© Don Davis/NASAWatch out! The object that slammed into Earth 66 million years ago wiped out the nonavian dinosaurs and many other groups but largely spared species in freshwater ecosystems, a disparity explained by a new study.
When an
asteroid or comet slammed into Earth about 66 million years ago, most of our planet's species were wiped out in a mass extinction - including entire groups such as the nonavian dinosaurs, marine reptiles such as mosasaurs, and their flying kin the pterosaurs. But not all ecosystems suffered equally, and the dramatic difference in survival rates between marine species and freshwater ones has been particularly puzzling. A new study weighs in on the long-standing riddle.
According to some estimates, about three-quarters of the plant and animal species on Earth disappeared during the end-of-the-Cretaceous dino-killing impact. But marine ecosystems lost only about half of their species, and freshwater environments lost a mere 10% to 22%, says William Lewis, an aquatic ecologist at the University of Colorado, Boulder.
For instance, only about 10% of the major groups of bony fish died out, but species from all six groups of turtles alive at the time - and from most if not all groups of amphibians - survived the impact. The disparity with marine ecosystems began to make sense, he notes, when researchers began thinking of the mass extinction as a one-two punch: the fiery aftermath of the extraterrestrial impact, followed by a "nuclear winter"-like cold spell triggered by the smoke, soot, and myriad other tiny particles flung high into the atmosphere.
In the wake of the impact, creatures in marine and freshwater ecosystems experienced three particularly strong stresses: starvation brought about by the collapse of the food chain (and especially by the lack of photosynthesis), the reduction or loss of dissolved oxygen in the water, and low temperatures. But in many cases,
species living in freshwater environments had advantages over sea creatures that bolstered their chances of survival, Lewis and his colleagues explain this month in the
Journal of Geophysical Research: Biogeosciences. Not that the animals living in lakes and rivers escaped unscathed, Lewis says: "A lot of them died, too, it's just that many species as a whole were able to persist until conditions returned to something near normal."
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