The Pull of the Recent
The apparent increase in marine biodiversity over the past 50 million to 100 million years is real and not just a false reading produced by the inconsistencies of the fossil record, says a team of paleontologists led by the University of Chicago's David Jablonski. This finding, published in the May 16 issue of the journal Science, may help scientists place the future of global biodiversity in its proper context.
"If you want to understand what's going to come in the future you need to understand the dynamics that led up to the biodiversity we see now," said Jablonski. There are approximately 1.4 million species that have been named by researchers, but the true number of species on earth may be anywhere between 5 and 30 million species. If the name of each species on Earth were put on a single recipe card, the box containing them all would stretch for over 6 miles. However, according to the World Wildlife Fund, the Earth is losing species at a rate not seen for 65 million years, since the extinction of the dinosaurs.
|400 million-year-old trilobite fossil. Trilobites dominated the as a marine fauna during the Cambrian Period. Charles Darwin noted: "Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they determine the present welfare, and, as I believe, the future success and modification of every inhabitant of this world."
By some measures, up to 50 percent of the increase in marine animal biodiversity during the past 50 million years can be attributed to what paleontologists call "the Pull of the Recent."
This is the idea, posed in 1979 by University of Chicago paleontologist David Raup, that the level of biodiversity is inflated in younger fossil deposits because sampling of the modern world is so much more complete than in the geologic past. But the Pull of the Recent accounts for as little as 5 percent of the biodiversity increase, at least for one well-preserved group.
"The results of this exciting study show how a thorough understanding of deep-time biotas and diversity places modern life into the correct perspective and provides a predictive capability for the future," said H. Richard Lane, director of the National Science Foundation (NSF)'s paleontology program, which funded the research. "These results can be applied to the study of natural processes and climate cycles in deep time, relating that to the modern situation, and using that knowledge to predict the future."
Scientists have long believed that diversity proliferated dramatically after the Paleozoic Era, which ended 250 million years ago, to the late present day. The work of James Valentine of the University of California, Berkeley, and a co-author of the Science article, pointed to a 10-fold increase.
The team studied bivalves (clams, scallops, oysters and mussels) to address the issue because they are one of the major contributors to marine animal biodiversity. In order to screen out a potential false reading for Cenozoic biodiversity, the team inventoried bivalve diversity in the youngest part of the geologic record. This would allow for assessment of the impact of the living bivalves by ignoring the biodiversity in modern oceans and building a diversity history based only on the fossil occurrences.
"This involved churning through a massive amount of the published paleontological literature of marine bivalves that lived during the last five million years," Jablonski said.
Complicating the task were the nomenclature changes that affected some types of bivalves. A single species might have been classified differently in each of four different papers published during the last 100 years as paleontologists' understanding of its evolutionary relationships improved, Jablonski said. Once the team members had standardized the classifications, they found that 906 of the 958 types (95 percent) of living bivalves they examined left a fossil record within the past 5 million years, as well as earlier in many cases.
|"Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck." -David Grinspoon
Image Credit: NASA
The possibility still existed that rocks deposited 5 million years ago were unusually rich and that they were distorting the fossil record. So the team conducted a second inventory of bivalves that plunged much deeper into the fossil record, back 65 million years ago to the days of the dinosaurs. The paleontologists still were able to recover 87 percent of the types of bivalves that lived through that interval, when some thought the record might be poorer. The high recovery rate supports claims that the lower diversity levels observed from this time are genuine and not artificially depressed by sampling or preservation.
During past periods life has simply sprouted wildly, in what some paleobiologists enthusiastically refer to as the 'explosive radiation of animals'. Buried in the sedimentary layers on both land and sea, previous fossil records show distinctly these innovative bursts. But such unbounded biological optimism contrasts markedly with sharp disturbances and interruptions, including seven mass extinction peaks in the last 250 million years, each spaced 20 to 60 million years apart.
"Skeptics would say, well, that's just bivalves. Maybe they're somehow unique," Jablonski said. But a similar recovery figure, 89 percent, applies to sea urchins, which researchers at London's Natural History Museum inventoried for the same period. "We've been talking about putting together a consortium of people to do exactly this kind of study with essentially all the major groups that make up the biodiversity increase," Jablonski said.
"It'd be a real boon for the field if we can get this under way, because it will simultaneously tackle the sampling question and put a huge chunk of the fossil record into a standardized evolutionary framework."
A 25 member scientific team, first organized by Dr. Charles Marshall (now at Harvard), has initiated a massive paleontological database in Santa Barbara, California, housed at the The National Center for Ecological Analysis and Synthesis (NCEAS). The team's fossil catalog comprises 8,500 collections and over 110,000 taxonomic occurrences, with an initial emphasis on the rich collections of marine invertebrates. Including 90% of all the animals on the planet, invertebrates encompass worms, sponges, mollusks and other species without a backbone. The analysis of the catalog spans 300 million years, an era that geologists classify as Middle Ordovician - Carboniferous (approximately 450 Million years ago) and Late Jurassic - Paleogene (approximately 150 Million years ago). Colleagues use statistics to try to correct biases in previous analyses, and to answer questions of whether modern diversity is real or a product of just more surviving modern fossils in the record.
Joining Jablonski and Valentine on the project were Kaustuv Roy, University of California, San Diego, and University of Chicago graduate students Rebecca Price and Philip Anderson.
Related Web Pages
Digital Zookeepers Take a Census
Biological Diversity: Fact or Artifact?
Evolution's Slow Recovery
New Species and Understanding Earth
The Paleobiology Database
NASA Workshop on Biodiversity
University of California Museum of Paleontology