TYPES OF FOSSILSAND WHAT THEY TELL US ABOUT THE

DINOSAURS

Fossils can be divided into two categories, fossilized body parts (bones, claws, teeth, skin, embryos, etc.) and fossilized traces, called ichnofossils (which are footprints, nests, dung, toothmarks, etc.), that record the movements and behaviors of the dinosaurs.

The four types of fossils are:

mold fossils (a fossilized impression made in the substrate - a negative image of the organism)

cast fossils (formed when a mold is filled in) trace fossils = ichnofossils (fossilized nests, gastroliths, burrows, footprints, etc.) true form fossils (fossils of the actual animal or animal part).

There are six ways that organisms can turn into fossils, including:

unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap)

permineralization=petrification (in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized)

replacement (An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron)

carbonization=coalification (in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed)

recrystalization (hard parts either revert to more stable minerals or small crystals turn into larger crystals)

authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).

BODY FOSSILS The most common body fossils found are from the hard parts of the body, including bones, claws and teeth. More rarely, fossils have been found of softer body tissues. Body fossils include:

Bones - these fossils are the main means of learning about dinosaurs. The fossilized bones of a tremendous number of species of dinosaurs have been found since 1818, when the first dinosaur bone was discovered. The first nearly-complete skeleton (of Hadrosaurus foulkii) was found in 1858 in New Jersey, USA.

Teeth and Claws - Sometimes a bit of a broken tooth of a carnivore is found with another dinosaur's bones, especially those of herbivores. Lots of fossilized teeth have been found,

including those of Albertosaurus and Iguanodon .

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Eggs , Embryos , and Nests - Fossilized dinosaur eggs were first found in France in 1869. Many fossilized dinosaur eggs have been found, at over 200 sites. Sometimes they have preserved parts of embryos, which can help to match an egg with a species of dinosaur. The embryo also sheds light on dinosaur development. The nests and clutches of eggs tells much about dinosaurs' nurturing behavior. A dinosaur egg was found by a 3-year-old child.

Skin - Some dinosaurs had thick, bumpy skin, like that of an alligator . A 12-

year-old girl discovered a T. rex's bumpy skin imprint, confirming that it had a "lightly pebbled skin."

Muscles, Tendons, Organs, and Blood Vessels - These are extremely rare because these soft tissues usually decay before fossilization takes place. Recently, a beautiful theropod fossil, Scipionyx, was found with many impressions of soft tissue preserved. Also rare are so-called dinosaur "mummies", fossilized imprints of dinosaur skin and other features. These are not real mummies in which actual animal tissue is preserved, but fossils that look a bit like mummies.

TRACE FOSSILS Trace fossils (ichnofossils) record the movements and behaviors of the dinosaurs. There are many types of trace fossils. Even the lack of trace fossils can yield information; the lack of tail-furrow fossils indicates an erect tail stance for dinosaurs that were previously believed to have dragged their tails.

Trackways (sets of footprints) - Dinosaur tracks, usually made in mud or fine sand, have been found at over 1500 sites, including quarries, coal mines, riverbeds, deserts, and mountains. There are so many of these fossils because each dinosaur made many tracks (but had only one skeleton) and because tracks fossilize well.

Fossil footprints have yielded information about:

o Speed and length of stride o whether they walked on two or four legs o the bone structure of the foot o stalking behavior (a carnivore hunting a herd of herbivores) o the existence of dinosaur herds and stampedes o how the tail is carried (few tail tracks have been found, so tails were probably held

above the ground)

Unfortunately, linking a set of tracks with a particular species is often virtually impossible.

Although there were many more plant-eating dinosaurs (sauropods and ornithopods) than meat-eating dinosaurs (theropods), many more footprints of meat-eaters have been found. This may be because the meat-eaters walked in muddy areas (where fottprints are more likely to leave a good impression and fossilize) more frequently than the plant-eaters).

Toothmarks - Toothmarks generally appear in bones .

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A Hadrosaur footprint.

Gizzard Rocks - Some dinosaurs swallowed stones to help grind their food (modern birds do this also). These stones, called gastroliths (literally meaning stomach-stones), have been found as fossils. They are usually smooth, polished, and rounded (and hard to distinguish from river rocks.)

Coprolite s (fossilized feces) - Coprolites yield information about the dinosaurs' diet and habitats. Coprolites up to 40 cm (16 inches) in diameter have been found, probably from a sauropod, considering its size. A huge theropod coprolite was recently found Sasketchewan, Canada. The only meat-eater large enough in that area at that time was Tyrannosaurus rex.

Burrows and Nests - Fossils of dinosaurs' burrows and nests can reveal a lot about their behavior.

http://people.uncw.edu/dockal/gly312/fossils/fossils.htm

Fossils, Fossilization and TaphonomyFossils 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 document.doc 3 of 17

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 123Bryozoans (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

To the left is a sand dollar from the Castle Hayne Limestone (Eocene) of southeastern North Carolina; in the center and on the

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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

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 viewGastropoda (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

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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 202Clams, 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 167Nautiloids, 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

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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 272Vertebrates. 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. 

 

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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.

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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

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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 from 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.

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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.

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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.

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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. 

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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. 

Return to the Table of Contents

TrackwaysTrackways are actually quite common.  They come from all periods of time and all sorts of

animals.  How do we know which animal made the tracks?  Paleontologists study foot bones of fossils to determine size, shape, and stride.  They also study living animals.  Current footprints show how hooves, foot pads, and claws compress mud.  Then the best candidate fossil is chosen as a match to the footprint.

The Tracks Tell a StoryOnce upon a very long time ago, dinosaurs lived near a limey mudflat close to a shallow inland

sea.  A herd of Pleurocoelus strolled across, their feet sinking deep into the soft mud, unaware of danger.  Stalking them, an Acrocanthosaurus, walked, crouched, and ran, hoping to catch a meal.  Nearby an Iguanodon watched both animals carefully, as it nibbled delicate ferns.

Soon afterwards another layer of mud gently filled the prints, preserving them. Many layers covered the first.  After millions of years, the calcium carbonate in the lime consolidated (cemented) forming limestone.  Each layer contained different amounts of minerals, making harder and softer layers of stone.  Millions of years passed again.  In 1908 a huge flood ripped through the Paluxy River removing several feet of stone and exposing the lower limestone prints.  The river will continue to erode the limestone tracks until none are left.  To preserve the existence of this wonderful story, paleontologists have removed some of the tracks.  You can see a long series at the Texas Memorial Museum at Austin.  Or go to Glen Rose to see the prints for yourself.

Mistaken IdentificationIn 1909, while wandering in a tributary of the Paluxy River, a teenager discovered giant turkey

tracks.  A local teacher had heard of tracks from New England and correctly identified these as dinosaur.  The next year two more teenagers found tracks in the River itself.  Accompanying the three-toed tracks were oblong tracks 15 to 18 inches long.  The boys described them as “giant man tracks.” 

In the 1930’s, locals cut out and pried up slabs of tracks to sell as curiosities.  One resident sold oblong “man tracks”.  But carving loose slabs was much easier than raising actual prints from the riverbed.  He added details not found in the originals to make them look more human.  These people were not promoting a scientific or theological agenda.  They only wanted to feed their families during the Great Depression.

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The first paleontologist to look at the tracks, R. T. Bird, arrived in 1938 after seeing a few carvings in New Mexico.  He found no human tracks but could not explain the oblong impressions.  He also discovered much larger tracks, belonging to a Sauropod.  While three-toed Theropod tracks had been seen before, this was the first report of Sauropod tracks.

Those supporting “man tracks” have misquoted his writings and used photos of the carved footprints as proof of humans living at the same time as dinosaurs.  Ironically, one of the main groups to publicly dispute the oblong tracks as human came from a team of Creationists.  They called some of the prints dinosaurian digits, others erosion oddities.  They found no evidence of human trackways.  They noted that many of the “man tracks” had actually been highlighted with oil to make them more humanlike.  No reputable researcher has ever found credible “man tracks” at the Paluxy River.

The three-toed tracks are accredited to Acrocanthosaurus, a common Theropod of the Early Cretaceous.  In hunting, Acrocanthosaurus used different modes of locomotion.  Scientists can tell if the animal was walking or running by the stride of the three-toed footprints.

The oblong tracks belong to this animal also.  Like birds, Theropod dinosaurs walk and run on their toes with the ankle and smallest toe held off the ground.  In 1979, G. J. Kuban concluded that the oblong tracks show a new way of walking.  Here the weight of the animal rests on the ball and heal of the foot instead of the toes.  In the oblong impressions, shallow toe marks and the smallest toe can often be distinguished.  Why would a Theropod do this?  Was it for better traction in the mud? Or did the animal crouch low while it stalked its prey.

What are fossils and why are they important?Fossils are the remains of ancient life, often buried under soil or water.  Fossils can be

bones, shells, plants, insects, or the tracks and impressions left by life.  Their age can range from thousands to billions of years old.  Most fossils are fragments of the original animal.  Finding complete specimens is considered rare.

Not all geologic layers contain fossils. To become fossilized, the carcass must be protected from scavengers and oxygen breathing microbes.  This can be accomplished by falling to the bottom of the sea, being silted with mud (marine or freshwater), covered by volcanic ash, falling into an acidic bog, or dieing in a dry cave.

Sedimentary rocks often contain fossils.  The most common, limestone, is composed of marine animals: forminifera, shells, urchins, corals, arthropods, the occasional fish, and marine reptile.  Inland, shales, siltstones, mudstones, and sandstones contain freshwater and land dwelling animals.

More than 90% of all fossils come from marine invertebrates.  Many help date layers and are called index fossils.  These were animals that were prolific for a time but then changed or died out. Some layers of dry land origin do not contain visible fossils. Scientists have recently begun using pollen and spores caught in the old soils as index fossils.  These microscopic fossils are in both land and sea layers, tying them together.  This way a more accurate timeline can be established.

Fossils Are Not All The SameActual bone, shell, and calcified structures --These are the hard parts of animals that were buried and preserved.

Petrification (petrifaction) – As an animal or plant slowly dissolves, minerals in the surrounding matrix grow in the small hollows.  Submersion in water rich in minerals is

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required.  This process can maintain detail at the cellular level.  This is the most common form of dinosaur and tree fossils one sees. 

Mummification – In very dry climates, soft tissue of animals is can be preserved.  This is very rare.

Coal – Plants buried in swamps do not decay quickly.  The material often becomes peat.  In the Carboniferous Period, great swampy forests covered the shorelines.  The climate cooled and the seas covered the land.  Layers of limestone deposited on top.  Compression, heat, and time transformed the plants' natural carbon compounds, preserving material as coal.  Plant and animal fossils can be quite detailed.

Films – A thin layer of carbon left after a plant or animal has been dissolved.  Delicate features, like leaves and feathers, can be perfectly preserved.

Amber – Resin of broadleaf conifers and legume trees naturally polymerizes (a chemical process) into a non-water-soluble material.  If a plants and animals become encase in the sticky goop, they are frozen in time as mummies.

Trace fossils – Fossils that are not part of the animal itself.

a. Natural impressions – A plant or animal part is pressed into soft mud leaving shallow marks.

b. Natural molds and casts – A mold is stone surrounding a fossil that has impression details.  A cast requires a hollow space that is filled with a secondary soil or mineral.  An external mold is where the fossil has dissolved and has been completely replaced.  An internal mold, usually a shell,  has the inside of the fossil filled, then the outside is dissolved or broken away.

c. Tracks – Footprints and tail drags from bugs to Sauropods are preserved in soft mud flats.

d. Burrows – Tunnels made by ancient animals that became filled with secondary sediments.  Generally, the fossil of the animal who made the tunnels are not found.

e. Coprolite – Petrified animal feces.  Many contain fossilized remains of what the animal ate.

 

How Does Wood Petrify? 

Wood must first be covered with such agents as volcanic ash, volcanic lava flow, volcanic mud-flows, sediments in lakes and swamps or material washed in by violent floods - by any means which would exclude oxygen and thus prevent decay. A number of mineral substances (such as calcite, pyrite, marcasite) can cause petrification, but by far the most common is silica. Solutions of silica dissolved in ground water infiltrate the buried wood and through a complex chemical process are precipitated and left in the individual plant cells. Here the silica may take a variety of forms; it may be agate, jasper, chalcedony or opal. The beautiful and varied colors of petrified wood are caused by the presence of other minerals that enter the wood in solution with the silica. Iron oxide stains

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the wood orange, rust, red or yellow. Manganese oxide produces blues, blacks or purple.

 

Mineralized fossil boneFossil bone can be mineralized in several ways. Permineralized fossils have their original pore space infilled with minerals. Permineralization is commonly confused with petrification, in which the original material of an organism is replaced with minerals, and the pore space is infilled with minerals. In other words, petrification is a combination of permineralization and replacement.

By far, permineralization is the most common type of preservation for most fossil bone, and even when petrification has occurred, there is almost invariably evidence that permineralization occurred first (otherwise, there would be no preservation of the original cavities in the bone!). So, if you are wondering what petrified bone looks like, imagine the bone material being replaced by other minerals, sometimes preserving the fine structure of the bone, sometimes not, and the open pore spaces infilled as seen here. I plan to eventually present some truly petrified bone eventually.

In either case, the boundary between the original, open pore space and the replaced material is quite obvious, because of variations in the shape and orientation of the crystals infilling the pores. In the case of the Haversian canals of bone, this is usually indicated by concentric growth of crystals from the inner surface of the canal towards the interior, often with clear radially-arranged crystals and/or layers of different minerals at early infilling versus later stages.

For more information on bone fossilization processes, including illustrations, see Reid (1996) and Hubert et al. (1996).

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