During the late Triassic period, when the terrestrial world was a single sprawling land mass called Pangaea, a dog-size plant-eating dinosaur perished near a river in the southern part of the continent. When the river flooded, its body was buried by sediment, with some bones still articulated as in life.
About 230 million years later, paleontologist Chris Griffin, then a doctoral student at the Virginia Polytechnic Institute and State University, spotted a thigh bone sticking out of a hill in the Cabora Bassa River Basin in what is now Zimbabwe. “I’ve got a dinosaur!” he called to his team.
“As soon as I dug that out, I knew that I was holding Africa’s oldest dinosaur,” says Griffin, now a postdoc at Yale University. “I had to sit down and breathe for a minute, because I thought, ‘There could be a lot more [bones] in there.’”
In the weeks that followed, Griffin and paleontologists Darlington Munyikwa and Michel Zondo of the Natural History Museum of Zimbabwe in Bulawayo unearthed a nearly complete skeleton. It turned out to be a new species of early dinosaur: Mbiresaurus raathi, which they describe today in Nature.
Though small by dinosaur standards at 1.8 meters long, the find has outsize implications for the early spread of dinosaurs, says Stephen Brusatte, a vertebrate paleontologist at the University of Edinburgh who was not involved in the study. “We’ve known next to nothing about the earliest dinosaurs in Africa,” Brusatte says. “It is one of the most important recent dinosaur discoveries anywhere in the world.”
Until now, the earliest known dinosaurs, also dating to about 230 million years ago, were found in Argentina and Brazil, with a few partial specimens from India. When the continents were gathered together to form Pangaea, those sites all lay at about 50° south, explains Diego Pol, a paleontologist at the Egidio Feruglio Paleontology Museum in Argentina who was not part of the team. Earth was warmer at the time, lacking icecaps, and climate models suggest that latitude on Pangaea had a wet, temperate climate with hot summers and cool, rainy winters. Researchers have suspected the first dinosaurs needed this type of climate, and that this limited their spread across the supercontinent. But to confirm that idea, they needed dinosaur fossils from other parts of the same climate belt.
Griffin’s team began its hunt with a geological map, tracing a Pangaean-era latitude line of 50°. They zeroed in on a shallow drainage in northern Zimbabwe where Munyikwa and Zondo knew other fossils had been found. “If dinosaurs are following this climate, then we should be able to find some of the oldest dinosaurs right here in southern Africa,” Griffin says they reasoned. “And we did.”
The M. raathi find, which was almost complete save for portions of the skull and forelimbs, “was just very amazing and exciting,” Munyikwa says. No bigger than a collie, M. raathi is named after Mbire, as the region was called during the 16th century Shona Empire, and a pioneering researcher who found fossils nearby. The dinosaur had a long tail, a smallish head, and small, triangular teeth, suggesting it favored plants.
The team also found fragments of bones from a large carnivorous dinosaur called a herrerasaurid, the first discovered in Africa. And it unearthed an array of other animal fossils, too: cynodonts, which are mammal relatives; armored crocodilian relatives called aetosaurs; and archaic reptiles called rhynchosaurs. Paleontologists have found similar creatures along the same climate band in South America and India.
Taken together, the fossils are the strongest evidence yet that the earliest dinosaurs and their relatives were constrained to a temperate climate belt bordered by arid ones, Pol says. “The assemblage was very similar to that of South America,” he says. Dinosaurs were restricted to their semihumid oasis for a few million years, until the arid regions to the north and south began to become wetter.
The rare find provides a welcome boost to Zimbabwe’s science that Munyikwa hopes will help attract more research funding. “This new species [shows] we have very important deposits,” he says. The fossils are now on display at the Natural History Museum of Zimbabwe and are a point of pride for the community and nation, he says.
The study notes that other specimens likely await discovery across the same Pangaean climate belt, offering a road map of sorts for other paleontologists on the hunt for early dinosaurs, says Kristi Curry Rogers, a vertebrate paleontologist at Macalester College. “Now it’s time for all the rest of us working in dinosaur paleobiology to get to work and discover some more early dinosaurs.”
Source: Science Mag