chapter 6 life on earth: what do fossils reveal?

Chapter 6Chapter 6

Life on Earth: What do Life on Earth: What do Fossils Reveal?Fossils Reveal?

Fossils Fossils

Fossils are the remains or traces of ancient life which have been preserved by natural causes in the Earth's crust.

Fossils include both the remains of organisms (such as bones or shells), and the traces of organisms (such as tracks, trails, and burrows - called trace fossils).

Fossil Preservation Fossil Preservation

To become preserved as a fossil, an organism must: • Have preservable parts. Bones, shells, teeth,

wood are more readily preserved than soft parts. • Be buried by sediment to protect the organism

from scavengers and decay. • Escape physical, chemical, and biological

destruction after burial (bioturbation, dissolution, metamorphism, or erosion).

Fossil Preservation Fossil Preservation

Organisms do not all have an equal chance of being preserved.

The organism must live in a suitable environment.

Marine and transitional environments are more favorable for fossil preservation. Higher rate of sediment deposition .

Types of Fossil PreservationTypes of Fossil Preservation

1. Chemical Alteration of Hard Parts

2. Imprints of Hard Parts in Sediment

3. Preservation of Unaltered Soft Parts

4. Trace fossils or Ichnofossils

5. Preservation of Unaltered Hard Parts

Preservation of Unaltered Hard PartsPreservation of Unaltered Hard Parts

The shells of invertebrates and single-celled organisms, vertebrate bones and teeth:a. Calcite (echinoderms and forams)

b. Aragonite (clams, snails, modern corals)

c. Phosphate (bones, teeth, conodonts, fish scales)

d. Silica (diatoms, radiolarians, some sponges)

e. Organic matter (insects, pollen, spores, wood, fur)

Chemical Alteration of Hard Parts Chemical Alteration of Hard Parts

a. Permineralization - filling of tiny pores

b. Replacement - molecule-by-molecule substitution of one mineral for another (silica or pyrite replacing calcite)

c. Recrystallization - aragonite alters to calcite

d. Carbonization - soft tissues preserved as a thin carbon film (ferns in shale)

Imprints of Hard Parts in SedimentImprints of Hard Parts in Sediment

• Impressions– External molds– Internal molds

• Cast

Preservation of Unaltered Soft Parts Preservation of Unaltered Soft Parts

• Freezing (wooly mammoths)

• Desiccation (drying or mummification)

• Preservation in amber • Preservation in tar (LaBrea tar pits)

• Preservation in peat bogs (Lindow man – England; Tollund man – Denmark)

Trace fossils or IchnofossilsTrace 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

Trace fossils or IchnofossilsTrace fossils or Ichnofossils

Trace fossils provide information about ancient water depths, paleocurrents, availability of food, and sediment deposition rates.

Tracks can provide information on foot structure, number of legs, leg length, speed, herding behavior, and interactions.

Biological classificationBiological classification

A system of binomial nomenclature (i.e., two names) is used to name organisms.

The first of the two names is the genus and the second name is the species.

Genus and species names are underlined or italicized.

Genus is capitalized, but species is not.

The Species The Species

A group of organisms that have structural, functional, and developmental similarities, and that are able to interbreed and produce fertile offspring.

The species is the fundamental unit of biological classification.

The SpeciesThe Species

Different species do not interbreed under natural conditions. Reproductive barriers between species prevent interbreeding.

Closely related (but different) species, such as the horse and the donkey CAN interbreed, but the offspring are usually sterile (unable to reproduce).

Hybrid mammalsHybrid mammals

• Mule (♂male donkey and ♀ horse) • Hinny (♂ horse and ♀ donkey)• Zedonk (♂ zebra and ♀ donkey)• Liger (♂ lion and ♀ tiger)• Tigon (♂ tiger and ♀ lion)• Wholphin (♂ false killer whale and ♀ bottlenose

dolphin; fertile)

Hybrids are usually sterile (unable to reproduce).

Cells Cells

All organisms are composed of cells. • Eukaryotic cells have a nucleus (or nuclei) and

organelles. – Organisms with this type of cell are called eukaryotes

(Domain Eukarya).

• Prokaryotic cells have no nucleus or organelles. – Organisms with this type of cell are called prokaryotes

(Domain Archaea and Domain Bacteria).

Taxonomy Taxonomy

Organisms are grouped based on their similarities into taxonomic groups or taxa.

DomainKingdomPhylum (plural = phyla)ClassOrderFamilyGenus (plural = genera)Species (singular and plural)

Classification of the human Classification of the human

Domain EukaryaKingdom AnimaliaPhylum ChordataClass MammaliaOrder PrimatesFamily HominidaeGenus HomoSpecies sapiens

There are six kingdoms of There are six kingdoms of organisms, grouped into three organisms, grouped into three

domains domains

1. Domain Eukarya

2. Domain Bacteria

3. Domain Archaea

Domain EukaryaDomain Eukarya

Organisms with eukaryotic cells (cells with a nucleus) • Kingdom Animalia (animals) • Kingdom Plantae (plants) • Kingdom Fungi (mushrooms, fungus) • Kingdom Protista (single-celled organisms)

Domain BacteriaDomain Bacteria

Organisms with prokaryotic cells (cells without a nucleus)

• Kindgom Eubacteria (bacteria and cyanobacteria or blue-green algae)

Domain ArchaeaDomain Archaea

Organisms with prokaryotic cells, but which are very unusual and quite different from Bacteria. Archaea tend to live under extreme conditions of heat, salinity, acidity.

• Kingdom Archaebacteria

Evolution Evolution

Evolution = change

• Organic evolution refers to changes in populations

• In biology, evolution is the "great unifying theory" for understanding the history of life.

EvolutionEvolution

Plants and animals living today are different from their ancestors because of evolution. They differ in appearance, genetic characteristics, body chemistry, and in the way they function.

These differences appear to be a response to changes in the environment and competition for food.

Lamarck's Hypothesis of EvolutionLamarck's Hypothesis of Evolution

Jean Baptiste Lamarck (1744-1829) observed lines of descent from older fossils to more recent ones, and to living forms.

He correctly concluded that all species are descended from other species.


Lamarck assumed that new structures in an organism appear because of the needs or " inner want " of the organism.

Structures acquired in this way were thought to be somehow inherited by later generations - inheritance of acquired traits.

The idea was challenged because there was no way to test for the presence of an "inner want".


Lamarck also suggested that unused body parts would not be inherited by succeeding generations.

The hypothesis was tested and rejected after an experiment in which the tails were cut from mice for twenty generations. The offspring still had tails.

Similarly, circumcision has been practiced for more than 4000 years with no change among newborn males.

Darwin's Natural Selection Darwin's Natural Selection

Charles Darwin and Alfred Wallace were the first scientists to assemble a large body of convincing observational evidence in support of evolution.

They proposed a mechanism for evolution which Darwin called natural selection.


Natural selection is based on the following observations:

1. More offspring are produced than can survive to maturity.

2. Variations exist among the offspring. 3. Offspring must compete with one another for

food, habitat, and mates. 4. Offspring with the most favorable characteristics

are more likely to survive to reproduce. 5. Beneficial traits are passed on to the next

generation.


Darwin's theory was unable to explain WHY offspring exhibited variability.

This was to come many years later, when scientists determined that genetics is the cause of these variations.

This principle can be stated as:

" the survival of the fittest ".

Inheritance, Genes, and DNA Inheritance, Genes, and DNA

Gregor Mendel (1822-1884) demonstrated the mechanism by which traits are passed to offspring through his experiments with garden peas. His findings were published in an obscure journal and not recognized by the scientific community until 1900.

Inheritance, Genes, and DNAInheritance, Genes, and DNA

Mendel discovered that heredity in plants is determined by what we now call genes. Genes are recombined during fertilization.

Genes are linked together to form chromosomes.

Mendel is known as the "Father of Genetics".

Chromosomes and DNAChromosomes and DNA

• Within the nucleus of each of our cells are chromosomes.

• Chromosomes consist of long DNA molecules (deoxyribonucleic acid).

• Genes are the parts of the DNA molecule that transmit hereditary traits.

The DNA molecule consists of two parallel strands, which resemble a twisted ladder.

The twisted strands are phosphate and sugar compounds, linked with nitrogenous bases (adenine, thimine, guanine, and cytosine).

DNADNA

The structure of the DNA molecule was discovered by Watson and Crick in 1953.

DNA carries chemically coded information from generation to generation, providing instructions for growth, development, and functioning.

Reproduction and Cell Division Reproduction and Cell Division

Reproduction in organisms may be:

• Sexual

• Asexual

• Alternation of sexual and asexual generations

All reproductive methods involve cell division.

Genetic RecombinationGenetic Recombination

New combinations of chromosomes result through sexual reproduction. One of each pair of chromosomes is inherited from each parent.

This sexual genetic recombination leads to variability within the species.

Asexual reproductionAsexual reproduction

• Binary fission - single-celled organisms that divide to form two organisms

• Budding – a bud forms on the parent that may:– Separate to grow into an isolated individual, or– Remain attached to the parent (colonial organisms).– Budding occurs in some unicellular and some

multicellular organisms.

• Spores shed by the parent (as in a seedless plant like moss or ferns) that germinate and produce male and female sex cells (leading to alternation of sexual and asexual generations).

Diploid and Haploid CellsDiploid and Haploid Cells

In a human cell there are 23 pairs of chromosomes. One of these pairs determines the sex of the individual.

• Diploid cells - cells with paired chromosomes.

• Haploid cells - sex cells (or gametes) with only one half of a pair of chromosomes. Example: egg cells or sperm cells

Cell divisionCell division

• Mitosis - Division of body cells of sexual organisms. Produces new diploid cells with identical chromosomes to the parent cells.

• Meiosis - Division of cells to form gametes or sex cells (haploid cells), with half of chromosomal set of the parent cell; occurs in a two-step process, producing four haploid gametes.

Recombination of GenesRecombination of Genes

• Fertilized egg forms when two gametes (egg and sperm) combine. Fertilized egg has paired chromosomes (diploid cell).

• Variation occurs because of the sexual recombination of genes.

• Genes are recombined in each successive generation.

Mutations Mutations

• Mutations are chemical changes to the DNA molecule.

• Mutations can be caused by: – Chemicals (including certain drugs), – Radiation (including cosmic radiation,

ultraviolet light, and gamma rays).

• Mutations may also occur spontaneously without a specific causative agent.

MutationsMutations

Mutations may occur in any cell, but mutations in sex cells will be passed on to succeeding generations.

Mutations produce much of the variability on which natural selection operates.

Causes of EvolutionCauses of Evolution

Evolution may involve change from three different sources:

• Mutations

• Gene recombination as a result of sexual reproduction

• Natural selection

Evolution in Populations Evolution in Populations

Evolution is a process of biologic change that occurs in populations.

• Population - A group of interbreeding organisms that occupy a given area at a given time.

• Gene pool - The sum of all of the genetic components of the individuals in a population.

Evolution in PopulationsEvolution in Populations

There is no exchange of genes between different populations because they are reproductively isolated.

Barriers keep their gene pools separate (distance, geographic barriers, reproductive barriers, etc.)

Geographic barriers Geographic barriers

• Isthmus of Panama, is a barrier between oceans and populations of marine organisms.

• Islands with isolated populations of land animals– Galapagos Island finches – Galapagos Island tortoises – Hawaiian Island honeycreepers (birds)

• Grand Canyon separates different species of animals living on opposite sides of the canyon

Reproductive barriers Reproductive barriers

• Ecological isolation - Populations inhabiting the same geographic area, but living in different habitats

• Temporal isolation - Populations that reproduce at different times (such as plants that flower in different seasons)

• Mechanical isolation - Incompatible reproductive organs due to differences in size, shape, or structure

• Gametic isolation - Fertilization is prevented by incompatible gametes

Speciation Speciation

• Speciation = The process through which new species arise.

• In the limited gene pools of an isolated population, over many generations, genetic differences may accumulate to the point that the different populations are no longer able to interbreed.

• At this point, the different populations would be considered separate species.

SpeciationSpeciation

Once a new species is established, segments of the population around the fringes of the population may undergo additional speciation.

With successive speciations, diverse organisms arise with diverse living strategies.

Adaptive Radiation Adaptive Radiation

The branching of a population to produce descendants adapted to particular environments and living strategies

Example of adaptive radiation Example of adaptive radiation

Hawaiian honey creepers

Bill shapes are adaptations to different means of gathering food.

Adaptive radiation and Adaptive radiation and MacroevolutionMacroevolution

Adaptive radiation can occur at higher taxonomic levels.

Example: Adaptive radiation of mammals at the beginning of the Cenozoic Era.

Macroevolution = Evolution above the species level

Modeling how evolution occurs Modeling how evolution occurs

The question is not whether evolution occurs, but rather, exactly how it occurs. What is the mechanism of evolution?

Modeling how evolution occursModeling how evolution occurs

• Phyletic gradualism - gradual progressive change by means of many small steps (old idea)

• Punctuated equilibrium - sudden changes interrupting long periods of little change (stasis). Most change occurs over a short period of time. (Stephen J. Gould and Niles Eldridge, 1977).

Phyletic gradualism

vs. Punctuated equilibrium

Fossil evidence provides support for both models.

Allopatric SpeciationAllopatric SpeciationPunctuated equilibrium model suggests that

evolution occurs in isolated areas around the periphery of the population (peripheral isolates).

Speciation may occur rapidly in these isolated areas. This is allopatric speciation.

When the new species expands or migrates from the isolated area into new areas, it looks like a sudden appearance in the fossil record.

Phylogeny - The Tree of Life Phylogeny - The Tree of Life

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.

CladogramsCladograms

Diagrams drawn to show ancestor-descendant relationships based on characteristics shared by organisms.

They show how organisms are related but do not include information about time or geologic ranges.

Cladogram of the horse ancestry

Stratophenic phylogeny arranges organisms in tree-like fashion with the most recently evolved individuals on the upper branches, and the older, ancestral forms on the lower branches.

Evolutionary tree of the horses and related organisms

Evidence for Evolution Evidence for Evolution

There are many lines of evidence that show that evolution has occurred. They come from both the fossil record and from biology.

Lines of evidence for evolution Lines of evidence for evolution cited by Darwin cited by Darwin

• Fossils provide direct evidence for changes in life in rocks of different ages.

• Homologous structures - Certain organs or structures are present in a variety of species, but they are modified to function differently.

• Modern organisms contain vestigial organs that appear to have little or no use. These structures had a useful function in ancestral species.

• Animals that are very different, had similar-looking embryos.

Other lines of evidence for evolution come from:

1. Genetics - DNA molecule

2. Biochemistry – similar in closely-related organisms, but very different in more distantly related organisms.

3. Molecular biology - sequences of amino acids in proteins

Evidence for Evolution from Evidence for Evolution from PaleontologyPaleontology

1. Horses

2. Cephalopods and other molluscs

3. Foraminifera and other microfossils

Many examples of gradual or sequential evolution in the fossil record, including:

Evolutionary change in Permian ammonoid cephalopods

Evidence for Evolution from BiologyEvidence for Evolution from Biology

Homologous structures - body parts with similar origin, history and structure, but different functions.


Vestigial organs suggest a common ancestry. Vestigial organs serve no apparent purpose, but resemble functioning organs in other animals.

Vestigial pelvis and femur of a whale in an Eocene fossil


Similarity of embryos of all vertebrates suggests a common ancestry.


Biochemistry - Chemicals (such as proteins, antigen reactions of blood, digestive enzymes, and hormone secretions) are more similar in related organisms.


DNA sequencing – If organisms appear to be similar on the basis of form, embryonic development, or fossil record, we can predict that they would have a greater percentage of DNA sequences in common, compared with less similar organisms.

This is proven to be correct in hundreds of analyses.

Fossils and Stratigraphy Fossils and Stratigraphy

How Do Fossils Reveal the Age of Strata?How Do Fossils Reveal the Age of Strata?

William Smith (late 1700's) discovered that certain rock units could be identified by the assemblages of fossils they contained.

This knowledge led to the Principle of Fossil Succession - Fossils occur in a consistent vertical order in sedimentary rocks all over the world.

The Geologic Time Scale is based on the appearance and disappearance of fossil species in the stratigraphic record.

Fossils can be used to recognize the approximate age of a unit and its place in the stratigraphic column.

Fossils can also be used to correlate strata from place to place.

Geologic range Geologic range

Geologic range = The interval between the first and last occurrence of a fossil species in the geologic record.

The geologic range is determined by recording the occurrence of the fossils in numerous stratigraphic sequences from hundreds of locations.

Using Fossils to Correlate Rock UnitsUsing Fossils to Correlate Rock Units

Use of Cosmopolitan and Endemic Use of Cosmopolitan and Endemic Species in CorrelationSpecies in Correlation

Cosmopolitan species have a widespread distribution.

Endemic species are restricted to a specific area or environment.

Cosmopolitan species are most useful in correlation

Pitfalls of Correlating with Fossils Pitfalls of 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

Index Fossils 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. Widely distributed (cosmopolitan)

3. Short geologic range (rapid evolution)

Biostratigraphic ZonesBiostratigraphic Zones

Biozone = A body of rock deposited during the time when a particular fossil organism existed.

A biozone is identified only on the basis of the fossils it contains.

Biozones are the basic unit for biostratigraphic classification and correlation.

Fossils Indicate Past EnvironmentsFossils Indicate Past Environments

Ecology and Paleoecology Ecology and Paleoecology

1. Ecology = Interrelationship between organisms and their environment.

2. Paleoecology = Ancient ecology; interaction of ancient organisms with their environment. Depends on comparisons of ancient and living organisms (modern analogs).

3. Ecosystem = Organisms and their environment - the entire system of physical, chemical, and biological factors influencing organisms.

Ecology and PaleoecologyEcology and Paleoecology

4. Habitat = Environment in which an organism lives.

5. Niche = Way in which the organism lives; its role or lifestyle.

6. Community = Association of several species of organisms in a particular habitat (living part of ecosystem).

7. Paleocommunity = An ancient community.

Marine Ecosystem 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

Marine EcosystemMarine Ecosystem

Pelagic realm

• Neritic zone = The water overlying the continental shelves.

• Oceanic zone = The water seaward of the continental shelves.


Benthic realm • Supratidal zone = Above high tide line • Littoral zone (or intertidal zone) = Between high

and low tide lines • Sublittoral zone (or subtidal zone) = Low tide line

to edge of continental shelf (about 200 m deep) • Bathyal zone - 200 - 4000 m deep• Abyssal zone - 4000 - 6000 m deep • Hadal zone - >6000 m deep; deep sea trenches.

Modes of Life of Marine AnimalsModes of Life of Marine Animals

Plankton - Small plants and animals that float, drift, or swim weakly.

• Phytoplankton - Plants and plant-like plankton, such as diatoms and coccolithophores

• Zooplankton - Animals and animal-like plankton, such as foraminifera and radiolaria

Modes of Life of Marine AnimalsModes of Life of Marine Animals

Nekton - Swimming animals that live within the water column

Benthic organisms or benthos - Bottom dwellers, which 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 SedimentsMarine Sediments

• Terrigenous sediment - from weathered rocks• Biogenous sediment – of biological origin

– Calcareous oozes - foraminifera, pteropods, and coccolithophores

– Siliceous oozes - radiolarians and diatoms – Phosphatic material – fish bones, teeth and

scales • Hydrogenous sediment - precipitated from sea

water - manganese nodules

Carbonate Compensation DepthCarbonate Compensation Depth

A depth in the oceans (about 4000-5000 m), which affects where calcareous oozes can accumulate.

Above the CCD (shallower than 4000-5000 m), the water is warmer, and CaCO3 is precipitated. Calcareous sediments (chalk or limestone) are deposited.

Carbonate Compensation DepthCarbonate Compensation Depth

Below the CCD (below about 4000-5000 m), water is colder, and CaCO3 dissolves. Clay or siliceous sediments are deposited.

Use of Fossils in Reconstructing Use of Fossils in Reconstructing Ancient Geography Ancient Geography

Environmental limitations control the distribution of modern plants and animals.

• Note locations of fossil species of the same age on a map

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

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

Land Bridges, Isolation and MigrationLand Bridges, Isolation and Migration

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

• Former land bridge(Bering Strait)

• Mountain barriers• Former ocean barriers between continents

Species Diversity and Geography Species Diversity and Geography

• High latitudes have low species diversity

• Low latitudes have high species diversity.

Species diversity is related to geographic location, particularly latitude.

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

Use of Fossils in the Interpretation Use of Fossils in the Interpretation of Ancient Climatic Conditions 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 Use of Fossils in the Interpretation of Ancient Climatic Conditionsof Ancient Climatic Conditions

4. Marine molluscs with spines and thick shells inhabit warm seas

5. Planktonic foraminifera vary in size and coiling direction with temperature

6. Shells in warmer waters have higher Mg contents

7. Oxygen isotope ratios in shells.

Overview of the History of Life Overview of the History of Life

Oldest evidence of life Oldest evidence of life

Remains of prokaryotic cells (blue-green algae or cyanobacteria) more than 3.5 billion years old. Found in algal mats and stromatolites.

Earliest Metazoan OrganismsEarliest Metazoan Organisms

Metazoans = multicellular organisms

• Trace fossils of metazoans about 1 billion years ago

• First body fossils of soft-bodied metazoans (worms, jellyfish, and arthropods) about 0.7 billion years ago

• Invertebrates with hard parts appeared in late Proterozoic or early Paleozoic.

Geologic ranges and relative Geologic ranges and relative abundances of fossil organismsabundances of fossil organisms

Early Paleozoic – Cambrian PeriodEarly Paleozoic – Cambrian Period

• Most animals were deposit and suspension feeders

• Trilobites • Brachiopods without hinged shells (inarticulates) • Small cap-shaped molluscs • Soft-bodied worms • Chitin-shelled arthropods • Reef-building archaeocyathids

Later in the Paleozoic Era Later in the Paleozoic Era

• Trilobites • Articulate (hinged)

brachiopods • Nautiloids • Crinoids • Rugose (horn) corals• Tabulate corals

• Branching twig-like bryozoans (moss animals)

• Vertebrates – Fishes – Amphibians – Reptiles

Mesozoic Era Mesozoic Era

• Modern scleractinian corals

• Bivalves• Sea urchins • Ammonoids

• Vertebrates – Dinosaurs – Primitive mammals – Birds

Cenozoic Era Cenozoic Era • Molluscs of many types (but no ammonoids) • Planktonic foraminifera • Sea urchins • Encrusting bryozoans • Barnacles • Vertebrates

– Age of mammals– Appearance of humans – Many other vertebrate groups

Extinctions Extinctions

Mass extinctions occurred at the ends of the following periods:

• Ordovician • Devonian - roughly 70% of marine invertebrates

extinct• Permian - the greatest extinction. More than

90% of marine species disappeared or nearly went extinct

• Triassic • Cretaceous - affected dinosaurs, other land

animals, and marine organisms; about 25% of all known animal families extinct

Evolutionary History of PlantsEvolutionary History of Plants

Evolutionary History of PlantsEvolutionary History of Plants

1. Earliest photosynthetic organisms were single-celled organisms in the Precambrian.

2. Green algae or chlorophytes may be the ancestors of vascular land plants.

3. Plants invaded the land in the Ordovician, reproducing with spores.

Evolutionary History of Plants – Evolutionary History of Plants – cont’dcont’d

4. First plants with seeds appeared in the Devonian. Gymnosperms (such as conifers). Had pollen.

5. Carboniferous coal swamps dominated by seedless, spore-bearing scale trees.

6. Flowering plants appeared in the Cretaceous. Angiosperms. Dominant plants today.

chapter 6 life on earth: what do fossils reveal?

Documents

fossils fossils

fossil preservation

ichnofossilstrace fossils

fossils reveal

species names

preservable parts

shale imprints of hard

traces of organisms