Several mass extinctions have occurred in the history of the Earth that have drastically changed life on the planet, and new evidence suggests that one of these effects may have been a large reduction in the size of sea creatures.
A paper, published in the journal Science, studied 1,100 fossils from a time between 419-323 million years ago. This time period spanned across the mass extinction event known as the end-Devonian, which ended 359 million years ago.
In the fossils dating back from before the end-Devonian, a progressive increase in the size of organisms was observed, but the fossils of organisms alive after the mass extinction showed a large reduction in body size. This is known as the Lilliput effect, in which small organisms become the dominant life form after extinction events. For about 40 millions years after the end-Devonian, the world’s oceans were ruled by small fish.
“Some large species hung on, but most eventually died out,” said Lauren Sallan of the University of Pennsylvania and primary author of the study. “So the end result is an ocean in which most sharks are less than a meter and most fishes and tetrapods are less than 10 centimeters, which is extremely tiny. Yet these are the ancestors of everything that dominates from then on, including humans.”
The reason for this is thought to be that smaller organisms have a reproductive advantage over larger ones after an extinction event because they can reproduce and increase in numbers faster. However, larger size fish can eventually evolve. Similar cases are well documented in forests when a disturbance, such as large-scale logging, occurs. Initially small plants like grasses are first to colonize and large trees and shrubs grow later.
“It doesn’t matter what is eliminating the large fish or what is making ecosystems unstable,” Sallan also said. “These disturbances are shifting natural selection so that smaller, faster-reproducing fish are more likely to keep going, and it could take a really long time to get those bigger fish back in any sizable way.”
This study contains the first concrete evidence of the Lilliput effect occurring after a mass extinction. Previously, there has been debate over whether it had occurred after the major mass extinction, but it is now clear that it has occurred after at least one mass extinction.