How did the dinosaurs spread across the world? A newly discovered fossil from Zimbabwe provides a new piece of the puzzle to answer this question. Here researchers identified the oldest known dinosaur in Africa, which lived around 230 million years ago. The fossil bears a striking resemblance to finds from South America and India - regions that were on the same latitude as Zimbabwe on what was then the supercontinent Pangea. The discovery supports the thesis that the distribution of early dinosaurs was determined by climate zones.
When the dinosaurs began to spread around 235 million years ago, all of the earth's landmasses were united into a single supercontinent: Pangea. There were no continental borders or high mountains at this time, so there were hardly any geographical barriers that could hinder their spread across Pangea. Nevertheless, previous fossil finds show that the distribution area of the early dinosaurs was initially limited. Very old fossils from 230 million years ago were only known from South America and India. Scientists have therefore hypothesized that climate belts consisting of temperate and overly dry zones controlled the spread of the dinosaurs.
Complete dinosaur skeleton
Guided by this hypothesis, a team led by Christopher Griffin from the Virginia Polytechnic Institute and State University in Blacksburg went in search of evidence. "If the dinosaurs initially spread under the influence of the climate zones of the time, then a similar aggregation as in South America and India - including the earliest dinosaurs - should be found in the Carnic deposits in southern central Africa," the researchers describe their reasoning. So they started excavations in Zimbabwe - and actually found what they were looking for. In the Mbire district of northern Zimbabwe, a previously unknown collection of fossils has been unearthed that dates to the Carnic Age around 230 million years ago.
In addition to many fossils of smaller animals such as early relatives of mammals and primeval reptiles, the researchers came across a surprise: they found an almost complete skeleton of a previously unknown dinosaur. "We never expected to find such a complete and well-preserved dinosaur skeleton," says Griffin. “When I found the femur, I knew immediately that it belonged to a dinosaur and I realized that I was holding the oldest dinosaur ever found in Africa. As I dug further and found the left hip bone right next to the left femur, I knew that a large part of the skeleton was probably still assembled in living position.”
Geographic gap filled
The animal was about two meters long, had a long neck and tail, and probably stood on two legs. Its head was relatively small and it had small, jagged teeth, so it was probably herbivorous or omnivorous. The researchers gave the newly discovered species the name Mbiresaurus raathi - named after the region where it was found and in honor of the paleontologist Michael Raath, who first discovered fossils in northern Zimbabwe.
"The discovery of Mbiresaurus raathi fills a critical geographic gap in the fossil record of the oldest dinosaurs and shows how well hypothesis-driven fieldwork can test predictions about the past," Griffin said. The age of the Mbiresaurus matches that of the oldest specimens from other parts of the world. Griffin explains: "The oldest known dinosaurs - from the Carnic stage of the late Triassic, about 230 million years ago - are extremely rare and have only been found in a few places worldwide, primarily in northern Argentina, southern Brazil and India." These regions were on the supercontinent Pangea as well as Mbire at about the 50th southern degree of latitude.
Climate belt specific spread
At that time, the climate on Pangea was characterized by a high concentration of carbon dioxide in the atmosphere and separate climatic belts with partly very humid and partly very dry conditions. In the region of the 50th degree of latitude, a temperate climate probably prevailed, while further north, towards the equator, there were hostile desert landscapes. "The distribution of the first dinosaurs is related to these climate barriers, and the spread of dinosaurs to the rest of the supercontinent was delayed until these barriers were relaxed," the authors write. "This suggests that the initial composition of the terrestrial faunas that survive to this day was determined by climatic influences."
Source: Christopher Griffin (Virginia Tech, Blacksburg) et al., Nature, doi: 10.1038/s41586-022-05133-x