Taphonomy of mollusk shells

This post concerns taphonomy = the scientific evaluation of what happens to shells and other organisms after they die. Taphonomy provides very practical information as to whether or not an assemblage of fossils (e.g., a layer of fossil shells) represents a life assemblage = the fossils died exactly where they lived in their original environment, or if it represent a death assemblage = the fossils have been moved by currents and or waves after death to a different spot from where they originally lived. A life assemblage is also called an in situ assemblage. A death assemblage is also called an exotic assemblage.  

The following photographs show examples of both assemblages.

In the middle of this photograph, there is a life assemblage of boring clams of late Pleistocene age (slightly older than 47,000 years ago) in southern California. The man's finger is pointing to a clam consisting of both valves and in its original life position. The clams lived in a rocky shoreline environment and bored into a deep-marine siltstone of Miocene age (about 6 to 7 million years old). The layer of shells directly above the boring clams represents a storm deposit that buried the boring clams. It is a death assemblage consisting of shells transported and deposited at this site about 47,000 years ago. Its clam shells are all single valves that were deposited after death in a horizontal position.

This photograph shows a life assemblage consisting mainly of the large oyster Crassostrea titan. They consist of both valves that are closed together in their original living position; in this case, vertical. The oysters formed a bed, one-oyster thick that lived in very shallow-marine waters in central California during the late Miocene.

The turritellid shells shown in above photograph were transported and deposited at this site by wave action; note that they show preferred orientation. The species is the middle Eocene Turritella andersoni lawsoni, which was discussed in one of my earlier posts.

The above photograph shows a death assemblage of late Pliocene mollusk shells that were transported and deposited after death as a result of a storm deposit with local channeling into the layer beneath it. The larger shells are Turritella cooperi, which was discussed in one of my earlier posts.

The above photograph shows a death assemblage of late Pliocene mollusk shells that were transported and deposited after death as a result of a storm deposit. Turritella cooperi, which was discussed in one of my earlier posts, is readily identifiable, whereas most of the other shells are too broken up and constitute "fossil hash."

Gari texta, an Eocene burrowing clam, 35 mm long.

This last picture shows a "butterflied" clam, called such because both valves are flattened out. Burrowing clams shaped like this one easily fall apart, because the valves get separated by even weak waves or currents. This particular clam, however, died near where it lived. Its "butterflied" condition indicates minimal transport after death.

Pseudofossils

Pseudofossils are not fossils at all. They are inorganic oddities that can resemble fossils. Five examples of pseudofossils are illustrated below:

top view of a concretion, 6 cm diameter

side view of the same concretion

concretion = hard, compact, rounded mass of mineral matter and silt or sand grains. The concretion generally forms by localized precipitation from aqueous solutions commonly around a nucleus consisting of some debris, including organic matter (like a leaf, shell, or bone, etc.) in the pores of a sedimentary or fragmental volcanic rock. The concretion weathers out differently than the surrounding rock because the concretion is commonly more tightly cemented. Although it is possible that a concretion might have a fossil at its center, there is no easy way to determine this unless you crack open the concretion.

dendrites, 15 cm in length

dendrites = a chemical stain consisting of an oxide of manganese that crystallized in a branching pattern resembling that of certain plants. Upon close scrutiny, one will see that the dendrites pattern is not seamlessly connected (i.e., unlike plants).

cone-in-cone from Pennsylvanian-age rocks in southern Oklahoma; entire rock 10 cm in width

cone-in-cone structure = occur in thin calcareous (limy) layers of some mudstones; the structure resembles a set of concentric circular cones fitting one into another in inverted positions (base upward, apex downward) and commonly separated by clay films. The formation of these structures is apparently because of pressure along conical shear zones in carbonate rocks.

cross section of a septarian nodule from Utah; slab is 16 cm in height

septarian nodule = roughly spheroidal body, commonly of impure carbonates, with a radiating series of crystal-lined cracks that intersect and form irregular polyhedral blocks with the body.

half of a geode, 15 cm diameter

geode = hollow to partly hollow, or in some cases a filled, subspherical body lined with crystals. The crystals are commonly quartz. Geodes are present in volcanic and certain sedimentary rocks.

What is a fossil?

Fossil = the remains or traces of an ancient organism preserved by natural processes in the Earth.  

[Note: the word fossil is derived from Latin, "fossilis," meaning dug up].

The definition of a fossil seems straightforward, but its component parts need some explanation.

remains = hard-body parts like shell, bone, teeth, wood, leaves, pollen; as well as soft-body parts like skin, hair, toenails, organs.

traces = indications of the behavioral activities of an organism (e.g., footprints, dwelling burrows, feeding trails, tracks, boreholes, tooth marks, arrowheads, pottery). Trace fossils are called  ichnofossils; in my last blog I mentioned that ichnology is the study of trace fossils.

ancient = must be older than 10,000 years (older than the Holocene). This age is admittedly arbitrary but it was picked by an international council because 10,000 years is beyond recorded history.

Question: Is a 2,000-year-old Egyptian mummy a fossil?
The answer is no: because it is too young, and it was preserved via a technique used by humans (this technique of mummification is not considered to be a "natural" process).

natural (i.e., non-human-related) processes: fossils are preserved by 1) burial in ice, amber (tree sap), asphalt, dry caves; 2) burial in sediment and later converted into hard rock via compaction and cementation; 3) imprints consisting of only carbonized residue of the organism (insects, leaves, and fish are commonly preserved in this manner).

What is Paleontology?

Paleontology is the study of ancient life, older than 10,000 years. The word "paleontology" is derived from three Greek words: "palaios," meaning ancient; "onto," meaning being; and "logy," meaning study of.

There are five main subdivisions of paleontology:

1. Invertebrate paleontology = study of animals with no backbone (e.g., sponges, corals, snails, clams, ammonites, trilobites).

2. Vertebrate paleontology = study of animals with a backbone (e.g., fish, dinosaurs, horses, elephants, humans).

3. Paleobotany = study of land plants.

4. Micropaleontology = study of microscopic organisms and microscopic parts of large organisms; palynology = study of pollen and spores, could be included in this subdivision.

5. Ichnology = study of tracks, trails, burrows, boreholes, tooth marks, and artifacts.

In recent years, a new subdivision, called Exopaleontology has been emerging. It deals with the evaluation of the possibility of finding evidence of ancient life on other planets. Examples of such life include blue-green bacteria (which construct cabbage-head-like structures called stromatolites); cold-desert soil bacteria; and hot-vent organisms. Some of these types of organisms can live in extremely harsh environments (very cold or very hot).

Geologic Time Scale

A geologic time scale is given here in order to help discussions
that will follow in my future posts.

For a more complete and much more detailed version, as well as one that is to scale, the link is HERE
It is from the Geological Society of America and is their 2012, version 4.0.

It is important to realize that the Precambrian accounts for about 7/8 of all geologic time. A way to visualize this is consider that if your  entire arm (from shoulder to wrist) and your hand represent all of geologic time, the Precambrian would extend from the shoulder to the wrist. The rest of geologic time is represented by your hand. 

A handy way to remember the sequence of the names of the periods of the geologic time scale involves the following mnemonic devices (by using the first letter of each period name in its respective position in the sequence):

For the Paleozoic:  "Certain old stones delight my paleontology professor."

For the Mesozoic and Cenozoic: "The jungle cat prowls nightly, quietly."