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)


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


Key terms(know definitions)

• Taphonomy• Biocenosis• Thanatocenosis• Necrolysis• Biostratinomy• Diagenesis • Lagerstätten


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


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


Eocene mammalwith partially preservedfur and flesh (Germany)

Carbonized Jurassic leaf


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


Life assemblage(biocenosis)

Death assemblage(thanatocenosis)

Total fossilassemblage

Fossils actuallydiscovered

Destruction of most soft tissues

Destruction of most hard tissues

Destruction of most fossils


• 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


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


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.)


Sources of bias

• Uneven preservation potential• Sampling bias


Uneven preservation potential

• Preservation potential of organisms is goverened by– Resistance to destruction

• Biological, mechanical, chemical• Hard parts vs. soft parts

– Habitat (during life)


Destruction

• Biologic destruction includes – Predation– Scavenging– Boring – Bacterial decay

Example:Radiograph of heavilybored gastropod


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


Abrasion experiment

gastropod

gastropod

coral

coralalga

(multitaxa)


Abrasion experiment(marine bivalves)

large shells

small shells


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


Destruction

• Chemical destruction varies with:– the original skeletal mineralogy of a fossil– the chemistry of subsurface fluids– temperature of burial environment


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


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


Distribution of modern deep sea pelagic sediments


Resistance to destruction

• Hard parts are much more likely to be preserved than soft parts (but soft parts and even pigments can be preserved)


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)


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)


Lagerstätten(Hunsrück Shale, Devonian of Germany)


Lagerstätten(Solnhofen Limestone, Germany)


Lagerstätten(Burgess Shale, Cambrian of Alberta)


Tully monster(Mazon Creek Shale,Pennsylvanian of Illinois)

Check out U-Haul websitehttp://www.uhaul.com/supergraphics/tully/the_graphic.html


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


Fossil species diversity vs sediment volume/exposure


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!


Sampling bias:Danish Miocene mollusks

Phyla 1Classes 3

Orders 12Families 44Genera 64Species 86Individual shells 2,954


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


Rarefaction curve[How many taxa would have been found had the sample been smaller?]


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”

fossils and evolution 870:125

Documents

fossil recordthe fossil

number of fossil species

knowthe fossil record

normal fossil preservation

described fossil speciesthus

study fossils

m197 shells

potential fossils