2.1 Introduction
Taphonomy is the study of postmortem processes on once-living organisms. In addition to determining the type and intensity of the processes and their role in preservation, taphonomy is a way to detect bias in the fossil record. For example, in a hypothetical fossil assemblage of shells certain questions can be asked, such as:
These are just some of the questions one could begin to ask about any assemblage of fossilized material. Indeed, it is documented that the relative abundance of species in a fossil assemblage may not be an accurate reflection of the relative abundances in the original assemblage of living populations.
2.1.1 Taphonomic processes
There are three major categories of taphonomic processes of alteration and destruction: physical processes, chemical processes and biological processes. Physical processes involve the mechanical breakdown of organic material via water and/or wind action (storms are an excellent example of a physical process). Chemical processes include any alteration of a material's mineralogy such as that discussed above, as well as any leaching of material by the surrounding water or air. Finally, biological processes, such as sponge or algal borings, can help to alter and eventually destroy potential fossil material. All three types of processes can act in concert at various amplitudes in any given situation. It is a taphonomist's job to look at the intensity and interactions of these processes and their effects on a fossil assemblage.
2.1.2 Fossil concentrations
Fossils can be concentrated in two major ways, first by physical processes mentioned above, such as storms and currents, or winnowing and deflation. Fossils are also concentrated by aggradation, which is a biological process in that it is the piling up of LIVE individuals, such as those found in oyster beds or coral reefs.
2.1.3 Konservat-Lagerstatten
This term was coined by German paleontologists. It means simply an exceptional preservation in the fossil record. Konservat-Lagerstatten represent a preservational endmember in the spectrum of fossilization. Not only are most of the hard and soft parts preserved, the assemblages in these types of deposits are probably the closest approximation to the abundance and diversity of the original assemblage. For Konservat-Lagerstatten to form, all taphonomic processes must be minimized. That is, physical, chemical and biological destruction must be kept to a minimum.
Some of the world's most famous fossil deposits happen to be Konservat-Lagerstatten. Faunas such as the Mazon Creek (Illinois), Solnhofen Limestone (Germany), La Brea (tar pits in Southern California; "La Brea tar pits" is a redundant name), insects and others in amber (the Baltic states, Dominican Republic), and Burgess Shale (Canada) are all good examples. The Burgess Shale is located in the Canadian Rockies (British Columbia). The shales and its fossils are dark black in color, suggesting anaerobic conditions (no oxygen) and the fine-grained nature of the sediment indicates quiet water deposition, because there is no disturbance from wave action or burrowing organisms in the sediment. The Solnhofen is also very fine grained. The complete skeletons (e.g. Archaeopteryx) preserved in the limestone indicate very quiet waters too.
Although these deposits give us some of our most spectacular fossil deposits, they are important for many other reasons. First of all, they represent a "snapshot" in time, because of probable rapid burial. Secondly, they provide previously unknown anatomical details that can be important from a systematic (evolutionary) point of view. They also can provide an additional test for environmental and diagenetic boundary conditions. And finally, the excellent time resolution may allow true biotic diversity for an assemblage to be observed. This may be the closest that paleoecologists can come to the conditions of modern ecology.
2.2 Exercises
2.2.1 Taphonomic grades
Some sedimentary environments are better than others when it comes to preserving fossils. The high energy conditions of a river channel or beach may grind and abrade bones or shells so that they are unidentifiable after only a short period of time. The quiet waters of swamps and lagoons, on the other hand, may permit the preservation of the delicate features of many hard parts.
The purpose of this exercise is to illustrate how the preservational condition of fossils (i.e. taphonomic grade) can vary from sample to sample, how taphonomic grades can be recognized and analyzed, and how that variation can be used to interpret the ancient environment of deposition. The material: The shells are specimens of the species Chione fluctifraga from the northern Gulf of California. They are all Recent in age. Chione fluctifraga lives buried a few centimeters below the surface of the sediment in the intertidal zone and in the shallow subtidal. After death, the shells rmay be eroded out of the sediment by waves and currents and then abraded and worn.
Taphonomic grades: It's easy to pick out some shells that show well preserved surface sculpture and growth lines; other shells have had these surface features almost completely worn away; others are in an intermediate condition.