To understand what caused the end-Cretaceous mass extinction, the first challenge was to recognize that it actually happened. This recognition, instigated by the discovery of a meteor impact and supported by fine-scale fossil studies, constituted a major scientific shift from a paradigm of gradual decline to one of abrupt cataclysm.
For a long time, paleontologists viewed the tremendous turnover from a world of dinosaurs (and ammonoids and belemnites) to a world of mammals (and nautiluses and squid) as both slow and inevitable. This perspective of the extinction went along with the perspective, at the time widely accepted, that dinosaurs themselves were slow and archaic. They were obviously doomed.
Then in 1980, an iconoclastic father–son duo, Luis and Walter Alvarez, championed a new theory of extinction involving a catastrophic meteor strike.1 As evidence for the meteor grew irrefutable, paleontologists remained puzzled over how such an event could have eradicated dinosaurs (etc.) while leaving mammals (etc.) untouched. Fine, there was an impact, they said, but it was just the last nail in the coffin. Dinosaurs were already deep in decline.
Ammonoids elicited a similar train of thought. For instance, the 1996 edition of the authoritative Ammonoid Paleobiology textbook tells us, “The ultimate extinction of the ammonoids was a continuous and long-lasting decline that can be traced over several million years.” This is followed by a decisive dismissal of the meteor strike: “There is no need to involve a major cosmic impact to explain the final decline of ammonoids.”2
But the acclaimed nautilus biologist and paleontologist Peter Ward has argued passionately for a sudden demise of ammonoids. In places where rock layers have been deposited continuously from many years before the asteroid impact to many years after, Ward and his collaborators have found that numerous ammonoid species fossilized abundantly right up to the impact —and then abruptly disappeared. Statistical analysis also showed that even those ammonoid species that disappeared from the fossil record well before the impact were likely to have actually survived to the end of the Cretaceous. “Everything looks more gradual than it was,” explains Matthew Clapham. “If you happen to pick a given rock, not every animal that lived then is fossilized in that rock. So ther are gaps within a species’ life span. And if there are gaps within, there must also be gaps at the ends. So not every species appears to go straight to the boundary.”3
After much scientific back-and-forth with various degrees of civility, the importance of the meteor impact in the end-Cretaceous extinction has been widely accepted. In the latest edition of Ammonoid Paleobiology (2015), Landman and colleagues write, “It is now generally accepted that the disappearance of ammonites . . . was due to the asteroid impact. However, the exact killing mechanism is still unknown.”4
So it was Mr. Asteroid in the Cretaceous Room with . . . the candlestick? The rope? The revolver?
In Search of the Smoking Gun
The extraterrestrial shock that life received at the end of the Cretaceous was compounded by another round of extreme volcanism, only a little less destructive than the one that ended the Permian. Both the explosion from without and the explosions from within Earth set off a cascade of environmental changes that could have killed animals by heat, by cold, or by acid.
My daughter’s preschool is embarking on a dinosaur theme as I write. Apparently they covered the Alvarez impact hypothesis at circle time; she came home with an enthusiastic description of the meteor “way up in outer space” that hit Earth and “tore off all the dinosaurs and plants.” It’s a compelling story, to be sure. I struggle to explain in four-year-old language that the meteor didn’t physically knock everything off the planet. Instead, it started fires, changed the climate, damaged the air and the seas . . . I’ve lost her, and I realize I don’t fully understand it myself.
There’s some comfort in good company. “I realized after fifteen years working on mass extinctions I still don’t know what causes them,” said David Bond, a paleontologist from the University of Hull in England, at the beginning of a talk at the 2016 meeting of the Geological Society of America.5 This opening statement was a bit glib, though, because he went on to explain quite convincingly what does cause nearly all mass extinctions: volcanism. And even though the end Cretaceous extinction was anomalously triggered by a meteor impact, it also involved some pretty epic volcanoes.
Illustrations in dinosaur books tend to have lots of volcanoes in the background. Such prehistoric scenes sometimes even show a terrified Tyrannosaurus in futile flight, like a Pompeiian fleeing Vesuvius. However, volcanoes were not popping up all over Earth and erupting on a daily or even a weekly basis, as the picture books might lead us to believe. End-Cretaceous volcanism was concentrated in India, which at this point was still an island on its way toward a dramatic slow motion collision with Eurasia. In fact, the volcanic flood basalts in India are very similar to the end Permian paving of Siberia, 19,000 square miles (500,000 km2; compared with the Siberian 772,000 square miles/2,000,000 km2) and erupting over the course of perhaps thirty thousand years (to the Siberian hundred thousand).6 While less impressive, it’s clearly volcanism on a grand scale and could have had similarly dramatic environmental impacts.
But . . . what about that meteor? Well, here’s the wild part: maybe the meteor caused the volcanoes.7 Some geologists suggest that the volcanic area in India was preloaded with magma, and the profound shock of a huge rock smashing into the planet brought this magma to the surface. It’s hard to know for sure whether this connection is real, but the extraterrestrial rock in question would certainly have been capable of triggering global earthquakes. It left a dent more than 110 miles (177 km) wide in the Yucatán Peninsula in Mexico, now called the Chicxulub (pronounced “cheek-shuh-loob”) crater. That’s big enough for the entire big island of Hawaii to fit inside, with room to spare. Scientists calculate that the crater-producing meteor must have been at least 6.8 miles (11 km) in diameter, which means it could have comfortably contained Mount Everest (if the great peak had existed back then —since India was still an island, Everest had not yet protruded from the planet).
The impact had impacts both short- and long-term on every habitat —except the deep sea. Earth’s surface was cooked for a few minutes by the reentry heat of particles that had been thrown out into space. Gases released into the atmosphere caused an “impact winter” in subsequent years (like the nuclear winter hypothesized by science fiction writers of the mid-1900s), as well as acid rai that dramatically changed the surface chemistry of the oceans.8
Obviously, if you happened to live at the end of the Cretaceous, you got a pretty raw deal. But the question that remains is how changes like these led to the specific changes we see in the fossil record. Why did some groups of animals disappear altogether, while others emerged relatively unscathed? One answer is simple chance. Lady Luck plays a huge role in extinction; each crisis is a new roll of the dice that sends some unfortunate species away from the table.
“Ammonoids almost bit it at every major extinction event,” Kathleen Ritterbush points out, which suggests that they were vulnerable.9 Though they finally did bite it at the end of the Cretaceous, it could have happened as easily at the end of the Permian or of the Triassic. The good luck that allowed them to survive as long as they did is perhaps more worthy of wonder than the bad luck that ended their days. Still, as Jocelyn Sessa notes, “It’s our human nature that we are going to look at this [extinction] event and look for a reason, look for explanations.”10
Excerpt from Monarchs of the Sea: The Extraordinary 500-Million-Year History of Cephalopods © Danna Staaf, 2017, 2020. Reprinted by permission of the publisher, The Experiment. Available wherever books are sold. experimentpublishing.com
- L. W. Alvarez, W. Alvarez, F. Asaro, and H. V. Michel, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” Science 208 (1980): 1095–1108. This is the explosive paper in which Walter and Luis Alvarez proposed that a meteor impact ended the dinosaurs (and ammonoids). For an outstanding breakdown of the turmoil and vitriol and wound licking that followed, read this account compiled by the science writer Ann Finkbeiner: http://www.lastwordonnothing.com/2013/11/11/what‑luis‑alvarez‑did/ (accessed January 28, 2017).
- Jost Wiedmann and Jürgen Kullman, “Crises in Ammonoid Evolution,” in Ammonoid Paleobiology, ed. Neil H. Landman, Kazushige Tanabe, and Richard Arnold Davis (Springer, 1996), 795–813.
- Matthew Clapham, Interview with the author, March 24, 2016.
- Neil H. Landman, Stijn Goolaerts, John W. M. Jagt, et al., “Ammonites on the Brink of Extinction: Diversity, Abundance, and Ecology of the Order Ammonoidea at the Cretaceous/Paleogene (K/Pg) Boundary,” in Ammonoid Paleobiology: From Macroevolution to Paleogeography, ed. Christian Klug, Dieter Korn, Kenneth De Baets, et al. (Springer, 2015), 497–553.
- David P. G. Bond, “The Causes of Mass Extinctions: How Can We Better Understand How, Why and When Ecosystems Collapse?” Paper presented at the GSA Annual Meeting, Denver, September 25–28, 2016.
- Scientists estimate that both the Siberian and the Indian flood basalts once covered far greater areas—about 580,000 square miles (1,500,000 km2) in India and up to nearly 3,000,000 square miles (7,000,000 km2) in Siberia.
- “Did Dinosaur-Killing Asteroid Trigger Largest Lava Flows on Earth?” http://www.sciencenewsline.com/news/2015050109530049.html (accessed November 12, 2016).
- Peter Schulte, Laia Alegret, Ignacio Arenillas, et al., “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous–Paleogene Boundary,” Science 327, no. 5970 (2010): 1214–1218.
- Kathleen Ritterbush, Skype interview with the author, August 28, 2015.
- Jocelyn Sessa, Skype interview with the author, January 21, 2016.