According to a report in Cornell Chronicle Online, the research casts doubt on the long-held theory that diatoms' success was tied to an influx of nutrients into the oceans from the rise of grasslands about 18 million years ago.
New evidence from a study led by graduate student Dan Rabosky of Cornell's Department of Ecology and Evolutionary Biology and the Cornell Laboratory of Ornithology takes into account a widespread problem in paleontology: that younger fossils are easier to find than older ones.
"We just tried to address the simple fact that the number of available fossils is colossally greater from recent time periods than from earlier time periods," Rabosky said. "It's a pretty standard correction in some fields, but it hasn't been applied to planktonic paleontology up till now," he added.
More than 90 percent of known diatom fossils are younger than 18 million years.
So, an unadjusted survey of diatom fossils suggests that more diatom species were alive in the recent past than 18 million years ago.
To find an ancient fossil, scientists first have to find ancient sediment - and that's no easy task because plate tectonics constantly shift the ocean floor, fossils and all.
Much of the seafloor is simply too young to sample.
So, Rabosky and co-author Ulf Sorhannus of Edinboro University of Pennsylvania controlled for how many samples had been taken from each million-year period of the Earth's history, going back 40 million years.
After reanalysis, the long-accepted boom in diatoms over the last 18 million years disappeared. In its place was a slow recent rise, with a much more dramatic increase and decline at the end of the Eocene epoch, about 33 million years ago.
With the new timeline, diatoms achieved their peak diversity at least 10 million years before grasslands became commonplace.
"If there was a truly significant change in diatom diversity at all, it happened 30 million years ago," Rabosky said.
According to Rabosky, although the new results don't explain the current prevalence of diatoms in the ocean, whatever led to diatoms' rise at the end of the Eocene, the tiny organisms may have contributed to the global cooling that followed.
"Why diatom diversity peaked for 4 to 5 million years and then dropped is a big mystery," Rabosky said. "But it corresponds with a period when the global climate swung from hothouse to icehouse," he added. (ANI)