A history with missing pages
The story of life on Earth is an epic, but paleontologists can only read a tiny fraction of it. The fossil record is very incomplete, a library with most of its books turned to dust before they could be read. This is the result of taphonomic bias. Soviet scientist Ivan Yefremov coined the term "taphonomy" in 1940 from the Greek words taphos (burial) and nomos (laws). It is the study of everything that happens to an organism after it dies—from decomposition and scavenging to burial and fossilization.
The transition from a living organism to a fossil is exceptionally unlikely. Estimates suggest that less than one-tenth of one percent of all species that have ever existed are known from fossils. For an organism to be preserved, it must be buried rapidly, protecting it from scavengers and the elements. This burial must also happen in an anoxic (low-oxygen) environment to slow down bacterial decay. These conditions are rare, meaning the vast majority of life vanishes without a trace. The processes that occur after death but before burial are called biostratinomy, while the changes that happen after final burial are known as diagenesis.
The filters of fossilization
The fossil record is filtered by several strong biases that shape our view of past life. The most significant is the hard-part bias. Organisms with bones, shells, teeth, and other mineralized parts have a much greater chance of preservation than soft-bodied creatures. This means the record overrepresents vertebrates and shelled invertebrates, while entire phyla of soft-bodied animals like jellyfish or slugs are almost invisible.
Environmental bias is also powerful. Organisms living in depositional settings, like shallow seas or river deltas where sediment accumulates, are far more likely to be buried and fossilized. Creatures from upland, erosional environments are seldom preserved. Tectonic activity adds another filter; fossils can be destroyed by heat, pressure, or subduction, where one tectonic plate is forced under another into the mantle.
Occasionally, however, unique conditions create sites of exceptional preservation called Lagerstätten (German for "storage place"). These sites show the soft tissues and delicate organisms that are usually lost. The 508-million-year-old Burgess Shale in Canada, for instance, preserves an array of bizarre soft-bodied Cambrian animals. The Solnhofen Limestone in Germany, famous for preserving the feathers of the 150-million-year-old Archaeopteryx, is another example. These rare sites help paleontologists understand just how much information is normally missing from the rock record.