The long-necked dinosaurs known as sauropods were the biggest animals to ever walk on land. But a stunningly complete skeleton found in northern Zimbabwe provides a reminder that these giants—some of which reached masses of more than 60 ton—had somewhat more modest beginnings.
The newfound fossil, unveiled today in the journal Nature, is the oldest definitive dinosaur discovered in Africa, dated to some 230 million years ago, during the Triassic period. The animal is also one of the earliest known ancestors to sauropods, the group that includes iconic long-necked giants such as Brachiosaurus and Brontosaurus. Even though the animal was nearing maturity when it died, experts estimate it would have been less than two feet tall at the hip.
With a small head, a set of leaf-shaped teeth, and a neck of modest length, “it’s almost like a generic dinosaur, if you had a kid draw a dinosaur and make it not a meat eater,” says Chris Griffin, a paleontologist at Yale University who conducted the research while a Ph.D. candidate at Virginia Tech.
Its scientific name, Mbiresaurus raathi, honors Mbire, a historical empire of Zimbabwe’s Shona people that once included the site where the fossil was unearthed. The dinosaur is also named for South African paleontologist Michael Raath, whose work in the area in the 1990s helped lead to its discovery.
Due to their age, Mbiresaurus and other fossils found alongside it shed light on how dinosaurs first arose and spread across ancient Earth during the Triassic, which stretched from 252 million to 205 million years ago. Bookended by two mass extinctions and filled with major climatic shifts, the Triassic was a critical period of transition for life on Earth. During this time, the ancestral lines of several key reptile groups first started to split from one another, giving rise to dinosaurs as well as crocodilians and the flying reptiles known as pterosaurs.
“This is when the magic happens,” says paleontologist Kimi Chapelle, a postdoctoral researcher at South Africa’s University of the Witwatersrand who wasn’t involved in the new study.
A new site of old dinosaurs
Decades’ worth of fossil finds suggest that dinosaurs first evolved about 245 million years ago in the southernmost regions of the ancient supercontinent known as Pangaea. Now split between Africa, South America, and India, the surviving landmasses from southern Pangaea contain fossils that reveal the first dinosaurs were nowhere near as big or diverse as they would later become. Smallish, scant, and scampering, they eked out a living for much of the Triassic in the shadows of a group of ancient crocodile cousins known as the pseudosuchians.
Though Pangaea was a single landmass, its climate varied widely. The poleward reaches of the supercontinent were lush and hospitable, but the tropical belts north and south of the Equator were hard living: hot, arid, and prone to fires.
“In Pangaea you can walk from the north pole to the south pole. There are no giant physical barriers we can see, like giant mountain chains,” says study co-author Sterling Nesbitt, a paleontologist at Virginia Tech and Griffin’s former Ph.D. adviser. “But there are some kinds of climatic barriers.”
Reconstructing how life responded to these barriers is tricky with clues from only a few scattered fossil sites. Based primarily on early Triassic specimens from Argentina and Brazil, paleontologists think that during the dinosaurs’ start, the wildlife was separated by deserts that flanked the Equator. That got Griffin thinking: Southern Africa has rocks just as old as those in Argentina and Brazil, so would these deposits also hold the world’s oldest dinosaurs?
Griffin visited Zimbabwe in 2015 and met a team of local paleontologists and museum staff who were keen to work together to find out. “We had been working in the area before, but with limited resources and also expertise,” says study co-author Darlington Munyikwa of the National Museums and Monuments of Zimbabwe. “It was very important for us to be a partnership.”
During this visit, Griffin stumbled across a 1992 paper co-authored by paleontologist Michael Raath that described sites in the Dande communal lands, a tribal tract in northern Zimbabwe near the Zambian and Mozambican borders. To Griffin’s delight, the sites Raath described bore the bones of a triangle-headed Triassic reptile called a rhynchosaur. Based on South American fossils, Griffin knew that rocks old enough to bear rhynchosaurs were also of the right age to capture the oldest dinosaurs.
Thanks in part to a grant from the National Geographic Society, Griffin returned in late July 2017 and retraced Raath’s footsteps with a team of local paleontologists. He recalls the cool breeze of a Zimbabwean winter’s morning rustling the foliage along dry riverbeds and stingless gnat-size bees known as mopane flies buzzing around their heads as the researchers got to work.
Almost immediately, they hit pay dirt. “This one area, the first day we were in the field, it was just an overwhelming amount of fossils—fossils just falling out of the rock,” Griffin says.
On the first field day, Zimbabwean paleontologist and study co-author Hazel Turavinga found a leg bone fragment that let the team know they were on the right track. Before lunchtime the next day, Griffin looked down and saw a fossilized femur sticking out of the ground. As he dug out more of the bone from the silky mudstone, he realized that the fossil was a dinosaur—and that the dinosaur’s hip was right next to the leg bone, suggesting much more of the animal lay below.
“At that time I had to sit down and breathe a little bit,” Griffin says.
From humble origins
Careful excavation revealed what is now Mbiresaurus in remarkable condition. Between the first skeleton and partial remains from a second individual, researchers are missing only a few vertebrae, hand bones, an ankle bone, and some parts of the skull. Parts of the skeleton, such as one of the feet, were even articulated as they would have been in life. With the site’s fossil potential now well-established, Griffin and his colleagues returned in 2019 for another three weeks of digging, again funded by the National Geographic Society.
Mbiresaurus stands out because it helps reveal the anatomy of the earliest sauropodomorphs, says Chapelle, an expert on this group of dinosaurs. For instance, the hip strikes Chapelle as especially primitive-looking when compared to those of later dinosaurs.
Hips matter quite a bit for dinosaurs. Since the 1880s the biggest fork in the dinosaur family tree has been defined by hip shape, and one of the key skeletal traits that defines dinosaurs can be found in the hip socket. The hip bone of Mbiresaurus has enough old-school features to place the animal near the base of the dinosaur family tree. “It’s an incredible specimen,” Chapelle says.
The site has also helped paleontologists learn more about the ecosystem that Mbiresaurus called home: a braided river system that flowed quickly enough to tumble rocks and form the pebbles of the Pebbly Arkose Formation, the specific rock layers in which Mbiresaurus was found.
Lush plants in the area nourished a diverse lot of scaly herbivores and omnivores, including the triangle-skulled rhynchosaurs, armored cousins of alligators called aetosaurs, and distant reptilian cousins of mammals called cynodonts. The team even found some bones of a predatory dinosaur called a herrerasaurid that would have stood nearly seven feet tall at the hip.
The animals of the Pebbly Arkose Formation closely resemble similarly aged fossils found in Argentina and Brazil. The match strongly suggests that areas in southern Pangaea along similar latitudes had the same animal groups, though not the same species. “It’s weird to dig up a bunch of bones in Africa and then, two years later, go to a museum and see everything you just dug up on display in a Brazilian museum,” Griffin jokes.
Using Mbiresaurus and other Triassic fossils, Griffin’s team analyzed how dinosaurs may have spread out across ancient Earth. Their findings support the idea that dinosaurs arose in the southernmost reaches of Pangaea, and the researchers estimate that the animals likely started expanding north after 230 million years ago.
The first to spread out were the theropods, the dinosaur group that later gave rise to two-legged predators like Tyrannosaurus as well as modern birds. Around roughly 220 million years ago, the sauropodomorph cousins of Mbiresaurus followed.
This timing lines up well with what’s known about the Triassic climate. Between 235 million and 230 million years ago, Earth experienced a couple million years of increased rain and humidity, a period called the Carnian pluvial event. During this time, Pangaea’s tropical deserts shrank and became more hospitable, which could have given dinosaurs an opportunity to migrate to new areas.
“It seems very clear that the first dinosaurs are not starting out taking over the planet. They’re constrained, they’re only in a certain area, they’re only in a certain environment, and only later in their evolution do they expand worldwide,” Griffin says.
Building a new legacy
Work on the Pebbly Arkose Formation’s fossils has just begun. So far the researchers have described only the bones of Mbiresaurus in detail, and they suspect some of the other reptiles found alongside it also represent newfound species.
The researchers are excited by the discoveries to come—especially now that they have shown that Africa contains fossils of the dinosaurs’ earliest days.
“Africa has been a place where we’ve gone back for the oldest lineages of humans and hominids and things like that, but dinosaurs haven’t been part of that because we just didn’t have that time period represented,” Nesbitt says. “It really brings Africa into the picture of dinosaur origins.”
Crucially, the team is taking steps to ensure the fossils found in the Pebbly Arkose Formation remain in Zimbabwe. Many are currently on loan to Virginia Tech for preparation and scanning, but once each fossil is ready, it will be returned to the Natural History Museum of Zimbabwe, where it will stay in perpetuity.
“It’s our heritage, that’s the bottom line.” Munyikwa says. “It should be, yes, accessible to everyone, but it needs to be deposited within our institution.”
