Washington, July 4 : An international team of researchers has determined that species over the years have come and gone at different rates than previously believed.
John Alroy of the University of California at Santa Barbara led the research team.
According to the researchers, diversity among the ancestors of such marine creatures as clams, sand dollars and lobsters showed only a modest rise beginning 144 million years ago with no clear trend afterwards.
This contradicts previous work showing dramatic increases beginning 248 million years ago and may shed light on future diversity.
"Some of the time periods in the past are analogies for what is happening today from global warming," said Jocelyn Sessa of Penn State.
"Understanding what happened with diversity in the past can help us provide some prediction on how modern organisms will fare. If we know where we have been, we know something about where it will go," she added.
Using contemporary statistical methods and a paleobiology database, the researchers report a new diversity curve that shows that most of the early spread of invertebrates took place well before the Late Cretaceous, and that the net increase through the period since, is proportionately small relative to the 65 million years that elapsed.
One key to the new curve is the Paleobiology Database, housed at the National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara.
Previous research was based on databases of marine invertebrate fossils that recorded only the first occurrence of an organism and the last occurrence of the organism. There was no information in between for the organism.
"Over 30 years ago, researchers looked at the curve they had and considered that perhaps diversity did not increase at all," said Mark E. Patzkowsky of Penn State.
"What researchers saw was the diversity curve leveled off for quite some time and then took off exponentially. However, diversity results are strongly controlled by sampling techniques," he added.
The new database allows researchers to standardize sample size because it includes multiple occurrences of each fossil.
Researchers can randomly choose equal samples from equal time spans to create their diversity curve.
This new curve uses 11 million-year segments, but the researchers hope to reduce the time intervals to 5 million years to match the interval of the previous curve, known as Sepkoski.
The data for this study contains 284,816 fossil occurrences of 18,702 genera that equals about 3.4 million specimens from 5384 literature sources.
"Comparing diversity through time is about how our world works, about the origin of species and how diversity changes with temperature," said Sessa.