| Geology 105 - Paleontology | ||||||
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Organisms make their skeletons from a variety of materials. Remember, "skeletons" include bone, the protein shell of crabs or insects, the sturdy shells of clams, or the massive rock secreted by coral.
A. Apatite (calcium triphosphate): This mineral makes up the bones and
teeth of vertebrates and the shells of inarticulate brachiopods. Calcium
phosphate looks chalky when new, but can darken to black when subjected
to heat.
A1. Teeth
A2. Brachiopod
B. Aragonite: Modern corals are aragonite, and mollusk shells (snails,
clams, Nautlius) are made of a mixture of aragonite and calcite. Plain aragonite
is chalky (think of the exterior of a clam shell). In a complex arrangement
with calcite and protein (called nacre), aragonite takes on the mother-of-pearl
appearance seen on the inside of mollusk shells. Aragonite is unstable over
geologic time and inverts to calcite. Aragonite is only found in young fossils
or in shells that have been surrounded by petroleum.
B1. MollusksC. Calcite: Calcite makes up the skeletons of extinct corals (rugose and tabulate), brachiopods, bryozoans, echinoderms, and formed a thin layer in the skeletons of trilobites. Calcite skeletons are typically gray, slightly translucent and sometimes shiny. Some organisms make skeletons of large calcite crystals, as in belemnoids and crinoids (each crinoid columnal is a single calcite crystal)
B2. Coral
C1. Brachiopods & bryozoans
C2. Crinoids, barnacle, belemnoid
C3. Coral
C4. Comparison of aragonite and calcite corals
D. Silica. Some microorganisms (diatoms, radiolarians) and some sponges make skeletons of silica. These are all very tiny skeletons, visible clearly only through a microscope. The silica typically looks glassy or whitish.
D. Radiolarians
E. Chitin: Arthropods have exoskeletons made of polysaccharide and protein.
Chitin skeletons do not fossilize well, and are typically preserved as carbon
films on impressions.
E. Crab skeleton
This does not mean the organism is unchanged, but that the original material of the organism has not been changed to another substance. The fossil may have lost water, or color, or the proteins of the soft tissue may have degraded.
Unaltered hard parts: teeth and very recent shells, bone or shell
encased in petroleum or in petroleum-containing sediments.
1A. Elephant tooth2. Shell material:
1B. Horse tooth
1C. Shark teeth
2A. Paleozoic brachiopods - calcite3. Bones: These bones are preserved in tar. The tar is impermeable, so no groundwater has reached the bones to alter them. Such preservation is remarkable rare.
2B. Modern limpets - aragonite with nacre (mother of pearl)
2C. Pleistocene clams - aragonite
3A & 3B. Bird bones from the LaBrea oil seep.
Unaltered hard and soft parts: mummification, freezing, encasement
in amber (fossilized tree sap). Very very rare, usually only very young
fossils.
4A. Peat5. Insects in amber: The entire organism - hard and soft parts - is preserved. However, the proteins break down when exposed to heat, so it is incredibly unlikely that any DNA remains intact (so much for Jurassic Park).
4B. Bog People
4C. Frozen mammoth
4D. Otzi the ice man
5A. Insect
5B. Cricket
6A & B. Bone
6C. Wood
7. Recrystallization: The original skeletal material has grown into new crystals. The process may be aragonite crystallizing to calcite, as in fossil snails, or it may be the growth of larger crystals of an existing mineral such as the large calcite crystals in the algae (note the large cleavage faces).
7. Algal pisolites.
8. Replacement: original mineral has been dissolved away and replaced
by a different mineral. Usually the original mineral was aragonite or calcite,
and it has been replaced by silica (in oxidizing and acidic conditions)
or pyrite (in reducing conditions, in the absence of oxygen). Under some
conditions, replacement happens on an atom-by-atom basis, and the fine structure
of the fossil is preserved in the new mineral. Recognizing replacement requires
that you first be able to recognize what mineral you are looking at, and
then that you know what the original skeletal material was.
Which mineral replaces the original is a function of the chemistry of the groundwater. The ammonoid (the round, snail-ish fossil) has been replaced by pyrite. Pyrite formation requires an absence of oxygen (or the iron would oxidize) in reducing conditions, usually acidic. Replacement by silica (silicification) also requires acidic conditions and an abundance of silica. The coral and algae have been silicified. Replacement by calcite is very rare. The original shell material would be silica or phosphate. Calcification requires alkaline conditions to dissolve the silica and induce deposition of calcite. We have no examples of calcified fossilization.
8A. Turritelid agate
8B. Silicified coral
8C. Pyritized ammonite
9-10. Carbonization: the soft parts of the organism were compressed and heated, driving off all the volatiles (H, N, O). A carbon film is left behind. Most common in plants, soft-bodied organisms, organisms with phosphate skeletons, organisms with chitin skeletons, and sometimes fish (under the right environmental conditions). Each organism here had a proteinaceous skeleton or framework (the trilobite skeleton also contained a layer of calcite). During lithification, pressure and high temperatures volatilized the N, H and O of the protein, leaving behind a black carbon film.
9A. Crab
10A. Calamites (Paleozoic plant)
10B. Sequoia needles.
11. Imprints are left when an organism is pressed into soft sediment. The original hard parts may be gone, dissolved after burial. Very shallow imprints are called impressions, as of the fern leaf. Larger organisms may leave molds or casts. A mold is the imprint left by the organism, and is a negative of the organism. Molds may be external - of the exterior of the shell, as on the slab of snail and clam external molds. If a shell fills with sediment that later hardens, it forms an internal mold ( a 3-D picture of the space inside the shell). If an external mold later fills with sediment or minerals, it can form a cast.
11A. Snails
11B. Trilobite
11C. Leaf
11D. Branch tip
11E. Fern
11F. Clam
11G. Pentamerid
11H. Ammonites
11I. Snails and clams
Trace fossils are other kinds of evidence that an organism existed. Trace
fossils include tracks, trails and footprints; burrows and other dwellings;
tools; coprolites (fossilized excrement); and chemical fossils, which is
chemical evidence of the existence of an organism. One of the most common
but least useful kinds of trace fossils is bioturbation, evidence that organisms
have churned through sediment. Bioturbation is recognized by the complete
lack of sedimentary structures such as laminations and cross-beds, or by
chaotic structures within the sediment. Unfortunately, it is usually impossible
to tie bioturbation to any specific organism.
12. Dwellings and structures: Some marine worms secrete calcareous tubes in which they live. The fossil is not of the organism, but of a structure it built. Even more elaborate dwellings have been fossilized. The photo shows a fossil termite mound. Even fossil bee hives have been found.
12A. Skolithose13. Tracks and trails: Paleontologists have found many fossilized tracks (see photos). The challenge is linking these tracks and trails back to specific organisms. This is much more easily done for fossil groups with living representatives than for extinct organisms.
12B. Worm tubes
12C. Wood with clam and worm borings
12D. Pictures of terrestrial trace fossils
13A. Reptile tracks14. Tools: Tools are usually associated with humans, but we can think of any object used for a specific function by an organism as a tool. Tools include gastroliths - the rocks ingested by some dinosaurs to help them grind their food (much like the gravel birds eat). The human-made tool displayed is from Ohio, and is only about 1500 years old.
13B trilobite tracks
13C. Picture of Glen Rose dinosaur tracks
14A. Human tools15. Coprolites: Fossilized excrement can be clues to the diets of extinct animals. The challenge is matching the coprolite to the organism. Even tiny organisms can leave coprolites. Look at the ammonite under the microscope. What you are looking at is the sediment that filled one chamber of a Nautilus-like animal (see the complete specimen). This particular chamber is filled entirely with the fecal pellets of some tiny marine worm that probably left no other fossil evidence of its existence. We can infer that the organism was a deposit feeder - ingesting sediment like an earthworm - and that it was millimeters in diameter - small enough to fit through the tiny holes in the chamber walls.
14B. Gastroliths
15A. Vertebrate coprolite
15B. Invertebrate fecal pellets
15C. Pseudocoprolite: may be just mud pushed through holes in a log.
16. Concretion: Fossils are frequently found in the middle of concretions (a nodule of well-cemented sediment in the middle of a relatively uncemented sedimentary rock). Indeed, the critter probably led to the formation of the concretion. As the soft parts of the creature decayed, they created a local change in the geochemistry of the rock that permitted the crystallization of cement (usually calcite) between the grains of the sediment. Note the discolored ring around the fossil, indicating a different chemical environment. Once formed, the concretion also protects the fossil from chemical destruction. Not all concretions contain fossils, though. The original creature that created the concretionary conditions may have been soft-bodied and decayed completely away, or the chemical change may have been caused by non-organic means.
17. Dendrites: Minerals can crystallize along cracks in rocks in patterns that mimic plants. You can recognize dendrites by their diagnostic pattern, and because they occur along cracks rather than in bedding planes. You would expect to find plant fossils in parallel layers, but you usually find dendrites along intersecting surfaces.
18. What is the mode of preservation? What is your evidence?
19. What is the mode of preservation? What is your evidence?
20. What is the mode of preservation? What is your evidence? What information
can you extract from this fossil?
21. These are both brachiopods (not clams). Without knowing anything about
the biology of brachiopods, choose the fossil that is preserved as original
shell material. What is your evidence (you should be able to figure this out
without looking it up).
22. Is this a mold or a cast? How do you know? What information does
this fossil tell you about the organism? What information has been lost?
23. What mode(s) of preservation is this (pay attention to the holes!)? What
kind(s) of organisms are fossilized here?
24. These are fossils of the same kind of organism, a crinoid. Crinoids
are very delicate creatures with many fine structures (see picture). In
one of these specimens the organisms are fragmented and jumbled; in the
other the animal is intact. How do you explain this difference in how the
organism was preserved?
25. What is the mode of preservation? What is your evidence?
26. What is the mode of preservation? What is your evidence?
27. Compare the modern and ancient nautiloid. How is the shell of the
ancient one preserved? What is your evidence? Note that the fossil also
includes internal molds of the chambers. The molds of the outer chambers
are made of mud. What mineral makes up the molds of the inner chambers?
Why are different chambers fossilized in different materials?
28. What mode of preservation is this? Some geologists doubt this is
a true coprolite, as the formation it was found in is filled with these
structures and with many petrified logs. Some paleontologists suggest this
is a pseudocoprolite, formed by mud pushed through a small hole in a log.
How could you tell a true coprolite from a pseudocoprolite?
29. What is the mode of preservation? What can you infer about the environment of deposition?