Supplemental Lecture (97/04/05 update) by Stephen T. Abedon (firstname.lastname@example.org)
Biases in the fossil record
- Chapter title: Fossilization
- A list of vocabulary words is found toward the end of this document
- The fossil record is imperfect. The fossil record is also our best window on the history of life on this planet. What happens when your best is less than ideal? You have to work that much harder to derive useful information. Consequently, paleontology, as with just about everything else associated with the sciences of evolutionary biology, is not easy to do.
- There are a number of basic principles which go a long way toward understanding interpretation of the fossil record. One is to understand just how fossilization occurs. Another is to be familiar with the various strategies for dating fossils. Of course, it also helps if you are an expert on the anatomy of those parts of organisms which tend to fossilize. Obviously you will not be held responsible here for this latter consideration. The object of this lecture is to familiarize you with the concepts of fossils and fossilization.
- To understand how it is that the history of life is thought to be understood, you have to understand fossils. In this lecture we will introduce the concept of fossilization, the dating of fossils, and dwell upon why it is that the fossil record is not perfect.
- Paleontology is the study of the past through the study of fossils.
- Mineralized dead organisms:
- What is a fossil? A fossil is some remain of a dead organism preserved in rock.
- This remain either represents part of the organism itself or some imprintation made by the organism's body.
- Often the remains of preserved organisms are not of the organism itself but instead of minerals deposited where, especially, hard parts of the organism previously existed. Thus, a fossil femur (a kind of bone) is a three dimensional "depiction" of a bone but consisting all or in part of not bone-like material.
- Mineralized hard parts, usually:
- Soft parts tend not to fossilize because they usually decay prior to mineralization.
- However, soft parts, under the right conditions, can also make impressions.
- Included among such imprints are those made during the act of locomotion such as fossil foot prints or various invertebrate trails.
- Convoluted series of events:
- The occurrence of a fossil depends on the occurrence of a sufficiently convoluted series of events that the likelihood of fossilization per dead individual is low.
- Fortunately for paleontologists, the many billions of years of earth's history has seen a vast accumulation of dead organisms.
- Bias toward numerous hard parts:
- A large fraction of organisms show a very low likelihood of fossilization due to a lack of sufficient hard parts such as bones or shells.
- Thus, the fossil record is biased toward organism's displaying hard parts.
- The fossil record is also biased, on a per individual basis, toward those individuals which display particularly robust hard parts (a large dinosaur, say, relative to a canary).
- Thus, the likelihood of fossilization is large when compared with the likelihood of fossilization of a given soft, small thing. The latter, however, tend to be more numerous among living populations, thus ballancing things out somewhat.
The mechanism of fossilization tend to occur nonrandomly about the landscape.
Fossilization in one area (or time) may be more likely than in other areas or at different times.
Fossilization thus is biased towards individuals who live in environments in which fossilization is more likely (e.g., not in tropical rain forests).
The fossil record itself tends to show severe gaps as groups moved into and out of areas, or as populations wax and wane within areas in which fossilization was more or less likely.
Fossils are rarely perfect
- Starts with burying:
- The key first step in the process of fossilization is the burying or otherwise entombing of an organism prior to that organism's complete decomposition.
- Often this entombment occurs as a corpse is buried under sediment at the bottom of a lake, stream, river, or sea. This may happen, for example, if an organism is swept away in a flood.
- Alternatively, organisms can be buried:
- under sand during storms
- under ash during volcanic eruptions
- in amber (polymerized tree sap)
- Followed by mineralization
- In time, with water seepage, the atoms associated with the once living organism are replaced by minerals which precipitate out of the water within the structures of the organism before these structures are lost.
- Additionally, the buried sediments containing the buried organism gradually turn to stone, thus entombing the fossil in stone as well as converting imprints to stone.
- Often burying occurs after decomposition or following predation.
- Consequently, only a section of an organism may have been buried, or sections may have been buried in an orientation different from that seen in the living organism (consider what a fossilized Thanksgiving turkey might look like if buried prior to Thanksgiving dinner versus following Thanksgiving dinner).
- Geologic distortion:
- A buried fossil may also be subject to distorting forces from the rock around it.
- These distortions can lead to reconstruction problems, i.e., ambiguities can exist with regard to spacial relationships.
Fossils are very often found at the surface during the course of weathering.
An exposed fossil likely has been subject to some degree of weathering and erosion.
Weathering results in a loss of parts of the fossil even given their successful and complete preservation up to the point of exposure.
Removal from rock:
Fossils very often have to be removed from the strata in which they are found.
Much skill and patience is often necessary to successfully and wholly extract a complex fossil and properly preserve and interpret all of the information, including positional information, plus various things often found associated with a given fossil.
- Sediment positional information:
- Key to untangling the history of life is the dating of fossils.
- The earliest means of dating fossils, still a quite powerful one, is by determining relative positions in layers of sediment.
- Crucial assumptions:
- Two basic assumptions are made in determining the relative age of strata:
- older strata is buried beneath younger strata (unless the rock itself has been flipped by geologic forces)
- fossils are found within the strata that formed contemporaneously with the death of the fossilized organism (an assumption which is obviously not necessarily true for organisms which tend to burrow in sediment)
Early geologists also assumed that sediments containing similar fossils were of similar ages, regardless of where on the surface of the earth the sediments were found (a technique known as biostratography).
Thus, quite accurate relative ages and durations of many geologic periods were determined from fossil information long before methods of direct dating were developed.
Indeed, many geologic periods were defined specifically in terms of the fossils present in strata.
Today used along with direct dating:
Relative dating is still commonly employed in paleontology today, usually in conjunction with direct dating.
Ideally a fossil is buried in a directly datable layer.
However, if a layer close above and another close below a fossil may be dated, it is possible to say that the layer in which the fossil was found is no older than the age of the layer below it and no younger than the age of the layer above it.
- Isotope determination:
- Determination of the date of fossils and/or the stratum in which they are found today relies extensively on direct age determination.
- This, more often than not, involves some kind of isotope determination.
- Radioisotope clocks:
- The idea is that isotopes of elements that are radioactive (radioisatopes) undergo radioactive decay at both a known rate and at rate which are independent of environmental conditions.
- Furthermore, some of these radioisotopes are instantaneously (at least in geologic terms) incorporated into certain types of rocks and/or may have the elements into which they decay instantaneously purged.
- Examples of methods of direct dating employing radioisotopes include:
- 14C dating
- the 40K-40Ar clock
Other methods of direct dating include:
- tree ring comparisons
- magnetic alignment with the earth's magnetic pole (which tends to switch from North to South from time to time; known as paleomagnitism)
Because of the relatively short 14C half-life, carbon dating is usable to date objects no older than about 50,000 years old.
- Input via cosmic ray bombardment:
- Carbon dating is possible because radiactive carbon, 14C ("carbon-14"), is produced as a consequence of bombardment of atmospheric carbon with cosmic rays.
- 14C decays at a constant rate (exponential decline). Consequently, 14C is available to organisms in a fixed ratio to 12C (the stable and most common carbon isotope).
- The clock:
- The ratio of 14C to 12C is fixed at death because not more carbon is taken up by the organisms after this point.
- Thereafter, the ratio of 14C to 12C declines as 14C decays (with the half-life of 14C equal to 5730 years). By measuring the ratio of 14C to 12C, one can infer the relative age of the remains of an organism.
- Longer half-lives for older objects:
- Radioisotopes other with significantly longer half-lives than that of 14C are are useful for dating older specimens.
- The half-life of 40K ("potassium-40"), for example, is equal to 1.28 x 109 years (1.3 billion).
- 40K decays into the radioactively stable 40Ar ("argon-40").
- Volcanic start of clock:
- Since argon is a gas, a rock upon melting is purged of all 40Ar.
- This sets the 40K-40Ar clock to zero.
- Rock melting occurs in the context of volcanic eruption.
- Thus a paleontologist often considers it lucky indeed if a fossil is located in strata near strata consisting of volcanic ash (i.e., a melted and resolidified product of volcanic eruption, one which is typically carried on the wind significant distances from the actual eruption).
- Absolute dating
- Biases in the fossil record
- Chronometric dating
- Direct age determination
- Direct dating
- Fossilization problems
- Relative dating
Practice question answers
- Discuss fossil dating. [PEEK]
- Give two reasons for why fossils are rarely "perfect." [PEEK]
- Details for which points may be awarded (not necessarily a complete list): "relative dating", only method available prior to invention direct dating methods, older strata found beneath newer, fossil as old as strata found in, strata with similar fossils considered same age, "direct dating", "radioisotopes" or equivalent, decay occurs at constant rates, "biostratography", 14C, 14C decays to 12C, ratio constant during life, dates only organic matter, death of organism fixes ratio (sets clock), half life 5730 years, 50,000 year maximum, 40K, 40K decays to 40Ar, clock set by purging of Ar (such as by melting rock), volcanic ash particularly good to find around fossil, half life 1.3 billion years, much longer than 50,000 year maximum, other types of direct dating.
- (i) the organism may be imperfect due to events occurring after death but prior to burial, (ii) in most cases soft body parts do not fossilize well, (iii) distorting forces from the rock surrounding the fossil can distort the fossil, (iv) upon exposure, fossils are subject to weathering, (v) the extraction of a fossil from rock can require great skill and perseverance.
- Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 423-427.