Jablonski, Kidwell and their colleagues based their study on an examination of museum specimens, combing the scientific literature and conducting statistical analyses. They determined the fossil occurrence of all 1,282 major types living today, then assessed why 24 percent of them failed to become fossils. In some cases, key factors were inextricably linked. "For example, deep-water species tend to have small bodies because there's so little food down there," Jablonski said. "Small body size turns out to be bad news for preservation in general, and thus deep-sea species are undercaptured."
Surprisingly, they found that burrowing clams living within sediments were no more likely to become fossils than similarly sized varieties that lived out in the open.
Perhaps fittingly, parasitic clams fared the worst. "They live inside the burrow of another animal, like a shrimp, or they live parasitically upon the soft tissues of another organism. These guys have a lousy fossil record," Kidwell said. "If you're living inside the tissues, or directly attached to the tissues of another organism and it dies, then you're attached to a corpse of decaying organic matter, which is not favorable to shell preservation."
The team also found that shell composition played virtually no role in distorting the bivalve fossil record, echoing the findings of a related study that Kidwell published in the Feb. 11, 2005, issue of the journal Science. That study showed, contrary to longstanding expectations, that clams with durable shells were not better represented in the fossil record than those more prone to dissolving.
The PNAS co-authors suggest three remedies for dealing with the weaknesses they've documented in the fossil record. First, leave out the poorly documented groups. "Set some kind of cutoff on the q
Source:University of Chicago