The Fossil Record

1
The Fossil Record

• CHAPTER 4

2
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.

3
Forms of Chemical Alteration(aids in
preservation)

• 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.

4
Imprints of Hard Parts in Sediment

• Impressions (molds)- imprints 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 is produced if the mold is later filled or
  covered by sediment and preserved.

5
Replacement of ammonite shell by pyrite
Mold and cast
6
Additional Methods of Fossilization

• Freezing mammoths in tundra
• Dessication mummification
• Amber trapped insects
• Tar/Asphalt LaBrea tarpits or peat bogs

7
Fossils!
Amber
LaBrea Tar Pits
Frozen Mammoth
8
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

9
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.
10
Dinosaur track
Worm burrows
11
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.

12
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
  offspring.
• Species are reproductively isolated from one
  another
• Example
• Goats and sheep do not interbreed in nature, so
  they are separate species
• In captivity they can produce fertile offspring!

13
Taxonomy

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

14
Five kingdoms of organisms

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

15
Proposed revision to current system

• Organisms are grouped into three superkingdoms or
  domains
• 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

16
Cells

• 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.

17
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

18
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
  descendants
• humans evolved from monkeys so monkeys should no
  longer exist

19
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

20
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
  extinct,
• and that change from one species to another
  occurred
• What was lacking, though, was a theoretical
  framework to explain evolution

21
Lamarck

• 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
  needs/wants
• Somehow are passed on to their descendants
• Lamarcks theory was widely accepted
• With more evidence, proved to be invalid

22
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

23
Lamarcks Theory of Inheritance

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

24
Darwin

• 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

25
Natural Selection

• Plant and animal breeders
• practice artificial selection by selecting those
  traits they deem desirable
• and then breed plants and animals with those
  traits
• 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

26
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

27
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

28
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

29
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

30
Which is favored in Natural Selection?

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

31
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
  arise,
• it would blend with other traits and would be
  lost
• The answer to these criticisms
• existed even then in the work of Gregor Mendel,
  but remained obscure until 1900

32
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)

33
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

34
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

35
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
  organisms.
• 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).

36
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

37
Evolution

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

38
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
  parent
• New variations arise by mutations
• change in the chromosomes or genes

39
Mutations

• Mutations result in a change of the hereditary
  information
• 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

40
Mutations

• 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

41
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

42
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.

43
Speciation

• The phenomenon of a new species arising from an
  ancestral species
• Involves change in the genetic makeup of a
  population
• 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

44
Allopatric Speciation

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

45
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

46
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.

47
Honey Creeper

• Bill/beak diversity--adaptive radiation

48
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)

49
Phylogeny

• 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.

50
Phylogeny

• 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
  relationships
• Build cladogram showing closeness of organisms

51
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).

52
Homology

• 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

53
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.
54
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.

55
Figure 4-22 (p. 124)The pelvis and femur of a
whale are vestigial organs.
56
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.

57
Evidence for Evolution

• Biochemistry provides evidence for evolutionary
  relationships
• 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

58
Fossils and Stratigraphy

• Establishing Age Equivalence of Strata with
  Fossils
• 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)

59
Fossils and Stratigraphy

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

60
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.

61
CHRONOSTRATIGRAPHY

• 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.

62
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
  indicate
• evolution or extinction
• changing environmental conditions that cause
  organisms to migrate into or out of an area

63
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)

64
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

65
Paleoecology

• 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)

66
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
  

67
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
  includes
• Supralittoral zone above the high tide line
• Littoral zone between the high and low tide
  lines
• 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.

68
Figure 4-31 (p. 131)Classification of marine
environments.
69
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

70
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
coral
Mobile epifauna Starfish Gastropod
Infauna- worm, bivalve
71
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.

72
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
  evaporation)

73
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

74
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
  Effect.
• 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
  nutrients
• Tides are generated by the effect of the Moon's
  gravity (and to a lesser extent, the Sun's
  gravity) on the oceans.

75
Figure 4-36 (p. 135)Major ocean surface
currents.
76
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
  C.
• 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.

77
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. Light penetration into the sea depends
  on
• 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).

78
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

79
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
  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)

80
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
  distributions.
• 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.

81
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.

82
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,
  stromatolites)
• Proterozoic 2.5-0.57 b.y. (fossil metazoans
  worms, coelenterates, arthropods

Modern stromatolites (blue-green algae)
83
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