Say an EKG machine is monitoring your heart, when it suddenly flatlines. You'd be keenly interested to know whether your heart had stopped or the machine had simply gone on the fritz. Paleontologists have faced a similar (if slightly less urgent) puzzle when it comes to the geologic record of life: does the fossil record we see reflect the state of ancient ecosystems, or is it just the readout from a defective instrument? A recent paper in Science gives reassuring support to the fidelity of the rock record.

It’s fascinating to study how species diversity has changed through time, since we can see the effects of major events in Earth’s past and watch evolution play out. It's literally reading the history of life on Earth. That’s a story we naturally want to know and tell. But fossils are difficult to come by—after all, less than one percent of extinct species are represented in the fossil record. As an imperfect recorder, we have to worry how much the evidence in the rocks is telling us about the organisms, and how much we're just seeing changes in the rocks themselves.

Prof. Shanan Peters, co-author of the recent paper, explained to Ars by e-mail that paleontologists have wrestled with this for a long time. "This goes right back to Darwin... The Origin of Species even has a chapter titled 'On the imperfection of the geological record,' with a subheading titled 'On the poorness of our paleontological collections.'"

A paper by David Raup in 1972 set out the problem for his fellow researchers. While it was tempting to interpret trends and patterns in the fossil record of diversity, Raup showed that it was important to look carefully for preservation biases. If the formation of sedimentary rocks decreases for a time, perhaps the fossil diversity would also decrease even if there was no change in the actual ecosystems.

"Raup’s novel comparison of global compilations of rock quantity to diversity estimates based on the stratigraphic ranges of fossil taxa yielded compelling correlations that did 'justify further investigation,'" Peters wrote. Since the paper was published, researchers have worked on ways to account for variations in preservation when estimating fossil diversity.

In this new study, Peters and co-author Prof. Bjarte Hannisdal re-examined the sampling bias in the rock record using sophisticated statistics that measure information transfer. "Basically [information transfer] allows us to do two things,” Peters explained. “First, it allows us to evaluate the statistical dependence between two variables (e.g., fossil diversity and temperature) beyond their mutual relationship to a third variable (e.g., sea level). In this sense, it is like a partial correlation, with the advantage that the variables don’t have to be linearly related. Second, it allows us to determine the directionality of coupling between two variables (e.g., does the information in temperature flow into diversity?)"

This provides a window into the nature of the correlation between rock formation and species diversity. Are trends in species diversity really caused by changes in rock formation, or are they both responding to some other factor? The researchers compared records of marine diversity, marine sedimentary rock formation, sea level, and isotopes of carbon, oxygen, sulfur, and strontium.

Carbon-13 tracks changes in the carbon cycle, such as sequestration in wetlands or volcanic emissions. Oxygen-18, which provides a proxy for temperature in ice cores, shows changes in climate in carbonate rocks. Sulfur-34 records how heavily oxidized the environment was, among other things. The ratio of strontium-87 to strontium-86 is used to examine changes in continental weathering and volcanism at mid-ocean ridges.

They found that our record of species diversity might not be as bad as we thought. Changes in sedimentation and species diversity do correlate, but that's because they both respond to other geological forces. "Raup was right, in that there is a strong correlation between the variable rock record and fossil diversity," wrote Peters, “but he was wrong about the meaning of that correlation. It exists because the biosphere actually responds to the environmental changes that are responsible for producing variability in the marine sedimentary rock record."

Our record of the history of life isn't strongly biased by preservation; it’s actually a pretty faithful telling. The results showed that the carbon cycle was a driver of climate (no surprise there) as well as the sulfur record, which was also related to sea level. Sea level exerted control on fossil diversity (probably due to the species-area effect), while the formation of marine sedimentary rocks is, of course, also related to sea level. But critically, sediment formation did not drive fossil diversity.

Peters suggested that some analytic techniques meant to adjust for changes in geology may have actually created their own biases. "This is so cool because paleontologists have been freaking out about the variability of the rock record and fossil record for a long time, and they have spent a great deal of energy trying to remove this variability analytically by sampling standardization techniques," Peters noted. "[Information transfer] analysis clearly shows that [those techniques do] a very good job of removing the signal of sea level, but that sea level actually contains an important signal! The great irony may end up being that the face-value fossil record is our best estimate of the true history of life."

Analyzing the relationships between all these parameters could tell us a lot about the mechanisms by which environmental changes have affected life on Earth. A major event that prominently affects many records is the formation and subsequent breakup of the supercontinent Pangaea, which involved the closing and re-opening of ocean basins. Many smaller-scale and less obvious events that affected fossil diversity remain poorly understood.

In the interim, the study suggests that paleontologists don't have to worry that their understanding of the history of fossil diversity comes from a dodgy translation. "In short," Peters wrote, "there is a really important signal about the state of the Earth that is captured quantitatively by Darwin’s geological incompleteness."

Science, 2012. DOI: 10.1126/science.1210695 (About DOIs).