The dino-killing asteroid smashed into an unfortunate spot and at the deadliest possible angle, and as a group of scientists have now concluded, it also struck Earth at a sensitive time of year—at least for animals living in the northern hemisphere.
The Chicxulub asteroid struck the Yucatan Peninsula some 66 million years ago, unleashing an impact winter that wiped out 75% of all species on Earth. It also ended the 165-million-year reign of the dinosaurs, allowing mammals to finally assert their dominance. For Cretaceous plants and animals living in what is today the western U.S. interior, the onset of this asteroid-induced winter occurred during the late spring, according to new research published in Scientific Reports.
Knowing the season during which the asteroid struck may not sound like a big whoop, but the new study deepens our understanding of this event and how it may have affected biological systems, both locally and at a the global scale.
“Time of year plays an important role in many biological functions such as reproduction, feeding strategies, host-parasite interactions, seasonal dormancy, and breeding patterns,” Robert DePalma, the senior author of the paper and a researcher from Florida Atlantic University, explained in a press release. “Hence, it is no surprise that the time of year for a global-scale hazard can play a big role in how harshly it impacts life.”
That the scientists were able to pinpoint not just the season but the stage of the season is pretty damned impressive. The discovery was made possible owing to the Tanis geological site—a remarkable sedimentary deposit from the Hell Creek Formation of North Dakota. The Tanis site features “a highly constrained sedimentological chronology” that’s “uniquely suited” to studying the “immediate post-impact events in a highly refined time-scale,” according to the study.
A paper co-authored by DePalma in 2019 identified the Tanis site as being ideal for this type of investigation. The lower of the two Tanis layers appears to be packed with debris from the impact as well as fossils of plants, trees, and animals that died on that apocalyptic day, while the upper layer was produced by accumulating falling ash, according to DePalma’s previous work.
For the new study, the scientists tracked patterns of growth as revealed in the Tanis fossils. By estimating the age of fish when they died (i.e. when they were buried by debris), DePalma and his colleagues were able to deduce the timing of the spawning season. Similar lines of evidence emerged by studying insect behavior, such as studying leaves damaged by bugs and the emergence of adult mayflies. This, plus other lines of evidence, pointed to the impact happening in the late spring.
“Field data collected at the site, after hard work that went into analyzing it, provided us with new incredibly detailed insight of not only what happened at the Cretaceous-Paleogene boundary, but also exactly when it happened,” said Anton Oleinik, a co-author of the study and a researcher at FAU. “It is nothing short of amazing how multiple lines of independent evidence suggested so clearly what time of the year it was 66 million years ago when the asteroid hit the planet.”
Importantly, these findings are the result of studying a single site. Future research at other locations would do much to strengthen the claims made in the new paper as well as the one from 2019 (that the Tanis site captures evidence of the Chicxulub impact is not a certainty). Also, having other teams inspect the Tanis site or analyze the sedimentary and fossil evidence gathered by DePalma’s team would also be a good idea, but that might not happen. As The New Yorker reported in 2019, DePalma works on private land—he pays the owning rancher—and he holds exclusive excavation rights to this part of the Hell Creek Formation. He subsequently retains oversight of any fossils pulled from the Tanis site, which means they can’t be studied freely by other scientists.
“I don’t know of another case though where someone keeps the location secret so only they can see it or study it,” Blair Schoene, a geoscientist at Princeton University, wrote in an email. “Makes it hard to verify their results, and so if that’s really how it’s being handled, it isn’t good scientific practice.”
In an email, DePalma told Gizmodo that the site’s location “was only kept confidential prior to the first publication, as is common practice regarding vulnerable material in the sciences” and that outside researchers “have visited the site nearly every dig season since work began, and/or continue to work on the material.” Paleontologist Neil Landman from the American Museum of Natural History told Gizmodo that he and a dozen professors and students toured the site in August 2019, and he noted that it’s not uncommon for access to be limited when a paleontological site is on private land.
If the interpretation presented in the new paper is correct, then the timing of the Chicxulub impact was seriously unfortunate. Late spring is a time for renewal, but on this occasion it was marred by apocalyptic destruction. The scientists say the effects of the asteroid “would have been amplified” for plants and animals in the northern hemisphere that were dependent on this season for growth and reproduction.
The scientists say mass deaths of young “would have been especially disastrous for species that took many years to reach breeding age or bred only under ideal conditions,” while the rapid onset of an impact winter, caused by copious amounts of debris in the atmosphere, would’ve made life particularly tough in ecosystems sensitive to seasonal shifts. As a result, extinction patterns in the northern hemisphere likely differed from those observed in the southern hemisphere, according to the paper. That alone is a very cool finding and something for future research to scrutinize.
So, some incredibly detailed results from what is an incredibly detailed geological layer. We still have so much to learn about what exactly happened on that most awful of awful days.
This article has been updated with comments from DePalma and Landman.