The discovery of a new fossil has given scientists an insight into an ancient group of animals that flourished when dinosaurs died out.
The find in New Mexico has been identified as a previously unknown species from a group of small, furry mammals that resemble rodents.
Known as multituberculates, they originated 100 million years before the dinosaurs were killed off and had distinctive sharp teeth.
They survived the events that led to the extinction of dinosaurs, and spread through what is now Asia and North America.
The new species, known as Kimbetopsalis simmonsae, has helped scientists update the mammals' family tree.
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Research revealed that the new fossil dates to about 500,000 years after the extinction and a study by teams at the University of Edinburgh, the New Mexico Museum of Natural History and Science and the University of Nebraska found that the fossil may be from a forebear to the biggest species in the multituberculates group, which weighed up to 100kg and is said to have resembled a large beaver.
Scientists hope studying the creatures will improve understanding of mass extinctions.
The group finally died out about 35 million years ago, when it was replaced by emerging rodents but it is unclear why multituberculates were superseded, the research team said.
Dr Thomas Williamson of the New Mexico Museum led the research and said the discovery was a "pleasant surprise".
"It helps fill an important gap in the record of this group of mammals," he said.
"It's interesting that this odd, now extinct group, was among the few to survive the mass extinction and thrive in the aftermath. It may be because they were among the few mammals that were already well-suited to eating plants when the extinction came.
"This new species helps to show just how fast they were evolving to take advantage of conditions in the post-extinction world."
Dr Steve Brusatte of the University of Edinburgh said: "We could think of Kimbetopsalis as a primeval beaver, which lived only a few hundred thousand years after the asteroid impact that killed the dinosaurs.
"The asteroid caused apocalyptic environmental change, but it seems like mammals began to recover pretty quickly afterwards. It was in this brave new world that our mammalian ancestors got their start."
The study of the fossil is published in the Zoological Journal of the Linnean Society.
