The large asteroid that crashed into the Earth 65.5 million years ago is best known for killing off the dinosaurs, but it also triggered a mass extinction in the oceans. Curiously, though, organisms living in inland rivers and lakes showed much lower rates of extinction.
Some scientists now think they can begin to explain this discrepancy: A new study proposes that the biological adaptations some organisms evolved to deal with the challenges of living in freshwater environments also helped shield them from the months of frigid darkness that followed the asteroid impact.
(Related: "What Killed Dinosaurs: New Ideas About the Wipeout.")
"[Freshwater] organisms are adapted to physical and chemical changes that go well beyond what marine organisms need to be adapted for," said study co-author William Lewis, a freshwater scientist at the University of Colorado.
For example, many freshwater creatures are adapted to annual freeze-thaw cycles and periods of low oxygen. Therefore, many of them can go dormant by burying themselves or their eggs in the mud, and thus would have been able to wait out the worst effects of the impact.
"We do see some dormancy in the marine environment," Lewis explained, "but it's unusual because it's not necessary for most organisms."
The findings, published online in the Journal of Geophysical Research—Biogeosciences, brings much-needed specificity to an area of research that has long been plagued by generalities, experts say.
"I think before that most people had concentrated on the collapse of the food chain to explain why certain groups went extinct," said Alison Murray, a paleontologist at the University of Alberta in Canada.
"In this current paper, the authors are developing that food chain collapse, but in a more detailed manner, examining different groups and determining which might survive a prolonged period without light and the corresponding loss of the photosynthetic organisms," Murray, who did not participate in the research, said in an email.
David Fastovsky, a paleontologist at the University of Rhode Island, agreed. "I've never seen a coherent statement of how we think [that mass extinctions] actually happen," Fastovsky said.
"But here it is, in living color on NBC, a model of how it actually might have worked."
Extinction in Two Parts
In earlier work, study first author Douglas Robertson, a geophysicist who is at the University of Colorado, showed that the asteroid that slammed into present-day Chicxulub, Mexico, likely triggered a global firestorm and hurled huge amounts of vaporized rock high above the atmosphere.
When the ejecta fell back to Earth a few hours later, it would have then reentered the atmosphere so fast that the heat of its descent would have caused the sky to glow red and tinder on the ground to burst into flames.
"The radiation and fires would have been fatal within hours to everything that was not sheltered underground or underwater," Robertson said. "Dinosaurs all died within a few hours of the impact."
Next, the mixture of dust and ash still in the air would have darkened the sky and plunged the planet into an "impact winter" lasting months to years. Plants and other organisms that relied on the sun's light for energy quickly died.
According to this model, the oceans were largely shielded from the initial burst of heat and fire, but soon after, entire groups of organisms, including the giant marine reptiles known as plesiosaurs and shelled, squid-like creatures called ammonites, became extinct when marine food chains collapsed. (See photos of prehistoric "sea monsters.")
About 20 years ago, however, scientists noticed that the extinction levels among freshwater creatures were more subdued: Whereas marine environments lost as much as half of their groups of creatures, the freshwater extinction rate was only about 10 to 20 percent.
At the time, some scientists, including Fastovsky and Peter Sheehan, a paleontologist at the Milwaukee Public Museum and a co-author on the new study, explained this curious pattern by pointing out that freshwater organisms are more accustomed to feeding off detritus, or dead organic matter.
During the impact winter, freshwater environments would have received a steady influx of dissolved organic matter that was regularly washed into rivers and streams from dead plants and animals on land. Those same flowing water sources would have also kept freshwater ecosystems well oxygenated.
Robertson and his team agree that detrital feeding was a factor in explaining why freshwater organisms better survived the dark winter that followed the impact. However, the ability to enter "dormancy and [greater availability of refuges] in freshwater are probably more important," Robertson said.
The challenge now for paleontologists will be to figure out ways to test the hypotheses outlined in the paper. "That's the bugaboo," Fastovsky said.
For example, according to the model developed by Robertson and his team, the collapse of the ocean food chain would have happened more quickly than in freshwater environments.
"That's something that logically follows from this model, but that's not a testable statement," in part because current scientific techniques are not yet capable of resolving time differences of months or a few years in the fossil record, Fastovsky said.
"I don't know that we have the resolution to do that," he said. "We might at some point."
Study co-author Robertson disagreed that the food chain collapse was faster in the oceans—he thinks the collapse occurred on similar timescales in both freshwater and marine environments, but that lakes and rivers recovered faster—but agreed that testing his team's model would not be easy.
For one thing, "the evidence for the survival of freshwater ecosystems comes entirely from the fossil record in a small region in Montana," Robertson explained. "It will be both important and difficult to find similar evidence elsewhere on the planet."
Follow Ker Than on Twitter.