Fossils, Fossilization and Taphonomy

Fossils represent the preserved remains of organism that once lived on or near the surface of the Earth. For something to be a fossil it must fulfill several conditions:

1. It must have been alive at some point.
2. It must now be dead.
3. It must have been dead long enough that bacteria are no longer interested in it (it no longer smells bad).
4. It must have been buried in sediment at some point.

Fossil are important in the study of sedimentary rocks because they or the one thing that really allows us to grasp some idea of what the environmental conditions were like at the time of deposition of a stratum of sediment. Their evolution through time also allows for an assignment of the relative position of that stratum in geologic time scale. They also can provide valuable information on the physical and chemical conditions that the stratum has experienced since deposition. Plus they can aid the structural geologists in the interpretation of the deformation of a region.

Recognition of Major Macroscopic Fossil Groups

A very important aspect of the training of a geologist it the learning of how to recognize and identify fossils. The best way to start this process is by looking at plates of fossils illustrated in the geologic literature. My favorite source of pictures and the book that I usually carry with me when heading away form the office to do geology is: Index Fossils of North America by Hervey W. Shimer and Robert R. Shrock 1944. This book is out of print, hard to find a copy of and real 'paleontologists' will tell you that it is dated. All of which is true but you still can not beat it for a field reference especially when all you are trying to do is get a handle on what you found and not trying to work out all the taxonomy of it. In the descriptions below of the various macroscopic fossil groups I have included links to scanned plates from this source for you educational benefit and pleasure. They are big files.

One question that students generally ask at this point is 'how far do I need to go in the identification of a fossil for this course?'  The answer is this: always take the identification process as far as you can with the time and references resources available. This extends into your professional career also. As a minimum you need to be able to place most of the fossils which you encounter into the various common groupings, brachiopods, gastropods, corals, etc. This you will find is not that difficult and later on you will also find that the microscopic fossils also are not really that difficult for the most part to deal with. Once you have taken paleontology you will have become very proficient at this task.

Brachiopods surprisingly belong to the Phylum Brachiopoda of which there are two divisions, the Articulata and the Inarticulata. Brachiopods appeared in the Cambrian, were devastated during the terminal Permian extinction event but continue to the present. All the Articulata secrete calcite whereas some of the Inarticulata secreted phosphorite, especially the Inarticulate Lingula. Lingula is strange in that it appeared in the lower Paleozoic and continued to the present with very little evolutionary changes, a true living fossil. Most folks when they think of brachiopods they think of the classic spiriferids but there is a fair amount a variability to the brachiopod form. Lot of students have problems differentiating the brachiopods from the pelecypods because they both look like how a sea shell should look but you can tell them apart by their symmetry. Shimer and Shrock Plate 123

Bryozoans (Phylum Bryozoa) represent a very large and morphologically very diverse group of organisms. They appeared in the Cambrian and are very common today in the marine evironment.  They have a colonial habit with the colonies known as zoaria (singular, zoarium). Individuals of the colony are referred to as zooids and each lives in its own zooecium (plural zooecia) which is some sort of tubular structure constructed of chitinous materal or calcite and/or aragonite. Each species differs from the next in the geometry of the zoarium and the geometry of the zooecium and within indivudual species there can be variation in the geometry of the zoarium but not in the zooecium hence all the taxonomy is based on the shape of the zooecium. Zooecia are always quite small, rarely greater than one millimeter in diameter but zoaria can exceed 50 cm.  Shimer and Shrock Plate 99

Photograph of bryozoans from the Red Wall Limestone (Mississippian) Uinta Mountains of Utah. Expanded view

Echinoderms (Crinoids, blastoids, sand dollars, starfish, sea biscuits, sea cucumbers, etc.) belong to the  Phylum Echinoderma. They first appeared in the Early Cambrian. Most produce a series of plates, ossicles, spines, and spicules which are generally firmly held together by tissue and composed of calcite. The crinoids and blastoids were sessile and held to the sea bed by a 'root'. Extending from this was a column which was made of stacked donut shaped ossicles (see center and right hand fossils in photograph to the right). Attached to this was a calyx. Attached to this were petal or feather like structures called brachioles or arms all of which were made up of small ossicles. When the animal died the tissue decayed and all you had was a pile of ossicles and rarely a calyx. The sand dollars, sea biscuits, sea urchins and heart urchins were mobile. The were constructed of plates of various shapes, and frequently attached to these were spines. All where constructed of  calcite and held together by tissue. Upon death they became brittle but generally did not disarticulate like their cousins the crinoids. Shimer and Shrock Plate 61

To the left is a sand dollar from the Castle Hayne Limestone (Eocene) of southeastern North Carolina; in the center and on the right are ossicles from the column of a crinoid from the Paleozoic of Missouri.

Corals. The corals belong to the  Phylum Coelenterata, and Class Anthozoa. There are three common subclasses: the Tabulata, Tetracorallia (Rugosa), and Scleractinia. The Tabulata and the Rugosa appeared in the latter portion of the Cambrian and disappeared with the terminal Permian extinction event. The Scleractinia took their place in the Triassic. The Tabulata and Rugosa are assumed to have secreted calcite, at least that is what their fossils suggest. The Scleractinia which are alive today secrete aragonite. Most corals occur as colonies however a large number of the Rugosa formed individual cup corals. To the right is a photograph of a polished slab of a colonial Rugosa, Hexagonaria from the Middle Devonian of Iowa. To the far right is Favosites, a Tabulata also from the Devonian of Iowa. The Scleractinia form much but necessarily all of the modern 'coral' reefs.  Shimer and Shrock Plate 25

Expanded view Expanded view

Gastropoda (snails)  are a class of the Phylum Mollusca. They appeared in the Cambrian and continue to the present with their diversity really exploding during the Cenozoic. Virtually all secreted aragonite, a few oddballs seem to have made calcite. Most have some sort of coiled form like those to the right. The near right is a 'nonmarine' snail from the Eocene of Wyoming. It is one of the oddballs that seems to be made of calcite. The snail on the far right is a marine snail form the Eocene of North Carolina. It is preserved as a steinkern (see below) since the organism secreted aragonite with is not very stable in the sedimentary rock enviroment. Shimer and Shrock Plate 199 Shimer and Shrock Plate 202

Clams, oysters, and scallops make up the Class Pelecypods of the Phylum  Mollusca. The Pelecypoda appeared in the Cambrian and are with us today, at least the local sea food mart here has them. The clams produced aragonite whereas the oysters and scallops secreted calcite. To the right are typical clams. Left hand one is Mercenaria from from the Neuse Formation (Pleistocene), Snows Cut, New Hanover County, NC. The center one is Glycymeris americana from the Waccamaw Formation (Pliocene) of eastern North Carolina. The right hand group are miniature clams form the Maquoketa Shale (Ordovician) of Dubuque, Iowa. Shimer and Shrock Plate 167

Nautiloids, Ammonoids, and Belemnites are members of the Phylum Mollusca, Class Cephalopoda. Cephalopods appeared in the Upper Cambrian and were once a large and diverse group, now they are very liminted and those that are around today that produce fossilizable hard parts are limited a single genus, Nautilus (photo to the right). Other modern cephalopods include squids, cuttlefish and octopuses. The nautiloids flourished in the early Paleozoic, Ordovician-Silurian. They have straight or coiled chambered shells with curved septa separating the chambers. One chamber joins the next on the exterior by a simple straight contact or suture. The Ammonoids flourished in the Mesozoic but did not survive the terminal Cretaceous extinction event. They have coiled shells with wrinkled septa and complex sutures. The Belemnites (belemnoids) had no exterior shell but formed a pen like devise know as a rostrum which was commonly the only thing to be preserved. Do not confuse cephalopods with gastropods!!! Shimer and Shrock Plate 242

Algae. Macroscopic fossil algae are not exactly common and we will deal with the algae in much greater detail when we take up carbonate rocks. At the right is a photograph of Receptaculities oweni from the Kimmswick Formation (Middle Ordovician) collected from near Hannibal, Missouri. It has been called a sponge, it has been referred to the corals, and most recently it has been considered a green algae. Which brings me to the point that the fossils never change but their taxonomy does and sometimes that creates problems in communication. Be aware that at differing times of publication the same organism with have different names and like R. oweni here even be assigned to very different groups. Sometimes a sedimentary petrologist needs to call upon the assistance of a paleontologist!

Trilobites belong to the  Phylum Arthropoda, Class Crustacea, Subclass Trilobita. They appeared in the Cambrian (and possibly earlier!!) and did not make it past the terminal Permian extinction event. Phylum Anthropoda includes organisms like shrimp and crabs, ostracods, barnacles, cockroaches and mosquitos. Trilobites seem to have secreted a calcite exoskeleton that was readily preserved in the geologic record. In the early part of the Paleozoic they are extremely important for biostratigraphic dating.  Shimer and Shrock Plate 272

Vertebrates. We belong to the vertebrates, so do the fish, lizards, snakes, toads and politicians. Vertebrates secrete a internal skeleton consisting of phosphorite (mineral apatite)  which can be retained in the fossil record. Some also produce teeth which are also apatite but generally more durable than bone and thus more readily retained in the sediment. When a vertebrate expires the bones generally dissarticulate, the scatter making identification challenging. They also are subject to the gnawing of other creatures and bacterial attack so that they are rarely pristine, and to make this worse they are very brittle and fragment easily. At the right is a fish from the Green River Formation (Eocene) of Wyoming.

Fossilization

Body Fossils represent the direct remains of an organisms that exists pretty much in the same condition that the organism produced it.  Most organisms that produce some sort biomineralized hard part could become a body fossil. Calcite is really the most stable mineral that organisms produce therefore those organisms that are calcite producers are the ones that commonly occur as body fossils. Aragonite is not as stable as calcite and aragonitic body fossils are really uncommon in sediments older than the Cenozoic.

Permineralization. The space that was occupied by the living tissue of an organism, or even the interior of cells, can after death be filled by a mineral precipitate. This then leaves a fossil with very exacting microscopic detail. Of the marine creatures the echinoderms are notorious for this method of fossilization. When living the echinoderm produces a test that is a maze of calcite crystals inter-grown with tissue with at least half the volume being tissue. Once the organism expires, the tissue decays leaving the test with a whole lot of open space. Calcite precipitates in this open space generally as an extension of the calcite crystals already present. For this reason a fossil sand dollar is quite heave relative to on that you might find on the beach. On land wood can be permineralized (opalized) in ideal circumstances. At the right is a picture of fossil log from Petrified Forest National Monument. Below is a photomicrograph of petrified wood from a similar situation in eastern Utah. Here the fossilization process starts with a forest being buried under a volcanic ash fall. The ash is volcanic glass which is chemically unstable and breaks down to soluble silica. This is carried by groundwater to the former cell cavities of the log where it is precipitated as opal or as quartz. The degree of textural preservation is quite remarkable right on down to being able to compare this fossil wood to modern wood and thus providing an identification of tree type.

Carbon Films and Impressions. If you look around you it becomes quickly apparent that there are a lot of organism which have no secreted hard parts like shell or bone yet these very infrequently also occur as fossils. The process requires swift burial within an oxygen depleted sediment. There the organic tissue does not get to decay. With time the organic material goes through the same process as the formation of coal, namely loss of volatile components and the resultant formation of a carbon rich film which retains quite well the form of the original organic tissue. To collect fossils of this type one only needs to find the proper rock which is generally a shale. Then split the shale parallel to bedding. The fossils when freshly split open will have the carbon film.  But this film is really fragile and is lost very quickly due to exposure to the elements or handling of the specimen. What you frequently end up with is an impression of where it was. At the right is an Eocambrian fossil preserved in the Burgess Shale as illustrated on the cover of Geology, January 1996, vol. 24, number 1. A very famous fossils of this type is Archaeopteryx, the dinosaur with feathers, from the Solonhoffen of Germany. The feathers are very well preserved in this manor. Other examples include the leaves of the Mazon Creek (Pennsylvanian) of Illinois and the fish of the Green River Formation (Eocene) of Wyoming.

Mold & Cast. Shells and other biomineralized hard parts that are not chemically stable in the encapsulating sediment or rock tend to dissolve with time leaving behind a hole or pore which is technically called a moldic pore. Aragonite is one mineral commonly produce by organism such as mollusk which is not stable and usually with time dissolves away. If the encapsulating sediment is already lithified then a mold will remain. Molds are difficult to collect and work with for once you have chipped away the matrix you have nothing left! But a wise petrologists will first pour latex or plaster in the mold and make a cast and then get rid of the matrix. Nature sometimes will do this for you forming natural cast of a fossil. check this out

Steinkern. Something that is similar to a cast is a steinkern but here the mold is the shell or skelletal material that the organism produced and the space within where the tissue was at becomes filled with sediment. This becomes lithified. Then the shell is lost or removed leaving behind a casting of the interior. Pelecypods and gastropods, both aragonite producers are notorious for this. On the far right is a steinkern of a pelecypod from the Castle Hayne Limestone (Eocene) of North Carolins. Next to it is a steinkern of a brachiopod, Pentamerites, from the Silurian of Iowa. This is unusual in that brachiopods produce calcite which is stable but here the calcite shell was surrounded by dolomite mud. Groundwater dissolved the calcite but did not remove the dolomite that filled the shell cavity.

Replacement. Frequently one encounters fossils where the mineralogy of the specimen is not the mineralogy that one would expect the organism to have produced. Furthermore the degree of preservation precludes the fossil being a cast. What has happened is that the original mineral has been replaced by a different mineral. This is a chemical process where as the primary mineral dissolves the secondary mineral precipitates in the vacated space. This takes place across a film of water that separates the primary form the secondary mineral so that there never is a wide gap or pore formed. To the right is an example of silicification of calcite where forams, which were once calcite have been preserved as quartz. Calcite commonly replaces aragonite, dolomite replaces calcite, and rarely pyrite will replace calcite.

Combination. It is possible for more than one process to be responsible for the preservation of a fossil. The example at the right is a photomicrograph of a dinosaur bone fragment from the Morrison  Formation (Jurassic) of Utah. The bone being composed of phosphorite would alone produce a body fossil but it has also been permineralized with the precipitation of quartz within all the positions formerly occupied by organic tissue thus greatly increasing the density of the fossil. If you have ever picked up an old cow bone you know such is quite light yet fossilized bone is dense.

Trace Fossils or Ichnofossils. These are the preserved evidence of the activities of organisms. they include tracks or foot prints, trails, feeding structures, dwelling structures like a bird nest, predation structures like holes drilled in a sea shell, and preserved fecal material. For many organism this is all the ever gets preserved in the geologic record. At the right are the tracks left by a trilobite that was marching across what is now Virginia during the Silurian. In this example the trilobite walked across a mud cover sea floor leaving behind the tracks. These were then later covered with another thin layer of mud and buried.

Taphonomy 

Taphonomy is the study of the nature of the occurrence of a fossil or fossils in the strata. The study entails trying to ascertain if the fossil is in its living or growth position or has suffered post mortem transport to some degree. It entails the examination of the relative positioning individual components of a fossil (articulation) or the relative positioning and association with other organisms (faunal association). It also entails evaluation of the orientation of individuals as the possible result of transport currents. 

Growth Position. Evaluation of the positioning of a fossil within its host sediment may provide clues to if the fossil is situated in the same orientation as it lived or if it has been transported from that situation. This involves a certain amount of uniformitarianism and a whole lot of background knowledge of how and were things live. This later item you can only gain by getting out in the world and examining how organism live in the wild. At the right is a photograph of a boring clam in growth position within its boring (arrow). The light colored patches below the arrow are encrustin bryozoans on the walls of abandoned borings. These are also within their growth position. The sample is from a modern submarine hard ground off the southeastern North Carolina coast.

Articulation refers to the degree to which individual parts are preserved attached to one another in the manor that they were when the organism was alive. In the simplest form there are the two valves of the pelecypods and brachiopods; upon death these can be detached from one another. Brachiopod fossils frequently are articulated whereas pelecypod fossils are disarticulated. The echinoderm plates usually become detached from one another shortly after death thus finding an articulated crinoid is a rare thing. Vertebrates generally have the skeletons dissasembled probably by predators and scavengers. At the right is a view of the Dinosaur Quarry at Dinosaur National Monument. The bones in the lower portion are the partially articulated hind quarters of a Apatasaur. Most of the other bones in the view are scattered odd and ends of many individuals.

Image: http://www.americansouthwest.net/utah/dinosaur/quarry.html

Faunal Association. How one organism relates spatially to another provides clues to the paleoecology of depositional situation. In the example at the right there is an oyster, Crassostrea virginica which is encrusted by a colonial gastropod or worm shell, Petaloconchus sp. Note that the gastropod encrusts both the outside (left photo) and inside (right photo) of the shell. This indicates that the oyster was already dead when the gastropods were living.

Abrasion is important to look for because it probably results from post mortem transport. Un-abraded fossils would have experienced littly post mortem transport and therefore are deposited close to where they lived and thus are reliable indicators of the environmental conditions. Greatly abraded fossils could have been transported great distance from where they lives and thus confuse the interpretation of the environmental setting. To the right are individual valves of the pelecypod Chione cancelatta. The left hand individual is pristine where the other illustrate progressive degrees of abrasion.

Orientation. In a flowing current (air or water) objects like organism, both dead and alive, will orient themselves such as to present the least amount of resistance to current flow. For elongate objects that are geometrically simple (think telephone pole) the long direction is oriented parallel to the current flow direction. More interesting shapes like that of clam shells also will have a preferred orientation but it may not be readily apparent how to relate the orientation of the shell to the orientation of the current. Concave objects like individual clam valves also tend to orient themselves such that the concave side is down. This is a valuable observation in structurally complex areas. At the right is a slab from the Red Wall Limestone (Mississippian) of Utah. Note the preferred orientation of the elongate echinoid spines. expanded view

Exercise #1: (To be done prior to lab time) Go to the library and go into the stacks where the geology and paleontology text books are kept and spend at least one hour looking at the pictures and plates of fossils. Especially look at the Journal of Paleontology. Make an annotated bibliography of everything that you looked at. This may sound dumb to you but it is a good thing to do because you will be expanding you background knowledge and you will be learning how to keep track of what you find. 

Exercise #2: Examine the collection of labeled fossils where the type, mineralogy, fossilization process and taphonomy have already been identified for you. Don't just check them out; use your hand lens and really study each sample. See if you can find things that are not on the attached label. Print out a copy of these Descriptions

Exercise #3: Examine the collection of 'new finds' and identify what it or they is/are, characterize the fossilization method/methods, and make any observations relative to taphonomy. 

The results of Exercise #1 and #3 will be written up as a Microsoft Word text file and emailed to me prior to next week's lab. Remember to use proper filename protocol. 

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