Rise of the Mammals
Colorado Discovery Rocks the World
Sixty-six million years ago, a meteorite larger than Mount Everest slammed into Earth and 75 percent of species went extinct in a geologic blink of an eye, including nonavian dinosaurs. For decades, scientists worked to determine how the planet and living creatures rebounded, yet continued to know very little about the recovery. Until now.
A remarkable new trove of fossils has been unearthed in Colorado—a discovery that reveals one of the most important but least understood chapters in the history of life. It’s an unprecedented look at the time when the great dinosaurs were replaced by our own group, the mammals, and the world as we know it began.
When the Cataclysm Struck
Sixty-six million years ago, a meteorite larger than Mount Everest slammed into Earth. This was the single worst day for life on our planet, when 75 percent of species went extinct in a geologic blink of an eye.
Obviously, the planet and living creatures rebounded. Humans are testament to the recovery of life. But how did it happen? For decades, scientists have tried to squeeze every bit of data possible out of the rocks from the first million years after the dinosaur extinction. But, the results of over 100 years of hammering away are just fragments of teeth, an occasional jaw, and an ultra-rare skull once every few decades. As a consequence, we don’t know much about the mammals that survived. Until now.
The discovery of an extraordinary treasure trove of fossils near Colorado Springs by Drs. Tyler Lyson and Ian Miller reveals in striking detail how life recovered after the catastrophic asteroid impact that wiped out the dinosaurs.
Their new research, published in Science magazine, paints a vivid portrait of the first million years after the impact.
The Light Bulb Moment
A monumental breakthrough occurred in 2016 when Lyson and Miller concentrated their efforts in an area called Corral Bluffs. They believed this Denver Basin locale showed promise because a handful of relatively complete vertebrate fossils had been found there over the decades.
So, they set their eyes to the ground, looking for bits of bone, the way they were trained to find fossils. They came up with only fragments.
However, Lyson was convinced they weren’t keying in on the right material, perhaps not seeing what was right in front of their noses. He recalled his fossil hunting experience with South African colleagues in the deserts of the Karoo, where the key to finding fossils was not searching for bone fragments but for a particular kind of rock called a concretion.
So what if the team searched Corral Bluffs for concretions instead of the usual bone? It was the light bulb moment that changed the game completely.
Cracking the Code
The team went back to the site, but this time they set their eyes on finding concretions. It wasn’t long before Lyson picked up a knobby, whitish-colored rock that looked more like a rotting loaf of bread than anything else. With a single, well-placed crack of his rock hammer, Lyson split open the concretion and saw the cross-section of a complete mammal skull staring back at him. He was completely stunned. After 20 years of combing the badlands of North Dakota for elusive fossils from just after the extinction of the dinosaurs, Lyson was holding the best fossil he’d ever found from this important slice of time.
Miller and Lyson looked around and saw the same unassuming concretions covering the ravines and gullies. In a frenzy, they gently cracked open concretions at their feet and in no time found four more complete mammal skulls. Paleontologists who study this time period go whole careers without finding one complete mammal skull from this time period; Miller and Lyson found four in just a couple of hours!
They had cracked the code.
Lyson and Miller and their excavation team spent about 10,000 hours in the field at Corral Bluffs, working through blazing sun, torrential rain, and even snowstorms. Close to a thousand vertebrate fossils have been unearthed from the site, from the skull that cracked the case—Carsioptychus coarctatus, an early relative of hoofed animals about the size of a pig—to skulls and skeletons of other mammals, turtles, and crocodilians, including many new species. They also found the most complete specimens of many known species from this time period.
The Plants and Mammals
Literally everything alive today can trace its ancestry back to the survivors of the asteroid impact, including humans. As a result, what we know today as the “modern world” was shaped by the recovery. The discoveries at Corral Bluff set the scene for the first one million years after the asteroid impact.
In the aftermath, ecosystems are completely devastated and extinction occurs everywhere on the planet, on land, and in the sea. All large animals are wiped out. The biggest animal to survive the mass extinction is a soft-shelled turtle weighing about 200 pounds; the largest surviving mammal weighs about a pound.
It's name means "ribbon lips"
- Age: First appeared 720,000 years after the K-T extinction.
- Size: 34-56 kg; comparable to a small capybara
- Belongs to an extinct group of mammals called “multituberculates,” arguably the most successful group of mammals that were around for over 150 million years.
- Fun facts: We hypothesize that they likely ate legumes and were river dwelling creatures. It was the largest multituberculate to ever live.
Named after the oblique folds found on the teeth.
- Age: First appeared 300,000 years after the K-T extinction.
- Size: ~30-47 kg
- One of the earliest relatives of living hoofed mammals (deer, cows, elephants, horses, etc.).
- Fun facts: This species represents the second major jump in body size, from 0.5 kg mammals that survived the K-T extinction, to ~6 kg mammals found ~100,000 years later, to Carsioptychus (~30kg), tipping the scales and 30 kg. Teeth suggest it was a hard object feeder (e.g. nuts); represents the first specialization in diet (from omnivore to herbivore) after the K-T extinction.
- Age: First appeared 680,000 years after the K-T extinction.
- Size: ~47-77 kg, about the size of a wolf.
- One of the earliest relatives of living hoofed mammals (deer, cows, elephants, horses, etc.).
- Fun fact: It has a large hole above its teeth, suggesting a large nerve, which means it had a very sensitive snout (and likely had long, dense whiskers (vibrissae) for sensing its surroundings).
- Age: First appeared 300,000 years after the K-T extinction.
- Size: ~4-6 kg, about the size of a raccoon.
- One of the earliest relatives of living hoofed mammals (deer, cows, elephants, horses, etc.).
- Fun fact: Likely belongs to a new species of extinct mammal.
EPI.51030
On the scorched landscape, a blanket of pioneering ferns, proliferating in sunshine where there were once shaded forests, quickly turns barren dirt into fields of green. They pave the way for a handful of other plants and the planet revegetates. Although the world is green again, this ecosystem has little diversity and endures for hundreds of thousands of years. The recovery is slow and patchy.
The field of ferns gives way to low-diversity forests, often completely dominated by palms. This weird palm world lasts for approximately 300,000 years. Within this palm-dominated forest is the first major jump in mammalian size—an increase in body mass of 30 times, from the shrew-size survivors to a raccoon-size trailblazer.
Within 300,000 years after the mass extinction, the forests start to regain their diversity, particularly walnut trees which may have provided an energy-rich food source for mammals. There’s a shift in mammals, from small omnivores to much larger herbivores tipping the scales at about 70 pounds. Legumes appear 700,000 years post extinction, as evidenced through the oldest bean pod found at the site. These “protein bars” provide a substantial food source for even larger herbivorous mammals, up to 100 pounds. At this point there is a 100-fold increase in body size compared to the tiny survivors of the asteroid. A comparable increase in body size in mammals will not occur for another 30 million years.
Because plants can’t get up and move, they reflect the climate in which they grew. In the Corral Bluffs section, there was a cooling leading up to the asteroid impact, and then a strong warming right afterward. These patterns link to the same patterns found elsewhere on Earth, including in the deep oceans. The whole global system—from the bottom of the deepest sea to the forests of the continents—is intricately tied and responds more or less in lockstep to climate change. In the Corral Bluffs section, we see that diversity of life, particularly the forests, increases as the climate warms. The plants also tell a story of climate change before and after the extinction.
Meet the Team
Denver Museum of Nature & Science
Dr. Tyler Lyson, curator of vertebrate paleontology, co-led the research team. Dr. Lyson broke the discovery wide open by following his curiosity and cracking open one of the many white rocks (called concretions) at the Corral Bluffs site, revealing an entire mammal skull from the period just after the asteroid impact that caused the extinction of the dinosaurs.
Dr. David Krause, curator of vertebrate paleontology, is an expert in fossil mammals and assisted with identification and determining body mass of the mammals.
Dr. James Hagadorn, curator of geology, studies how the planet has changed over time and is researching how the Corral Bluffs fossils formed.
Dr. Sarah “Gussie” Maccracken, curator of paleobotany, has been a key collaborator in elucidating the fossil plant story. Her research focuses on the feeding patterns of insects on fossil leaves in the Cretaceous throughout the American West.
Research Associates Drs. Antoine Bercovici and Farley Fleming are experts on fossilized pollen and together they analyzed over 37,000 pollen grains to help paint a vivid picture of the plant ecosystems.
Research Associate Dr. Ken Weissenburger is a geologist who helped tie the fossil plant and animal localities to the timeline.
National Geographic
Dr. Ian Miller, former curator of paleobotany and now Chief Science Officer at National Geographic, co-led the research team and spearheaded the collection of fossil plants from the Corral Bluffs area, along with the analysis of more than 6,000 specimens to determine the relationship between plant and animal diversity in the years immediately following the asteroid impact.
The City University of New York (CUNY), Brooklyn College and the Graduate Center
Dr. Stephen Chester, assistant professor of anthropology and archaeology, is a paleontologist whose research focuses on fossil mammals after the K-T extinction. Dr. Chester and his lab, including PhD candidates and several undergraduate students, assisted with identification and descriptions of mammals, analysis of CT data, and interpretation of the sensory ecology of the earliest mammals after the K-T extinction.
The University of New Hampshire
Dr. William Clyde, professor of earth sciences, and his master’s student Anthony Fuentes used magnetostratigraphy to precisely date the rocks that entomb the fossils. The work conducted by Mr. Fuentes formed the basis for his master’s thesis.
University of Washington
Dr. Greg Wilson, professor in the Department of Biology, is a mammalian paleontologist. He and his lab helped identify and describe mammals, analyzed CT data, and interpreted the sensory ecology of the earliest post K-T extinction mammals.
The Smithsonian National Museum of Natural History
Dr. Kirk Johnson, Sant Director of National Museum of Natural History, Smithsonian Institution, and Dr. Rich Barclay, research scientist, are both paleobotany experts who assisted with the fieldwork and description of the fossil plants and paleoecological inferences drawn from the paleobotanical data.
Wesleyan University
Matthew Butrim, recent master’s of science graduate from Wesleyan University, studied the mass of fossil leaves across the K-T boundary in the Corral Bluffs site. His research helped the team understand how the world of plants rebounded after the extinction that killed the dinosaurs.
The Colorado College
Ben Lloyd, recent graduate from The Colorado College, assisted Dr. Ian Miller with the analysis of the over 6,000 fossil leaves from the Corral Bluffs study area.
United States Geological Survey
The Museum collaborated with United States Geological Survey’s National Unmanned Aircraft Systems Project to gather high-resolution elevational and photogrammetry data to create a 3D model of the research area. The USGS team mapped each fossil locality and timeline (i.e., radiometric ash dates, paleomagnetic reversal locations, and the K-T boundary) into a high-resolution 3D digital model.
Howard Hughes Medical Institute
The Museum collaborated with the Howard Hughes Medical Institute (HHMI) to film the discovery and ensuing science, producing Rise of the Mammals, a NOVA production by HHMI Tangled Bank Studios for WGBH Austin. HHMI and the Museum use the assets for outreach purposes, telling the story of the scientific discovery to audiences around the world.
The City of Colorado Springs/The State of Colorado
Approximately one-half of the land surface on which the fieldwork is taking place is the property of the City of Colorado Springs Parks, Recreation, and Cultural Department. As a result, the Museum collaborates closely with the City to gain access to the property and to keep them informed on the science. The Office of the State Archaeologist grants the right to collect and deposit the fossils at the Museum.
Private Land Owners
Museum extends a special thanks to the private land owners of Corral Bluffs who helped make this discovery possible.
Publications
Bertrand, O. C., Shelley, S. L., Williamson, T. E., Wible, J. R., Chester, S. G. B., Flynn, J. J., Holbrook, L. T., Lyson, T. R., Meng, J., Miller, I. M., Püschel, H. E., Smith, T., Spaulding, M., Tseng, Z. J., & Brusatte, S. L. 2022. Brawn before brains in placental mammals after the end-Cretaceous extinction. Science, 376 80-85. DOI: 10.1126/science.abl5584
Butrim, M. J., Royer, D. L., Miller, I. M., Dechesne, M., Neu-Yagle, N., Lyson, T. R., Johnson, K. R., and Barclay, R. S. 2022. No consistent shift in leaf dry mass per area across the Cretaceous–Paleogene boundary. Frontiers in Plant Science, 13.894690. DOI: 10.3389/fpls.2022.894690
Brownstein, C. D. and Lyson, T. R. 2022. Giant gar from directly above the Cretaceous–Palaeogene boundary suggests healthy freshwater ecosystems existed within thousands of years of the asteroid impact. Biology Letters, DOI: 10.1098/rsbl.2022.0118
Petermann, H., Lyson, T. R., Miller, I. M. & Hagadorn, J. 2022. Crushed turtles: Proxies for lithification and burial depth histories. Geosphere, GES02513.1. DOI: 10.1130/GES02513.1
Lyson, T. R., Petermann, H., & Miller, I. M. 2021. A new plastomenid trionychid turtle, Plastomenus joycei, sp. nov. from the earliest Paleocene (Danian) Denver Formation of south-central Colorado, USA. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2021.1913600
Lyson, T. R., Miller, I. M., Bercovici, A. D., Weissenburger, K., Fuentes, A. J., Clyde, W. C., Hagadorn, J. W., Butrim, M. J., Johnson, K. R., Fleming, R. F., Barclay, R. S., Maaccracken, S. A., Lloyd, B., Wilson, G. P., Krause, D. W., and Chester, S. G. B. 2019. Exceptional continental record of biotic recovery after the Cretaceous–Paleogene mass extinction. Science 366, 977-983. DOI: 10.1126/science.aay2268
Fuentes, A. J., Clyde, W. C., Weissenburger, K., Bercovici, A., Lyson, T. R., Miller, I. M., Ramezani, J., Schmitz, M. D., and Johnson, K. R. 2019. Constructing a timescale of biotic recovery across the Cretaceous–Paleogene boundary, Corral Bluffs, Denver Basin, Colorado. Rocky Mountain Geology 54, 133-153. DOI: 10.24872/rmgjournal.54.2.133
Lyson, T. R., Petermann, H., & Miller, I. M. 2021. A new plastomenid trionychid turtle, Plastomenus joycei, sp. nov. from the earliest Paleocene (Danian) Denver Formation of south-central Colorado, USA. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2021.1913600
Lyson, T. R., Petermann, H., Toth, N., Bastien, S., & Miller, I. M. 2021. A new baenid turtle, Palatobaena knellerorum, sp. nov. from the earliest Paleocene (Danian) Denver Formation of south-central Colorado, USA. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2021.1925558
Krause, D. W., Hoffmann, S., Lyson T. R., Dougan, L. G., Petermann, H., Tezca, A., Chester, S. G. B., & Miller, I. M. 2021. New skull material of Taeniolabis taoensis (Multituberculata, Taeniolabididae) from the early Paleocene (Danian) of the Denver Basin, Colorado. Journal of Mammalian Evolution.