Jaw discovery suggests modern mammals originated in southern hemisphere – ScienceAlert

Jaw discovery suggests modern mammals originated in southern hemisphere – ScienceAlert

It has taken more than two decades and one pandemic for paleontologists to piece together the fossilized remains of the early ancestors of mammals and discover that their evolution, which gave rise to modern humans, may have started in the southern hemisphere, and not in the north, as scientists have long thought.

Analysis of a small collection of tiny fossilized jaws with distinctive back teeth upends our understanding of when and where modern mammals evolved the head, according to the team of researchers who produced it.

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

He was part of the team that, in 1997, after 23 years of research, announced that they had found on an Australian beach a mammal jaw with strange teeth, such as had only been seen inside Europe and North America. The jaw was of a small shrew-like creature and dated back to the Cretaceous period when dinosaurs also wandered

As the years passed, more mammal jawbones Mesozoic they were discovered: in Madagascar, Argentina, India and, more recently, in Australia.

Each of these specimens, measuring an inch or less, had distinctive back teeth. According to the latest analysis that reviews them, the oldest fossil predates those found in the northern hemisphere by about 50 million years.

“This surprising series of discoveries has completely changed our theory of mammalian evolution. In fact, it turns our ideas about mammalian evolution on its head,” Rich. he says.

The adolescent teeth in question are called tribosphenic molars, which interlock at the top and bottom to cut, crush, puncture and grind plant food and insect prey.

The tiny tribosphenic molars in the jaw of an early Cretaceous mammal found in Australia. (James Alcock/Australian Museum)

During the pandemic, esteemed palaeontologists Tim Flannery and Kris Helgen, chief scientist at the Australian Museum, had the idea to review the three Australian tribosphenic mammal fossils, the most recent of which Rich described in 2020 – and began scouring the scientific literature to see what else they could find.

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

From there, they mapped out an alternative origin story for the mammals, whose ancestors may have jumped between the southern continents when they merged into a supercontinent called Gondwana about 125 million years ago before heading is in the north

Based on the age of the fossils and their anatomical similarities, the team believes they represent the earliest ancestors of marsupials (such as Australia’s koalas and wombats) and placentals (which includes humans), which group as Therian 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.” explain Heglen “Once they arrived in Asia, they diversified rapidly, filling many ecological niches.”

Map showing the location of tribosphenic mammal fossils found on the southern continents that formed Gondwana.
Fossils of tribosphenic mammals found on the southern continents, shown here as Gondwana. (Flannery et al., Alcheringa2022)

Researchers suggest that the specialized molars of our early mammalian ancestors may have been the key to their evolutionary success. But the evolution of the first mammals that survived the dinosaurs did fascinated scientists for a long time and will undoubtedly continue to attract ongoing scrutiny.

In paleontology, like 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 led scientists to believe that the ancestors of placentals and marsupials arose in the north and spread to the south.

However, research shows that the fossil record may be biased by who looks where. For now, all we have to challenge this long-held theory of where mammals originated is this small collection of tiny teeth, and it took several decades to find even these seven specimens.

Grayscale reconstructions of dentitions of Mesozoic tribosphenic mammals found in the Southern Hemisphere.
Dentary reconstructions of Mesozoic tribosphenic mammals found in the Southern Hemisphere. (Flannery et al., Alcheringa2022/Australian Museum)

“It is the most important piece of palaeontological research, from a global perspective, that I have ever published, but it may take some time to find full acceptance among researchers in the northern hemisphere.” he says Wandering

It even took a long time to accept the conclusions of the analysis. “I resisted the conclusion as much as I could, but the evidence is compelling,” Flannery said he said Australian Geographic’s science and environment editor, Karen McGhee.

In fact, not all paleontologists are convinced. While Flannery and team are hailing this new revelation as a massive discovery that alters our understanding of mammal evolution, Flinders University palaeontologist Gavin Prideaux he says their conclusions are based on “the smallest, shittiest fragments” of fossilized teeth.

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

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

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