Researchers in England have discovered the fossilized teeth of humanity's earliest mammalian ancestors — and the creatures were not what you might think. 

The teeth belonged to small, furry, rat-like animals that shared the planet with the dinosaurs, 145 million years ago. 

Two teeth from two different — yet similar — animals were found. Researchers were able to determine that one creature was likely a nocturnal burrower that ate insects.

The second, likely, was both an insectivore and a herbivore. 

The teeth were found by Grant Smith, an undergraduate student at the University of Portsmouth in England, who was sifting through rocks on the coast of Dorset.

While Smith knew that he'd found the remains of something mammalian, he wasn't quite sure what. He contacted his supervisor, Dave Martill.

Prehistoric rat-like teeth

The fossilized teeth were found in southern England, on the coast of Dorset. (University of Portsmouth)

"We looked at them with a microscope but, despite over 30 years' experience… we needed to bring in a third pair of eyes and more expertise," Martill said in a statement.

They reached out to Steve Sweetman, a research fellow at the university who studies prehistoric microvertebrates. 

"I was amazed," Sweetman, who is the lead author of the resulting paper, published this month in the journal Acta Palaeontologica Polonica, told CBC News.

"It was a jaw-dropping moment."

Spectacular find

He instantly knew the teeth were a spectacular find.

They belong to a branch of animals known as eutherians, and are part of the order of placentals, animals that give live birth after nourishing embryos through a placenta.

"These are the ancestors of placentals, and therefore the ancestors of us, and creatures as diverse as the pygmy shrew and the blue whale," Sweetman said. "It's quite an exciting thing in the science of paleontology." 

A study published in the journal Nature in 2013 had suggested a new species found in China — Juramaia sinesis — was the oldest eutherian-placental. 

"Subsequent studies have said that's almost certainly not the case," Sweetman said. "But there's no question from the morphology of the teeth that ours are."

The new species have been named Durlstotherium newmani and Dulstodon ensomi.

Eutherian rat-like creatures placentals

Steve Sweetman, Grant Smith and Dave Martill, authors of the study. (University of Portsmouth)

Sweetman said they will continue to look for more samples in the area but notes that, since the search for such fossils has been going on so long, that they may have just been a lucky find.

Finding another could take years. 

Sweetman says learning about small creatures provides a greater picture of prehistoric times — it's important not to just look at the giant dinosaurs that once traversed the land.

"When you start looking carefully, sifting the sediments and finding the tiny things, that's when the world really comes to light."