Jawbone Discovery Suggests Modern Mammals Native to the Southern Hemisphere: ScienceAlert

Jawbone Discovery Suggests Modern Mammals Native to the Southern Hemisphere: ScienceAlert

Jawbone Discovery Suggests Modern Mammals Native to the Southern Hemisphere: ScienceAlert

It has taken more than two decades and one pandemic in front of paleontologists to unite the fossilized remains of mammals’ earliest ancestors and discover that their evolution that gave rise to modern humans may have begun in the southern hemisphere — and not in the north, as scientists have long believed.

The analysis of a small collection of tiny fossilized jawbones with distinctive back molars is changing our understanding of when and where modern upside-down mammals evolved, according to the team of researchers who produced it.

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

He was part of the team that announced in 1997, after 23 years of searching, that they had found a mammal jawbone with strange teeth, which had only been seen in Europe and North America. The jawbone belonged to a small shrew-like creature and dated back to the Cretaceous Period dinosaurs also wandered.

As the years passed, more mammalian jawbones emerged from the Mesozoic era were discovered: in Madagascar, Argentina, India and again, most recently, in Australia.

Each of these specimens, measuring an inch or less, had distinctive back molars. According to the latest analysis reviewing them, the oldest fossil predates the fossils found in the Northern Hemisphere by some 50 million years.

“This astonishing set of discoveries has completely changed our long-held theory of mammalian evolution. Indeed, it turns our ideas about mammalian evolution upside down,” Rich say.

The tiny teeth in question are called tribosphenic molars, which mesh above and below to cut, crush, pierce and grind plant food and insect prey.

Jawbone Discovery Suggests Modern Mammals Native to the Southern Hemisphere: ScienceAlert
The small tribosphenic molars on the jaw of an early Cretaceous mammal found in Australia. (James Alcock/Australian Museum)

During the pandemic, esteemed paleontologists Tim Flannery and Australian Museum Chief Scientist Kris Helgen had the idea of ​​revisiting Australia’s three tribosphenic mammal fossils – the most recent of which Rich described in 2020 – and began digging through the scientific literature to see what else they could find.

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

From there, they mapped out an alternate origin story for mammals, whose ancestors could have jumped between southern continents some 125 million years ago when they converged into a supercontinent called Gondwana before moving north.

Based on the age of the fossils and their anatomical similarities, the team believes they represent the earliest ancestors of marsupials (such as the Australian koalas and wombats) and placentals (which include humans), which are grouped as Theric 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,” explains Heglen. “Once they arrived in Asia, they quickly diversified and filled many ecological niches.”

Map showing the location of tribosphenic mammalian fossils found on the southern continents that made up Gondwana.
Tribosphenic mammalian fossils found on the southern continents, shown here as Gondwana. (Flannery et al., Alcheringa2022)

The researchers suggest that the specialized molars of our earliest mammalian ancestors may have been key to their evolutionary success. But the evolution of early mammals that survived the dinosaurs has long fascinated scientists and will undoubtedly continue to look critically.

In paleontology, as in any science, the weight of evidence speaks volumes. And for more than 200 years, the diversity of mammals living in 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 may be skewed by who looks where. For now, all we have to dispute this long-standing theory of where mammals come from is this small collection of tiny teeth — and it’s taken decades to find even those seven specimens.

Grayscale reconstructions of Mesozoic tribosphenic mammalian teeth found in the Southern Hemisphere.
Reconstructions of Mesozoic mammalian tribosphenic dentaries found in the Southern Hemisphere. (Flannery et al., Alcheringa2022/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 before it is fully accepted by researchers in the Northern Hemisphere,” say flannery.

In fact, it took him a long time to accept the findings of the analysis. “I resisted the conclusion as long as I could, but the evidence is compelling,” Flannery said told Karen McGhee, Australian Geographic science and environment editor.

Indeed, not all paleontologists are convinced. While Flannery and team view this new revelation as a huge discovery that will upend our understanding of mammalian evolution, paleontologist Gavin Prideaux of Flinders University argues say their conclusions are based on “the tiniest, nastiest little shards” of fossilized teeth.

As he told the Sydney Morning Herald, another interpretation could be one convergent evolution: that these tribosphenic molars evolved at similar times in a few separate places. “The jury is still out,” he said say.

The study is published in Alcheringa: an Australasian Journal of Paleontology.

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