Since the mid-19th century, scientists have questioned the extent to which the fossil record accurately depicts increases and decreases through time in biodiversity. A new database and analysis now indicates that, if not properly corrected, the fossil may provide a misleading signal of biodiversity changes during important intervals of geological time.
In a report published Tuesday, May 15 in the Proceedings of the National Academy of Sciences (PNAS), a team of 25 researchers, including UC geologist Arnold Miller and four of his graduate students, report that a major increase in marine biodiversity during the late Mesozoic and Cenozoic Eras may be an artifact of sampling and not nearly as extensive as once believed. The paper is dedicated to the late Jack Seposki who is listed as a coauthor and who developed the most widely known biodiversity graph depicting the large increase.
The lead author of the study is John Alroy of the National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara, California where the database is located. It is an outgrowth of numerous efforts to get a better understanding of how different factors affect what is found in the fossil record.
For example, scientists have known for many years that different geological time periods yield different numbers of fossils simply because the amount of sedimentary rock formed is different. That's known as the "rock volume" problem. More rocks mean more fossils, but, in reality, not necessarily more biodiversity.
The first step in resolving the conflicts among the various diversity curves was collecting more data. Miller has been working since the early 1990s on tracking occurrences of marine genera within the Ordovician Period of the Paleozoic Era, during which global marine diversity exhibited perhaps its most protracted increase ever. An analysis he published in 1996 showed that the nature and timing of the large jump in Ordovician global biodiversity were affected significantly by variations in sampling from interval to interval through the period.
The PNAS analysis goes much further, extending this approach more broadly through geological time. It uses four distinct analytical methods and a database that has grown to include more than 8,500 collections and over 110,000 taxonomic occurrences.
"Basically, we were asking the question 'How meaningful is the raw global diversity trajectory extracted from fossil record?'" explained Miller. "So we looked directly at the samples from various venues around the world and used statistical analyses for sampling standardization to give us roughly equal samples from interval to interval."
The results of some of the analyses turned up very different patterns from the ones shown on "industry standard" global diversity curves. Instead of a sharp increase in biodiversity in the most recent geological time periods, there was very little change in overall diversity over time, at least in some of the "sampling standardized" reconstructions.
"The results are very tantalizing," admitted Miller, "but in no way are we ready to claim that the huge increase is not a biological reality."
What the team of researchers really wants is to expand their database even more. Alroy is already working on a terrestrial version covering mammalian evolution in North America. There is also a great need for data from sites outside North America and Europe.
"The heart of our work is the database. It's morphing into something much more than a basic list of genera and species, and we hope that it will be a valuable research resource for the entire paleobiological community" said Miller.
Alroy notes that the database "should facilitate a wide array of future paleoecological and marcoevolutionary analyses" because it also contains detailed information about the location, environment and rock type where the fossils were found.
The Paleobiology Database can be found online at http://flatpebble.nceas.ucsb.edu/public/.
The research was supported by the National Science Foundation, NASA's Exobiology program, and NCEAS. UC graduate students who were co-authors on the paper included Karen Bezusko, Donna Jones, Phil Novack-Gottshall, and Andrew Webber. Other co-authors include: Charles Marshall (Harvard University), Richard Bambach (Virginia Tech), and David Raup (University of Chicago).
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