Life would have been particularly hot and dry if dinosaurs really did emerge near the equator
Mark Witton/The Trustees of the Natural History Museum, London
Dinosaurs may have first evolved close to the equator, not in the far south of the southern hemisphere as previously thought. A modelling study suggests they originated in a region that covers what is now the Amazon rainforest, Congo basin and Sahara desert.
“When you consider the gaps in the fossil record and the evolutionary tree of dinosaurs, it could very likely be a centre point for where dinosaurs originated,” says Joel Heath at University College London.
Dinosaurs evolved some time during the Triassic Period, which ran from 252 to 201 million years ago, but Heath says there is “pretty huge” uncertainty about where and when. The oldest known fossils of these animals are about 230 million years old, but they are distinct enough to suggest that dinosaurs had already existed for a few million years. “There must have been a lot going on in terms of dinosaur evolution, but we just don’t have the fossils,” he says.
At this time, Earth looked very different. All the continents were joined into a single supercontinent called Pangaea, which was shaped like a C with its middle straddling the equator. South America and Africa were in the southern hemisphere segment of this, where they fitted together like jigsaw pieces. The earliest known dinosaurs are from southern parts of those two continents, in places such as modern Argentina and Zimbabwe – so this was thought to be their point of origin.
To learn more, Heath and his colleagues built computer models to work backwards in time from the oldest known dinosaurs to the origin of the group. They created several dozen versions, to take into account uncertainties such as gaps in the fossil record, possible geographic barriers and ongoing doubts over how the earliest dinosaurs were related to each other.
Most of these simulations concluded that dinosaurs first appeared near the equator, with just a minority supporting the southerly origin.
Palaeontologists have tended to assume that dinosaurs couldn’t have originated near the equator, says Heath, partly because there are no early dinosaur fossils from that region. What’s more, it was a challenging place to live. “It was very, very dry and very hot,” he says. “The dinosaurs were thought to have not been able to survive in those sorts of conditions.”
However, most of the models say otherwise. “It’s suggesting things that we didn’t actually think were possible in the past,” says Heath.
Instead, the lack of early dinosaur fossils from near the equator may have a more prosaic explanation. Palaeontologists have tended to dig in North America and Europe, and more latterly China. “There are lots of areas in the globe that are quite neglected,” says Heath. He adds that geologists haven’t found many rocks of the right age in the regions relevant to the study’s findings that they can excavate. “They might not be exposed in a way that we can easily study them.”
However, a piece of evidence in support of Heath’s idea has recently emerged. On 8 January, researchers led by David Lovelace at the University of Wisconsin-Madison reported that they had found the oldest known dinosaur from the northern part of Pangaea. They discovered a species that is new to science called Ahvaytum bahndooiveche, a sauropodomorph related to long-necked dinosaurs like Diplodocus that evolved later. The team found it in the rocks of the Popo Agie Formation in Wyoming, dated to 230 million years ago.
If dinosaurs were already in the north and south of Pangaea that long ago, the equatorial middle can’t have been closed off to them, says Heath. “They have to have been crossing that region.”
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