Boulder, Colo., USA: Sixty-six million years ago, the dinosaurs had a really bad day when a colossal asteroid impact spurred their extinction. But even though those reptilian megafauna get all the attention, the devastation was just as bad, if not worse, for tiny marine single-celled organisms called foraminifera, or forams. These creatures, which make elaborate shells of calcium carbonate that are well preserved in the fossil record, serve as key clues for geologists working to understand the impacts of environmental change in Earth's history.
"You can identify all these species of forams, and you can reconstruct in great detail how ocean ecosystems have changed in response to changing climate, changing ocean circulation, or changing tectonic configurations," says Dr. Chris Lowery, a paleoceanographer at the University of Texas at Austin. "That really helps us to understand how these ocean ecosystems respond to these changes, and helps us understand, then, how they can respond in the future."
Ninety percent of foraminifera species died off during the mass extinction event that ended the dinosaurs' reign. The sudden disappearance of foraminifera species at the end-Cretaceous mass extinction and the reappearance of new species just after are a primary way geologists identify the event in marine rocks around the world. But just how quickly those new species evolved in the wake of the asteroid impact has long been debated by scientists.
Lowery and his colleagues' new research, published today in the journal Geology, in a paper titled "New species evolved within a few thousand years of the Chicxulub Impact," determined that the rise of new species was remarkably quick, occurring in some places in less than two thousand years. For reference, in normal evolutionary conditions it takes around two million years on average for a new species to develop.
"When you take away all the constraints," asks Lowery, "how quickly can you get new species forming? The answer seems to be very, very fast. And that's important to understand throughout Earth history—it's also important for us." Lowery says that as anthropogenic warming of Earth's climate potentially moves the planet toward another mass extinction, looking to the past record of how species respond to rapid change could help us understand what might happen in the near future.
To figure out how long after the asteroid impact new foraminifera species originated, the team measured the amount of an isotope of helium, 3He, in sediments deposited just after the asteroid at six sites around the world. 3He only occurs on Earth through a steady delivery of interstellar dust and wasn't increased by the material brought in by the asteroid. In sediments deposited slowly, there's a higher concentration of 3He. When sediments are deposited more quickly, the isotope is diluted. By precisely measuring the 3He concentration in the rock layer between the asteroid impact and the first new species, the scientists were able to determine how long it took for the novel foraminifera to emerge.
Previously, the geologic time scale for this period was determined by extrapolating between the known ages of magnetic reversals hundreds of thousands of years before and after the asteroid impact. That approach, though, doesn't account for changes in the rate of sedimentation during one of the most catastrophic events in Earth history. In this paradigm, the new foram species emerged about 10,000 years after the impact. That's still remarkably quick in terms of normal rates of evolution, but still an order of magnitude slower than what Lowery and his team found.
"Why does this matter? It helps us understand how quickly evolution can occur," says Lowery. "It helps us understand how the environment changed after the impact and how brief and extreme that disturbance actually was."
Geology: https://doi.org/10.1130/G53313.1
About the Geological Society of America
The Geological Society of America (GSA) is a global professional society with more than 17,000 members across over 100 countries. As a leading voice for the geosciences, GSA advances the understanding of Earth's dynamic processes and fosters collaboration among scientists, educators, and policymakers. GSA publishes Geology, the top-ranked geoscience journal, along with a diverse portfolio of scholarly journals, books, and conference proceedings—several of which rank among Amazon's top 100 best-selling geology titles.
