldest dinosaur fossils from Africa. Dating to 230 million years ago, or around 20 million years before Musankwa, the fossils reveal that Mbiresaurus was another bipedal sauropodomorph related to Musankwa.
After these two species, sauropodomorphs and their evolutionary descendants became bigger and made more use of their front legs. Ledumahadi mafube, a 200-million-year-old species from Zimbabwe that Choiniere and colleagues described in 2018, may have been the first sauropod ancestor to walk on four legs. Weighing in at around 12 tons, its size meant that the species pretty much needed to be quadrupedal, Choiniere says.
Meanwhile, Vulcanodon karibaensis, another 200-million-year-old species from Zimbabwe, may have been the first sauropod ancestor to fully adapt to life on four legs. It’s likely one of the first true sauropods, Choiniere adds, unlike the sauropodomorphs that preceded it.
Still, it would take another 60 to 80 million years after Musankwa for true sauropod giants, including Brontosaurus and Brachiosaurus, to appear.
Read More: Why Are Herbivore Dinosaurs Important, and What Can They Teach Us About Evolution?
Lives of Musankwa
While the earliest sauropodomorphs were likely omnivorous, Choiniere and his colleagues believe that Musankwa had already developed a fully herbivorous diet. They likely fed on the plants around bodies of water, just like elephants and rhinoceros do in Zimbabwe today.
“It’s analogous to the current ecosystem,” Choiniere says.
Instead of modern crocodiles, however, the Late Triassic was dominated by other kinds of predators, like Metoposaurus — an amphibian the size of a small crocodile that could have eaten juvenile Musankwa. “They are like these giant devil frog-looking things,” Choiniere says. “They have a head that looks like a giant toilet seat.”
Also waiting in ambush was Phytosaurus, a “really monstrous animal” that looked a lot like a 30-foot-long crocodile, Choiniere adds. And there may have also been freshwater sharks and giant lungfish that preyed on wading or drinking herbivores.
“Two hundred million years ago, kind of the same story is being played out [in Zimbabwe], just the actors are different,” Choiniere says. “Musanka is evolving in ecosystems that aren’t dino-dominated.”
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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.
Source : Discovermagazine
