Download Chapter 1 Preservation and the fossil record

Fossils and Evolution
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• Review syllabus
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Text
Supplemental resources
Objectives
Tests and grading
Trip to Ashfall (NE)
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Ch. 1—Key concepts to know
• The fossil record is incomplete. Biological, mechanical and
diagenetic agents all destroy potential fossils.
• Normal fossil preservation is favored by rapid burial of
durable hard parts.
• Exceptional preservation occurs via rapid burial in finegrained sediments under low oxygen conditions.
• The fossil record is biased because of differential
preservation and uneven sampling. It is biased in favor of:
1) durably skeletonized organisms; 2) marine organisms;
3) geologically recent organisms
• Because of biases, knowledge of past life is far better at
higher taxonomic levels than at lower taxonomic levels
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Key terms
(know definitions)
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Taphonomy
Biocenosis
Thanatocenosis
Necrolysis
Biostratinomy
Diagenesis
Lagerstätten
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Why study fossils?
• Fossil = remains or traces of a once-living
organism
• Paleontology = the study of fossils
• Importance of paleontology
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Biostratigraphy (age dating of rocks)
Evolution
Paleoecology/paleoenvironmental interpretation
Paleogeography/paleobiogeography
Simple fascination
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Modes of preservation
1. Unaltered remains (frozen mammoths; insects in
amber; unaltered shells & bones)
2. Permineralization (infilling of void spaces)
3. Replacement (molecule by molecule
substitution)
4. Impressions
5. Carbonization
6. Molds / casts
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Eocene mammal
with partially preserved
fur and flesh (Germany)
Carbonized Jurassic leaf
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Taphonomy = science of fossilization
• Many steps in the process of fossilization, with significant
removal of specimens at every step
• Once fossilized, the odds of being collected are low (uplift
and exposure; weathering; discovery; chance, etc.)
Life assemblage
(biocenosis)
Necrolysis
(scavenging, decay)
Death assemblage
(thanatocenosis)
Biostratinomy
(break-up, scattering and
shallow burial of remains)
Initial
fossil assemblage
Diagenesis
(deep burial, recrystallization,
dissolution, metamorphism, etc.)
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Final
fossil assemblage
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Life assemblage
(biocenosis)
Destruction of most soft tissues
Death assemblage
(thanatocenosis)
Destruction of most hard tissues
Total fossil
assemblage
Destruction of most fossils
Fossils actually
discovered
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Quality of the fossil record
• The fossil record is highly biased
– Number of fossils is but a fraction of the
number of once-living plants and animals
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Fossilization is a rare event!
• Some estimates:
– > 4,500,000 living species of plants and animals
– 250,000 described fossil species
– Thus, all described fossil species represent < 5% of the
total number of living species
– Yet, fossil record covers billions of years and today’s
biota is but a snapshot
– If preservation were truly efficient, then number of
fossil species should dwarf number of extant species
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Standing crop in ¼ m2
(offshore Japan)
½m
•197 shells (~ 200)
•Average lifespan = 2
years, thus:………
•1000 empty shells in 10
years
•100,000,000 empty shells
in 1 m.y.
•A stack of shells 1000 m
high if a layer of 1000
shells is 1 cm thick (actual
sedimentary thickness is
~320 m/m.y.)
½m
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Sources of bias
• Uneven preservation potential
• Sampling bias
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Uneven preservation potential
• Preservation potential of organisms is
goverened by
– Resistance to destruction
• Biological, mechanical, chemical
• Hard parts vs. soft parts
– Habitat (during life)
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Destruction
• Biologic destruction includes
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Predation
Scavenging
Boring
Bacterial decay
Example:
Radiograph of heavily
bored
gastropod
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Destruction
• Mechanical destruction includes breakage
and abrasion due to particle interactions
caused by wind, waves, currents
– Some shells and bones are more resistant to
abrasion and breakage than others
– Different sizes of the same shells vary in their
resistance to abrasion and breakage
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Abrasion experiment
(multitaxa)
gastropod
coral
alga
coral
gastropod
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Abrasion experiment
(marine bivalves)
large shells
small shells
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Durability of vertebrate bones
• Durability is governed by bone density and
thickness; also by surface area-to-volume ratio:
– Least durable
• Ribs, vertebrae, breastbone, hip (part), shoulder blade, fingers,
toes
– Intermediate
• Thigh, shin, upper and lower arms, ankles and wrists, hip (part)
– Most durable
• Teeth, jaws, skull
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Destruction
• Chemical destruction varies with:
– the original skeletal mineralogy of a fossil
– the chemistry of subsurface fluids
– temperature of burial environment
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Relative chemical stability
<-- low stability
fossil groups
radiolarians
diatoms
sponges
calcareous algae
calcareous forams
mollusks
cnidarians
opaline
silica
aragonite
high stability -->
hi-Mg
calcite
low-Mg
calcite
most
oysters
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apatite
(CaPO4)
organic
tabulates
rugosans
scleractinians
stromatoporoids
echinoderms
brachiopods
bryozoa
arthropods
conodonts
vertebrates
graptolites
dinoflagellates
pollen/spores
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Destruction
• Chemical stability vs. temperature and
pressure
– Silica is more stable in cold water
– Carbonate is more stable in warm water and
under low pressures
• Dissolution occurs under high pressure and low
temperature conditions
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Distribution of modern deep sea
pelagic sediments
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Resistance to destruction
• Hard parts are much more likely to be
preserved than soft parts (but soft parts and
even pigments can be preserved)
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Environment and preservability
• Best preservation generally occurs in calm,
aquatic environments
• Exceptional preservation occurs in finegrained sediments in the absence of oxygen,
(“biologically inert” burial conditions)
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Environment and preservability
• Lagerstätten (“Mother lode”) = deposits that
contain large numbers of unusually well preserved
fossils
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Burgess Shale (Cambrian, Canada)
Hunsrück Shale (Devonian, Germany)
Mazon Creek Shale (Pennsylvanian, Illinois)
Solnhofen Limestone (Jurassic, Germany)
Baltic amber (Oligocene, Germany)
La Brea tar deposits (Pleistocene, California)
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Lagerstätten
(Hunsrück Shale, Devonian of Germany)
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Lagerstätten
(Solnhofen Limestone, Germany)
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Lagerstätten
(Burgess Shale, Cambrian of Alberta)
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Tully monster
(Mazon Creek Shale,
Pennsylvanian of Illinois)
Check out U-Haul website
http://www.uhaul.com/supergraphics/tully/the_graphic.html
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Sampling bias
• Fossil record is best in most recent
geologic systems
– Younger rocks are less likely to be covered or
obscured by other rocks
– Younger rocks are less likely to have been
eroded, metamorphosed or subducted
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Fossil species diversity vs sediment
volume/exposure
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Consequences of preservation
and sampling bias
• Knowledge of past life is far better at higher
taxonomic levels than at lower taxonomic
levels
– In a given sample, you’d only need to look at a
small number of specimens to find all of the
phyla present, but you’d have to look at a lot of
specimens to find all of the species present!
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Sampling bias:
Danish Miocene mollusks
Phyla
Classes
Orders
Families
Genera
Species
Individual shells
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2,954
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Sampling bias:
Danish Miocene mollusks
• If sample size were larger, then more
species and possibly more genera might
have been found, but probably no more
classes or phyla
• If sample size were smaller, then fewer
genera and species would have been found,
but probably no fewer classes or phyla
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Rarefaction curve
[How many taxa would have been found had the sample been smaller?]
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Conclusions
• Every assemblage of fossils represents an
extremely biased sample of the organisms
once living in an area
• Lack of fossils in a rock cannot be taken to
mean that organisms were not living in the
area
– “Absence of evidence is not evidence of
absence”
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