An asteroid really was the dominant cause of the most recent mass extinction, and it did not spare creatures living around Antarctica, despite their distance from the impact site, marine fossils suggest.
Dinosaurs were already on the decline when an asteroid formed the Chicxulub crater, but hundreds of thousands of species, from all classes of the animal and plant kingdom, were wiped out by the collision and its immediate aftermath.
Nevertheless, some evidence suggests polar species were less affected. University of Leeds PhD Student James Witts has refuted this with a study of marine fossils at Seymour Island off the Antarctic Peninsula. In the process, Witts has added further evidence that it was the asteroid, rather than events preceding it, that did the most to transform the planet.
Seymour Island is one of the few places that has provided us with a record of Antarctic life in the late Cretaceous Era. Six thousand mollusk fossils have been extracted from a thousand meters (3,300 feet) of sedimentation laid down over 4 million years, neatly including the time of the impact.
"Most fossils are formed in marine environments, where it is easy for sediment to accumulate rapidly and bury parts of animals, such as bones, or bodies of creatures with a hard shell. For a dinosaur or other land animal to become fossilized, a series of favorable events are needed, such as for bones to fall into stagnant water and be buried rapidly to prevent decomposition, or be washed out to sea by rivers,” said Witts in a statement. "This means that marine fossils are generally much more abundant.”
Seymour Island has one of the best collections of marine fossils in the world. Along with vertebrates such as giant Mosasauruses, it preserves a stunning record of molluscs from small bivalves to Diplomoceras, an ammonite that grew to 2 meters (7 feet) long.
However, species diversity crashed 66 million years ago, with 67 percent of mollusk species and 43 percent of genera vanishing suddenly, similar to elsewhere in the world. "Our research essentially shows that one day everything was fine – the Antarctic had a thriving and diverse marine community – and the next, it wasn't,” Witts said. “Clearly, a very sudden and catastrophic event had occurred on Earth.”
Not everyone suffered, however. Fish skeletons become far more common immediately above the layer of iridium that marks when the asteroid hit.
Volcanic activity, or some other force, may have started the non-Avian dinosaurs' decline, but the extinctions Witts observed are too sudden to fit these theories. “This is the strongest evidence from fossils that the main driver of this extinction event was the after-effects of a huge asteroid impact, rather than a slower decline caused by natural changes to the climate or by severe volcanism stressing global environments," Witts said.
Moreover, the findings demonstrate the effect was truly global – refuting theories that the clouds of smoke from fires the asteroid started were restricted to tropical and temperate regions.