How incomplete is the fossil record?

Scientists use a number of tools to fill in these missing links. They depend heavily on cladistics, a method of hypothesizing relationships among organisms. Think of it as creating a family tree with blank spaces for unknown ancestors. They also turn to molecular sequencing in which they reveal the hierarchy of relationships among different organisms by comparing their molecular details.

Think of a family tree again, only this time using DNA evidence to figure out what goes where on the chart. That's the fossil record: the preserved remains of ancient life forms imprisoned in the Earth itself. By charting where fossils occur in rock layers, scientists can note when organisms existed in geologic time.

The fossil record, however, is quite incomplete. Here's one major reason why: Sediment has to cover organic material in order for the long fossilization process to begin. Most organisms decompose before this can happen.

Fossilization odds increase if the organism happened to exist in large numbers or lived in or around sedimentary rocks. For example, trilobites, ancient marine arthropods, met both criteria, so they're a rather common fossil species. The Tyrannosaurus rex, however, is far rarer. It was large and land-dwelling, and as a top predator made up a far smaller percentage of the population.

Even though abundant fossils form in stone, they're far from impervious. Like all rocks, they erode, melt, and fragment. Factor in all the fossils we haven't uncovered alongside the fossil evidence we can't decipher properly (due to partial fossilization or insufficient technology), and the fossil record gets even spottier.

So like the mineralized bones themselves, the fossil record is an incomplete framework that scientists flesh out through additional methods. While cladistics, molecular sequencing, and the fossil record all present different data sets, systematic biologists generally find similar patterns of diversification in all three. In other words, the three methods complement each other and paint a congruous picture of what the tree of life should look like.

Fossil records grow more incomplete the further back in time we attempt to look. Organisms that are more recent don't appear either. For example, freshwater mollusks of the class Bivalvia suffer up to 45 percent incompletion in some subclasses [source: Valentine et al.]. Ancient animals that represent important links in the fossil record also remain unaccounted for, such as the last common ancestors connecting entire phyla. Research into the fossilization process continues to illuminate just how much of the record we're missing.

So, taken on its own, the fossil record is considerably lacking in many areas. Yet like fingerprints at a crime scene, it's just one piece of the puzzle. Fossils, cladistic and molecular sequencing work together to form a larger picture that correctly documents the evolution of life [source: Benton et al.].

Explore the links on the next page to learn even more about fossils and evolution.