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Colorado Fossils Document Life’s Rebound After Last Extinction

mammal skulls collected from Corral Bluffs
Mammal skulls collected from Corral Bluffs (Left to right: Loxolophus, Carsioptychus, Taeniolabis, Eoconodon). | HHMI Tangled Bank Studios

A wealth of new discoveries from the fossil-rich rocks of central Colorado's Corral Bluffs reveal previously unknown details about how Earth's plants and animals recovered in the wake of the Cretaceous-Paleogene extinction, according to a new study published in the October 25 issue of Science.

The well-preserved and unusually complete fossil examples of the region's mammals, reptiles and plants provide an unparalleled record of rapidly rebounding terrestrial ecology and species diversity over the first million years after the planet's fifth and most recent mass extinction.

Detailed records of post-mass extinction recovery may provide important insights for predicting ecosystem recovery following catastrophic extinction events, including today's global wave of disappearing species that some researchers call the "Sixth Extinction."

Nearly 66 million years ago a wayward asteroid fell from the sky and in the ensuing cataclysm, more than 75% of all life on Earth was erased. The Cretaceous-Paleogene extinction drastically altered the shape and direction of life on the planet. Most of Earth's modern biodiversity arose out of the ashes of the event.

"The meteorite that struck Earth 66 million years ago was the biologic reset button," said Tyler Lyson, the study's lead author and paleontologist at the Denver Museum of Nature and Science. "The age of reptiles ends and our age, the age of mammals begins."

All living things today — humans included — have ancestors that survived and thrived after the extinction event. However, fossils of the plants and animals that inherited Earth after the fallout are extremely rare or fragmentary, and life at the beginning of the new era is shrouded in mystery.

Lyson and a team of researchers studied an extensive, continuous fossil outcrop located in the Denver Basin, a fossil-rich region in the eastern shadow of the Colorado Rockies, which contained an exceptionally dense and diverse collection of vertebrate and plant fossils spanning the first million years of the post-extinction era. According to Lyson, the discovery provided the ability to study, for the first time and in unmatched detail, the recovery of an entire terrestrial ecosystem following the extinction event.

"We found that the recovery of the plants and animals are intricately intertwined and that the stepwise recovery was quite fast," said Lyson.

In the first millennia after the extinction, the climate of what is now central Colorado was warm and wet. Devastated forests were replaced by verdant blankets of ferns, which thrive in disturbed environments, and eventually palm forests. Small crocodiles and turtles, like the 200-pound soft-shelled Axestemys — the largest terrestrial creature known to survive the impact — lived along the banks of fast-flowing rivers rolling from the slowly uplifting hills destined to eventually become the Rocky Mountains.

While large-bodied mammals disappeared, smaller rat-sized mammals survived in the underbrush. These species quickly diversified and filled the newly vacated ecological niches eventually growing to their pre-extinction body sizes within 100,000 years.

Subsequent forest diversification continued to impact early mammal evolution. Large, herbivorous mammals with teeth adapted for eating hard objects appeared at the same time the walnut and pecan tree diversified 300,000 years after extinction. Similarly, another increase in body size occurred 700,000 years after extinction. Mammal species nearly 100 times larger than their pre-extinction ancestors were identified in the record alongside the fossilized remains of bean pods from the world's very first legumes.

"We've dubbed this the 'protein bar world' where the protein rich legumes may have helped fuel the larger bodied mammals that appear at this period," said Lyson.

These periods of diversification also correlate with periods of climate warming, suggesting a dynamic interplay between warming and ecosystem change during the recovery following extinction.

"It is an open question of whether climate is a main driver of the recovery, or better yet, diversification of these ecosystems in the first 1 million years after the extinction. However, these ecosystems are incredibly complicated," said co-author Ian Miller, also at the Denver Museum of Nature and Science. "Even with this new and incredible dataset, it is an imperfect view into these ancient worlds."

Despite this, Lyson and Miller believe that the close link between climate and diversification is compelling and likely played a central role in post-extinction recovery.

"Building a record that includes fossil animals and plants from any time period and dating those biotic records precisely is sort of the holy grail in our business — we called it a paleontological trifecta," said Miller. "For it to happen during this all-important 1 million years following the [Cretaceous-Paleogene] boundary is unheard of."

Life on Earth bounced back — albeit in a different trajectory — almost immediately on a geological timescale after the catastrophic impact that ended the rule of dinosaurs on Earth.

However, while life finds a way, extinction is final, the authors noted.

"As we begin to understand the sixth mass extinction, the one we are in now, we should remember that while Earth will recover, life as we know it will be lost forever," said Miller.

[Credit for accompanying image of CGI-rendered Taeniolabis mammal: HHMI Tangled Bank Studios]