Washington:
Scientists on Thursday revealed that the mass extinction event that wiped out the dinosaurs about 66 million years ago gave rise to modern ‘age of fish’. And, so far 188 meteorite impact sites on Earth have been confirmed so far, and 340 are still awaiting discovery.
Researchers determined that the world’s most numerous and diverse vertebrates - ray-finned fish - began their ecological dominance of the oceans 66 million years ago, aided by the mass extinction event that killed off dinosaurs.
They analysed the microscopic teeth of fish in sediment cores around the world and found that the abundance of ray-finned fish teeth began to explode in the aftermath of the mass die-off of species, which was triggered by an asteroid strike in the Yucatan Peninsula.
Scientists refer to this episode as the Cretaceous-Paleogene (K/Pg) extinction event.
Ninety-nine per cent of all fish species in the world - from goldfish to tuna and salmon - are classified as ray-finned fishes.
“We find that the extinction event marked an ecological turning point for the pelagic marine vertebrates,” said researchers.
“The K/Pg extinction appears to have been a major driver in the rise of ray-finned fish and the reason that they are dominant in the open oceans today,” they said.
In cores from numerous ocean basins, researchers found that while the numbers of sharks remained steady before and after the extinction event, the ratio of ray-finned fish teeth to shark teeth and scales gradually rose, first doubling then becoming eight times more abundant 24 million years after the extinction event.
Now there are 30,000 ray-finned fish species in the ocean, making this class the most numerically diverse and ecologically dominant among all vertebrates on land or in the ocean.
Scientists had known that the main diversification of ray-finned fish had happened generally between 100 million and 50 million years ago.
“The diversification of fish had never been tied to any particular event. What we found is that the mass extinction is actually where fish really took off in abundance and variety,” said Elizabeth Sibert from the Scripps Institution of Oceanography in California.
“What’s neat about what we found is that when the asteroid hit, it completely flipped how the oceans worked. The extinction changed who the major players were,” said Sibert.
Sibert and Professor Richard Norris believe that some key changes in the oceans might have helped ray-finned fish along.
Large marine reptiles disappeared during the mass extinction, as did the ammonites, an ancient cephalopod group similar to the chambered nautilus.
Those species, the researchers believe, had been either predators of ray-finned fish or competitors with them for resources.
“What’s amazing is how quickly fish double, then triple in relative abundance to sharks after the extinction, suggesting that fish were released from predation or competition by the extinction of other groups of marine life,” Norris added.
A total of 188 meteorite impact sites on Earth have been confirmed so far, and 340 are still awaiting discovery, a first-of-its-kind study has found.
Meteorite impacts have shaped the development of the Earth and life repeatedly in the past.
The extinction of the dinosaurs, for instance, is thought to have been brought on by a mega-collision at the end of the Cretaceous period.
In comparison to the more than 300,000 impact craters on Mars, the mere 188 confirmed craters on Earth seem almost negligible, researchers said.
Moreover, 60 of them are buried under sediments. Advances in remote sensing have not led to the expected boom in crater discoveries: An average of only one to two meteorite craters are discovered per year, most of them already heavily eroded.
The probability of a meteorite impact on Earth is not fundamentally different than on Mars. However, the Earth’s surface changes much more quickly.
As a result, the craters remain visible for a much shorter period of time, meaning that many less of them are detectable today.
“The main challenge of the study was to estimate the long-term effect of erosion, which causes craters to disappear over time,” said Dr Stefan Hergarten from the University of Freiburg in Germany.
The life span of a crater depends on the rate of erosion and its size. Large craters can achieve a life span of several 100 million years, depending on the region in which they are located.
On the other hand, large impacts are much rarer than small impacts.
The solution was to compare the amount of confirmed craters of different sizes, calculate the expected frequency of the impacts on the basis of the known probabilities, and combine this information to infer the rates of erosion.
“A surprising, initially sobering finding we made was that there are not many craters of above six kilometres in diameter left to discover on the Earth’s surface,” said Hergarten.
In the case of smaller craters, on the other hand, the scientists found the current list to be far from complete:
Around 90 craters with a diameter of one to six kilometres and a further 250 with a diameter of 250 to 1000 meters are still awaiting discovery.
While there are undoubtedly still a number undiscovered large craters buried deep under sediments, they are much more difficult to detect and confirm, researchers said.
