Fossils where they don’t belong? We didn’t look hard enough

Anthony Ham

In 1996, paleontologists made a startling discovery in northwestern Madagascar. Among dinosaur bones and sandy sediment there emerged a 167-million-year-old tiny jaw fragment with three teeth. It belonged to Ambondro mahabo, a species that was 25 million years older than any mammal of its kind ever found. And it wasn’t supposed to be there. At the time, what was known of the fossil record pointed overwhelmingly to the conclusion that modern mammals’ forerunners arose in the Northern Hemisphere.

“The prevailing wisdom suggested that we shouldn’t find something like that from the time interval we were sampling, nor from the Southern Hemisphere,” said John Flynn, the paleontologist who led that dig and is now the Frick curator of fossil mammals at the American Museum of Natural History in New York. It takes more than a single fossil to overturn an entire theory of evolution. But a review of existing fossil holdings published last year in the journal Alcheringa sought to turn decades of paleontological wisdom on its head. After an exhaustive study of skulls, jaws and teeth, a team of Australian paleontologists presented their conclusion that modern mammals originated in the Southern Hemisphere.

Their findings have set off an impassioned debate, revealing a North-South divide. Defenders of the Northern Hemisphere hypothesis highlight weaknesses they see in the latest findings. In response, supporters of the Southern Hemisphere origin, like Dr. Flynn, say it is time for paleontologists to grapple with the argument that their field’s understanding of natural history may be slanted toward the half of the world where scientists have carried out the most digs.

“In the Southern Hemisphere, these are just places that haven’t been explored by paleontologists,” Dr. Flynn said. “There has been a long-term, overall bias in the system toward a Northern Hemisphere perspective, partly because that’s where the scientists came from. And it leads you to interpret a lot of things in the light of that bias.”

At the heart of the dispute are the primitive early forerunners to modern placental and marsupial mammals. Known as tribosphenic mammals, they were “little shrew-like creatures that would have weighed about as much as a mouse,” said Tim Flannery, an independent Australian paleontologist and one of the authors of the recent review paper. Although sophisticated for their time, they were a very basic version of mammals as we know them today. Dr. Flannery compared them to the Ford Model T “of modern or placental mammals.”

Dr. Flannery and company point to geographic arguments in favor of the idea that early mammals could have arisen in the Southern Hemisphere. The larger the land mass, the greater the likelihood of major evolutionary activity occurring. When mammals were emerging, Gondwana encompassed Africa, India, Australia and South America and was far larger than Laurasia in the Northern Hemisphere. “A lot was happening there,” Dr. Flannery said, noting the emergence of songbirds and raptors on Gondwana during the age of dinosaurs. “We’ve just added this extra twist that we think the mammals were also evolving here.”

Ham is a journalist with NYT©2023