8/29 – 9/3/02 Taphonomy
Taphonomy - the study of fossilization and its
effects
Hard parts
mineralogy and stability: calcite, aragonite, opaline silica,
calcium phosphate, chitin,
combinations
distribution among organisms: molluscs, brachiopods, corals,
sponges, arthropods, echinoderms,
vertebrates
Recrystallization, replacement,
permineralization
Fossils without hard parts
preserving soft tissues:mummification, freezing, amber, carbon
films
·
extraordinary fossil deposits: “Fossil-lagerstätten”
trace fossils: tracks and trails =>fossil behavior The fate of hard
parts
·
governed by their composition, construction, and
environment of deposition
·
agents of destruction
chemical, physical, biological
·
agents of confusion transportation - movement within and
between habitats time averaging- the accumulation of remains over
time
bioturbation - mixing fossils because of burrowing
organisms
recycling - erosion and redeposition of older
fossils
Jello, bone, clam, leaf, crab claw
Which is most likely to become a fossil
What determines the odds of fossilization?
Not all organisms are fossilized, and not all of an
organism is fossilized. Those having all or mostly soft
parts don’t make it. Those with hard parts, most often
don’t get the soft parts also preserved. Though note
the impression of soft parts on hard parts, a theme to be
explored as we go through the fossil groups in
lecture and in lab.
The subdiscipline of paleontology devoted to studying all
the nasty things that happen to a potential fossil
between the time it dies and the time it’s discovered
is
Taphonomy
- the study of how fossils and assemblages of fossils are preserved,
altered, or destroyed.
The post-mortem life of potential fossils.
From the greek word taphos, meaning grave. The study of
entombment
Since the most common fossils are those that were once the
hard parts of some living organism, I want to
consider them first.
Note that hard parts
can be either internal (as in vertebrates) or external (as with the crab or
clam).
Common types of hard parts
1. Calcium carbonate
CaCO3. Occurs in two mineral
forms:
Aragonite: very common. Most
mollusks (clams, snails, cephalopods), most living corals. -This
mineral
form of calcium carbonate is not stable; it either
dissolves or
recrystallizes to the stable mineral form, calcite,
over time. It’s rare to find original aragonite in rocks
older than Triassic.
Calcite: very common. The
stable form of calcium carbonate. Comes in two chemical
varieties:
low
magnesium calcite - a relatively pure form, in which Mg makes up less
than 4% (Mg substitutes for
Ca in the crystal lattice). Common component of
brachiopods, bryozoans, oysters (molluscs), two extinct
groups of corals.
·
high
magnesium calcite. More than 4% Mg substituting for Ca. Echinoderms
and a few other groups.
2.
Calcium phosphate - several varieties of this compound occur as minerals
commonly called apatite: a calcium hydroxy
phosphate. Most common is the mineral dahlite. Ca10(PO4)6CO3.H20. The major mineral constituent in vertebrate
bone.
3. Opal.
Hydrous silica of amorphous structure. Rather soluble in water, so rarely
survives unaltered. Some algae (diatoms), some sponges.
4. Chitin.
An organic material - a polysaccaride that forms long fibers. Makes up the hard
parts
of most arthropods- crab shells and the like. Often
subject to bacterial decay or even consumption by
other organisms. Sometimes all that’s left is a
carbonized film on the rocks.
5.
Combinations - some organisms have hard parts of more than one type.
Perhaps the most common combination is that of chitin and calcium carbonate.
Trilobites, for example, have an exoskeleton made up of chitin that was
impregnated with high Mg calcite.
Very often hard parts are not directly preserved. Their original
mineralogy may be recrystallized to a more
stable form (example: aragonite => calcite)
The original mineralogy may be replaced (examples: calcite
=> silica; calcite =>pyrite),
The original hard parts may be permineralized: all the open
space in the hard parts may be filled in with
a mineral and the original is then dissolved. You are going
over these in lab this week.
Casts and molds
Non-hard part fossils
We might think of these as falling in two
categories:
sediments and rocks. These are rare, but often important,
because of the opportunity to look at the softtissues
of fossils.
·
deposits which contain a lot of fossils in which
soft-tissues are preserved or recorded are sometimes
called, in German,
Fossil-Lagerstätten, or, roughly translated, “fossil
bonanzas”. Some famous ones are: Precambrian
Ediacaran fauna of Australia (soft-bodied)
Cambrian Burgess Shale of Canada
Mazon Creek Fauna of Illinois
Jurassic Solnhofen Limestone of Germany (first
bird)
Paleogene amber deposits in the Dominican Republic (insects,
mushrooms)
Rodent middens. Records of Pleistocene vegetation in desert
regions
of them are mobile, or in some other way active, and affect
their surroundings. Dinosaur tracks and trails
are sometimes preserved; the bore holes of algae and
sponges are preserved in shells, the trails of snails
across mudflats. Coprolites. Show recent and K example.
Fossil behavior. More about trace fossils in
a couple of lectures
The fate of hard parts
What then happens to hard parts after death; what controls
their fate; why are some things preserved and
not others?
1. original
composition (chemistry, mineralogy)
2. mode of
construction (how hard part is built - sturdy or fragile?)
3.
environment of deposition (physical, chemical and biological activity
affecting the hard part)
Agents of destruction
- processes that destroy hard parts
organic matrix within which the hard parts were secreted
decays.
Three things control a hard part’s susceptibility to
chemical destruction:
a. original mineralogy:
opal, high mg calcite, aragonite, calcite, calcium phosphate; in decreasing
order of solubility.
c. hard part construction.
Hard parts that are porous, or thin and delicate, present a large
surface
area to any dissolving fluids. Dense, compact ones will
last longer than lacy, flimsy ones. Here’s an
example from a real simple lab study:
The Cholla Bay Weight Loss Plan
·
we put several different shells from the Gulf of
California in acid (calcium carbonate dissolves in
acid) baths and pulled them out at different intervals and
measured how much weight had been lost.
dissolution rate
(%original wt per hour)
surface area/weight
The point here is not only that fossils are being destroyed
(through solution), the destruction is selective.
Dense, compact shells dissolve more slowly that thin
shells.
c.
environment of preservation. The ground water in acid soils and swamps
may dissolve
potential fossils. Weathering on the outcrop (acid
rain) may dissolve fossils made of calcium carbonate.
For example, the solubility of calcium carbonate
increases with increasing pressure and decreasing
temperature. Watch what happened to calcitic
microfossils (foraminifera) at different depths in the
Pacific Ocean.
Preweighed samples were placed at different depths
along a buoy cable in the central Pacific.
The colder, deeper water dissolved more microfossils.
These microfossils live, in fact in the upper 100
meters of so of the water column, whether the water is
in the deep ocean or in the shallows near shore.
Where would you expect to find microfossils more
frequently?
tossed around in the surf and all that. Abrasion can
destroy surface details and breakage can be so extensive that the hard parts are
no longer recognizable.
a. Hard part
mineralogy - little or no effect
b. hard part
construction. Very important. Well-constructed hard parts will survive longer
than flimsy ones.
c.
environment of preservation. Quiet water environments (deep water,
swamps, lagoons) are
more likely to protect against physical destruction than
high-energy environments (surf zone beaches,
river channels, etc.)
Chave’s experiment with shells in a tumbling barrel.
(Figure from today’s reading) Hard parts, quartz sand
or chert pebbles, water. Rotate the barrel, inspect
frequently
Doesn’t just demonstrate the obvious, that hard parts get
destroyed, but that physical destruction is also
selective. What if you only found Nertia.. Would that be an
unbiased assemblage? How could you tell?
organisms. Hyenas crushing bones, sponges or algae boring
into shells (pass around Dosinia)
a. Effect of
original mineralogy not well studied. I expect little biol. destruction of opal,
calcium
carbonate is probably equally probable. Calcium
phosphate has organic goodies in it, so prob often
consumed. Chitin all organic, so prob, very freq.
consumed. All based on hardness and palatability, not
on experiment.
b. hard part
architecture. Probably plays a role again. Hard parts with a lot of exposed
surface are get lots of borers, those with sturdy shells probably don’t. Again,
little study
c.
environment of preservation. Potential fossils that are removed from a
biologically active zone will stand a better chance - quick burial, or low
oxygen conditions.
An example: Neumann’s experiments in Bermuda
took slabs of the coral limestone that make up the
island,
weighed ‘em
wired onto these “carpets” of the boring sponge Cliona
set them out in the harbor for 33 to 100 days
removed the Cliona carpets (with
hydrogen peroxide)
weighed the slabs
recorded weight loss of 1000 to 6000 g/m2/100 days
approx equal to -1.4 cm/yr; 1 m per 70 yrs (counteracted by
continued production)
---of course, chemical, physical and biological destruction
may not act alone. Breaking up a shell exposes
more surface area for biological or chemical destruction.
Biological destruction may weaken a shell,
making it more liable for physical destruction.
Classroom experiment
Summary
Destruction occurs, its rate may depend on three factors
(mineralogy, environment, architecture), and
destruction is selective.
Agents of confusion. even if the hard parts are preserved,
they may still be affected by processes that can
move them from their original place or preserve them with
other species that lived several generations or
thousands of years apart. I call these agents of
confusion.
1.
Transportation. One of the first that probably comes to mind.
Tidal currents, streams, oceanic
currents, mudflows, etc., can all cause transportation
of hard parts away from where they lived. The
result: a. unnatural assemblages of species (Logs and clams
preserved together)
b. size sorting. smaller remains more likely to be removed,
bigger stuff can stay behind as a
lag (placer). Selective removal of juveniles or of
small-bodied species
c. physical
destruction during transportation.
2. Time
averaging.
Peterson’s study: Mugu Lagoon and Tijuana
Slough
sampled living molluscs and sampled dead
remains
Mugu 35 live over ten months of study, 49 dead
Tijuana 28 live over ten months, 39 dead
What gives? The collection of hard parts represents the
accumulation of species, remains over
some period of time. Thus, species may not have lived at
exactly the same time, even if they are found
together. Environments fluctuate over time. These are, in
some crude sense, averaged together in the
deposit of fossils.
The amount of time-averaging probably varies a lot. Some
collections of fossils may represent
“instants” in time; others may represent thousands of
years. Cholla Bay time-averaging of 4,000 years.
3.
Bioturbation. Hard parts can be displaced after their death by
other organisms. Bioturbation refers to
the churning about of sediment by burrowing organisms.
This is usually only local displacement from
life position to some other posture, or moving up or
down in the sediment.
4.
Recycling. Some hard parts are eroded and deposited several
times before final burial. This recycling
can mix together hard parts of very different ages and
environments. Chesapeake Group (Miocene)
fossils washing out with Recent shells along the coast
of Maryland/Virginia. Age diffs of 15 MY, shark’s
teeth and other open continental shelf species
together with living species of the bay.