life on earth: what do fossils reveal?

LIFE on EARTH: LIFE on EARTH: What Do Fossils Reveal?What Do Fossils Reveal?

CHAPTERS 3 and 7CHAPTERS 3 and 7

Fossils • Fossils are the remains or traces of prehistoric

organisms which have been preserved by natural causes in the Earth's crust. Evidence of past life.

• Most common in sedimentary rocks. Some in pyroclastic materials, especially ash.

• Useful for determining relative ages of strata. Also environments of deposition.

• Fossils provide some of the evidence for organic evolution. Many fossils are of organisms now extinct.

Fossils

• Fossils include both: The remains of

organisms. The traces of

organisms.

• Remains of organisms are called body fossils. Mostly durable skeletal elements such as bones,

teeth and shells.

Body Fossils

Rarely we might find entire animals preserved by freezing or mummification.

• Indications or traces of organic activity including tracks, trails, burrows, and nests are called trace fossils.

Trace Fossils

• The fossil record is the record of ancient life preserved as fossils in rocks.

• The fossil record is incomplete because of: Bacterial Decay Physical Processes (Weathering and Erosion) Scavenging Metamorphism

• In spite of this, fossils are quite common.

Fossil Record

How Do Fossils Form?To become preserved as a fossil,

an organism typically must: 1. Have preservable parts:

Hard parts (bones, shells, teeth, wood) Only rare preservation of soft parts

(muscle, skin, internal organs)2. Be buried rapidly by sediment:

Protects the organism from decay3. Escape physical, chemical, and

biological destruction after burial: Escape destruction by burrowing

(bioturbation), dissolution, metamorphism, or erosion.

4. Live in a suitable environment: Marine and transitional environments

with higher rates of sediment deposition are more favorable for fossil preservation.

Types of Fossil Preservation

1. Preservation of Unaltered Hard Parts

2. Chemical Alteration of Hard Parts

3. Imprints of Hard Parts in Sediment

4. Preservation of Unaltered Soft Parts

5. Trace Fossils or Ichnofossils

Preservation of Unaltered Hard Parts • Hard Parts may be preserved unaltered:

Shells of invertebrates and single-celled organisms Vertebrate bones and teeth

www.clas.ufl.edu/.../gly3603c/fossils.html paleo.cortland.edu/.../preservation.htm

Chemical Alteration of Hard Parts: Petrification by Permineralization

Permineralized bone in which the porous marrow cavities have been filled with mineral matter.

• Permineralization: Filling of pores (tiny holes) in bone or shell by the deposition of minerals from solution.

www.geol.umd.edu/.../lectures/104fossils.html

Unaltered bone with porous marrow cavities.

The shell of an extinct marine organism known as an amminoid. In this 160-million-year-old Jurassic fossil, the original calcium carbonate skeleton has been completely replaced with the mineral pyrite.

Chemical Alteration of Hard Parts: Petrification by Replacement

Replacement: Molecule-by-molecule substitution of another mineral of different composition for the original material.

www.geol.umd.edu/~jmerck/ASTR380/evolution.html

www.stat.wisc.edu/.../Fossils/fossils.html

• Recrystallization: • The original crystals forming

the shell undergo a change in form and size, but the composition remains unchanged.

• Many modern shells are made of aragonite, a metastable form of calcium carbonate (CaCO3) that will alter or recrystallize to calcite.

Chemical Alteration of Hard Parts: Petrification by Recrystallization

www.nasmus.co.za/PALAEO/jbotha/paleo101.htm

Imprints of Hard Parts in Sediment

• Impressions or molds are the imprints of an organism (or part of an organism) in the sediment. External Molds are imprints

of the outside of a shell. Internal Molds are imprints

of the inside of the shell.

• A cast may be produced if a mold is filled with sediment or mineral matter.

Cast

Mold

Amminoid Casts

Preservation of Soft Parts• Soft parts may be preserved as fossils by:

Freezing (wooly mammoths in Siberian tundra) Desiccation (drying or mummification) Preservation in tree sap (amber – Preserves delicate

organisms and insects – like in Jurassic Park) Preservation in tar (LaBrea tar pits) Preservation in peat bogs (Lindow Man – England;

Tollund Man – Denmark) Carbonized Imprints in fine-grained sediment

Preservation of Soft Parts by Freezing

Baby mammoth dug from permafrost (permanently frozen soil) in northeast Siberia. The mammoth stood about 3 feet tall at the shoulders, was covered with reddish hair, and was probably only several months old. Radiocarbon dating indicates it died 44,000 years ago.

Preservation of Soft Parts by Amber

Preservation of Soft Parts by Tar

Preservation of Soft Parts by Peat Bogs

Preserved torso, arms, and head of the 2000-year-old Lindow Man. This example of preservation of soft tissue was found in a peat bog in 1984 at Lindow Moss, England. The lower half of the body was destroyed by an excavation machine.

Preservation of Soft Parts by Carbonized Imprints

Fossil Wasp – Victim of a ash showers from the Colorado volcano eruption about 30 mya (Fossil Beds National Monument, Florissant, Colorado).

Fossil Seed Fern from rocks of Pennsylvanian age (~300 mya).

• Carbonization: Preserves soft tissues of plants or animals as a thin carbon film after pressure from overlying sediments squeezes out the liquid and gases. Usually in fine-grained sediments (shales). If the carbon is removed, a fossil impression remains that replicates the surface of the organism.

Preservation of Soft Parts by Carbonized Imprints

Fossilized fish with fleshy parts – USGS (public domain)

Trace Fossils or Ichnofossils

• Tracks

• Trails

• Burrows – in soft sediment

• Borings – in hard material

• Root marks

• Nests

• Eggs

• Coprolites

• Bite marks

Markings in the sediment made by the activities of organisms

Dinosaur Tracks, Morrison Formation

Trails in Red Triassic Siltstone, Virginia Burrows probably made by worms

Dinosaur Footprint in Limestone

• A land-dwelling beaver, Paleocastor, made this spiral burrow in Nebraska.

Trace Fossils

Trace Fossils or Ichnofossils• Trace fossils provide information about:

Ancient Water Depths Paleocurrents Availability of Food Sediment Deposition Rates

• Tracks can provide information about: Diet Foot Structure Number of Legs Leg Length Speed Herding Behavior Reproductive Behavior Interactions

• A coprolite is a type of trace fossil consisting of fossilized feces that may provide information about the size and diet of the animal that produced it.

Trace Fossils

• Fossilized feces (coprolite) of a carnivorous mammal. Specimen measures about 5 cm

long and contains small fragments of bones.

Figure 6-10 (p. 110)Dinosaur trackways arranged to indicate the passage of a biped (identical three-toed imprints) whose tracks were crossed by a quadruped having larger rear than front feet (typical of many quadruped dinosaurs). Claw imprints on the biped suggest that it was a predator.

What indicates the quadruped crossed the area after the biped?

Fossils Indicate Past Environments

Use of Fossils in Reconstructing Ancient Geography

• Environmental limitations control the distribution of modern plants and animals:

Each species has a definite range of conditions for living and breeding, and generally, it is not found outside that range.

Ancient organisms had similar restrictions on where they could survive.

Paleontologic Correlation • Cosmopolitan Species are found almost

everywhere; they are not restricted to a single geographic location in their environment. Cosmopolitan species have been especially

useful in establishing the contemporaneity of strata (correlation).

• Endemic Species are confined to a restricted area in the environment in which they live. Endemic species are generally good indicators

of the environment where the strata were deposited.

Index Fossils

• Index Fossils (or guide fossils) are useful in identifying time-rock units and in correlation.

Characteristics of an Index Fossil: 1. Abundant

2. Easily Identified

3. Widely Distributed (cosmopolitan)

4. Short Geologic Range (rapid evolution or extinction rates)

Use of Fossils in Reconstructing Ancient Geography

• Environmental limitations that control the distribution of modern plants and animals include:

Marine Ecosystem Realm Chemistry of the Water Movement of Ocean Water Water Temperature Depth Light Carbonate Compensation Depth (CCD) Land Bridges and Barriers (Mountains and Oceans) Latitude

Classification of marine environments(After Hedgspeth, UJ. W., ed. 1957. Treatise of Marine Ecology and

Paleoecology. Geological Society of America Memoirs 67(1): 18.)

Marine Ecosystem

• The ocean may be divided into two realms: Pelagic Realm

– The water mass lying above the ocean floor

Benthic Realm – The bottom of the sea

• Where and how animals and plants live in the marine ecosystem

Marine Ecosystem – Planktonic/Nektonic

Floaters:

Plankton

Jelly Fish Swimmers (Nekton): Fish

Cephalopod

Benthos: d-k

Sessile Epiflora:

Seaweed Sessile Epifauna:

Bivalve

Coral Crinoid

Marine Ecosystem – Benthos

Mobile Epifauna: Gastropod Starfish

Infauna:

Worm Bivalve

Ancient Marine Environment

The Chemistry of Sea Water

• Salinity – A measure of the total dissolved solids in water Salinity is measured in parts per thousand (ppth or o/oo) by weight

• Salinity terms for various types of water: • Normal Ocean Water = 35 ppth or 3.5%

A salinity of 35 ppth means that there are 35 pounds of salt per 1000 pounds of sea water.

• Freshwater = about 5 ppth to less than 1 ppth. • Brackish Water = sea water with less than about 30 ppth. • Hypersaline Water = more than 250 ppth.

Typically in lakes in arid areas, or in enclosed areas like lagoons or isolated seas in arid areas.

Water Temperature and Depth

• Water temperature varies with latitude and depth: Near the poles, water may be at or near

freezing. Near the equator, it may be as much as 28 Cº. Surface waters are generally the warmest,

because they are warmed by the Sun. Temperature decreases with depth. At great ocean depths, temperatures may be

just above freezing.

Light• The well-illuminated

water near the surface of the ocean is called the photic zone.

• Light is used by certain organisms in the water for photosynthesis.

• Therefore, photosynthetic organisms are restricted to the near-surface waters.

• Clarity of the Water (or conversely, the amount of suspended sediment in the water). Interdependence of Photosynthesis

and Respiration

Carbonate Compensation Depth or CCD

• The Carbonate Compensation Depth or CCD is a particular depth in the oceans, which particles of calcium carbonate from micro-organisms are dissolved as fast as they descent through the water column.

• 4-5 km, varying from place to place• The CCD affects where calcareous

sediments/oozes may or may not accumulate.

Carbonate Compensation Depth or CCD

• Above the CCD, water is warmer, and precipitation of CaCO3 is greater than dissolution. Calcarous plankton can be found in the water column, and on the

bottom. Bottom sediments can consist of calcareous sediments forming chalk or

limestone.

• Below the CCD, water is colder, and CaCO3 tends to dissolve (dissolution is greater than precipitation). Tiny shells of CaCO3 dissolve, and do not accumulate on the bottom if

water is deeper than the CCD.

• Below the CCD, the bottom sediments consist of: Clay Silica shells of plankton (diatoms, radiolarians)

Carbonate Compensation Depth (CCD): (1) Calcium carbonate accumulates along parts of the midoceanic ridge that are above the CCD. (2) The accumulated layer is then carried away as the lithospheric plates diverge from the ridge. (3) When a given region of the seafloor has reached depths below the CCD, calcium carbonate no longer is deposited, but clay and siliceous remains of radiolaria and diatoms may accumulate.

Species Diversity and Geography

• High Latitudes: Low species diversity. Large numbers of

individuals.

• Low Latitudes: High species diversity. Fewer individuals

within each.

Species diversity is related to geographic location, particularly latitude:

Species Diversity and Geography

• Likely because relatively fewer species can adapt to the rigors of polar climate.

As a general rule, species diversity increases toward the equator:

Species Diversity and Geography

• The equator offers: A stable input of

solar energy. Less duress from

changing seasons. More stable food

supply.

Warm areas place less stress on organisms and provide opportunities for continuous uninterrupted evolution, encouraging more variety.

Species Diversity and Geography:Land Bridges, Isolation, and Migration

Migration and dispersal patterns of land animals can indicate the existence of:

• Former Land Bridges(Bering Strait)

• Mountain Barriers• Ocean Barriers

• Closing and Opening of Ocean Basins.

Use of Fossils in the Interpretation of Ancient Climatic Conditions

Fossils can be used to interpret paleoclimates (ancient climates):

1. Fossil spore and pollen grains can tell about the types of plants that lived, which is an indication of the paleoclimate.

2. Plant fossils showing aerial roots, lack of yearly rings, and large wood cell structure indicate tropical climates.

3. Presence of corals indicates tropical climates.

Use of Fossils in the Interpretation of Ancient Climatic Conditions

4. Marine molluscs (clams, snails, etc.) with spines and thick shells inhabit warm seas.

5. Planktonic organisms vary in size and coiling direction with temperature, for example the foraminifer Globorotalia.

6. Compositions of the skeletons, for example shells, have higher magnesium contents in warmer waters.

7. Oxygen isotope ratios in shells. Oxygen16 evaporates easier than Oxygen18 because it is lighter. O16 falls as precipitation and gets locked up in glaciers, leaving sea water enriched in O18 during glaciations. Shells that are enriched in O18 indicate times of glaciation.

Fossils and Stratigraphy

• Principle of Fossil Succession:• Time periods and certain ROCK units can be

recognized by its fossil content.• Fossil species appear and disappear throughout

the stratigraphic record. • The Geologic Time Scale is based on these

appearances and disappearances. • Each of the Eras ends with a mass extinction.• Period boundaries coincide with smaller

extinction events, followed by appearances of new species.

Example of a range chart showing the ranges of late Cretaceous ammonite cephalopods (chambered mollusks) from the Lopez de Bertodano Formation, Seymour Island, Antarctic Peninsula. (From Macellari, E. E. 1986. J. Paleontol. Mem. 18, Part 2.)

The Geologic Range • Geologic Range –

The interval between the first and last occurrence of a fossil species in the geologic record. Determined by recording the

occurrence of the fossils in numerous stratigraphic sequences from hundreds of locations.

Figure 6-34 (p. 143)Geologic ranges of three genera of foraminifera.

The interval between A and B is the total range zone of Assilina.

The interval between X and Y could be designated the Assilina-Heterostegina concurrent range zone.

Biostratigraphic Zones

Figure 6-30 (p. 140)Use of geologic ranges of fossils to identify time-rock units.

• This diagram illustrates how geologists use geologic ranges of fossils to identify time-rock units.

• In Region 1, geologists identify time-rock systems of strats O, D, and M (Ordovician, Devonian, and Mississippian).

• In Region 2, they identify units O and D, and an older unit C (Cambrian).

• In Region 3, they locate unit S (Silurian) between units O and D.

• They can put the information together to come up with a composite geologic column and time-rock units C, O, S, D, and M.

• Note the geologic ranges of the three fossils plotted beside the composite section.

Limiting Factors on Correlating with Fossils

• Appearances and disappearances of fossils may indicate: Evolution Extinction Changing environmental conditions that cause

organisms to migrate into or out of an area. Reworked Fossils (weathering, erosion,

transport, re-lithification).

Use of Fossils in Reconstructing Ancient Geography & Paleogeographic Maps

Paleogeographic Maps are interpretive maps which depict the geography of an area at some time in the past, for example, maps showing the distribution of land and sea in the past.

Paleogeographic Maps

• Constructing a Paleogeographic Map:

1. For the selected area, collect all available data that show the occurrence of the selected time-rock unit.

2. Note locations of fossil species of the same age on a map.

3. Interpret paleoenvironment for each region using rock types, sedimentary structures, and fossils.

4. Modern coral reefs occur in the tropics, within 30o north and south of the equator. Ancient coral reefs likely had similar distributions.

Paleogeographic Maps

• Constructing a Paleogeographic Map:

5. Plot locations of non-marine (terrestrial) deposits using locations of land-dwelling organisms such as dinosaurs or mastodons, fossilized tracks of land animals, and fossils of land plants. It may also be possible to recognize various non-marine environments.

6. Plot the environments to produce a paleogeographic map for that time interval.

Figure 6-46 (p. 150)Major land and sea regions in North and South America during the Carboniferous Period.

Fossils allow use to infer the locations of ancient seafloors, land areas, and coastlines

(Adapted from Ross, C. A. 1967. J. Paleontol. 41(6): 1341-1354.)