While fossils of the glamorous dinosaurs get most of the attention, they are too few and far between to answer the most important questions about the evolution of life on Earth. So for decades, paleontologists have turned to humble shelled creatures like sea urchins or mollusks, whose enormous numbers and geographic range make their fossils a much better marker of the history of life. But paleontologists also have worried that some of those fossils decayed much faster than others, so that the remains collected today might misrepresent the true proportions of past life, thereby throwing off their conclusions.
To test this suspicion, Kidwell carefully analyzed the fossil record of marine bivalves, a diverse group of shellfish that includes scallops, oysters, cockles and mussels. Her study ruled out longstanding predictions that patterns in the fossil record would be dictated by differences in shell toughness among evolutionary lineages.
All of her analyses on how shell composition might distort the bivalve record over the past 500 million years showed either no effect, or the opposite effect, compared to what was expected if shell preservation was the most important factor. "The broad outlines of the bivalve record can be counted on as a reliable picture of evolutionary history," said Kidwell, whose work was supported by a grant from the National Science Foundation.
Nevertheless, there is no question that initially abundant, hard shells can be lost to the fossil record.
"Shells can be crushed by predators or storm waves, disintegrated by acids and microbes in the sediment that buried them, or dissolved during deep burial or by ground water during continental uplift. The older the rocks, the more likely some form of destruction becomes," she said. "Based on short-term experiments, some shell types should be more prone to destruction than others. So the question is, how do these losses add up over the long term?"
To find out, Kidwell amassed an inventory of the makeup of living and fossil shells from the scientific literature. "All bivalve shells are composed of calcium carbonate," she said, "the same material as in bird eggshells. But bivalves can produce two different mineral forms of carbonate and, beyond that, can form mineral crystallites in diverse shapes and sizes and pack them into a huge variety of fabrics. These differences in reactivity could distort the signal from the fossil record."
Oysters make shells of an especially stable form of calcium carbonate, for example, whereas cockles more readily dissolve when immersed in ground water. Mussels use both mineral forms but have much more organic material in their shells than do scallops, which also are composed of two minerals, and thus disintegrate relatively rapidly due to loss of that organic matrix.
"The concern has been that all these differences in preservability could add up to a fossil record where cockles and mussels and their equivalents have been preferentially lost or thinned out from the record, leaving a record that favors organisms that happened to secrete the most durable shells," Kidwell said.
But despite decades of concern about these effects, they turned out to be minor, Kidwell found. She combined her world
wide inventory of shell composition with another massive database of the geologic ranges of all known bivalves, which had been built by the late Jack Sepkoski (also of the University of Chicago), and is available online via the NSF's Paleobiology Database Project. Then she checked the data for patterns that were more likely to be shaped by fossil preservation than by true evolutionary dynamics.
"There was no sign that preservability was running the show," she said. In fact, if anything, variations having shells that seemed least likely to be preserved actually survived longer than forms with the toughest shells.
"This frankly surprising result might have several explanations--a lot of redundancy in the fossil record at the scale of individual beds, for example, or maybe some compensatory remineralization and stabilization of the highly reactive shell types," Kidwell said. "But whatever the ultimate causes, the study is fundamentally good news for paleontologists and evolutionary biologists.
"The fossil record isn't complete--the vast majority of individual bivalve shells are undoubtedly recycled back into calcium, carbon and oxygen. But even though there's a lot of potential for that recycling to shortchange the completeness of the record of some bivalve groups relative to others, there's no evidence that long-term patterns are actually biased. The fossil record appears to be capturing this important aspect of the biological story." And because the bivalves cover suc
h a wide range of shell types, these results suggest that shelly mainstays of the fossil record, such as snails, sea urchins and corals, may have comparably unbiased records, Kidwell said.