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The Fossil Record


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Title: The Fossil Record

The Fossil Record

Fossil Preservation
  • Preservation as a fossil usually requires
  • Preservable parts. Hard parts (bones, shells,
    teeth, wood) have a much better chance at being
    preserved than do soft parts (muscle, skin,
    internal organs).
  • Rapid burial by sediment. Burial protects body
    parts from decay.
  • No physical, chemical, and biological destruction
    after burial. Most organismal remains are
    destroyed by burrowing (bioturbation),
    dissolution, metamorphism, or erosion.

Forms of Chemical Alteration(aids in
  • Carbonization preserves soft tissues of plants or
    animals as a thin carbon film.
  • Permineralization -filling of pores (tiny holes)
    in bone or shell by the deposition of minerals
    from solution.
  • Replacement- molecule-by-molecule substitution of
    a different mineral for the original material.

Imprints of Hard Parts in Sediment
  • Impressions (molds)- imprints of an organism in
    the sediment.
  • External molds are imprints of the outside of a
  • Internal molds are imprints of the inside of the
  • A cast is produced if the mold is later filled or
    covered by sediment and preserved.

Replacement of ammonite shell by pyrite
Mold and cast
Additional Methods of Fossilization
  • Freezing mammoths in tundra
  • Dessication mummification
  • Amber trapped insects
  • Tar/Asphalt LaBrea tarpits or peat bogs

LaBrea Tar Pits
Frozen Mammoth
Trace Fossils
  • Ichnofossils are the markings in the sediment
    made by the activities of organisms.
  • Indicators of movement in the sediment
  • Tracks, trails burrows
  • Nests
  • Coprolites
  • fossilized feces

Figure 4-11 (p. 111)Traces that reflect animal
behavior (A) crawling traces, (B) resting
traces, C) dwelling traces, (D) grazing traces,
and (E) feeding traces.
Dinosaur track
Worm burrows
The Rank and Order of Life
  • A system of binomial nomenclature is used
  • The first of the two names is the Genus and the
    second name is the species.
  • The genus and species names are underlined or
    italicized. The name of the genus is capitalized,
    but the name of the species is not.

Classification of Organisms
  • The Species
  • A group of organisms that have structural,
    functional, and developmental similarities, and
    that are able to interbreed and produce fertile
  • Species are reproductively isolated from one
  • Example
  • Goats and sheep do not interbreed in nature, so
    they are separate species
  • In captivity they can produce fertile offspring!

  • Linnaean taxonomic groups
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

Five kingdoms of organisms
  • Animalia (animals)
  • Plantae (plants)
  • Monera (bacteria and blue-green algae)
  • Fungi (mushrooms, fungus)
  • Protista (single-celled
  • organisms)

Proposed revision to current system
  • Organisms are grouped into three superkingdoms or
  • Bacteria - Kingdom Monera including cyanobacteria
    (blue-green algae)
  • Archaea - sometimes called archaebacteria as
    different from the bacteria as the eukaryotes are
    from the prokaryotes
  • Eukarya - animals, plants, fungi, and protists

  • All organisms are composed of cells.
  • There is a fundamental difference between
    organisms based on the type of cells
  • Cells with a nucleus (or nuclei) are eukaryotic
    cells. Organisms with this type of cell are
    called eukaryotes.
  • Cells without a nucleus are prokaryotic cells.
    Organisms with this type of cell are called
    prokaryotes. Kingdom Monera (bacteria) only.

Why Study Evolution?
  • Evolution involves inheritable changes in
    organisms through time
  • Fundamental to biology and paleontology
  • Paleontology is the study of life history and
  • evolution as revealed by fossils
  • Evolution is a unifying theory that explains a
    collection of facts
  • Evolution provides a framework for discussion of
    life history in later parts of the term

Misconceptions about Evolution
  • Many people have a poor understanding of the
    theory of evolution and hold a number of
    misconceptions, which include
  • evolution proceeds strictly by chance
  • nothing less than fully developed structures are
    of any use
  • there are no transitional fossils so-called
    missing links connecting ancestors and
  • humans evolved from monkeys so monkeys should no
    longer exist

Evolution Historical Background
  • Evolution, the idea that todays organisms have
    descended with modification from ancestors that
    lived during the past
  • Usually attributed solely to Charles Darwin,
  • Seriously considered long before he was born (by
    some ancient Greeks and by philosophers and
    theologians during the Middle Ages)
  • Nevertheless, the prevailing belief in the 1700s
    was that Genesis explained the origin of life

Evolution Historical Background
  • During the 18th century, naturalists were
    discovering evidence that could not be reconciled
    with literal reading of Scripture
  • In this changing intellectual atmosphere,
    scientists gradually accepted a number of ideas
  • the principle of uniformitarianism,
  • Earths great age,
  • that many types of plants and animals had become
  • and that change from one species to another
  • What was lacking, though, was a theoretical
    framework to explain evolution

  • Jean-Baptiste de Lamarck
  • (1744-1829) is best remembered for his theory of
    inheritance of acquired characteristics,
  • According to this theory,
  • new traits arise in organisms because of their
  • Somehow are passed on to their descendants
  • Lamarcks theory was widely accepted
  • With more evidence, proved to be invalid

Lamarcks Theory
  • Lamarks theory was totally refuted
  • Decades later
  • Discovered that genes are the units of heredity
  • Cannot be altered by any effort by an organism

Lamarcks Theory of Inheritance
  • Ancestral short-necked giraffes stretched their
    necks to reach leaves high on trees
  • Their offspring were born with longer necks

  • In 1859, Charles Robert Darwin (1809-1882)
  • published On the Origin of Species
  • In it he detailed his ideas on evolution
  • formulated 20 years earlier
  • and proposed a mechanism for evolution

Natural Selection
  • Plant and animal breeders
  • practice artificial selection by selecting those
    traits they deem desirable
  • and then breed plants and animals with those
  • thereby bringing about a great amount of change
  • Observing artificial selection
  • gave Darwin the idea that a process of selection
    among variant types
  • in nature could also bring about change

Darwin and Wallace
  • Darwin and Alfred Russel Wallace (1823-1913)
  • A natural process was selecting only a few
    individuals for survival
  • Darwins and Wallaces idea
  • called natural selection
  • was presented simultaneously in 1859

Natural SelectionMain Points
  • Organisms in all populations
  • posses heritable variations such as
  • size, speed, agility, visual acuity,
  • digestive enzymes, color, and so forth
  • Some variations are more favorable than others
  • some have a competitive edge
  • in acquiring resources and/or avoiding predators
  • Not all young survive to reproductive maturity
  • Those with favorable variations are more likely
    to survive and pass on their favorable variations

Naturally Selected Giraffes
  • According to the Darwin-Wallace theory
  • giraffes long neck evolved because ancestors
    with longer necks had an advantage and reproduced
    more often

Survival of the Fittest
  • Natural selection is commonly referred to as
    survival of the fittest
  • This is misleading because
  • natural selection is not simply a matter of
    survival but involves differential rates of
    survival and reproduction

Which is favored in Natural Selection?
  • One misconception about natural selection
  • Only biggest, strongest, fastest animals survive
  • These characteristics might provide an advantage
  • Natural selection may favor
  • the smallest if resources are limited
  • the most easily concealed
  • those that adapt most readily to a new food
  • those having the ability to detoxify some
  • those that are able to withstand extreme heat
  • Etc.

Limits of Natural Selection
  • Natural selection works on existing variation in
  • a population
  • It could not account for the origin of variations
  • Critics reasoned that should a variant trait
  • it would blend with other traits and would be
  • The answer to these criticisms
  • existed even then in the work of Gregor Mendel,
    but remained obscure until 1900

Mendel and the Birth of Genetics
  • During the 1860s, Gregor Mendel
  • Performed a series of controlled experiments
  • True-breeding strains of garden peas
  • Strains that when self-fertilized always display
    the same trait, such as flower color
  • Traits are controlled by a pair of factors, now
    called genes
  • Genes occur in alternate forms alleles
  • One may dominate
  • One allele from each parent (pair)

Importance of Mendels Work
  • The factors (genes) controlling traits
  • do not blend during inheritance
  • Traits not expressed in each generation
  • may not be lost
  • Therefore, some variation in populations
  • results from alternate expressions of genes
  • Variation can be maintained

Genes and Chromosomes
  • Complex, double-stranded helical molecules
  • of deoxyribonucleic acid (DNA)
  • called chromosomes
  • are found in cells of all organisms
  • except bacteria,
  • which have ribonucleic acid (RNA)
  • Specific segments of DNA
  • are the basic units of heredity (genes)
  • The number of chromosomes
  • varies from one species to another
  • Example fruit flies 8 humans 46 horses 64

Reproduction and Cell Division
  • Reproduction in organisms may be
  • sexual
  • asexual
  • alternation of sexual and asexual generations
  • Asexual reproduction can occur through
  • binary fission (cells that split in two) - only
    among single-celled organisms
  • budding in which the parent " sprouts" an
    appendage that may separate to grow into an
    isolated individual, or remain attached to the
    parent (as in colonial organisms). Budding occurs
    in some unicellular and some multicellular
  • spores shed by the parent (as in a seedless
    plant) that germinate and produce male and female
    sex cells (leading to alternation of sexual and
    asexual gererations).

Evolution evolving
  • During the 1930s and 1940s,
  • paleontologists, population biologists,
  • geneticists, and others developed ideas that
  • merged to form a modern synthesis
  • or neo-Darwinian view of evolution
  • They incorporated
  • chromosome theory of inheritance
  • into evolutionary thinking
  • They saw changes in genes (mutations)
  • as one source of variation

  • They completely rejected Lamarcks idea of
    inheritance of acquired characteristics
  • They reaffirmed the importance of natural

Where does Variation Arise?
  • Evolution by natural selection works on variation
    in populations most of which is accounted for by
    the reshuffling of alleles from generation to
    generation during sexual reproduction
  • The potential for variation is enormous
  • Thousands of genes with several alleles, and with
    offspring receiving 1/2 of their genes from each
  • New variations arise by mutations
  • change in the chromosomes or genes

  • Mutations result in a change of the hereditary
  • Mutations that take place in sex cells
  • Inheritable
  • Chromosomal mutations
  • affecting a large segment of a chromosome
  • Point mutations
  • individual changes in particular genes
  • Mutations are random with respect to fitness
  • they may be beneficial, neutral, or harmful

  • If a species is well adapted to its environment,
  • most mutations would not be particularly useful
  • and perhaps would be harmful
  • But what was a harmful mutation
  • can become a useful one if the environment changes

What Causes Mutations?
  • Some mutations are induced by mutagens
  • agents that bring about higher mutations rates
    such as
  • some chemicals
  • ultraviolet radiation
  • X-rays
  • extreme temperature changes
  • Some mutations are spontaneous
  • occurring without any known mutagen

Evolutionary Terminology
  • Population - a group of interbreeding organisms.
  • Gene pool - the sum of all of the genetic
    components in a population.
  • Speciation - the origin of new species.

  • The phenomenon of a new species arising from an
    ancestral species
  • Involves change in the genetic makeup of a
  • May bring about changes in form and structure
  • During allopatric speciation,
  • species arise when a small part of a population
  • becomes isolated from its parent population
  • This involves the creation of geographic barriers

Allopatric Speciation
  • Geographic barriers may form across parts
  • of a central populations range,
  • thereby isolating small populations that speciate

Allopatric Speciation
  • A few individuals
  • May reach a remote area and no longer exchange
    genes with the parent population
  • This out-migration can lead to the formation of a
    peripheral isolate that gives rise to a new
    species while the parent population persists
    without change

Adaptive radiation
  • The branching of a population to produce many
    species through many separate speciation events.
  • The descendant species are each adapted to
    particular environment and living strategies.

Honey Creeper
  • Bill/beak diversity--adaptive radiation

Gradual or Rapid Evolution?
  • We ponder whether evolution occurs in jumps or in
    a gradual progression
  • Phyletic gradualism - gradual progressive change
    by means of an almost infinite number of small,
    subtle steps (traditional idea)
  • Punctuated equilibrium - sudden changes
    "punctuating" (or interrupting) long periods of
    little change, termed stasis (Gould and Eldridge,
    1977). Most change occurs over a short period of
    time (new idea)

  • Phylogeny the sequence of organisms placed in
    evolutionary order.
  • Diagrams called phylogenetic trees are used to
    display ancestor-descendant relationships.
  • Branches on the tree are called clades.

  • Stratophenetic-
  • Traditional view, where evolutionary tree shows
    succession of life through time, dependant on
    fossil record
  • Cladistic-
  • Organisms are analyzed based on characteristics
    they share to determine ancestor-descendant
  • Build cladogram showing closeness of organisms

Evidence for Evolution
  • Known examples of sequential evolution, for
    example the Cenozoic fossil horses.
  • Evolution of the lower foreleg in horses from the
    Eocene (left) to the modern horse (right).

  • Body parts with similar origin, history and
    structure, without reference to function.
  • Homologous organs and bone configurations have a
    common origin and ancestry (toes of land-dwelling
    mammals vs. bat wings). Many natural examples.
  • Result of variations/adaptations to environment

Figure 4-21 (p. 123)Skeleton of right forelimb
of several vertebrates to show similarity of
structure. Key c, carpals h, humerus m,
metacarpals r, radius u, ulna, 1-5, digits.
Evidence for Evolution
  • Vestigial organs suggest a common ancestry.
  • Vestigial organs serve no apparent purpose, but
    resemble functioning organs in other animals.
  • Example fossil whales have useless pelvic bones
    (and occasionally rear feet) resembling those in
    other mammals.
  • All mammals have similar structures whether they
    are used or not.

Figure 4-22 (p. 124)The pelvis and femur of a
whale are vestigial organs.
Embryo Evidence
  • Embryos of all vertebrates VERY similar and
    suggest a common ancestry.
  • Example Gill slits in human embryos
  • Evidence of homologous organs, vestigial organs,
    and embryology
  • Gill slits are found in the embryos of all
    vertebrates because they descend from a common
    ancestor, the fish, in which these structures
    first evolved.
  • Example Human embryos also have a well-defined
    tail, which is visible by the fourth week of
    development, and reaches maximum length at the
    sixth week. The tail then shortens, becoming
    vestigial as the coccyx.

Evidence for Evolution
  • Biochemistry provides evidence for evolutionary
  • Blood chemistry is similar among all mammals
  • Humans blood chemistry is related
  • most closely to the great apes
  • then to Old World monkeys
  • then New World monkeys
  • then lower primates such as lemurs
  • Biochemical test support the idea
  • that birds descended from reptiles
  • a relationship also evidenced in the fossil record

Fossils and Stratigraphy
  • Establishing Age Equivalence of Strata with
  • Principle of Biologic Succession (or fossil
    succession) - states that fossils occur in a
    consistent vertical order in sedimentary rocks
    all over the world. (William Strata Smith)

Fossils and Stratigraphy
  • Fossils can be used to recognize the approximate
    age of a unit and its place in the stratigraphic
  • They can also be used to correlate strata from
    place to place.

The Geologic Range
  • Geologic range - the interval between the first
    and last occurrence of a fossil species in the
    geologic record.
  • It is determined by recording the occurrence of
    the fossils in numerous stratigraphic sequences
    from hundreds of locations.
  • Ranges are well known for some species, and
    poorly known for others.

  • Time-rock units-chronostratigraphy
  • 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.

Paleontologic Correlation
  • Cosmopolitan species are found almost everywhere
    they are not restricted to a single geographic
    location in their environment.
  • Endemic species are confined to a restricted area
    in the environment in which they live.
  • Appearances and disappearances of fossils may
  • evolution or extinction
  • changing environmental conditions that cause
    organisms to migrate into or out of an area

Index Fossils
  • Index fossils or guide fossils are useful in
    identifying time-rock units and in correlation.
  • Characteristics include
  • abundant and easily identified
  • widely distributed (cosmopolitan)
  • organisms with short geologic ranges (rapid
    evolution or extinction rates)

Biostratigraphic Zones
  • Biozone a body of rock that is identified only
    on the basis of the fossils it contains. They are
    the basic unit for biostratigraphic
    classification and correlation
  • Types of biozones
  • range zone total range (on geologic time scale)
    of one taxa
  • assemblage zone the part of the stratigraphic
    column containing an assemblage or set of several
    associated fossils that coexist
  • concurrent range zone the rock where the ranges
    of two (or more taxa) overlap

  • Relation of ancient organisms to their
    environment or ecosystem (selected physical,
    chemical and biological factors).
  • Study community
  • Organisms coexisting in a specific ecosystem
  • Each with a niche (role in its habitat)

Trophic level (feeding heirarchy)
  • Primary Producers or autotrophs - produce their
    own food through photosynthesis, and supply food
    and energy for other organisms.
  • Consumers or heterotrophs - cannot produce their
    own food and must eat.
  • Herbivores - heterotrophs that eat plants
  • Carnivores - heterotrophs that eat herbivores and
    other carnivores
  • Other feeding modes
  • Decomposers and Transformers - bacteria and fungi
    which break down organic matter converting it
    into a form which can be utilized by other
    organisms (nutrients)
  • Parasites - derive nutrition from other organisms
    without killing them
  • Scavengers - derive nutrition from dead organisms

The Marine Ecosystem
  • The ocean may be divided into two realms
  • Pelagic realm the water mass lying above the
    ocean floor. It can be subdivided into
  • Neritic zone the water overlying the
    continental shelves
  • Oceanic zone the water seaward of the
    continental shelves
  • Benthic realm the bottom of the sea, which
  • Supralittoral zone above the high tide line
  • Littoral zone between the high and low tide
  • Sublittoral zone continuously submerged zone,
    from low tide line to the edge of the continental
    shelf (about 200 m deep)
  • Bathyal zone (200 - 4000 m deep)
  • Abyssal zone (4000 - 6000 m deep)
  • Hadal zone (more than 6000 m deep) - the extreme
    depths found in the deep sea trenches. Note that
    the deepest point in the oceans is in the Mariana
    Trench, 11,033 m deep.

Figure 4-31 (p. 131)Classification of marine
Marine ecosystem modes of life
  • Planktonic - small plants and animals that float,
    drift, or swim weakly (plankton)
  • Phytoplankton - plants and plant-like plankton,
    such as diatoms and coccolithophores
  • Zooplankton - animals and animal-like plankton,
    such as foraminifera and radiolaria
  • Nektonic - swimming animals that live within the
    water column (nekton)
  • Benthonic or benthic - bottom dwellers, whch may
    be either
  • Infaunal - living beneath the sediment surface
    they burrow and churn and mix the sediment, a
    process called bioturbation
  • Epifaunal - living on top of the sediment surface

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

Plankton Jelly fish
Nekton fish cephalopod
Sessile epiflora seaweed
Sessile epifauna bivalve
Mobile epifauna Starfish Gastropod
Infauna- worm, bivalve
Physical Constraints within the Ecosystem
  • The Chemistry of Sea Water
  • Nearly all water contains dissolved chemicals.
    Even rain water. These dissolved chemicals are
    called "salts."
  • Salinity a measure of the total dissolved
    solids in water. Salinity is measured in parts
    per thousand (ppt or o/oo) by weight.

Various Salinity Levels
  • Normal ocean water 35 ppt or 35 o/oo or 3.5. A
    salinity of 35 ppt means that there are 35 pounds
    of salt per 1000 pounds of sea water.
  • Freshwater about 5 ppt to less than 1 ppt
  • Brackish water sea water with less than about
    30 ppt (input of freshwater)
  • Hypersaline water more than 250 ppt (typically
    in lakes in arid areas, or in enclosed areas like
    lagoons or isolated seas in arid areas) (through

The Chemistry of Sea Water
  • Essential to Life
  • Carbon dioxide (listed as part of HCO3) - used by
    marine plants amount varies with photosynthesis
  • Nitrogen - used in proteins and nucleic acids
  • Phosphorus - a component of DNA and RNA, and
    molecules used in metabolism
  • Sulfur - used in proteins and other molecules an
    energy source for Bacteria and Archaea

Movement of Ocean Water
  • Waves are generated as the wind blows over the
    surface of the water.
  • Currents are the unidirectional flow of water.
  • Surface currents
  • Coriolis Effect. Currents in the northern
    hemisphere tend to be deflected toward the right
    (or clockwise), and currents in the southern
    hemisphere tend to be deflected to the left (or
    counter clockwise) as a result of the Coriolis
  • Surface currents have an affect on the climate -
    transporting warm waters to northern latitudes,
    for example.
  • Thermohaline currents are initiated at the ocean
    surface by temperature and salinity conditions.
    Gravity acts to pull colder (or more saline)
    denser water downward, displacing less dense
    water upward.
  • Upwelling- Along edge of continents-brings
  • Tides are generated by the effect of the Moon's
    gravity (and to a lesser extent, the Sun's
    gravity) on the oceans.

Figure 4-36 (p. 135)Major ocean surface
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 degrees
  • Surface waters are generally the warmest, because
    they are warmed by the Sun.
  • Temperature decreases with depth.
  • A zone of rapid temperature decrease with depth
    in a water mass is called the thermocline.
  • At great ocean depths, temperatures may be just
    above freezing.

  • 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. Light penetration into the sea depends
  • Sun angle
  • Atmospheric conditions
  • Clarity of the water (or conversely, the amount
    of suspended sediment in the water)
  • In some areas, light may penetrate as deep as 200
    m or more, but generally there is light adequate
    to support photosynthesis only in the upper tens
    of meters of the sea (to perhaps 100 m).

Types of Sea Floor Sediments
  • Terrigenous sediment
  • Mineral grains from weathered continental rocks
  • Fine-grained sediment (clay, mud)
  • Accumulates slowly (5000 to 50,000 years to
    deposit 1 cm)
  • Color may be black, red or brown
  • Biogenous (or Organic) sediment
  • Calcareous oozes - form chalk in waters less than
    about 4000-5000 m
  • Siliceous oozes
  • Phosphatic material
  • Hydrogenous sediment
  • Minerals that precipitate from sea water by
    chemical reactions.
  • Example manganese nodules

Carbonate compensation depth or CCD
  • The Carbonate Compensation Depth or CCD is a
    particular depth in the oceans (4-5 km, varying
    from place to place), which affects where
    calcareous oozes may or may not accumulate.
  • 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
  • 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)

Use of Fossils in Reconstructing Ancient Geography
  • Environmental limitations control the
    distribution of modern plants and animals.
  • Locations of fossils--use to construct
    paleogeographic maps
  • Example Modern coral reefs occur in the tropics,
    within 30o north and south of the equator.
    Ancient coral reefs likely had similar
  • Example 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.

Fossils for Paleoclimate Interpretations
  • Fossil spore and pollen grains can tell about the
    types of plants that lived, which is an
    indication of the paleoclimate.
  • Presence of corals indicates tropical climates
  • Plant fossils showing aerial roots, lack of
    yearly rings, and large wood cell structure
    indicate tropical climates
  • Marine molluscs (clams, snails, etc.) with spines
    and thick shells inhabit warm seas
  • Planktonic organisms vary in size and coiling
    direction according to temperature, for example
    the foraminifer Globorotalia
  • Compositions of the skeletons, for example shells
    in warmer waters have higher magnesium contents
  • 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.

Overview of History of Life
  • Precambrian Eons (b.y. billion years)
  • Hadean 4.6-3.8 b.y. (no fossil record)
  • Archean 3.8-2.5 b.y. (bacteria, algae,
  • Proterozoic 2.5-0.57 b.y. (fossil metazoans
    worms, coelenterates, arthropods

Modern stromatolites (blue-green algae)
Overview of History of Life
  • Phanerozoic Eras (0.57-0.0 b.y.)
  • Paleozoic marine invertebrates, first
    vertebrates, land plants
  • Mesozoic more marine invertebrates, land
    vertebrates, plants
  • Cenozoic modern mammals, flowering plants
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