exercise 6 fossils—part 1 fossil preservation; trace...
TRANSCRIPT
6–1
Exercise 6 Fossils—Part 1
Fossil preservation; trace fossils; stromatolites
FOSSIL PRESERVATION Fossils are preserved by three main methods: (1) unaltered soft or hard
parts, (2) altered hard parts, and (3) trace fossils.
(1) Unaltered fossils include such rarities as frozen wooly mammoths and insects preserved in amber. Normally, preservation of soft tissue requires
rapid but gentle burial in an oxygen-free sedimentary environment where
bacterial decay cannot occur. Because such conditions are uncommon,
preservation of soft tissue occurs only rarely. Much more common examples
of unaltered fossils are those in which hard skeletal material has been
preserved with little or no change. Many marine invertebrate fossils and
microfossils are typically preserved in this manner.
(2) Alteration of hard parts occurs when original skeletal material is permineralized, replaced, carbonized or dissolved.
Replacement refers to the very slow removal of original skeletal material and substitution by a secondary compound. For example, the calcite of a
brachiopod shell may be replaced on a molecule-by-molecule basis by silica or
pyrite. Remarkably, the replaced fossil may retain most or all of the fine
detail present in the original. This is analagous to rebuilding a brick wall, one
brick at a time. Imagine starting with a red brick wall and then replacing
each red brick with a yellow one. Once all the original bricks have been
replaced, the wall is still intact and unchanged, except in its composition.
Permineralization occurs in porous substances such as bone and wood. In this type of preservation, minerals fill the pore spaces in the original structure
to make it heavier and more durable. The original bone or wood material may
be preserved, or it may be replaced or recrystallized.
Carbonization is a type of preservation in which the remains of an organism are preserved as a thin film of carbon. Leaves, fish and graptolites are
commonly preserved in this way.
6–2
Dissolution is the complete removal of original skeletal material. When a
shell dissolves it may leave a void in the enclosing sediment or rock. This void
is in effect a three-dimensional negative of the original object, and it is termed a mold. An impression is a very shallow, or two-dimensional, mold. A
cast is an exact replica of the original object that is formed by secondary
filling of a mold.
(3) Trace fossils are discussed separately on following pages.
Preservation Examples
1. Examine these examples of impressions. Note that impressions
commonly preserve exquisite details, as in the fish and angiosperm leaves.
What is the grain size of the surrounding rock matrix?
Do you think impressions could be preserved in coarse grained sediment?
2. Study these examples of carbonization. In each case, the dark patches
are remnants of organic carbon that were never oxidized (decayed). Under
what conditions might this kind of preservation occur?
Graptolites are extinct planktonic, colonial animals that secreted an organic
shell similar in composition to your fingernails. The colonies commonly are
preserved as two-dimensional impressions, almost always black (indicating
carbonization of the of the organic matter).
3. These are examples of molds. The snails and brachiopods are preserved
as internal molds: i.e., lithified sediment that filled the insides of the shells. The trilobite is preserved as an impression or shallow external mold, along with some original shell material. Note that one brachiopod still has original
shell material preserved in places.
4. Examine these casts. Obviously the trilobites and crinoid are plaster
casts made from clay molds of actual fossils. The root casts were formed
6–3
when sediment filled in voids left by decayed roots. The sediment then
lithified to produce these fossil structures.
5. These are examples of permineralization. Permineralization occurs when
minerals are deposited in the void spaces of vascular tissue, usually wood or bone. The source of the minerals is usually percolating ground water. Dissolved minerals are deposited in the vascular spaces much as “hard water”
deposits form in houshold plumbing.
6. Various examples of replacement, in which the original mineral
composition of a shell has been replaced by some secondary mineral. Note
that SiO2 is a very common replacement mineral. It usually occurs as chert,
but if red in color, then it’s called “jasper.” Replacement by pyrite (“fool’s
gold”) is less common, but spectacular!
7. Probably the most common mode of preservation is simply the
preservation of unaltered or slightly altered original shell or bone! Examine
these examples of slightly altered teeth (Bison and shark) and brachiopod shells. There’s also an example of amber—fossil tree sap! Make sure you
check out the insects preserved in amber in the display cabinet across the
hall.
TRACE FOSSILS Trace fossils are not the remains of actual organisms, but evidence of the
activities of organisms, such as tracks, trails, burrows, footprints, borings,
and even such improbable items as excrement and vomit. Trace fossils are
sometimes called “ichnofossils” and the study of trace fossils is called
“ichnology.”
Environmental Range and Morphology
Trace fossils are not particularly useful biostratigraphically, but they are
very good environmental indicators. In modern marine environments, both
the orientation and complexity of traces change in predictable ways with
increasing water depth.
6–4
Traces in intertidal areas are mostly simple, vertical tubes and burrows in
which organisms seek protection from subaerial exposure. In contrast,
traces in the very stable deep ocean environments are mostly horizontal and
highly elaborate, reflecting the feeding behaviors of animals that extract
nutrients from the muddy bottom. Traces in intermediate (neritic)
environments are intermediate in complexity between intertidal forms and
bathyal/abyssal forms, and they include vertical, horizontal and intermediate
orientations.
Distinctive assemblages of modern traces correspond to specific
bathymetric zones. Such assemblages are known as “ichnofacies.” By
applying uniformitarianism, ancient ichnofacies are assumed to have
developed in roughly the same water depths as their modern counterparts.
Horizontal burrow (left) of sediment-ingesting organism, and vertical, U-shaped borrow for protection or escape.
6–5
Horizontal worm burrows on the surface of a rock. Vertical worm tubes in rock, as seen in profile.
Elaborate, fan-shaped horizontal feeding traces preserved on the surface of a rock
6–6
Trace fossil assemblages and bathymetry. Note that intertidal and shallow marine traces are simple, vertical structures, whereas bathyal and abyssal traces are complex and horizontal.
Trace Fossil Examples
1. The specimens represent a variety of worm burrows, mostly horizontal
types. Note the range in size among the different kinds.
What is the grain size of the sediment in the surrounding rock matrix?
Do you think burrows could be preserved in coarse grained sediment?
In what water depth were the samples likely formed?
6–7
2. Examples of calcareous worm tubes. These tubes were secreted by
worms on the shells of other organisms. Presumably the tubes afforded
protection from predators and/or subaerial exposure. Why are the tubes
considered trace fossils and not actual “body fossils”?
3. Dinosaur “gizzard stone.” This smooth, polished stone probably was
carried for months or years in the stomach of a dinosaur. Dinosaurs carried
such stones to help in the digestion of fibrous plant material, much as
certain birds do today.
4. Yep, it’s fossil poop, no kidding. The general term for this kind of trace
fossil is “coprolite,” from the Greek root kopros, meaning dung. Be sure to check out other examples in the display cabinet across the hall.
5. AND, while you’re at the display cabinet, check out the dinosaur footprint, also an example of a trace fossil. What type of preservation is
exhibited by this fossil?
6. Tracks. The upper surface of this slab of sandstone is covered by the
traces of an animal (or animals) who probably was feeding in the sediment.
Many invertebrates are so-called “deposit feeders” because they extract
nutrients from ingested sediment. In this case, the track-maker was
probably an arthropod or worm — who knows? — as evidenced by bilateral symmetry of the traces.
6–8
STROMATOLITES Stromtatolites are structures formed by alternating layers of
cyanobacterial filaments and fine-grained sediment. The alternating
bacterial and sediment layers are easily seen in modern stromatolites, but in
fossil specimens the bacteria are usually not preserved and laminations are
the result of subtle differences in color and texture.
Sketch illustrating alternating layers of cyanobacterial and sediment. Each sediment layer is typically 1mm or so thick.
Cyanobacterial filaments are coated with sticky mucus that traps fine sediment.
Stromatolites often develop as discrete columns or domes, but shapes can
range from low mounds to complex branching structures.
Sketch illustrating growth of columnar stromatolites. Bacterial filaments must grow up through sediment layers in order to receive sunlight for photosynthesis.
Sediment layer
Bacterial layer
6–9
Modern stromatolites exposed at low tide in Shark Bay, Western Australia (note hammer for scale)
Vertical profile through Early Proterozoic stromatolites (~2.0 billion years old), District of Mackenzie, Canada
Vertical profiles through branching stromatolites: left, Cambrian (Alberta, Canada); right, Eocene (Wyoming).
6–10
Stratigraphic Range
The oldest known stromatolites are from South Africa in rocks dated as
~3.2 billion years old (Archean Eon). Stromatolites became very abundant
and large (up to 6m in height!) during the Proterozoic Eon, especially from
about 1.6 to 1.0 billion years ago. They declined in abundance in the late
Proterozoic Eon and early Paleozoic Era (Phanerozoic Eon), but they still
persist today in a few areas. 4.6 billion years ago 2.5 0.543
Environmental Range
Modern stromatolites occur in tropical to subtropical, carbonate depositional
environments. They usually form in supratidal, intertidal, and shallow
subtidal marine and lake settings—typically under conditions that are too
harsh for other organisms. For example, they occur in hypersaline lagoons
and in areas dominated by strong currents.
Archean, Proterozoic and early Phanerozoic stromatolites probably
developed in tropical and subtropical normal marine shelf environments. The
dramatic decline in abundance of stromatolites during late Proterozoic and
early Phanerozoic time probably is related to the evolutionary origin of
gastropods and other grazing organisms that may have fed on cyanobacterial
mats.
Stromatolite Examples
What is the relationship between stromatolites and the presence of oxygen in Earth’s early atmosphere?
Archean Eon Proterozoic Eon Phanerozoic Eon
6–11
1. How would you describe the overall shape of this stromatolite?
Make sure you are able to see the laminations and follow them from one side
of the specimen to the other. Apart from the laminations, are other internal
structures present?
What is the composition (rock type) of the enclosing rock matrix in which
the stromatolite is contained?
Are fossils present in the enclosing rock
matrix?__________________________
If yes, what are
they?_______________________________________________
If no, why
not?_____________________________________________________
Draw a sketch of the stromatolite in the space provided below:
6–12
2. How would you describe the overall shape of this stromatolite?
What is the mineralogic composition of this specimen?
What is the approximate average thickness of individual laminations?
3. These specimens are examples of stromatolites that were originally
preserved as limestone, but later were recrystallized (altered) to dolomite.
Note that the quality of preservation is not as good as in specimens 1 and 2.
Make sure you can see the laminations, and note the other fossil (probably a
mollusk) in one corner of the larger rock sample.
4. Can you see laminations in this specimen?
What is the mineralogic composition of this stromatolite?
What inference(s) can you make about mineralogy and quality of
preservation?
What is the overall shape of this specimen?
6–13
5. This one exhibits a general tabular shape, as opposed to domal or
columnar. What do you suppose accounts for the slight differences in color
between adjacent laminations?
6. Note the excellent state of preservation exhibited by this specimen of
Precambrian age (older than 543 million years!). Note also the very thin
laminations.
7. Examples of branching or “digitate” stromatolites from Precambrian
rocks in Minnesota. Note the fine structure of the stromatolites. The
original calcium carbonate in these specimens has been replaced by red
chert (“jasper”). The rocks themselves are from “banded iron formations,”
which have special significance in Precambrian geology and as a major source
of iron ore.