Fossilization and the fossil record
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.
- Body fossil: Actual part of organism.
- Trace fossil: Evidence of the organism's activity as a living creature. These include:
Oviraptor nest - Wikipedia
- Nest sites
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 chemically altered. Major modes of preservation include:
- 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.
Recrystallized brachiopod - Wikimedia Commons
- 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.
Pyritized ammonite - New York State Museum
- 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 (right), originally aragonite (CACO3) replaced by pyrite (FeS2), a mineral that no organism secretes directly.
Petrified Forest NP, AZ
Stenopterygius Ichthyosaurs: a day in the life
- Soft tissue preservation: Rare but interesting when it happens.
Taphonomy - The process of becoming a fossil: From the birth of the organism to discovery by a paleontologist, fossils go through four general stages.
- Decomposition: destruction of soft tissues by bacteria
- Dissociation: body 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
- Dissolution: hard parts chemically altered into soluble substances
- Compaction: remains crushed by overlying sediment
- Recrystallization: ground water & minerals enter, form crystals & disrupt remains
Field excavation from Bureau of Land Management
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 than do jellyfish 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 material, 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. For example:
- In the Middle - late 19th century, paleontology was well-funded big science, so museums and universities equipped large field crews to recover large specimens for their displays.
- 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.
- By the mid 20th century, dinosaurs were considered irrelevant to the big issues of mammalian origins, and their remains were ignored.
- 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.
Fossils are bound to the study of the rock record in two ways:
- Fossils were instrumental in the discovery and understanding of geologic time.
- The significance of fossils as records of evolution is lost if we don't understand their place in time.
Recall that fossils were crucial to the establishment of the Geologic Time Scale. Are all fossils equally useful? No, you have to use the right fossils:
- Index fossils: Fossils of organisms that existed for short perios of geologic time but were geographically widespread. Example: Ammonites, Shelled cephalopods that evolved quickly (so each species lasted only a few million years, but whose remains were distributed worldwide in many environments. E.G.: Sphenodiscus lenticularis - an ammonoid.
Lingula sp., a brachiopod, from Wikipedia
- Facies fossils: Fossils of organisms that endured for long periods of geologic time but were linked to a specific environment. Example: Lingula, a brachiopod living only in lagoonal mud-flats that has changed very little in the last 500 million years. E.G. Modern (left) and ancient (right) examples of the brachiopod Lingula.
Of course microscopic index fossils can be superabundant, even in small rock samples, so they are particularly useful.
Sphenodiscus lenticularis, an ammonoid.