The buzz over the causes of the mass extinction 65 million years ago, at the
Cretaceous-Tertiary (K/T) boundary, is getting louder. In the past 20 years,
many geologists have accepted the idea that a massive asteroid, or some other
large projectile, crashed off the Yucatán Peninsula in Mexico and triggered
the demise of the dinosaurs and 75 percent of life on Earth through ensuing
climate changes. Now, a paleontology graduate student has found evidence in
the survival of tiny honeybees that could be another sting to that theory.
Although the impact theory has come under scrutiny in recent years, many scientists
have proposed that in the aftermath of the K/T impact, dust and debris swirled
in the atmosphere for about 10 years, choking out sunlight, halting photosynthesis
and cooling the planet. Models of this "impact winter" have estimated
that global temperatures plummeted 7 to 12 degrees Celsius (13 to 22 degrees
Fahrenheit), freezing out ecosystems and exterminating three out of every four
species at the K/T boundary. In the oceans, it may have taken up to 2,000 years
for temperatures to return to previous levels.
Jacqueline Kozisek, a graduate student in vertebrate paleontology at the University
of New Orleans, recognized that there were not many specifics of temperature
change from the models, so she went looking for K/T survivors that might give
some clues about environmental conditions following the impact. She focused
on tropical honeybees because they have modern descendents with which to compare
them, and their very survival meant extreme temperature changes could not have
dominated the entire planet.
Based on fossil records, bees first evolved during the Late Cretaceous. While
many species were omnivorous, the tropical honeybee, Cretotrigona prisca,
relied mainly on nectar from flowering plants and never stored any food. Specimens
captured in amber show few changes between then and now. After the giant impact
65 million years ago, the only way the tropical bees could have survived was
if the tropics and the flowers did too.
"The tropics are such a fragile environment," Kozisek says, and these
ecosystems would have been the ones most easily affected by the impact and a
long, dark and cold impact winter. Fossil specimens support that the tropical
honeybee endured the collision, and not just in an isolated refuge. Based on
environmental requirements for tropical bees today, temperatures could not have
dropped more than 7 degrees Celsius after the impact, says Kozisek, who presented
her results at the the Geological Society of America's annual meeting in November.
Matthew Huber of Purdue University, who has co-authored a paper on the impact
winter, says that such local variations among greater global temperature changes
make sense. "It is quite conceivable, based on the modeling that I did,"
Huber says, "that bees living near the coasts in the tropics would experience
significantly above-freezing temperatures for several years during even a total
impact winter."
"I'm definitely not trying to say a nuclear [impact] winter didn't happen,"
Kozisek says. She hopes other scientists will perform similar analyses of K/T
survivors to help "flesh out" the specific consequences of the impact
winter and create a better picture of post-impact environmental conditions.
"We need more studies such as this to constrain the magnitude of and severity
of the global cooling episode," Huber says. Similar research should continue
to clear the haze of the K/T extinction.
Joshua Zaffos
Geotimes contributing writer
Back to top
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers |