The discovery of the jawbone suggests the presence of modern mammals that originated in the southern hemisphere

It took more than two decades for one pandemic Paleontologists To piece together the fossilized remains of the earliest mammalian ancestors and find that their evolution that gave rise to modern humans may have begun in the southern hemisphere — not in the north as scientists have long thought.

Analysis of a small group of tiny fossilized jaw bones bearing distinct posterior teeth upends our understanding of when and where modern mammals evolved cephalopods, according to the team of researchers who produced it.

Paleontologist Thomas Rich of Museums Victoria is a longtime fossil hunter and co-author of the new study.

He was part of the team that, in 1997, after 23 years of searching, announced that they had found on an Australian beach a mammalian jawbone with strange teeth, the likes of which had only been seen in Europe and North America. The jawbone was from a small shrew-like creature that dates back to the Cretaceous period when dinosaurs also roamed.

As the years went by, more Mesozoic mammalian jawbones were discovered: in Madagascar, Argentina, India, and again, recently, in Australia.

Each of these specimens, measuring an inch or less, has distinct posterior teeth. According to the latest analysis revisiting them, the oldest fossil predates those from the Northern Hemisphere by about 50 million years.

“This amazing series of discoveries has completely changed our old theory of mammalian evolution. In fact, it has turned our ideas about mammalian evolution upside down,” Rich says.

The small teeth involved are called tribosphenic molars, which interlock from above and below to cut, crush, puncture, and grind plant food and insect prey.

Small tribosphenic molars on the jaw of an early mammal of the Cretaceous period, found in Australia. (James Alcock / Australian Museum)

During the pandemic, esteemed paleontologists Tim Flannery and Chris Helgen, chief scientist at the Australian Museum, had the idea to revisit the three Australian fossils of tribosphenic mammals — most recently described by Rich in 2020 — and began sifting through the scientific literature to look at them. What can they find.

They realized that these strange teeth united early mammal fossils found in the southern hemisphere and that the Argentine specimen was the oldest in the group, millions of years older than any early mammal fossils found in the north.

From there, they plot an alternate origin story for mammals, whose ancestors could have hopped between southern continents when they joined together at a supercontinent called Gondwana about 125 million years ago before heading north.

Based on the age of the fossils and their anatomical similarities, the team believes they represent the ancestors of early marsupials (such as koalas and Australian wombats) and placental animals (which includes humans), which were grouped together as bull mammals.

“Our research indicates that Theria evolved in Gondwana, thriving and diversifying there for 50 million years before migrating to Asia during the early Cretaceous period,” explains Hegglin. Once in Asia, they diversified rapidly, filling many ecological niches.

Map showing the location of fossils of tribosphenic mammals found on the southern continents that formed Gondwana.
Trifosphene mammal fossils have been found on southern continents, shown here as Gondwana. (Flanery et al., shringa2022)

The researchers suggest that the specialized molars of our early mammalian ancestors may have been key to their evolutionary success. But the evolution of early mammals that outlived dinosaurs has long impressed scientists and will no doubt continue to attract close scrutiny.

In paleontology, like any other science, the weight of evidence speaks volumes. For more than 200 years, the diversity of mammals that inhabit the northern hemisphere and the abundance of fossils found there have led scientists to believe that the ancestors of placentals and marsupials originated in the north and spread south.

However, research shows that the fossil record can be skewed by who is looking for its location. For now, all we have to challenge this ancient theory about where mammals originated from is this tiny set of tiny teeth—and it took several decades to find even those seven specimens.

Gray scale reconstructions of teeth of Mesozoic Tribosphenic mammals found in the Southern Hemisphere.
Reconstruction of teeth of Mesozoic Tribosphenic mammals found in the Southern Hemisphere. (Flanery et al., shringa2022 / The Australian Museum)

“It is the most important piece of paleontological research, from a global perspective, that I have ever published, but it may take some time to find full acceptance among Northern Hemisphere researchers,” says Flannery.

He even took a long time to accept the results of the analysis. “I’ve resisted the conclusion as long as I can, but the evidence is compelling,” Flannery told Australian Geographic’s science and environment editor, Karen McGhee.

In fact, not all paleontologists are convinced. While Flannery and his team regard this new discovery as a monumental discovery that turns our understanding of mammalian evolution on its head, Flinders University paleontologist Gavin Prideaux says their conclusions are based on “smaller and smaller tooth fragments.”

As he told The Sydney Morning Herald, another explanation could be for convergent evolution: that these tribosphenic molar teeth evolved in a few separate places at similar times. “The jury is still out,” he says.

The study has been published in Alcheringa: An Australian Journal of Paleontology.

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