The Cretaceous-Paleogene (K/Pg) extinction event wiped out up to 75% of ALL species on earth, including the non-avian dinosaurs! Why the “non-avian” distinction and what does it mean? The fossil record indicates that birds (avians) evolved from dinosaurs by the end of the Jurassic Period, meaning birds ARE dinosaurs. Birds obviously survived the K/Pg mass extinction, but what happened to the all of the “non-avian” dinosaurs? The K/Pg mass extinction was rapid, global, and severe. Evidence from 66 million-year-old rocks around the world support various theories and scientists today continue to debate whether the K/Pg mass extinction was due to a single cause, or to multiple causes. In fact, DIG teachers help scientists answer this very question during their time at the DIG Field School. Let’s examine each of these ideas and the evidence supporting them.
Single Cause Theory (or the Asteroid Impact Theory)
If you ask most people how the dinosaurs went extinct they would probably tell you it was from an asteroid impact. This idea was first proposed in 1980 by Luis and Walter Alvarez, a nobel-prize winning physicist father and geologist son team from UC Berkeley. In the 1970s Walter was a young professor doing fieldwork in Italy in sediment that straddles the Cretaceous-Paleogene boundary. In sediments older than 66 million years ago (mya), he discovered abundant marine microfossils (including various forms of plankton), above these was a distinct clay layer lacking any fossils, and in the layers above the clay, (younger than 66 mya), there were very few microfossils. Further investigation revealed that the clay layer contained extremely high concentrations of iridium (30x normal levels!), a rare earth element found in high concentrations in asteroids and comets. Walter’s father, Luis, suggested this “iridium anomaly” could be due to some sort of extraterrestrial impact, and the hunt was on for more evidence to support this theory for non-avian dinosaur extinction, or the “Alvarez Hypothesis.”
A few pieces of evidence to corroborate an extraterrestrial event were found in Italy and ultimately worldwide. Shocked quartz results from impact shock waves (extraterrestrial, nuclear bomb) penetrating quartz crystals with such force that the crystalline structure is disrupted. Produced at the site of K/Pg impact, shocked quartz would have floated into the atmosphere, and dispersed across the globe. Additionally, the impact would have been forceful enough to liquefy nearby rock and sand, creating tiny “glassy spherules” that would also have been dispersed globally.
It appeared the rocks were telling Walter the story of how the dinosaurs met their fate! At the site of the DIG Field School, rocks tell the same story. We find a clay layer at the K/Pg boundary, with shocked quartz, glassy spherules, and high levels of iridium, as well as a coal layer immediately above the clay that contains high levels of spores from ferns. This “fern spike” is a common indicator of “primary succession” following an ecological disturbance, and even today, as on Mount St. Helens, ferns are the first to colonize devastated areas. At the K/Pg boundary, the fern spike tells us plants were growing again and the environment was recovering after the mass extinction.
There was ample evidence to suggest an extraterrestrial impact, but where was the crater? Separately but at approximately the same time, geophysicists searching for oil off the coast of the Yucatan Peninsula region in the Gulf of Mexico found a 110-mile wide circular feature. Working with geologist Alan Hildebrand, they determined it was a crater that was the result of an asteroid impact, and they named it the Chicxulub Crater, after a nearby town.
Based on the size of the enormous crater, it is estimated that the asteroid was 6-miles wide! Such a large impact would have had approximately the energy of 100 trillion tons of TNT, or about 2 million times greater than the most powerful thermonuclear bomb ever tested. An impact of this size would have produced many cascading environmental effects in addition to distributing iridium, glassy spherules, and shocked quartz globally. First, the collision of the asteroid with the earth’s crust likely triggered earthquakes, tsunamis, and wildfires. In some places deposits from this time preserve giant trees that suggest these monster tsunamis from the Gulf of Mexico penetrated all the way to Texas and Brazil! Second, the impact would have ejected huge amounts of debris and rock into the atmosphere, which would have globally darkened the skies and cooled the planet for approximately a year, ultimately inhibiting photosynthesis and collapsing ecosystems dependent upon plants
Scientists generally agree that this enormous impact was a significant contributing cause to the K/Pg mass extinction. However, many scientists argue that evidence of environmental change and disturbance BEFORE the impact suggests the asteroid impact was not the ONLY cause, but was potentially one of MANY causes resulting in such a devastating extinction event.
Multiple Causes Theory
During the last few million years of the Cretaceous, and the last ~10 million years that non-avian dinosaurs were in existence, the earth was a very dynamic place. Volcanic activities in India, known as the Deccan Traps, were erupting 1.5 million square kilometers of lava (thats half the size of India!) and releasing huge amounts of dust and sulfurous gases into the atmosphere. These factors caused decreases in sunlight, as well as global cooling that would have affected plant-dependent food chains worldwide before the K/Pg mass extinction. Fossil evidence from our field sites suggests that during these Deccan Trap eruptions in India, ecosystems in the Hell Creek of northeastern Montana were stressed. Stressed ecosystems are analogous to the whole pile of straw on a camel before the last piece added (the asteroid) will break its back. Much of our work during the field season and during the DIG is designed to investigate which groups of animals were suffering decreased numbers before the asteroid impact (like dinosaurs and mammals), and which animals were thriving or doing just fine (like amphibians).
Another contributing factor was regression of the Western Interior Seaway (remember the Western Interior Seaway?) that we can trace in the changing rock formations in northeastern Montana. As the Western Interior Seaway regressed, or receded, areas that were once marine or near shore would have dried up and been replaced by more inland ecosystems and different species. Also during this time, global climate change was also occurring and the once warm, mild climate became more varied. So although the asteroid impact at ~66 mya would have had a major effect on the planet and its inhabitants, the geological and biological evidence suggests a much more complicated explanation for the end of the non-avian dinosaurs. Will YOU help us find more evidence to answer these questions?
Regardless of the exact cause, the K/Pg mass extinction wiped out three quarters of the species on earth and led to a major transition in floras and faunas. Not all groups were affected equally, but non-avian dinosaurs, pterosaurs, ammonites, and many mammals went completely extinct during this time. A loss of different species at various levels of the food chain can result in empty “ecological niches,” and with these previously occupied ecological niches now open, other groups can evolve to fill them. What followed the K/Pg mass extinction were a series of radiations (rapid diversification of organisms resulting from environmental change), with Cenozoic mammals ultimately replacing the niches left empty by the Mesozoic non-avian dinosaurs. We can even think of today’s tigers, cows, and rodents as the modern day version of the Mesozoic’s theropod carnivorous dinosaurs, Triceratops, and multituberculates, respectively. Most dinosaurs went extinct 66 mya, but a few survived and have been very successful. If you had chicken for dinner last night, you ate one!
*If you are interested in learning more about Walter Alvarez’s scientific journey and his process of science, the Understanding Science website has an excellent interactive narrative describing his discovery.