What Killed the Dinosaurs?

April 9, 2019

5-minute read

What caused the mass extinction of dinosaurs? Ask Science looks at two new studies that blame volcanoes and asteroids. So, how did the dinosaurs meet their end?

325 – What Killed the Dinosaurs?

Your browser doesn’t support HTML5 audio

Paleontologists have long searched for clues as to what caused the mass extinction of the large dinosaurs. How did hundreds of species vanish so abruptly, leaving us only their fossilized remains as clues that they ever existed? Such a devastating event would have required a sudden onset of dramatic changes in the Earth’s climate.

This event in time is known as the K-Pg boundary, or the Cretaceous-Paleogene boundary, a marker between an era when dinosaurs roamed the Earth and a world more like today. We can trace the Earth’s history to study this time period by delving into deeper and deeper rock layers which each carry an imprint of the events that occurred when those rocks were once at the surface. The thin layer of rock at the K-Pg boundary marks the end of the Mesozoic Era and, along with it, the end of most of the Mesozoic species, including all non-flying dinosaurs.

Believe it or not, we are still learning new things from these ancient rocks, both as scientists discover new sites and see technological improvements in the equipment used to study them. Already this year, two exciting results have weighed in on two possible causes for the abrupt climate change at the K-Pg boundary: volcanoes and asteroids. So, how did the dinosaurs meet their end?

The large cloud of dust kicked up by the impact would have been thick enough to coat the planet and block out sunlight, first killing off plants, then the animals that fed on plants, then the animals that fed on those animals.

Evidence for an Asteroid Impact

An asteroid impact, given a large enough asteroid, could have caused fast and thorough devastation. The large cloud of dust kicked up by the impact would have been thick enough to coat the planet and block out sunlight, first killing off plants, then the animals that fed on plants, then the animals that fed on those animals.

There are four leading pieces of evidence that suggest a massive and dramatic asteroid impact led to the dinosaurs’ demise:

First, a massive crater believed to mark the asteroid’s impact, known as the Chicxulub Crater, has been found buried underneath the ground in the Yucatan Peninsula in Mexico. The asteroid big enough to make the crater would have been ~7 to 50 miles across and would have dated back to precisely the time of the fall of the dinosaurs.
Second, sand that originated in the ocean is found deposited around the Chicxulub Crater, which is consistent with a giant tsunami, which would have been triggered by an impact, carrying the sand inland.
Third, elements like iridium which are rare here on Earth but more common in asteroids, are found deposited in layers of rock that date back to the days when dinosaurs roamed the Earth.
Fourth, massive impacts are expected to form quartz crystals due to the intense pressure, and fragments of such crystals are found embedded in rock layers age dated to about 65.5 million years ago, which matches the date of the K-Pg boundary and the timeline set by the Chicxulub Crater.

The paleontology community (and lovers of dinosaurs everywhere) has been abuzz this week with talk of new results published in the Proceedings of the National Academy of Sciences that claim to have found much of this evidence and more all together in one site which is being called a snapshot of “the day the dinosaurs died.”

Tanis, North Dakota

So what makes this recent discovery different and exciting? In the past, a few markers of the K-Pg boundary have been uncovered at a time: a few shark teeth here, a handful of fossils there. But in the Tanis site in North Dakota, named after the storied lost ancient city of Egypt, paleontologists believe they have discovered an entire ecosystem all in one place.

The site sits in an outcropping of ancient rock layers known as the Hell Creek Formation from mostly the Upper Cretaceous period. That means those rocks witnessed the final days of dinosaurs on Earth. So what did paleontologists find there?

For starters, they found impact debris and those quartz crystals we mentioned earlier. They also found tektites, which are hunks of glass that scientists think formed in the atmosphere in the moments post-impact from coalescing pieces of molten Earth that were sprayed into the sky. That glass then rained down on the flora and fauna below. Interestingly, the tektites at the Tanis site form a thicker layer than has been observed at other sites, suggesting that water on the surface at the site may have been sloshing back and forth in order to distribute them.

The site also serves as a tomb for a large group of fish that got thrown ashore upon impact and have gills full of related debris. The remains of other sea-dwelling creatures are also found at the Tanis site which the authors claim may be evidence that an inland sea was suddenly forced upstream at the time of impact, gushing into the banks of Tanis and combining sea and river creatures into one area.

The volcanoes known as the Deccan Traps were much larger than present-day volcanoes and spent roughly 1 million years erupting, spewing more than 135,000 cubic miles of lava.

Lead author of the study, Dr. Robert DePalma, a PhD student at the University of Kansas and curator of the Palm Beach Museum of Natural History, suggests there is enough to study at the site to last scientists for decades to come.

Interestingly, as is their job, some scientists remain skeptical of the extent to which we can draw conclusions from the Tanis site. For example, a piece in National Geographic highlights the exciting potential of the findings, but also points out the unusual timeline of an article in The New Yorker which was published before the journal article and made more claims than the peer-reviewed piece. So is this find too good to be true? My answer might sound familiar if you’re a frequent Ask Science listener: We need to do more research to find out!

Volcanoes in Ancient India

Most people have heard of the theory that an asteroid ended the dinosaurs. But was the asteroid working alone? Scientists have suggested since the 1980s that ancient Indian volcanoes could have worked with an asteroid impact to help kill off the dinosaurs and two new studies published earlier this year in the journal Science have laid out a clear timeline of how that might have happened. The volcanoes known as the Deccan Traps were much larger than present-day volcanoes and spent roughly 1 million years erupting, spewing more than 135,000 cubic miles of lava, over a time period that straddles the timing of the event that created the Chicxulub Crater. Interestingly, both teams of researchers that authored the studies agree on that timing estimate.

The teams disagree, however, on the details of that timing. One group reported more eruption activity from the volcanoes in the ~100,000 years before the suspected asteroid impact which would have put a burden on ecosystems just in time to make them extremely vulnerable at the time of the asteroid’s arrival. The other study, however, found the eruptions were more dominant later, with 75% of their lava being spewed post-impact. In that scenario, the volcanoes would not have been as big of a factor in ending the dinosaurs reign. Instead, the impact may have inspired some of the continued eruptions.

This may seem like a pretty large disagreement, but the teams do agree on one important fact: the timing of the Deccan Traps volcanoes is very nearly coincident with the impact event suspected to have caused the extinction of the dinosaurs. So either way, the volcanoes have more to tell us about why the dinosaurs disappeared.

And although our dating techniques continue to improve, the event marked by the K-Pg boundary is likely to remain a challenge to study because it occurred over a timeframe of days. Rocks and fossils, our only way of probing this time period, on the other hand, like to tell us stories on time scales of thousands of years.

Until next time, this is Dr. Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.

Dinosaur image courtesy of Shutterstock.