A trove of thousands of newly discovered fossils could provide a window into the earliest days of mammalian life, soon after the cataclysmic extinction event that wiped out giant reptiles some 66 million years ago.

Unearthed by a team of paleontologists in Colorado’s Corral Bluffs, the new fossils will help scientists understand the interactions between flora, fauna and climate which allowed mammals to develop from small rodent-like animals to the splendor of diversity seen in the wild today.

“We provide the most vivid picture of recovery of an ecosystem on land after any mass extinction,” Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science, told the New York Times.

In a paper published on Thursday in Science, Lyson’s team explains how Corral Bluffs – located some 80 miles south of Denver – contains an “unusually complete” fossil record, offering unique insight into the “drivers and tempo of biotic recovery during the poorly known first million years” after the mass extinction of the dinosaurs, known as the K-T or “K-Pg event” for short.

In addition to dinos, K-Pg wiped out a number of larger mammals as well, leaving mostly rat-sized creatures to roam a scorched Earth. The new fossils show how mammals began to bounce back, growing in size and diversity over the millennia following the extinction. After a brief 300,000 years – a mere blip on the evolutionary timeline – the pig-sized Carsioptychus appeared on the scene, while Eoconodons, about the size of a wolf, started thriving another 400,000 years later.

“You’re going from a very small dog that you’d see on the streets on New York City to a very large wolf within those hundreds of thousands of years,” Stephen Chester, a mammalian paleontologist at Brooklyn College and an author on the paper, told the Times.

The research team also gathered thousands of samples of fossilized leaves and ferns – important sources of food – as well as over 37,000 pollen grains, which could answer a number of other questions about the rise of mammals on Earth.

A geoscientist at the University of Florida, Courtney Sprain, marvelled at the detail and completeness of the fresh fossil collection, noting that the level of preservation was “amazing.”

“That’s one of the really spectacular things about this paper, just how amazing the preservation is and how good the record is for [seeing] a variety of different changes following a mass extinction,” she told the Times.

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