New fossils reveal secrets of the origin of modern mammals

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It was a team of paleontologists in southern England who discovered fossilized teeth that may belong to mammals that are ancestors of most modern mammals.

Paleontologists at the University of Porthsmouth in southern England have discovered a pair of fossilized teeth in the Dorset region, which they believe may belong to the oldest placental mammal species, the ancestors of the vast majority of mammals, including humans.

The fossils were in rocks of the Cretaceous period, of the Mesozoic era, that is, they are rocks that are between 66 and 145 million years. The fossilized teeth belong to species called Durlstodon ensomi and Durlstotherium newmani. These newly discovered species are part of the Eutheria group, and the mammals of this group are placental, meaning that their offspring develop completely within the mother’s body. This is the most successful group of mammals on the planet, including a diversity of species ranging from the blue whale to humans.

The discovery of these teeth happened a bit by chance, so told Steven Sweetman, the investigator who led the study, to The Guardian. Grant Smith, a graduate student, was examining samples of rocks from the Cretaceous period “in the hope of finding some interesting remains”, Sweetman said. When he discovered the fossils, Grant knew he had mammals in his hands, yet he did not know the ones he had were so important and unique.

“My jaw dropped”, said Sweetman, who said he realized “immediately that he was looking at mammal remains from the Lower Cretaceous”. Mammalian teeth are complex and have unique specializations, so it is only a few teeth for paleologists to realize which group of mammals or species they belong to, even if it is never seen before. According to the researchers, these animals were most likely nocturnal and their structures were similar to those of a mouse.

This finding was published in the “Acta Palaeontologica Polonica”, a publication of paleontology and paleobiology. These fossils may or may not be the oldest specimens of an Eutheran mammal. Since in 2011, a group of Chinese researchers discovered a fossil in the formation of Tiaojishan’s Jurassic rocks which they claim to be the earliest Eutheran specimen of which there is knowledge. The fossil belongs to a species called juramaia, and, according to the researchers, is 160 million years old. But this statement is disputed, and the discovery of Sweetman and the rest of the team is not.

Another study released this week also suggests that the first mammal species were all nocturnal, having come to live during the day when the dinosaurs died out.

This study, published in Nature Ecology and Evolution, may also indicate why there are few modern mammals that are diurnal.

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