The long-necked dinosaurs, known as sauropods, were the largest animals that ever walked on land. But the discovery of a stunningly complete skeleton in northern Zimbabwe reminds us that the origins of these behemoths — some of which weigh more than 60 tons — are much milder.
The newly discovered fossils published today in the journal Nature are the oldest dinosaurs found in Africa, dating back to the Triassic period, about 230 million years ago. This animal is also one of the ancestors of the earliest known sauropods, including iconic giraffes such as brachiosaurus and thunderstorms. Although the animal was nearing maturity at the time of its death, experts estimate that its hips are less than two feet tall.
Chris Griffin, a paleontologist at Yale University who conducted the study while studying for his Ph.D. at Virginia Tech, said: "It has a small head, a pair of leaf-shaped teeth, and a neck length, "If you ask a child to draw a dinosaur and make it not eat meat, it's almost like a normal dinosaur."
Because of their age, Mbileiosaurus and other fossils found along with it reveal how dinosaurs first appeared and spread on paleo-Earth during the Triassic period (from 252 million to 205 million years ago). The Triassic, ended by two mass extinctions, was accompanied by major climate change and was a critical period for the transition of life on Earth. During this time, the ancestors of several key reptile groups began to separate from each other, producing flying reptiles such as dinosaurs, crocodiles, and pterosaurs.
Fossil discoveries over the decades suggest that dinosaurs first evolved about 245 million years ago at the southernmost tip of the ancient supercontinent Panmagneum. The surviving southern pangaea, divided into Africa, South America, and India, contain fossils that suggest that the earliest dinosaurs were nowhere near as large and diverse as their later ones. For most of the Triassic, they lived in the shadow of a group of ancient crocodile cousins known as false crocodiles, small in stature and sparse in stature, running around and barely making ends meet.
Although the Pancontinent is a single continent, its climate change is significant. The polar regions of this supercontinent are lush and habitable, but life is difficult in the tropics north and south of the equator: hot, arid and prone to fires.
"In Pan continent, you can go from the North Pole to the South Pole. We don't see huge physical barriers, like huge mountains," said study co-author Sterling Nesbitt, a paleontologist at Virginia Tech and a former doctoral supervisor at Griffin. "But there are also some climatic obstacles."
Recreating life's response to these obstacles is difficult, and only a few scattered fossil sites provide clues. Based on early Triassic specimens, mainly from Argentina and Brazil, paleontologists believe that at the beginning of the dinosaurs, wild animals were separated by deserts on both sides of the equator. This caused Griffin to think: the rocks of southern Africa are as old as the rocks of Argentina and Brazil, so will these deposits also have the world's oldest dinosaurs?
Griffin visited Zimbabwe in 2015 and met a team of local paleontologists and museum staff who were eager to find answers through collaboration. "We've been working in this area before, but resources and expertise are limited," said Darlington Munikova, co-author of the study and of the National Museum of Zimbabwe and The Monument. "Partnerships are very important to us."
During the visit, Griffin stumbled upon a 1992 paper co-authored by paleontologist Michael Raath describing the site of land in Dande Commune, a tribal area in northern Zimbabwe near the border between Zambia and Mozambique. To Griffin's delight, the skeleton of a Triassic triangular-headed reptile called Beaked Dragon was found in the site described by Russ. Based on fossils from South America, Griffin knew that rocks large enough to house beaked dragons were also suitable for capturing the oldest dinosaurs.
With funding from the National Geographic Society, Griffin returned in late July 2017 to trace Russ's footsteps with a team of local paleontologists. He recalls the cool breeze of a Zimbabwean winter morning, rustling leaves along the dry riverbed, and when the researchers began their work, the thornless little bees known as the mopane flies buzzed over their heads.
They found the treasure almost immediately. Griffin said: "When we first went to this area, we found a lot of fossils – fossils that fell from the rocks.
On the first day of the field trip, Zimbabwean paleontologist and co-author Hasel Tulavenga found a leg bone fragment that let the team know they were going in the right direction. The next day, just before lunch, Griffin looked down and saw a fossil femur sticking out of the ground. As he dug more bones out of the smooth mudstone, he realized that the fossil was a dinosaur — the dinosaur's hip was right next to the leg bone, suggesting there were more animals beneath.
Careful digging revealed the current Mbileisurus, which is in very good condition. Between the first bone and the partial remains of the second, the researchers lost only some vertebrae, hand bones, an ankle bone and some parts of the skull. Certain parts of the bones, such as a foot, are even connected as they were when they were alive. With the site's fossil potential now confirmed, Griffin and his colleagues returned again in 2019 for a three-week excavation with funding from the National Geographic Society.
Mbiresaurus stands out because it helped reveal the anatomy of the earliest sauropods, which Chapelle said, an expert on this type of dinosaur. For example, Chapelle's hips look particularly primitive compared to later dinosaurs.
The hips were important to dinosaurs. Since the 1880s, the largest branch in the dinosaur family tree has been defined as a hip shape, and one of the key skeletal features that define dinosaurs can be found in the acetabular molars. Mbiloon's hip bone had enough old-school features to bring it close to the bottom of the dinosaur family tree. "It's an incredible specimen," Chabel said.
The site also helped paleontologists learn more about what Mbileilong called home: a braided river system that flowed fast enough to cause rocks to tumble to form pebbles of the Arkose Formation, a special rock formation in which Mbileisurus was found.
The lush vegetation nourished many scaly herbivores and omnivores, including a beaked dragon with a triangular skull, an armored cousin of a crocodile, and a distant relative of mammals, Reptile, a close cousin of cannitosaurs. The team even found the bones of a predatory dinosaur known as herrerasaurid, which had hips nearly 7 feet high.
Animals in the Pebbly Arkose group are very similar to similar fossils found in Argentina and Brazil. This match strongly suggests that southern Pancontinent has the same fauna along similar latitudes, although not identical species. Griffin joked: "It's weird to dig up a bunch of bones in Africa and then two years later, to go to a museum and see everything you just dug up on display in a Brazilian museum."
Using Mbilon and other Triassic fossils, Griffin's team analyzed how dinosaurs spread out on ancient Earth. Their findings support the idea that dinosaurs appeared at the southernmost tip of the Pan continent, and the researchers estimate that the animals may have begun to expand north 230 million years ago.
The first to expand outward were theropod dinosaurs, a dinosaur group that later evolved bipedal predators like Tyrannosaurus rex as well as modern birds. About 220 million years ago, Mbilon's cousin, sauropods, followed close behind.
This time coincides well with the known Triassic climate. Between 235 million and 230 million years ago, the Earth experienced millions of years of rainfall and humidity increases, a period known as the Cal Year Rainfall Event. During this time, the tropical desert of the Pancontinent shrank and became more habitable, which may have given dinosaurs the opportunity to migrate to new areas.
"It's clear that the first generation of dinosaurs didn't start to take over the planet. They are limited, they live only in specific regions, only in specific environments, and only in the later stages of evolution will they expand to the whole world. Griffin said.
Research on pebble Alcos formation fossils has only just begun. So far, the researchers have only described in detail the bones of Mbilon, and they suspect that other reptiles found along with it also represent newly discovered species.
The researchers are excited about the upcoming discovery, especially now that they have shown that Africa has fossils of early dinosaurs.
"Africa has always been a place where we can go back to the oldest lineages like humans and hominids, but dinosaurs weren't part of that because we just didn't have representatives of that period," Nesbitt said. "It really brought Africa into the picture of the origins of dinosaurs."
Crucially, the team is taking steps to ensure that fossils found in the pebble Alcors formation remain in Zimbabwe. Many fossils are currently leased to Virginia Tech for preparation and scanning, but once each fossil is ready, they will be returned to the Zimbabwe Museum of Natural History and left there forever.