Palaeontologists have uncovered the remains of some of humanity's oldest ancestors along the fossil-rich Jurassic Coast in the UK. The team found two teeth belonging to rat-like animals that date back 145 million years, meaning they're among the earliest creatures on our particular branch of the tree of life to have been discovered.

There are three kinds of mammals alive today: marsupials like the kangaroo, which raise their young in a pouch; monotremes like the platypus, which lay eggs; and placental mammals like humans, which give birth to live young. This last group, also known as eutherians, are by far the most widespread mammals on Earth, encompassing everything from the tiniest shrew to the gigantic blue whale.

The two newly-discovered species, dubbed Durlstotherium and Durlstodon, are claimed to be the earliest known eutherians, dating back about 145 million years to the height of the reign of the dinosaurs. On studying the teeth, palaeontologists from the University of Portsmouth found they appear far more advanced than they should be for their age.

"The teeth are of a type so highly evolved that I realized straight away I was looking at remains of Early Cretaceous mammals that more closely resembled those that lived during the latest Cretaceous – some 60 million years later in geological history," says Steve Sweetman, co-author of a study describing the find.

The owners of the teeth were small and rat-like, with the scientists suggesting they may have been nocturnal, burrowing creatures. Judging by their shape, the team believes that one most likely ate insects, while the larger species probably spiced up its diet with some plants as well.

"The teeth are of a highly advanced type that can pierce, cut and crush food," says Sweetman. "They are also very worn which suggests the animals to which they belonged lived to a good age for their species. No mean feat when you're sharing your habitat with predatory dinosaurs!"

The teeth were first discovered by undergraduate student Grant Smith while sifting through rock samples collected from the Jurassic Coast. The age of the surrounding rock is what indicates just how ancient these animals were, but while the team is quick to call their find the "oldest mammals related to mankind," the title is up for contention.

Juramaia, a specimen described in 2011, was uncovered in China and dates back 160 million years – some 15 million years earlier than the two English fossils. Although the Portsmouth team points out that both the age and classification of Juramaia is up for debate, the overall scientific consensus still seems to give the title of oldest eutherian to the Chinese specimen.

The past is extremely murky and we may never really know the truth, but at the very least, Durlstotherium and Durlstodon are the oldest eutherian specimens discovered in that part of the world.

The research was published in the journal Acta Palaeontologica Polonica.

Source: University of Portsmouth

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