Sixty-six million years ago, an asteroid slammed into what is today the Yucatan peninsula of Mexico. The impact was so forceful that it kicked a tremendous amount of debris out of the atmosphere, which then rained back down, blanketing the Earth's surface with a layer of dust.
All that debris re-entering the atmosphere created a pulse of heat so strong that it set the world on fire.
As @laelaps describes so vividly in her book, The Last Days of the Dinosaurs, this heat -- which only lasted a few hours--was lethal to most animals on the Earth's surface.
Most of those that survived were the ones that could hide in burrows or in the wet protection of lakes, swamps, or the oceans. And then they had to contend with a blasted, charred landscape where little plant life remained except as seeds or roots in the soil.
The survivors weren't out of the proverbial woods yet. The ash of these global fires, and the dust from the impact, then severely cooled the planet in an "impact winter" that lasted for years. Most plants lacked the light they needed to thrive. Food was scarce.
The recovery of life in the aftermath of this mass extinction is one of the greatest stories of resilience in Earth's history.
And we owe it, in part, to the humble fern.
The fossil record is like a book read in the rocks. Some pages (rock layers) are really detailed, with lots of information. Others are missing -- sometimes entire chapters. Sometimes, individual pages are really thick, and the words are blurry -- you can only make out fragments.
The K-Pg impact is not one of those blurry, thick layers where the geologic timeline is muddy.
It's one really bad afternoon recorded in a visible layer of dust, ash and charcoal. You can even touch it, in places like New Mexico, where the rock outcrops are easily accessible.
That bright line in the rock marks the end of the old world (dinosaurs, cycads and conifers) and the beginning of the new (flowering plants and mammals, which rapidly filled the niches left by dinosaurs).
No impact, no us. It's a debt we can't repay, but we can pay it forward.
So, how did life recover after the impact? If you sample the rocks right above the K-Pg impact layer, you can recreate the timeline of how plants recovered after the fires, the thick blanket of ash, the darkness, the acid rain, and the cold.
One group of plants held on in this harsh environment, as indicated by their abundance in the fossil record in the years, centuries, and even millennia after the impact event: FERNS.
Ferns' success after a catastrophe has even been seen in modern environments. They're often the first species to return after volcanic eruptions.
So, what's their secret? Is it their ability to disperse easily, via tiny spores? Or is it something about fern biology that helps them thrive after disasters?
To test this, my colleagues Jarmila Pittermann, Emily Sessa, Regan Dunn, and Ellen Currano and I were funded by NASA Exobiology to bridge paleontology with modern experiments, growing ferns and other plants in the pre- and post-impact environment.
Last fall, the experimental "asteroid" hit the greenhouse.
In a greenhouse in Santa Cruz, Jarmila Pittermann recreated the post-impact environment (low light, acid rain, cold) and then measured how ferns and other plants responded. You can read about the experiment in this fantastic article in the LA Times by Corrine Purtill:
We're hoping that our research will shed light on why ferns were so successful following disasters, which can help us to understand how to manage ecosystems following major upheavals (stay tuned for a really cool paper on this by postdoc Lauren Azevedo-Schmidt!).
Finally, my first toot made it sound like TODAY is the anniversary of the K-Pg asteroid impact. While the geologic record of that event is very precise, we couldn't possibly know when, exactly, the impact happened.
Or could we?
A 2022 paper led by Melanie During looked at the fossils of fishes that died in the impact event, and found growth patterns and other evidence consistent with the fish dying during the northern hemisphere springtime!
This is consistent with an earlier study by Jack Wolfe, who experimented on the leaves of aquatic plants from a K-Pg site in Wyoming. Based on their growth patterns, he concluded that the Chixulub asteroid like hit the Earth in...[drumroll]...
June.
A LARGE bolide impact, such as that thought to have occurred at the Cretaceous/Tertiary (K/T) boundary, should produce large amounts of light-attenuating debris, thereby causing an 'impact winter'1–3. Because of thermal buffering in the oceans, evidence for a brief (1–2 months2–4) impact winter would be found only in terrestrial environments. Aquatic leaves in the K/T boundary section near Teapot Dome, Wyoming, preserve structural deformation that can be duplicated experimentally in extant aquatic leaves by freezing. Reproductive stages reached by the fossil aquatic plants at the time of death suggest that freezing took place in approximately early June. Both the existence of the structurally deformed plants and the high abundance of fern spores occur in a horizon containing sparse impact debris, but below the horizon containing abundant impact debris; I therefore suggest that the lower horizon represents debris and effects from a large, distant bolide impact, and the upper horizon represents a small, nearby bolide impact.
Professor, I've missed your paleoclimate posts since I had left Twitter, glad to have them (and you!) back in here! 🤩
@christoph_STCmicrobeblog It's scientifically interesting because the seasonal timing of the extinction may actually have influenced the trajectory of the recovery of ecoystems (as Melanie's paper explores).
"June" just means "6-8 weeks after the spring equinox and a few weeks before the summer solstice."
If you're going to quibble about seasonality, you might as well take issue with "Wyoming" and "Mexico," which also didn't exist. :)
@JacquelynGill
I get it 👍
and concerning the K/T line I mostly think of the bottagione gorge near gubbio/umbria (simply bc it's closer to home 😁 and I love umbria for other reasons too)
Cool.
I'm betting that my dad's garden nemesis and my woodland friend the horsetails would have done OK, though IDK why only Equisetum is still around.
We seem to be trying to do that by clearing a lot of niches for whatever comes next. 😡
@JacquelynGill
"No impact, no us. It's a debt we can't repay, but we can pay it forward."
wise words (not really science·y tho).
Q : do you and your colleagues consider it likely that ferns were preceeded by lichens & mosses? I have frequently seen the most inhospitable places (rock) covered by lichens and, at more humid spots, mosses. ferns only if there was a tiny bit of soil at least (me🙋🏻♂️=microbiologist free-styling as botanist here)
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