Evolution will adapt a given population more accurately to contain the knowledge needed to survive. 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 impact of the large asteroid that crashed into present-day Chicxulub, Mexico, likely triggering a global firestorm and hurled huge amounts of vaporized rock high above the atmosphere 65.5 million years ago, killing off the dinosaurs.
In 1990, dramatic support for this theory came from cosmochemist Alan Hildebrand's revelation of a 65 million year old, 112 mile wide ring structure shown in the image above that is still detectable under layers of sediment in the Yucatan Peninsula region of Mexico. The outlines of the structure, called the Chicxulub crater (named for a local village), are visible in the above representation of gravity and magnetic field data from the region. In addition to having the right age, the crater is consistent with the impact of an asteroid of sufficient size (6 to 12 miles wide) to cause the global disruptions. Regardless of the true cause of the K-T event, it is fortunate that such impacts are presently believed to happen only about once every 100 million years.
The "heat pulse" caused by re-entering ejected matter would have reached around the globe, igniting fires and burning up all terrestrial organisms not sheltered in burrows or in water, he said. "The kinetic energy of the ejected matter would have dissipated as heat in the upper atmosphere during re-entry, enough heat to make the normally blue sky turn red-hot for hours," said Robertson.
Scientists have speculated for more than a decade that the entire surface of the Earth below would have been baked by the equivalent of a global oven set on broil. The evidence of terrestrial ruin is compelling, said Robertson, noting that tiny spheres of melted rock are found in the Cretaceous-Tertiary, or KT, boundary around the globe.
The spheres in the clay are remnants of the rocky masses that were vaporized and ejected into sub-orbital trajectories by the impact. A nearly worldwide clay layer laced with soot and extra-terrestrial iridium also records the impact and global firestorm that followed the impact. The spheres, the heat pulse and the soot all have been known for some time, but their implications for survival of organisms on land have not been explained well, said Robertson. Many scientists have been curious about how any animal species such as primitive birds, mammals and amphibians managed to survive the global disaster that killed off all the existing dinosaurs.
"[Freshwater] organisms are adapted to physical and chemical changes that go well beyond what marine organisms need to be adapted for," said William Lewis, a freshwater scientist at the University of Colorado. Many freshwater creatures are adapted to annual freeze-thaw cycles and periods of low oxygen when many of them go dormant by burying themselves or their eggs in the mud. "We do see some dormancy in the marine environment," Lewis added, "but it's unusual because it's not necessary for most organisms."
"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, published online in the Journal of Geophysical Research—Biogeosciences, 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.
In his earlier work, Robertson, showed that when the impact debris 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." The dust and ash still in the air would have darkened the sky and plunged the planet into an "impact winter" lasting months to years killing off 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.
About 20 years ago, scientists began to notice 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. Meanwhile, Fastovsky and Peter Sheehan, a paleontologist at the Milwaukee Public Museum and a co-author on the new study, observed 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. However, the ability to enter dormancy in freshwater was probably more critical to survival, Robertson added.
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.
Robertson belives that thinks the collapse occurred on similar timescales in both freshwater and marine environments, but that lakes and rivers recovered faster—but agreed that testing the model would not be easy because the evidence for the survival of freshwater ecosystems comes entirely from the fossil record found in a small area in Montana. "It will be both important and difficult to find similar evidence elsewhere on the planet," he concluded.
The Daily Galaxy via http://solarsystem.nasa.gov/news/display.cfm?News_ID=8396 and National Geographic
Image credit: courtesy V.L. Sharpton, LPI