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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.

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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

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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.

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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.

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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

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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?

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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.

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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

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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).

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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

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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:

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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?

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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.