Fossils and Evolution 870:125
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Transcript of Fossils and Evolution 870:125
Fossils & Evolution—Chapter 1 1
Fossils and Evolution870:125
• Review syllabus– Text– Supplemental resources– Objectives– Tests and grading– Trip to Ashfall (NE)
Fossils & Evolution—Chapter 1 2
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 fine-
grained 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
Fossils & Evolution—Chapter 1 3
Key terms(know definitions)
• Taphonomy• Biocenosis• Thanatocenosis• Necrolysis• Biostratinomy• Diagenesis • Lagerstätten
Fossils & Evolution—Chapter 1 4
Why study fossils?
• Fossil = remains or traces of a once-living organism
• Paleontology = the study of fossils• Importance of paleontology
– Biostratigraphy (age dating of rocks)– Evolution– Paleoecology/paleoenvironmental interpretation– Paleogeography/paleobiogeography– Simple fascination
Fossils & Evolution—Chapter 1 5
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
Fossils & Evolution—Chapter 1 6
Eocene mammalwith partially preservedfur and flesh (Germany)
Carbonized Jurassic leaf
Fossils & Evolution—Chapter 1 7
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)
Death assemblage(thanatocenosis)
Necrolysis(scavenging, decay)
Biostratinomy(break-up, scattering andshallow burial of remains)
Initialfossil assemblage
Diagenesis(deep burial, recrystallization,
dissolution, metamorphism, etc.)
Finalfossil assemblage
Fossils & Evolution—Chapter 1 8
Life assemblage(biocenosis)
Death assemblage(thanatocenosis)
Total fossilassemblage
Fossils actuallydiscovered
Destruction of most soft tissues
Destruction of most hard tissues
Destruction of most fossils
Fossils & Evolution—Chapter 1 9
• The fossil record is highly biased– Number of fossils is but a fraction of the
number of once-living plants and animals
Quality of the fossil record
Fossils & Evolution—Chapter 1 10
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
Fossils & Evolution—Chapter 1 11
Standing crop in ¼ m2 (offshore Japan)
½ m
½ m
•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.)
Fossils & Evolution—Chapter 1 12
Sources of bias
• Uneven preservation potential• Sampling bias
Fossils & Evolution—Chapter 1 13
Uneven preservation potential
• Preservation potential of organisms is goverened by– Resistance to destruction
• Biological, mechanical, chemical• Hard parts vs. soft parts
– Habitat (during life)
Fossils & Evolution—Chapter 1 14
Destruction
• Biologic destruction includes – Predation– Scavenging– Boring – Bacterial decay
Example:Radiograph of heavilybored gastropod
Fossils & Evolution—Chapter 1 15
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
Fossils & Evolution—Chapter 1 16
Abrasion experiment
gastropod
gastropod
coral
coralalga
(multitaxa)
Fossils & Evolution—Chapter 1 17
Abrasion experiment(marine bivalves)
large shells
small shells
Fossils & Evolution—Chapter 1 18
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
Fossils & Evolution—Chapter 1 19
Destruction
• Chemical destruction varies with:– the original skeletal mineralogy of a fossil– the chemistry of subsurface fluids– temperature of burial environment
Fossils & Evolution—Chapter 1 20
Relative chemical stabilityopaline
silica aragonite hi-Mg calcite
low-Mg calcite
apatite (CaPO4) organic
most oysterstabulatesrugosansscleractinians
?? ??
fossil groups
<-- low stability high stability -->
calcareous foramsmollusks
diatoms
cnidarians
brachiopodsbryozoa
arthropods
pollen/spores
radiolarians
calcareous algae
echinodermsstromatoporoids
dinoflagellates
sponges
graptolites
conodontsvertebrates
Fossils & Evolution—Chapter 1 21
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
Fossils & Evolution—Chapter 1 22
Distribution of modern deep sea pelagic sediments
Fossils & Evolution—Chapter 1 23
Resistance to destruction
• Hard parts are much more likely to be preserved than soft parts (but soft parts and even pigments can be preserved)
Fossils & Evolution—Chapter 1 24
Environment and preservability
• Best preservation generally occurs in calm, aquatic environments
• Exceptional preservation occurs in fine-grained sediments in the absence of oxygen, (“biologically inert” burial conditions)
Fossils & Evolution—Chapter 1 25
Environment and preservability
• Lagerstätten (“Mother lode”) = deposits that contain large numbers of unusually well preserved fossils– 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)
Fossils & Evolution—Chapter 1 26
Lagerstätten(Hunsrück Shale, Devonian of Germany)
Fossils & Evolution—Chapter 1 27
Lagerstätten(Solnhofen Limestone, Germany)
Fossils & Evolution—Chapter 1 28
Lagerstätten(Burgess Shale, Cambrian of Alberta)
Fossils & Evolution—Chapter 1 29
Tully monster(Mazon Creek Shale,Pennsylvanian of Illinois)
Check out U-Haul websitehttp://www.uhaul.com/supergraphics/tully/the_graphic.html
Fossils & Evolution—Chapter 1 30
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
Fossils & Evolution—Chapter 1 31
Fossil species diversity vs sediment volume/exposure
Fossils & Evolution—Chapter 1 32
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!
Fossils & Evolution—Chapter 1 33
Sampling bias:Danish Miocene mollusks
Phyla 1Classes 3
Orders 12Families 44Genera 64Species 86Individual shells 2,954
Fossils & Evolution—Chapter 1 34
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
Fossils & Evolution—Chapter 1 35
Rarefaction curve[How many taxa would have been found had the sample been smaller?]
Fossils & Evolution—Chapter 1 36
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”