Fossils show life’s recovery after dino-killing asteroid strike

An overhead shot of the prepared mammal skull and lower jaw fossils retrieved from Corral Bluffs. (Credit: HHMI Tangled Bank Studios)

A newly discovered collection of fossils reveals how life recovered after the asteroid impact that wiped out the dinosaurs 66 million years ago at the end of the Cretaceous Period.

The unprecedented find—thousands of exceptionally preserved animal and plant fossils from the first million years after the catastrophe—shines light on how life emerged from one of Earth’s darkest hours.

Scientists unearthed this new record from the first million years after the asteroid impact in the region around Colorado Springs, Colorado, which forms part of a larger geological province known as the Denver Basin. The find includes both plants and animals, painting a portrait of the emergence of the post-dinosaur world.

A partial skull fossil of a mammal sits on a black surface
A cranium of a Taeniolabis taoensis, a herbivorous mammal, uncovered at the Corral Bluffs fossil site. Taeniolabis appears approximately 700,000 years after the end-Cretaceous mass extinction, approximately at the same time as the world’s oldest legume fossil. (Credit: HHMI Tangled Bank Studios)

“I’ve been working on this time period and on these mammals for 21 years, and this is truly an exceptional window into this pivotal event in life history,” says coauthor Greg Wilson, a professor of biology at the University of Washington and curator of vertebrate paleontology at the Burke Museum of Natural History & Culture. “We change over from terrestrial ecosystems dominated by dinosaurs to those that become dominated by mammals in a geological blink of an eye.”

Fossils show how life changed

“The course of life on Earth changed radically on a single day 66 million years ago,” says lead author Tyler Lyson, the curator of vertebrate paleontology at the Denver Museum of Nature & Science. “Blasting our planet, an asteroid triggered the extinction of three of every four kinds of living organisms. While it was a really bad time for life on Earth, some things survived, including some of our earliest, earliest ancestors.”

A moment of serendipity pointed the way to these rare fossil finds. Lyson, who had been looking for post-impact vertebrate fossils without success, took inspiration from a fossil that had been sitting in a museum drawer and fossil-hunting techniques used by colleagues in South Africa. In the summer of 2016, he stopped looking for glinting bits of bone in the Denver Basin and instead zeroed in on egg-shaped rocks called concretions.

Cracking open the concretions, Lyson and coauthor Ian Miller, curator of paleobotany and director of Earth and space sciences, found fossils such as the skulls of mammals from the early generations of survivors of the mass extinction. Since most of what is understood from this era is based on tiny fragments of fossils, such as pieces of mammal teeth, finding a single skull would be exceptional. Lyson and Miller found four in a single day and more than a dozen in a week. So far, they’ve found fossils from at least 16 different mammalian species.

A big comeback

Wilson participated in excavations at Corral Bluffs, a site just east of Colorado Springs. He and coauthor Stephen Chester, an assistant professor at the City University of New York’s Brooklyn College, worked to identify the mammal fossil that the team found, calculate changes in body size over time, and analyze the diversity of species after the asteroid impact.

The researchers determined that just 100,000 years after the cataclysm, mammalian diversity had approximately doubled. At 300,000 years after the impact, the maximum body mass of mammals had increased threefold, and mammals were evolving specialized diets, possibly in response to changes in plant diversity.

The plant fossils appear relatively flat sitting on a dark gray surface, with leave patterns showing in some
An overhead view of selected plant fossils retrieved from Corral Bluffs. (Credit: HHMI Tangled Bank Studios)

These findings illustrate how the Denver Basin site also is adding evidence to the idea that the recovery and evolution of plants and animals were intricately linked after the asteroid impact. The researchers collected more than 6,000 leaf fossils as part of the study to help determine how and when Earth’s forest rebounded after the mass extinction event. Combining a remarkable fossil plant record with the discovery of the fossil mammals allowed the team to link millennia-long warming spells to specific global events, including massive amounts of volcanism on the Indian subcontinent.

These events may have shaped the ecosystems half a world away. For example, the research team saw another increase in body size among mammals at about 700,000 years post-impact, which coincided with the evolution of legumes, then a new type of plant. Additional analyses of these fossils, as well as the unearthing of new specimens, will only deepen scientists’ understanding of this critical period in Earth’s history and the evolution of mammals that came before humans.

“Our understanding of the asteroid’s aftermath has been spotty,” Lyson explains. “These fossils tell us for the first time how exactly our planet recovered from this global cataclysm.”

The research appears in the journal Science.

Additional coauthors are from the Denver Museum of Nature & Science; the University of New Hampshire; the Smithsonian Institution’s National Museum of Natural History; Wesleyan University; the University of Maryland; and Colorado College. Funding for the research came from the Lisa Levin Appel Family Foundation, M. Cleworth, Lyda Hill Philanthropies, David B. Jones Foundation, ML and SR Kneller, T. and K. Ryan, and JR Tucker as part of the Denver Museum of Nature & Science No Walls: Schools initiative.

Source: University of Washington