Fossil and paleontology
Fossils: Are the remains or traces of past life. Paleontology is the study of past life. We encounter fossils in sedimentary rocks because, unlike other rocks, sedimentary rocks form where life lives.
Definition: A fossil is any trace of an animal's body or behavior that becomes part of the rock record.
Two types of fossil:
Applications of fossils to geological problems
- Body fossil: Actual part of organism.
- Trace fossil: Evidence of the organism's activity. These include:
- Preservation: 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. Major modes of preservation include:
- No alteration: Materials preserved in rock record unchanged.
- Permineralization: Pore space in material filled in with mineral cements. Common in porous material like wood or vertebrate bone.
- Replacement: Original components have been dissolved and replaced by precipitates.
- Carbonization: Soft tissues compressed into two dimension al carbon film
- Mold: Original material dissolved away but not replaced, leaving a mold or void.
- Cast: What we get if a mold is secondarily filled with sediment.
Paleobiology: Of course fossils tell us about the animals they represent. Potential information includes:
- Feeding and locomotor adaptations
- Metabolic strategies (from chemistry)
When many specimens are available
- Growth strategies
- Population ecology
- Position in food web (from gut contents, coprolites, chemistry, etc.)
What do they tell us about geological issues?
Biostratigraphy: The formal name of the application of William Smiths's principle of faunal succession to the identification of rocks. Based on the identification of:
- Index fossils: Globally widespread over a short period of time, thus wherever you see one, you know WHEN you are. Example: ammonites.
Of course, as parts of the rock record, fossils, like rocks are records of the environments in which they lived and died.
- Taphonomy: The study of the effects of transport and burial on fossils - fossils as sediment. During transport, burial, and diagenesis, a fossil is subjected to the same forces as other clasts. Thus they can be revealing of the environment in which they lived and died. When many fossils are present in one layer, their
are all informative. E.G.: Orientation of long elements can be plotted on a rose diagram to indicate ancient current flow.
- size distribution
- relative abundance of species
- Interaction of individual organisms with one another and their physical environment.
- Facies fossils: Fossils that occur over a long span of time but only in specific environments. Allow you to know what kind of depositional environment you are - like a living sedimentary structure. Example: Lingula.
- Ecology of communities of organisms with their environment and each other can be reconstructed when:
- We are reasonably sure (based on taphonomy) that fossils are preserved where they lived.
- We consistently find the same critters occurring together in separate locations.
Organisms that live together often interact ascommunities either because they depend on one another or by coincidence. In the fossil record, we often see similar communities at different times with analogous structures despite their being made up of different critters: Reefs, are a geologically significant example.
- Paleontologists reconstruct ancient climate by comparison with modern analogs. Example: In the modern world -
In the ancient world we can infer ancient environments from fossils:
Tends in biological diversity over time:
As we noted, biostratigraphy addresses the appearances and disappearances of individual species from the fossil record. Paleontology also addresses the more general changes in biological diversity over time. Some patterns include:
- The rise and decline of major evolutionary faunas.
- Major mass extinction events.
Note, especially, the Permo-Triassic extinction event marking the end of the Paleozoic Era. The scary one that almost sterilized the Earth. Contemporary research indicates that this happened over a very brief geologic interval (tens to hundreds of thousands of years). Possible causes include:
- Asteroid impact - Controversial and unlikely to have been the major cause
- Eruption of the Siberian traps - mammoth volcanic eruptions on an unprecedented scale. Triggered a cascade of effects:
Organisms dies of combined effects of climate change and CO2 poisoning.
- Erupted through major coal deposit, adding significant CO2 to the atmosphere.
- Prompting major episode of global warming