This post was written by Nathan Roden.
If you ask almost any kid today how the dinosaurs died, they’ll tell you an asteroid killed them, but this didn’t used to be the leading theory. When you look at key papers about the asteroid impact the kids are referring to, you’ll learn that it defined the transition from the Cretaceous to the Paleogene (K-Pg) boundary ~66 million years ago. Before the discovery of the asteroid, there wasn’t a single agreed upon theory on what caused the 5th global mass extinction. Not only did this mass extinction kill off the dinosaurs, but it also allowed the age of the mammals to begin. But how did scientists theorize about the 5th mass extinction before the asteroid discovery?
In 1825, scientists first discovered that dinosaurs had mysteriously died out. Georges Cuvier, a zoologist deemed the “founding father of paleontology,” first observed a mass extinction event/major changes in fauna between the Mesozoic and Cenozoic eras (Benton, 1990). In the rock records, there was no evidence of dinosaurs above the K-Pg boundary layer, so Georges theorized that something must have caused a wide variety of species to suddenly disappear 66 million years ago.
Between 1825-1979, multiple hypotheses were proposed by many scientists about how the extinction event happened. The four previous global mass extinctions have mostly occurred because of severe volcanism that filled the oceans and/or atmosphere with carbon dioxide (CO2). Some researchers proposed a massive volcanic eruption as the cause for the 5th extinction. Other researchers proposed a supernova that hit the earth with rays of harsh radiation, changing sea levels (Benton, 1990). Despite all the interesting and sometimes outlandish hypotheses, geologists and paleontologists could not agree on how the dinosaurs mysteriously died 66 million years ago.
It wasn’t until 1980 that the famous Alvarez paper (Alvarez, 1980) first theorized that the dinosaurs had died out due to an asteroid impact. When they looked at the rock layers around 66 million years old in Denmark, New Zealand, and Italy, they realized that something was different about the clay and rocks. When looking at the impact layer, the K-Pg boundary, around the globe and performing chemical analysis on the clay, they discovered elevated levels of iridium within the thin clay layer. In most places, iridium is quite rare in the earth’s crust, often less than 0.1 ppb (parts per billion); the fact that there were elevated levels of iridium in this layer was quite the anomaly. Since iridium is more common in asteroids, they believed that a massive asteroid killed off all the dinosaurs, spreading a layer of asteroid particles worldwide to create the boundary layer. The only flaw they found in their paper is that they did not know where the crater was that caused the mass extinction. Because of that, this paper was highly debated within the geology community for many years to determine the theory’s legitimacy.
Geologists searched the globe for years for a crater that was not only 66 million years old but big enough to cause a global mass extinction. It wasn’t until 1990 that Hildebrand and colleagues rediscovered the Chicxulub crater in Mexico. After looking at magnetic and gravity field anomalies within the area, they looked at oil drill cores to determine the age of the area. They discovered that the crater was about the same size that Alvarez and colleagues (1980) calculated that would have caused a layer of ash and meteorite material to cover the earth, and it was also around the same age as the dinosaur extinction. With those two important data points, Hildebrand and colleagues believed they had managed to find the crater that wiped out the dinosaurs. Because of this rediscovery, further evidence supporting the asteroid impact theory became more widely accepted within the geology community.
Over the years, more tests have been done by a wide variety of scientists to determine how a massive asteroid wiped out a large portion of the earth’s species. It wasn’t until 2010 that 41 scientists from a wide range of backgrounds concluded in a review that the crater hypothesis was the best hypothesis based on all of the evidence given (Schulte et al. 2010).
This discovery took several scientists from a wide range of fields to get a good conclusive answer to solve the mystery of the K-Pg mass extinction. Paleontologists were able to determine the age of the K-Pg boundary, geochemists did the analytical research on the rocks and clay, physicists determined the scale of the asteroid to cause the mass extinction, biologists determined how the dinosaurs died, atmospheric scientists hypothesized what the weather was like immediately after the asteroid impacts, and the list goes on. This discovery would be impossible for a single scientist to single-handedly take on the research to provide all of the necessary evidence to make this hypothesis a theory. Even still to this day, we are still learning more about the K-Pg layer, the asteroid that hit the earth, what happened to the dinosaurs around the time of the impact, and how the atmosphere was affected after the collision.
Alvarez et al., (1980). “Extraterrestrial cause for the Cretaceous-Tertiary Extinction”. Science. Volume 208, Number 4448. https://websites.pmc.ucsc.edu/~pkoch/EART_206/09-0305/Alvarez%20et%2080%20Science%20208-1095.pdf
Hildebrand et al., (1991). “Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico”. Geology. Volume 19, pg. 867-871. https://pubs.geoscienceworld.org/gsa/geology/article/19/9/867/205322/Chicxulub-Crater-A-possible-Cretaceous-Tertiary
Benton, M. J. (1990). “Scientific methodologies in collision: the history of the study of the extinction of the dinosaurs”. Evolutionary Biology. Volume 24, pg. 371–400. https://cpb-eu-w2.wpmucdn.com/blogs.bristol.ac.uk/dist/5/537/files/2019/08/1990Collision.pdf
Schulte, Peter et al., (2010). “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous–Paleogene Boundary”. Science. Volume 327, Number 5970), pg. 1214-1218. doi10.1126/science.1177265.
Izett et al., (1990). “The Cretaceous-Tertiary (K-T) boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals: Implications concerning the K-T boundary impact-extinction theory” USGS, Open File Report 87-606. https://pubs.usgs.gov/of/1987/0606/report.pdf