Fossilization and Information Loss
We would much rather have a living organism to study than a fossil, which is incomplete and ugly by comparison. The living biota, however, gives us only the vanishingly thin time slice of the modern world. To appreciate the processes that made that biota, we have to study the imperfect remains of ancient creatures.
Definition: A fossil is any trace of an organism's body or behavior that becomes part of the rock record.
Two types of fossil:
- Body fossil: Actual part of organism.
- Trace fossil: Evidence of the organism's activity as a living creature. These include:
Modes of Preservation: Generally only the hard parts of organisms are preserved (although there are some exceptions). Despite what you have heard, organismal remains do not have to be altered in any way to be regarded as fossils. And yet, they often are altered by diagenesis. Major modes of preservation include:
Becoming a fossil: From the birth of the organism to discovery by a paleontologist, fossils go through four general stages.
- Unaltered: Materials preserved in rock record without chemical change.
- Petrifaction: Alteration through the diagenetic addition of new minerals. Encompasses a range of alteration including:
- Permineralization: Pore space in material filled in with mineral cements. Note that the original material (or its durable fraction) is still present. Common in porous material like wood or vertebrate bone.
- Recrystallization: Some minerals are metastable in nature, eventually transforming into something else. A common example: aragonite (a form of CACO3 with needle-shaped crystals, secreted by many animals in their skeletons) eventually recrystallizes as calcite (the blocky morph of CACO3) When this occurs, many small-scale details of the fossil are lost, even though the same material is present.
- Replacement: Original components have been dissolved and replaced by precipitates. Recognizable when the chemistry of the fossil is not what one would expect of the original material. E.G.: Ammonoid shell, originally aragonite (CACO3) replaced by pyrite (FeS2), a mineral that no organism secretes directly.
- Mold: Original material dissolved away but not replaced, leaving a "negative" or void.
- Cast: What we get if a mold is secondarily filled with sediment to form a positive image but lacking any internal structure.
- Soft tissue preservation: Rare but interesting when it happens:
- Carbonization (AKA "distillation"): Soft tissues preserved when they are deposited in an environment that protects them from mechanical destruction and decomposition by bacteria. Usually form a two dimensional carbon film. Occurs in anoxic depositional environments and in amber.
- Freezing: Rare and not really a long-term mode of preservation, yet we have interesting fossils of cold-adapted animals from the Pleistocene Epoch. These must ahve become frozen immediately after death.
- Dessication: When soft tissues are dessicated then buried before they can decompose, they can be preserved as molds or casts.
Biotic stage: Birth to death. The organism grows whatever tissue is capable of being preserved then dies. It has done its part. Whether it will become part of the fossil record depends on whether its carcass can remain intact long enough to be buried. The quicker the better because.....
Interment stage: Death to final burial. The carcass is exposed to:
- Decomposition: destruction of soft tissues (mostly by bacteria)
- Disassociation: hard parts become separated
- Abrasion: surface details of hard parts lost
- Breakage: hard parts degraded into fragments
- Winnowing: fragments sorted by size due to moving water
Diagenetic stage: Final burial to discovery. Once buried, the remains are officially fossils, however their existence is still perilous. Diagenesis results in:
- Dissolution: hard parts chemically altered into soluble substances
- Compaction: remains crushed by overlying sediment
- Recrystallization: ground water & minerals enter, form crystals & disrupt remains
Investigative stage: Discovery to ultimate destruction. Every day, fossils are unearthed by erosion, only quickly to be destroyed by it. To enter the fossil record, as scholars understand it, a fossil must be exhumed (usually by natural processes), discovered, and described.
Filters and Biases
The selection of fossils for inclusion in the fossil record is non-random. Each stage imposes its filters.
- Presence of hard tissues: Soft-bodied organisms rarely make it into the record because their bodies are thoroughly destroyed after death. Creatures like oysters have a better chance because they have durable hard parts.
- Presence of large robust parts: The femur of a hippo and the rib cage of a hummingbird are made of the same substance, but the former can simply withstand much more mechanical force without being destroyed. Thus, large organisms with robust elements are more common in the record.
- Habitat: Organisms that live and die in non-depositional environments (especially those with high kinetic energy like mountain streams) are much less likely to be preserved as fossils than those who live and die in places like meandering river floodplains and shallow carbonate shoals.
- Rapid burial: Organisms that are buried rapidly endure fewer destructive forces while exposed on the ground.
- Mechanically benign burial environment: Organisms that come to rest in sediment deposited by relative quiet waters are more likely to survive as fossils.
- Biologically benign burial environment: Organisms that come to rest in places inaccessible to scavengers and decomposers are more likely to survive as fossils.
- Clast size: Organisms that are buried in fine sediment will be preserved in greater detail than those buried in coarse sediment.
- Groundwater chemistry: Depending on the chemistry of groundwater, a fossil might be preferrentially preserved or totally dissolved.
- Tectonically benign environment: Organisms that are buried in geologically active regions are more prone to destruction while beneath the ground. We know far more fossils from the geologically stable Colorado Plateau than from the active volcanic arcs of southern Mexico.
- Ease of discovery: We are more likely to discover an exhumed fossil in an arid region with little plant cover, such as the American southwest, than in heavily forested regions.
- Scientific vogues and fashions: Sad to say, scientists' and collectors' concepts of what was valuable is subject to its own biases. In the mid 20th century, dinosaurs were considered irrelevant to the big issues of mammalian origins, for instance. In early 20th century South Africa (blessed with some of the world's best vertebrate fossils) collectors became head-hunters, collecting well preserved vertebrate skulls while leaving the postcranial remains in the ground.
- Ultimate destruction: Our museums won't last forever. Consider the fate of many excellent collections in Germany during the Third Reich - reduced to bombed ruins. We embrace the study and dessemination of information on fossils with a sense of urgency.