27.4C: Post-Cambrian Evolution and Mass Extinctions - Biology

The post-Cambrian era was characterized by animal evolution and diversity where mass extinctions were followed by adaptive radiations.

Learning Objectives

  • Differentiate among the causes of mass extinctions and their effects on animal life

Key Points

  • During the Ordovician period, plant life first appeared on land, which allowed aquatic animals to move on to land.
  • Periods of mass extinction caused by cataclysmic events like volcanic eruptions and meteor strikes have erased many genetic lines and created room for new species.
  • The largest mass extinction event in earth’s history, which occurred at the end of the Permian period, resulted in a loss of roughly 95 percent of the existing species at that time.
  • The disappearance of some dominant species of Permian reptiles and the warm and stable climate that followed made it possible for the dinosaurs to emerge and diversify.
  • Another mass extinction event caused by a meteor strike and volcanic ash eruption occurred at the end of the Cretaceous period, bringing the Mesozoic Era to an end and pushing dinosaurs into extinction.
  • The disappearance of dinosaurs led to the dominance of plants, which created new niches for birds, insects, and mammals; animal diversity was also brought on by the creation of continents, islands, and mountains.

Key Terms

  • Cenozoic: a geologic era about between 65 million years ago to the present when the continents moved to their current position and modern plants and animals evolved
  • mass extinction: a sharp decrease in the total number of species in a relatively short period of time
  • Cretaceous: the last geologic period within the Mesozoic era from about 146 to 65 million years ago; ended with a large mass extinction

Post-Cambrian Evolution and Mass Extinctions

The periods that followed the Cambrian during the Paleozoic Era were marked by further animal evolution and the emergence of many new orders, families, and species. As animal phyla continued to diversify, new species adapted to new ecological niches. During the Ordovician period, which followed the Cambrian period, plant life first appeared on land. This change allowed formerly-aquatic animal species to invade land, feeding directly on plants or decaying vegetation. Continual changes in temperature and moisture throughout the remainder of the Paleozoic Era due to continental plate movements encouraged the development of new adaptations to terrestrial existence in animals, such as limbs in amphibians and epidermal scales in reptiles.

Changes in the environment often create new niches (living spaces) that contribute to rapid speciation and increased diversity. On the other hand, cataclysmic events, such as volcanic eruptions and meteor strikes that obliterate life, can result in devastating losses of diversity. Such periods of mass extinction have occurred repeatedly in the evolutionary record of life, erasing some genetic lines while creating room for others to evolve into the empty niches left behind. The end of the Permian period (and the Paleozoic Era) was marked by the largest mass extinction event in Earth’s history, a loss of roughly 95 percent of the extant species at that time. Some of the dominant phyla in the world’s oceans, such as the trilobites, disappeared completely. On land, the disappearance of some dominant species of Permian reptiles made it possible for a new line of reptiles to emerge: the dinosaurs. The warm and stable climatic conditions of the ensuing Mesozoic Era promoted an explosive diversification of dinosaurs into every conceivable niche in land, air, and water. Plants, too, radiated into new landscapes and empty niches, creating complex communities of producers and consumers, some of which became extremely large on the abundant food available.

Another mass extinction event occurred at the end of the Cretaceous period, bringing the Mesozoic Era to an end. Skies darkened and temperatures fell as a large meteor impact expelled tons of volcanic ash, blocking incoming sunlight. Plants died, herbivores and carnivores starved, and the mostly cold-blooded dinosaurs ceded their dominance of the landscape to more warm-blooded mammals. In the following Cenozoic Era, mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs. Birds, the warm-blooded offshoots of one line of the ruling reptiles, became aerial specialists. The appearance and dominance of flowering plants in the Cenozoic Era created new niches for insects, as well as for birds and mammals. Changes in animal species diversity during the late Cretaceous and early Cenozoic were also promoted by a dramatic shift in earth’s geography, as continental plates slid over the crust into their current positions, leaving some animal groups isolated on islands and continents or separated by mountain ranges or inland seas from other competitors. Early in the Cenozoic, new ecosystems appeared, with the evolution of grasses and coral reefs. Late in the Cenozoic, further extinctions followed by speciation occurred during ice ages that covered high latitudes with ice and then retreated, leaving new open spaces for colonization.

Biology (Browse)

Dehydration Synthesis
Benefits of Carbohydrates
Registered Dietitian
Fats and Oils
Omega Fatty Acids
Types and Functions of Proteins
Amino Acids
Primary Structure of Protein
Secondary Structure of Protein
Tertiary Structure of Protein
Denaturation and Protein Folding
DNA Double-Helix Structure
Trans Fat
The Evolutionary Significance of Cytochrome c

Light Microscopes
Electron Microscopes
Components of Prokaryotic Cells
Cell Size
The Plasma Membrane
Rough Endoplasmic Reticulum (RER)
The Golgi Apparatus
Flagella and Cilia
Extracellular Matrix of Animal Cells
Intercellular Junctions
The Nucleus
Animal Cells versus Plant Cells
Intermediate Filaments

Structure and Function of Plasma Membrane

Fluid Mosaic Model
How Viruses Infect Specific Organs
Selective Permeability
Channel Proteins
Carrier Protein
Tonicity in Living Systems
Electrochemical Gradient
Primary Active Transport
Secondary Active Transport (Co-transport)
Membrane Fluidity
Receptor-mediated Endocytosis

Carbohydrate Metabolism
Metabolic Pathways
Anabolic and Catabolic Pathways
Free Energy
Energy Types
Endergonic Reactions and Exergonic Reactions
Activation Energy
The First Law of Thermodynamics
The Second Law of Thermodynamics
ATP: Adenosine Triphosphate
Molecular Regulation of Enzymes
Drug Discovery
Feedback Inhibition in Metabolic Pathways
Induced Fit and Enzyme Function

Electron Carriers
ATP Structure and Function
First Half of Glycolysis (Energy-Requiring Steps)
Second Half of Glycolysis (Energy-Releasing Steps)
Outcomes of Glycolysis
Breakdown of Pyruvate
Citric Acid Cycle
Electron Transport Chain
Lactic Acid Fermentation
Alcohol Fermentation
Carbohydrate, Protein, Lipid Metabolic Pathways
Pathways of Photosynthesis and Cellular Metabolism
Regulatory Mechanisms of Cellular Respiration
Main Structures and Summary of Photosynthesis

Basic Photosynthetic Structures
The Two Parts of Photosynthesis
Photosynthesis at the Grocery Store
Understanding Pigments
How Light-Dependent Reactions Work
The Calvin Cycle
Evolution of Photosynthesis
Absorption of Light
The Energy Cycle

Forms of Chemical Signaling
Intracellular or Cytoplasmic Receptors
Cell-Surface Receptors
Signaling Molecules
Binding Initiates a Signaling Pathway
Methods of Intracellular Signaling
Gene Expression
Cell Death
Cellular Communication in Yeasts

Genomic DNA
Eukaryotic Chromosomal Structure and Compaction
The Cell Cycle
Regulation at Internal Checkpoints in Cell Cycle
Positive Regulation of the Cell Cycle
Negative Regulation of the Cell Cycle
Tumor Suppressor Genes
Binary Fission
Mitotic Spindle Apparatus

Meiosis and Sexual Reproduction

Meiosis I
Meiosis II
Comparing Meiosis and Mitosis
Life Cycles of Sexually Reproducing Organisms

Mendel’s Experiments and Heredity

Mendelian Crosses
Garden Pea Characteristics Revealed the Basics of Heredity
Who is the Father of Genetics?
Phenotypes and Genotypes
The Punnett Square Approach for a Monohybrid Cross
The Multiple Alleles
Multiple Alleles Confer Drug Resistance in the Malaria Parasite
The X-Linked Traits
The Product Rule and Sum Rule in Genetics
The Test Cross Distinguishes the Dominant Phenotype
Mendel’s Law of Independent Assortment
Mendel’s Forked-Line Method
Linked Genes Violate the Law of Independent Assortment

Modern Understanding of Inheritance

The Genetic Maps
Chromosome Identification
Karyograms to Identify Chromosomal Aberrations
Chromosome Number Disorders
Sex Chromosome Nondisjunction in Humans
The Chromosome 18 Inversion

DNA Structure and Function

Historical Basis of DNA’s Modern Understanding
DNA Structure and Sequencing
DNA Sequencing Techniques
Neanderthal Genome: How Are We Related?
DNA Packaging in Cells
Basics of DNA Replication
DNA Replication in Prokaryotes
The DNA Replication in Eukaryotes
Telomere Replication
The Central Dogma: DNA Encodes RNA RNA Encodes Protein
The Genetic Code Is Degenerate and Universal
Prokaryotic Transcription
Which Has More DNA: Kiwi or Strawberry?
Initiation of Transcription in Prokaryotes
Prokaryotic Termination Signals
The Three Eukaryotic RNA Polymerases
The Three Eukaryotic RNA Polymerases
RNA Polymerase II Promoters and Transcription Factors
The mRNA Processing
DNA Repair
Processing of tRNAs and rRNAs
Ribosomes and Protein Synthesis
The Mechanism of Protein Synthesis

Prokaryotic versus Eukaryotic Gene Expression
The trp Operon: A Repressible Operon
Catabolite Activator Protein (CAP): A Transcriptional Activator
The lac Operon: An Inducible Operon
Epigenetic Control
The Promoter and the Transcription Machinery
Eukaryotic Enhancers and Transcription
Alternative RNA Splicing
Control of RNA Stability
Chemical Modifications, Protein Activity, and Longevity
Cancer and Gene Regulation

Biotechnology and Genomics

DNA and RNA Extraction
Nucleic Acid Fragment Amplification by Polymerase Chain Reaction
The Molecular Cloning
Genetic Diagnosis and Gene Therapy
Production of Vaccines, Antibiotics, and Hormones
Transgenic Plants
Transformation of Plants Using Agrobacterium tumefaciens
The Genetic Maps
Whole-Genome Sequencing
Applying Genomics
Genomics and Proteomics
Basic Techniques in Protein Analysis
Cancer Proteomics
Reproductive Cloning

Evolution and the Origin of Species

Charles Darwin and Natural Selection
Field Biologist
Processes and Patterns of Evolution
Evidence of Evolution
Misconceptions of Evolution
Species and the Ability to Reproduce
What is Speciation?
Allopatric Speciation
Adaptive Radiation
Sympatric Speciation
Reproductive Isolation
Habitat Influence on Speciation
Varying Rates of Speciation
Reconnection After Speciation

The Evolution of Populations

Population Genetics
Hardy-Weinberg Principle of Equilibrium
Genetic Variance
Genetic Drift
Environmental Variance
Frequency-Dependent Selection
What is Sexual Selection?

Phylogenies and the History of Life

Phylogenetic Trees
Limitations of Phylogenetic Trees
Taxonomic Classification Levels
Misleading Appearances Among Organisms
Why Does Phylogeny Matter?
Building Phylogenetic Trees
Horizontal Gene Transfer in Prokaryotes
Horizontal Gene Transfer in Eukaryotes
Genome Fusion and Eukaryote Evolution
Phylogenetic Ring of Life Models

Discovery and Detection of Virus
Evolution of Viruses
Viral Morphology
Virus Past Systems of Classification
The Baltimore Classification System
Steps of Virus Infections
Plant Viruses
Animal Viruses
Vaccines and Antiviral Drugs for Treatment
The Prions

Prokaryotes: Bacteria and Archaea

Microbial Mats
Microbes Are Adaptable
Unculturable Prokaryotes
The Ecology of Biofilms
The Prokaryotic Cell
The Cell Wall of Prokaryotes
The Reproduction in Prokaryotes
The Evolution of Prokaryotes
The Needs of Prokaryotes
Prokaryotes and the Carbon Cycle
Prokaryotes and the Nitrogen Cycle
The Bubonic Plagues
Emerging and Re-emerging Diseases
Biofilms and Disease
Antibiotics: Are We Facing a Crisis?
Methicillin-resistant Staphylococcus aureus (MRSA)
Microbes on the Human Body
Early Biotechnology: Cheese, Bread, Wine, Beer, and Yogurt
Cooperation between Bacteria and Eukaryotes: Nitrogen Fixation
The Foodborne Diseases
Using Prokaryotes to Clean up Our Planet: Bioremediation

Characteristics of Eukaryotes
The Mitochondria
Secondary Endosymbiosis in Chlorarachniophytes
Protists Metabolism
Protist Life Cycles
Green Algae: Chlorophytes and Charophytes
Slime Molds
What is Opisthokonta?
The Rhizaria
Alveolates: Dinoflagellates, Apicomplexians, and Ciliates
The Excavata
The Primary Producers of Food
The Plasmodium Species
Plant Parasites

Fungi Cell Structure and Function
Fungi Growth
Fungi Nutrition
Fungi Reproduction
Fungi Asexual Reproduction
Fungi Sexual Reproduction
Chytridiomycota: The Chytrids
Zygomycota: The Conjugated Fungi
Ascomycota: The Sac Fungi
Basidiomycota: The Club Fungi
Asexual Ascomycota and Basidiomycota
Fungi as Decomposers and Recyclers
Fungus/Plant Mutualism
Coevolution of Land Plants and Mycorrhizae
Fungus/Animal Mutualism
Plant Parasites and Pathogens
Animal and Human Parasites and Pathogens
The Importance of Fungi in Human Life
Dutch Elm Disease

Algae and Evolutionary Paths to Photosynthesis
Plant Adaptations to Life on Land
Alternation of Plant Generations
Sporangia in Seedless Plants
Evolution of Land Plants
Vascular Tissue: Xylem and Phloem
Phylum Lycopodiophyta: Club Mosses
Phylum Monilophyta: Class Equisetopsida (Horsetails)
Phylum Monilophyta: Class Polypodiopsida (True Ferns)
Landscape Designer
The Importance of Seedless Plants

The Evolution of Gymnosperms
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
The Evolution of Angiosperms
Flowers and Fruits as an Evolutionary Adaptation
Building Phylogenetic Trees
The Life Cycle of a Conifer
The Diversity of Gymnosperms
The Life Cycle of an Angiosperm
The Diversity of Angiosperms
The Flowers
The Fruit
Animals and Plants: Herbivory
Animals and Plants: Pollination
Testing Attraction of Flies by Rotting Flesh Smell
The Importance of Seed Plants in Human Life
Biodiversity of Plants

Introduction to Animal Diversity

Complex Tissue Structure of Animals
Processes of Animal Reproduction and Embryonic Development
The Role of Homeobox (Hox) Genes in Animal Development
Animal Characterization: Body Symmetry
Animal Characterization: Embryological Development
Presence or Absence of a Coelom
Embryonic Development of the Animal’s Mouth
The Evolution of the Coelom
Constructing an Animal Phylogenetic Tree
Modern Advances in Phylogenetic Understanding
The Cambrian Explosion of Animal Life
The Pre-Cambrian Animal Life
Post-Cambrian Evolution and Mass Extinctions

The Morphology of Sponges
Physiological Processes in Sponges
Phylum Cnidaria
The Class Anthozoa
The Class Scyphozoa
The Class Cubozoa
The Class Hydrozoa
Phylum Platyhelminthes
Physiological Processes of Flatworms
Diversity of Flatworms
Phylum Rotifera
Phylum Nemertea
Phylum Mollusca
Classification of Phylum Mollusca
Phylum Annelida
Classification of Phylum Annelida
Phylum Nematoda
Phylum Tardigrada
Phylum Arthropoda
Phylum Arthropoda Morphology
Subphylum Chelicerata
Subphylum Myriapoda
Subphylum Crustacea
Subphylum Hexapoda
Superphylum Deuterostomia
Phylum Echinodermata
C. elegans: Linking Developmental Studies
Parasitic Nematodes
Classes of Echinoderms

The Characteristics of Chordata
Subphylum Vertebrata (Craniata)
Jawless Fishes: Superclass Agnatha
Class Myxini: Hagfishes
Class Petromyzontida: Lampreys
Gnathostomes: Jawed Fishes
Class Chondrichthyes: Cartilaginous Fishes
Osteichthyes: Bony Fishes
Characteristics of Amphibians
Evolution of Amphibians
Urodela: Salamanders
Anura: Frogs
The Characteristics of Amniotes
The Paleozoic Era and the Evolution of Vertebrates
The Evolution of Amniotes
The Characteristics of Reptiles
The Evolution of Reptiles
Archosaurs: Dinosaurs
Archosaurs: Pterosaurs
The Characteristics of Birds
The Evolution of Birds
The Evolution of Flight in Birds
The Characteristics of Mammals
The Evolution of Mammals
The Living Mammals
The Characteristics of Primates
The Evolution of Primates
Human Evolution
The Very Early Hominins
Early Hominins: Genus Homo
Early Hominins: Genus Australopithecus
Humans: Homo sapiens

Plant Form and Physiology

The Plant Stem
The Plant Tissues
The Plant’s Stem Anatomy
Growth in Plant Stems
The Plant Vascular Tissue
Plant Stem Modifications
Plant Root Growth and Anatomy
Plant Root Modifications
The Structure of a Typical Leaf
The Leaf Structure and Function
Plant Adaptations in Resource-Deficient Environments
Water Potential of Plants
Solute Potential of Plants
Pressure Potential of Plants
Movement of Water and Minerals in the Xylem
Control of Transpiration in Plants
Transportation of Photosynthates in the Phloem
Translocation in Plants: Transport from Source to Sink
Plant Responses to Light
The Phytochrome System and the Red/Far-Red Response
The Blue Light Responses
Plant Responses to Gravity
Plant Growth Responses
Plant Responses to Wind and Touch
Defense Responses against Herbivores and Pathogens

The Chemical Composition of Plants
Plant Macronutrients and Micronutrients
Soil Composition
Physical Properties of the Soil
Soil Scientist
Nitrogen Fixation: Root and Bacteria Interactions
Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
Plant Nutrients from Other Sources

The Flower Structure
Male Gametophyte (The Pollen Grain)
Female Gametophyte (The Embryo Sac)
Sexual Reproduction in Gymnosperms
Pollination by Insects
Pollination by Deception
Double Fertilization in Plants
Development of the Plant Seed
Seed Germination
Development of Fruit and Fruit Types
Fruit and Seed Dispersal
Asexual Reproduction
Artificial Methods of Asexual Reproduction
Plant Life Spans

The Animal Body: Basic Form and Function

Physical Anthropologist
The Limits on Animal Size and Shape
Animal Bioenergetics
Animal Body Planes and Cavities
Animal Epithelial Tissues
Animal Connective Tissues
The Animal Cartilage
Animal Bones
Animal Blood
Animal Muscle Tissues
Animal Endotherms and Ectotherms
Animal Heat Conservation and Dissipation
Neural Control of Thermoregulation

Animal Nutrition and the Digestive System

Herbivores, Omnivores, and Carnivores
Invertebrate Digestive Systems
Vertebrate Digestive Systems
The Oral Cavity
Small Intestine
Accessory Organs of the Digestive System
Organic Precursors
Essential Nutrients
Food Energy and ATP
The Digestion of Carbohydrates
Lipid Digestion
Protein Digestion
The Digestive Phases
Hormonal Responses to Food

The Parts of a Neuron
Types of Neurons
Neuron Resting Membrane Potential
Neuron Action Potential
Synaptic Transmission
Brain-Computer Interface
Synaptic Plasticity
The Central Nervous System
Spinal Cord
Sympathetic Nervous System
Parasympathetic Nervous System
Sensory-Somatic Nervous System
Alzheimer’s Disease
Parkinson’s Disease
Attention Deficit Hyperactivity Disorder (ADHD)
Mental Illnesses

Transduction Function of the Sensory System
Somatosensory Receptors
Tastes and Odors
Reception and Transduction
Transduction of Sound
Vestibular Information
Anatomy of the Eye
Transduction of Light
Trichromatic Coding
Retinal Processing
Vision Higher Processing

Intracellular Hormone Receptors
Plasma Membrane Hormone Receptors
Hormonal Regulation of the Excretory System
The Dangers of Synthetic Hormones
Regulation of the Female Reproductive System
Regulation of Blood Glucose Levels by Thyroid Hormones
Hormonal Control of Blood Calcium Levels
Hormonal Regulation of Growth
Fight-or-Flight Response
Hormonal Regulation of Stress
Regulation of Hormone Production
Hypothalamic-Pituitary Axis
Anterior Pituitary Gland
Posterior Pituitary Gland
Thyroid Gland
Parathyroid Glands
Adrenal Glands
Organs with Secondary Endocrine Functions

The Musculuskeletal System

Hydrostatic Skeleton
The Skull
The Vertebral Column
The Thoracic Cage
The Pectoral Girdle
The Upper Limb
The Pelvic Girdle
The Lower Limb
Evolution of Body Design for Locomotion on Land
Compact Bone Tissue
Spongy Bone Tissue
Endochondral Ossification
Bone Remodeling and Repair
Fibrous Joints
The Movement at Synovial Joints
Types of Synovial Joints
Types of Muscle Tissue
Skeletal Muscle Fiber Structure
Sliding Filament Model of Contraction
ATP and Muscle Contraction
Excitation–Contraction Coupling
Control of Muscle Tension

Animal Gills
Insect Tracheal Systems
Mammalian Systems
Lungs: Bronchi and Alveoli
Respiratory System Protective Mechanisms
Gas Pressure and Respiration
Lung Volumes and Capacities
Gas Exchange Across the Alveoli
Avian Respiration
The Mechanics of Human Breathing
The Work of Breathing
Lung Resistance and Compliance
Dead Space: V/Q Mismatch
Factors That Affect Oxygen Binding
Transport of Carbon Dioxide in the Blood
Carbon Monoxide Poisoning

Circulatory System Architecture
Circulatory System Variation in Animals
Red Blood Cells
White Blood Cells
Platelets and Coagulation Factors
Blood Types Related to Proteins
The Cardiac Cycle
Arteries, Veins, and Capillaries
How Blood Flows Through the Body
Plasma and Serum
Structure of the Heart
Blood Circulation Diversity in Vertebrates
Blood Pressure

Osmotic Regulation and Excretion

The Need for Osmoregulation
Transport of Electrolytes Across Cell Membranes
Osmoregulators and Osmoconformers
Dialysis Technician
Kidney Structure
Kidney Function and Physiology
Kidney Tubular Re-absorption and Secretion
Flame Cells of Planaria and Nephridia of Worms
Malpighian Tubules of Insects
Nitrogenous Waste in Terrestrial Animals: The Urea Cycle

Body Physical and Chemical Barriers
Pathogen Recognition
Cytokine Release Effect
Phagocytosis and Inflammation
Natural Killer Cells
The Complement System
Antigen-Presenting Cells
B Lymphocytes
T and B Lymphocytes
Helper T Lymphocytes
Cytotoxic T Lymphocytes
Mucosal Surfaces and Immune Tolerance
Immunological Memory
Primary Centers of the Immune System
The Antibody Structure
The Antibody Classes
Antibody Functions
Affinity, Avidity, and Cross Reactivity

Animal Reproduction and Development

Asexual Reproduction
Sexual Reproduction
Animal External Fertilization
Animal Internal Fertilization
The Evolution of Reproduction
Male Reproductive Anatomy
Female Reproductive Anatomy
Sexual Response During Intercourse
Male Hormones
Female Hormones
The Ovarian Cycle and the Menstrual Cycle
Reproductive Endocrinologist
Human Gestation
Labor and Birth
Contraception and Birth Control
Human Fertilization
Cleavage and Blastula Stage
Are Designer Babies in Our Future?
Vertebrate Axis Formation

Levels of Ecological Study
Organismal Ecology
Population Ecology
Community Ecology
Ecosystem Ecology
Energy Sources
The Effect of Temperature on Living Organisms
The Importance of Water to the Terrestrial Organism
Abiotic Factors Influencing Plant Growth
Tropical Wet Forest
Subtropical Deserts
Temperate Grasslands
Temperate Forests
Boreal Forests
Arctic Tundra
Abiotic Factors Influencing Aquatic Biomes
Coral Reefs
Global Decline of Coral Reefs
Estuaries: Where the Ocean Meets Fresh Water
Lakes and Ponds
Rivers and Streams
Climate and Weather
Current and Past Drivers of Global Climate Change

Population and Community Ecology

Population Size and Density
Population Research Methods
Species Distribution
Parental Care and Fecundity
The Survivorship Curves
Single versus Multiple Reproductive Events
Drosophila Research
Population Exponential Growth
Carrying Capacity and the Logistic Model
Density-Dependent Regulation
Why Did the Woolly Mammoth Go Extinct?
Life Histories of K-selected and r-selected Species
Human Population Growth
Age Structure, Population Growth, and Economic Development
Consequences Human Population Growth
Predation and Herbivory
Defense Mechanisms Against Predation and Herbivory
Competitive Exclusion Principle
Characteristics of Communities
Invasive Species
Community Dynamics
Innate Behaviors: Movement and Migration
Communication within a Species
Altruistic Behaviors
Finding Sex Partners in Animal Kingdom
Conditioned Behavior
Cognitive Learning

The Ecology of Ecosystems
Food Chains and Food Webs
Three-Spined Stickleback
Research into Ecosystem Dynamics
How Organisms Acquire Energy in a Food Web
Productivity Within Trophic Levels
The Transfer of Energy Between Trophic Levels
Modeling Ecosystems Energy Flow: Ecological Pyramids
Consequences of Food Webs: Biological Magnification
The Biogeochemical Cycles
The Water (Hydrologic) Cycle
The Carbon Cycle
The Nitrogen Cycle
The Phosphorus Cycle
The Chesapeake Bay
The Sulfur Cycle

Conservation Biology and Biodiversity

The Biodiversity Crisis
Types of Biodiversity
Current Species Diversity
Patterns of Biodiversity
Biodiversity Change Through Geological Time
The Five Mass Extinctions
The Pleistocene Extinction
Estimates of Present-Time Extinction Rates
The Holocene Extinction
Human Health
The Agricultural Diversity
Wild Food Sources
Animal Habitat Loss
Preventing Habitat Destruction with Wise Wood Choices
Overharvesting of Resources
Exotic Species
Climate Change
Measuring Biodiversity
Changing Human Behavior
Conservation of Wildlife and Ecosystem Preserves
Habitat Restoration
The Role of Captive Breeding

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The Cambrian Explosion of Animal Life

The Cambrian period, occurring between approximately 542–488 million years ago, marks the most rapid evolution of new animal phyla and animal diversity in Earth’s history. It is believed that most of the animal phyla in existence today had their origins during this time, often referred to as the Cambrian explosion ([link]). Echinoderms, mollusks, worms, arthropods, and chordates arose during this period. One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among the first animals to exhibit a sense of vision ([link]abcd).

An artist’s rendition depicts some organisms from the Cambrian period. These fossils (a–d) belong to trilobites, extinct arthropods that appeared in the early Cambrian period, 525 million years ago, and disappeared from the fossil record during a mass extinction at the end of the Permian period, about 250 million years ago.

The cause of the Cambrian explosion is still debated. There are many theories that attempt to answer this question. Environmental changes may have created a more suitable environment for animal life. Examples of these changes include rising atmospheric oxygen levels and large increases in oceanic calcium concentrations that preceded the Cambrian period ([link]). Some scientists believe that an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space for larger numbers of different types of animals to co-exist. There is also support for theories that argue that ecological relationships between species, such as changes in the food web, competition for food and space, and predator-prey relationships, were primed to promote a sudden massive coevolution of species. Yet other theories claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility and complexity of animal development afforded by the evolution of Hox control genes may have provided the necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period. Theories that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons for the massive animal diversification, as well as explain why it happened when it did. There is evidence that both supports and refutes each of the theories described above, and the answer may very well be a combination of these and other theories.

The oxygen concentration in Earth’s atmosphere rose sharply around 300 million years ago.

However, unresolved questions about the animal diversification that took place during the Cambrian period remain. For example, we do not understand how the evolution of so many species occurred in such a short period of time. Was there really an “explosion” of life at this particular time? Some scientists question the validity of the this idea, because there is increasing evidence to suggest that more animal life existed prior to the Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as well. Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian period continued well into the following Ordovician period. Despite some of these arguments, most scientists agree that the Cambrian period marked a time of impressively rapid animal evolution and diversification that is unmatched elsewhere during history.

View an animation of what ocean life may have been like during the Cambrian explosion.


* Synapomorphic traits are those which have appeared only in the last common ancestor for the first time, and not in more primitive organisms. This helps researchers find out which ancestral organism first evolved a particular character, that is now seen in different species or populations. It helps them establish evolutionary relationships between different groups of organisms, like birds, reptiles, and mammals, that show similar properties.

* The category of 'land plants' includes different groups, like the Coleochaete (a type of algae), liverworts (flowerless plants), conifers, and angiosperms (flowering plants). Despite this, only liverworts, conifers, and angiosperms show a multicellular sporophyte (spore-producing stage), while the Coleochaete does not. This is a synapomorphy passed down to them by the last ancestor that they shared.

* The genus Homo includes all great apes, such as Homo erectus (upright man), Homo neanderthalensis (neanderthals), and Homo sapiens (modern man). Despite the differences between them, they all shared the similar property of having large brain-cases, indicating their higher intelligence. This is because, their most recent and common ancestor―Australopithecus―evolved this trait for the first time, and passed it down to them. Of these species, we are the only ones still surviving today.

* Hypothesis 1 requires six evolutionary changes and Hypothesis 2 requires seven evolutionary changes, with a bony skeleton evolving independently, twice. Although both fit the available data, the parsimony principle says that Hypothesis 1 is better — since it does not hypothesize unnecessarily complicated changes.

* This principle was implicit in the tree-building process we went through earlier with the vertebrate phylogeny. However, in most cases, the data are more complex than those used in our example and may point to several different phylogenetic hypotheses. In those cases, the parsimony principle can help us choose between them.

•Measures of relative nodal support can give some confidence
-Values >70 for bootstrap or jackknife

It is the opposite of divergent evolution, where related species evolve different traits.

On a molecular level, this can happen due to random mutation unrelated to adaptive changes see long branch attraction.

In cultural evolution, convergent evolution is the development of similar cultural adaptations to similar environmental conditions by different peoples with different ancestral cultures.

An example of convergent evolution is the similar nature of the flight/wings of insects, birds, pterosaurs, and bats.

All four serve the same function and are similar in structure, but each evolved independently.

Some aspects of the lens of eyes also evolved independently in various animals.

Convergent evolution is similar to, but distinguishable from, the phenomena of evolutionary relay and parallel evolution.

Evolutionary relay refers to independent species acquiring similar characteristics through their evolution in similar ecosystems, but not at the same time (e.g. dorsal fins of extinct ichthyosaurs and sharks).

Parallel evolution occurs when two independent species evolve together at the same time in the same ecospace and acquire similar characteristics (extinct browsing-horses and extinct paleotheres).

•Duplications WITHIN α and β globin lineages lead to more copies of each

•Each gene product slightly different
-Fetal product has higher affinity for oxygen, etc

•Each subfamily has own trajectory
-β-globin genes monophyletic
-α-globin genes monophyletic

•Orthology: genes are related by common descent
-Can be used to reconstruct an accurate phylogeny for that particular gene

* Orthologs are homologous genes that are the result of a speciation event.

* Paralogs are homologous genes that are the result of a duplication event.

-May be in conflict across different data sets-Biological signal for: hybridization, incomplete lineage sorting, horizontal gene transfer

•Earth Sciences
-geology, paleontology, climatology, limnology-geography and historical geography-plate tectonics, glaciation

•Climatic shifts correlated with continental movements-Drying trends and glaciation

* Wallace had already accepted evolution when he began his travels in 1848 through the Amazon and Southeast Asia. On his journeys, he sought to demonstrate that evolution did indeed take place, by showing how geography affected the ranges of species. He studied hundreds of thousands of animals and plants, carefully noting exactly where he had found them. The patterns he found were compelling evidence for evolution. He was struck, for example, by how rivers and mountain ranges marked the boundaries of many species' ranges. The conventional explanation that species had been created with adaptations to their particular climate made no sense since he could find similar climatic regions with very different animals in them.

* Wallace came to much the same conclusion that Darwin published in the Origin of Species: biogeography was simply a record of inheritance. As species colonized new habitats and their old ranges were divided by mountain ranges or other barriers, they took on the distributions they have today

•Similarity of Southern fauna across continents

•Role of continental drift in vicariance

1) The species evolved independently on separate continents - contradicting Darwin's theory of evolution.
2) They swam to the other continent/s in breeding pairs to establish a second population

* . connects together and renders intelligible a vast number of independent and hitherto unexplained facts. The natural system of arrangement of organic beings, their geographic distribution, their geological sequence.

* Granted the law, and many of the most important facts in Nature could not have been otherwise, but are almost as necessary deductions from it, as are the elliptical orbits of the planets from the law of gravitation.Wallace, 1855

* A species:
1.has a definite site or region of origin
2.achieves a broader distribution by dispersal
3.Becomes modified and gives rise to descendant species in the various regions to which it migrates

* In order to better understand an organism, ecologists try to determine what sort of niche it would have in the absence of such competition. They might ask, 'How would a lion behave if there were no other predators competing for a zebra's flesh?' or 'What would a stand of water lilies look like if there was no duckweed living in the same area of the pond?' A fundamental niche is the term for what an organism's niche would be in the absence of competition from other species.

* The male red-winged blackbird's mating call can be heard in the marshes in early spring. At that time, they hold the prime real estate in the marsh. However, as the season progresses, the more aggressive tri-color blackbirds move in. The tri-colors take over the best territory and force the red-wings to choose the leftovers. The entire marsh represents the red-winged blackbirds' fundamental niche.

* Geological Data (Elevation)

•Islands closer to a large land mass are more species-rich than remote islands.

•Mountainous (high-altitude) islands are more species-rich than low-altitude islands.

ŜSN> ŜSF where L= large, S= small, N= near, F= far

* Model also predicts this relationship among turnover rates:

* For example, flower mites are wingless and so use foraging bees to travel to new flowers. When bees enter a flower to collect nectar or pollen the mites climb on to the bee. The bee then flies to the next flower and the mites climb off the bee.

* The Hawaiian islands were formed as the Pacific Plate moved over a volcanic "hotspot"

* Northern and eastern faces of the islands receive more rain than the southern and western faces

* The evolution of a key adaptation
- A key adaptation usually means an adaptation that allows the organism to evolve to exploit a new niche or resource. A key adaptation may open up many new niches to an organism and provide the opportunity for an adaptive radiation. For example, beetle radiations may have been triggered by adaptations for feeding on flowering plants.

* Release from competition/vacated niches
- Lineages that invade islands may give rise to adaptive radiations because the invaders are free from competition with other species. On the mainland, other species may fill all the possible ecological niches, making it impossible for a lineage to split into new forms and diversify. On an island, however, these niches may be empty. Extinctions can also empty ecological niches and make an adaptive radiation possible. For example, open niches vacated by dinosaur extinctions may have allowed mammals to radiate into these positions in the terrestrial food web.

* 1,100 native plant species are descended from 300 founder species.

*Scientists believe that the entire silversword family probably descended from a member of the sunflower family, similar to Muir's Tarweed, from California. The barbed fruit of this tarweed may have been carried to Hawai`i on the feathers of a bird. Since this tarweed came from an alpine shrubland, it most likely became established in Hawai`i in a similar kind of habitat.

* Since so few organisms successfully colonized Hawai`i, many diverse habitats were available. Over time, descendants of the tarweed slowly adapted to many of these habitats. Beneficial mutations enabled the plant forms to change and become quite different from the ancestor. The end result was extensive and spectacular adaptive radiation.

* Today, plants of the silversword family occupy every terrestrial habitat in Hawai`i--from wet forests to dry forests and from near sea level to alpine shrublands. Although these plants are still closely related (all species are able to hybridize, they often look extremely different from one another.

* The silversword family tree is divided into three genera: Argyroxiphium (5 species), Wilkesia (2 species), and Dubautia (21 species). Dubautia species are the most diverse. All members of the silversword family are endemic to Hawai`i 82% are to a single island. Approximately one half of these single island endemic species are further limited in distribution, often growing only in one area or microclimate. It is a very special `ohana!

Adaptive radiation

Allopatric Speciation

  • Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.
  • This is called adaptiveradiation because many adaptations evolve from a single point of origin, causing the species to radiate into several new ones.
  • Island archipelagos like the Hawaiian Islands provide an ideal context for adaptiveradiation events because water surrounds each island which leads to geographical isolation for many organisms.
  • The Hawaiian honeycreeper illustrates one example of adaptiveradiation.
  • The honeycreeper birds illustrate adaptiveradiation.

Loss of Biodiversity

  • The cichlids of Lake Victoria are the product of an adaptiveradiation.
  • An adaptiveradiation is a rapid (less than three million years in the case of the Lake Victoria cichlids) branching through speciation of a phylogenetic tree into many closely-related species typically, the species "radiate" into different habitats and niches.
  • The Galápagos finches are an example of a modest adaptiveradiation with 15 species.
  • The cichlids of Lake Victoria are an example of a spectacular adaptiveradiation that includes about 500 species.

Post-Cambrian Evolution and Mass Extinctions

  • The post-Cambrian era was characterized by animal evolution and diversity where mass extinctions were followed by adaptiveradiations.
  • As animal phyla continued to diversify, new species adapted to new ecological niches.
  • Continual changes in temperature and moisture throughout the remainder of the Paleozoic Era due to continental plate movements encouraged the development of new adaptations to terrestrial existence in animals, such as limbs in amphibians and epidermal scales in reptiles.
  • Plants, too, radiated into new landscapes and empty niches, creating complex communities of producers and consumers, some of which became extremely large on the abundant food available.
  • In the following Cenozoic Era, mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs.

The Fossil Record and the Evolution of the Modern Horse

  • Early horse ancestors were originally specialized for tropical forests, while modern horses are now adapted to life on drier land.
  • Successive fossils show the evolution of teeth shapes and foot and leg anatomy to a grazing habit with adaptations for escaping predators.
  • The fossil record shows several adaptiveradiations in the horse lineage, which is now much reduced to only one genus, Equus, with several species.
  • The species depicted are only four from a very diverse lineage that contains many branches, dead ends, and adaptiveradiations.
  • One of the trends, depicted here, is the evolutionary tracking of a drying climate and increase in prairie versus forest habitat reflected in forms that are more adapted to grazing and predator escape through running.

Characteristics and Evolution of Primates

  • All primates exhibit adaptations for climbing trees and have evolved into two main groups: Prosimians and Anthropoids.
  • All primate species possess adaptations for climbing trees, as they all descended from tree-dwellers.
  • This arboreal heritage of primates has resulted in adaptations that include, but are not limited to: 1) a rotating shoulder joint 2) a big toe that is widely separated from the other toes and thumbs, that are widely separated from fingers (except humans), which allow for gripping branches and 3) stereoscopic vision, two overlapping fields of vision from the eyes, which allows for the perception of depth and gauging distance.
  • Due to this reproductive isolation, New World monkeys and Old World monkeys underwent separate adaptiveradiations over millions of years.

Extremophiles and Biofilms

  • Prokaryotes are well adapted to living in all types of conditions, including extreme ones, and prefer to live in colonies called biofilms.
  • These adaptations, along with others, allow bacteria to be the most abundant life form in all terrestrial and aquatic ecosystems.
  • Other bacteria and archaea are adapted to grow under extreme conditions and are called extremophiles, meaning "lovers of extremes."
  • Because they have specialized adaptations that allow them to live in extreme conditions, many extremophiles cannot survive in moderate environments.
  • Other extremophiles, like radioresistant organisms, do not prefer an extreme environment (in this case, one with high levels of radiation), but have adapted to survive in it.

Plant Adaptations to Life on Land

  • Plants adapted to the dehydrating land environment through the development of new physical structures and reproductive mechanisms.
  • As organisms adapted to life on land, they had to contend with several challenges in the terrestrial environment.
  • The organism is also subject to bombardment by mutagenic radiation because air does not filter out the ultraviolet rays of sunlight.
  • The most successful adaptation solution was the development of new structures that gave plants the advantage when colonizing new and dry environments.
  • Discuss how lack of water in the terrestrial environment led to significant adaptations in plants

Heat Conservation and Dissipation

  • For example, vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body.
  • Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, thus, warming blood that returns to the heart.
  • This adaption, which can be shut down in some animals to prevent overheating the internal organs, is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds.
  • In contrast, similar adaptations (as in dolphin flukes and elephant ears) can help cool endotherms when needed.

The Origins of Archaea and Bacteria

  • Early earth had a very different atmosphere (contained less molecular oxygen) than it does today and was subjected to strong radiation thus, the first organisms would have flourished where they were more protected, such as in ocean depths or beneath the surface of the earth.
  • It is probable that these first organisms, the first prokaryotes, were adapted to very high temperatures.
  • Early earth was prone to geological upheaval and volcanic eruption, and was subject to bombardment by mutagenic radiation from the sun.
  • The evolution of Archaea in response to antibiotic selection, or any other competitive selective pressure, could also explain their adaptation to extreme environments (such as high temperature or acidity) as the result of a search for unoccupied niches to escape from antibiotic-producing organisms.

Introduction to Light Energy

  • The sun emits an enormous amount of electromagnetic radiation (solar or light energy).
  • The electromagnetic spectrum is the range of all possible frequencies of radiation .
  • Each type of electromagnetic radiation travels at a particular wavelength.
  • The sun emits energy in the form of electromagnetic radiation.
  • All electromagnetic radiation, including visible light, is characterized by its wavelength.
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Cambrian explosion

Once a saturation point was reached for the reactions in rock and water, oxygen was able to exist as a gas in its diatomic form. The burrows opened up new ecological niches beneath the sea floor as water and oxygen could now get into the sediment layers.

Bulletin of the National Museum of Natural Science 10, Characteristics that are compared may be anatomicalsuch as the presence of a notochordor molecularby comparing sequences of DNA or protein.

At the time, Darwin pointed to the imperfection of the fossil record as his only defence, arguing complex animal life must have lived long before the Cambrian, but traces of that life had not yet been found.

Unfortunately for the hypothesis, the last worldwide glaciation seems to have ended around million years ago – nearly 90 million years before the first signs of the Cambrian explosion in the fossil record which was followed by another major regional glaciation around million years ago.

Fossilization is a rare event, and most fossils are destroyed by erosion cambriwnne metamorphism before they can be observed.

The Ordovician environment

During the Ordovician Period, four major continents were present and separated by three major oceans. Although the positions of these continents are frequently updated with new evidence, current understanding of their position is based on paleomagnetic evidence, fossil markers, and climatically sensitive sediments, such as evaporite minerals. The craton (stable interior portion of a continent) of Laurentia—made up of most of present-day North America, Greenland, and part of Scotland—straddled the Equator and was rotated approximately 45° clockwise from its present orientation. The craton made up of Siberian and Kazakhstania (which is also called Siberia-Kazakhstan) lay east of Laurentia, along and slightly north of the Equator. The Iapetus Ocean separated these two landmasses on the south from the Baltica craton, which included present-day Scandinavia and north-central Europe. The microcontinent of Avalonia—made up of England, New England, and maritime Canada—was positioned to the west of Baltica and also faced Laurentia across the Iapetus Ocean. The Paleotethys Sea separated Avalonia, Baltica, and Kazakhstan from the supercontinent of Gondwana, which consisted of Africa, South America, India, Arabia, China, Australia, Antarctica, Western Europe, the southeastern United States, and the Yucatán Peninsula of Mexico. This immense supercontinent straddled both the South Pole, located then in what is now northwest Africa, and the Equator, which then crossed present-day Australia and Antarctica. In this position, Africa and South America were rotated nearly 180° from their present orientation. A single body of water, the Panthalassic Ocean, covered almost the entire Northern Hemisphere and was as wide at the Equator as the modern Pacific Ocean.

Exploring macroevolution using modern and fossil data

Macroevolution, encompassing the deep-time patterns of the origins of modern biodiversity, has been discussed in many contexts. Non-Darwinian models such as macromutations have been proposed as a means of bridging seemingly large gaps in knowledge, or as a means to explain the origin of exquisitely adapted body plans. However, such gaps can be spanned by new fossil finds, and complex, integrated organisms can be shown to have evolved piecemeal. For example, the fossil record between dinosaurs and Archaeopteryx has now filled up with astonishing fossil intermediates that show how the unique plexus of avian adaptations emerged step by step over 60 Myr. New numerical approaches to morphometrics and phylogenetic comparative methods allow palaeontologists and biologists to work together on deep-time questions of evolution, to explore how diversity, morphology and function have changed through time. Patterns are more complex than sometimes expected, with frequent decoupling of species diversity and morphological diversity, pointing to the need for some new generalizations about the processes that lie behind such patterns.

1. Introduction

When George Gaylord Simpson presented his model for adaptive radiations in 1944 [1], he was keenly aware of the difficulties of marrying modern and fossil data. Using examples from vertebrates, he outlined cases where the acquisition of a key adaptation, such as feathers and flight in birds, provided the stimulus for rapid diversification. Some adaptive radiations were triggered by opportunities instigated by external processes, such as a change in climate, or clearing of ecospace by a mass extinction event.

Adaptive radiations are a key element of macroevolution, encapsulating all the broad-scale, deep-time components of the expansion of species numbers, expansion of the range of habitats occupied by life and expansion of the breadth of novel adaptations both in terms of morphology and function. These are patterns of change, and a criticism of macroevolution has sometimes been that there are no models or processes, and it is simply microevolution writ large. There are in fact three models for evolutionary radiations, and these will be explored.

The most important recent advance has been the development of new tools for the investigation of macroevolution in a phylogenetic context. These tools can handle phylogenetic trees based on genomic or morphological data, and can include or exclude fossil taxa. Improvements also in the precision of geological dating, the stability of phylogenies, and care in extracting data from fossils have all helped to answer questions which a few years ago would have seemed beyond the reach of an analytical approach.

The aim of this paper is to explore key questions in macroevolution, and especially to highlight the substantial opportunity for advance at the moment, as methods and data improve massively, and as ways are found to bridge between living and extinct organisms, between biology and palaeobiology.

2. Models for macroevolutionary processes

The three models for macroevolution are broad-scale and all encompassing. They are at a different level from particular models that apply to individual radiations, for example, whether the diversification of mammals after the extinction of the dinosaurs followed an early burst, trend, Brownian motion or other model. The three process models for macroevolution are the ecospace, macromutation and developmental models.

The Simpsonian ecospace model [1,2] includes an early phase of rapid expansion, during which the new adaptation and the new ecological opportunity are tested and explored to the limit (figure 1). Then, in the early phases of the radiation, some marginal forms might die out, under selection, because they have in some way ‘overshot’ the possibilities of the new key adaptation. Available ‘adaptive space’ then causes further sorting of lineages during ‘normal’ times of evolution, after the initial explosion, and numerous initial lineages are weeded down to the really successful ones. In the ecospace model, variations in ecological opportunity control the success of major new morphologies and this produces a pattern that mimics differential introduction of innovations. There are several aspects of this model: radiations are ‘driven’ by key adaptations/innovations, whether they enter previously vacated ecospace or conquer new habitats, and they rapidly expand and overshoot, and then there is extinction/weeding out as many early lineages fail, so leaving gaps in ecospace/morphospace. Under the ecospace model, new species may emerge either into new, unexploited ecospace, or subdivide existing niches by a process of specialiazation.

Figure 1. Simpson's [1] ecospace model for adaptive radiation, showing initial explosive evolution into new sectors of ecospace, followed by extinction of intermediates, and reduction of diversity to those lineages that occupy habitable ecospace (shown by blank areas), located between forbidden ecospace (stippled).

The macromutation model, much discussed in Simpson's day, and occasionally revitalized under different titles (reviewed, [3]), proposes that many clade origins were abrupt and dramatic, and produced by a genetic or genomic revolution. Such ideas of macromutations have attracted attention at times, but I assume that macroevolution is a part of Darwinian evolution, as do most others [3], and that there is probably no need for genetic or developmental revolutions. That is not to say that fundamental genomic reorganizations are not associated with major clades—indeed novelties that characterize particular clades are doubtless associated with the acquisition of unique, and perhaps stable, genomic characters such as transcription factors, their regulatory elements and post-transcriptional regulators such as micro RNAs, but these are associated with changes that characterize the clades, namely phenotypic characters, and are driven by natural selection. For example, the phenotypic characteristics of certain clades may arise through heterochrony, such as the origin of many cranial characters in birds through paedomorphosis [4], and these may be associated with particular regulatory genes. Furthermore, the origin of birds is a good example that was once cited as evidence for macromutation. Surely, it was argued, birds emerged rapidly in geological time, and they were from the start exquisitely well adapted, aerodynamically perfect organisms, and so intermediate stages could not be imagined? However, new evidence rules out the need for genetic revolutions. Palaeontologists at times favoured wholesale, macromutational changes in cases where fossils were absent: however, the phenotypic gap between dinosaurs and birds has been filled with many new fossils since 1995, and these show that the unique plexus of morphological and physiological changes that distinguish birds from dinosaurs and crocodiles, was acquired piecemeal over a span of 60 Myr, from Coelophysis to Archaeopteryx [5].

Finally, the developmental model (or ‘genomic reorganization model’) posits [6–8] a model of macroevolution in which developmental patterning has become increasingly resistant to modification, and so the potential for innovation has reduced through geological time. The proposal that genetic and developmental constraints were less restrictive at the time of the Cambrian Explosion, and then genomic regulatory networks became increasingly established and ‘hardened’ against subsequent change throughout the Phanerozoic, is not borne out by evidence from morphological disparity: major new bauplans have emerged repeatedly through post-Cambrian evolution, and within those bauplans, disparity typically expands rapidly with the origin of each new subclade. Erwin [8] and colleagues subsequently modified their view to suggest that, whereas gene regulatory networks became fixed in the Cambrian, downstream regulatory systems remained flexible to substantial changes in developmental patterns within constraints of the fundamental body plans.

The ecospace and developmental macroevolutionary models make different predictions about the evolution of life. The ecospace model says that diversification and innovation continue unabated through geological time, when environmental opportunity or novel adaptations permit. The developmental model proposes that there have been stepwise limits to the extent of morphological change. This means that much innovation would have been exhausted in the Palaeozoic, and later evolution would then have consisted of tinkering or specialization within a limited number of bauplans [8]. Such a view of long-term ‘exhaustion’ of adaptive opportunities would seem to be at odds with evidence that life has massively diversified in the past 100 Myr, expanding in terms of species numbers, disparity and functional breadth [2,9–11], but others argue that life in the sea at least reached its maximum diversity 500 Myr ago and has remained at a steady equilibrium level ever since [12,13]. These contrasting viewpoints go to the heart of a number of fundamental debates in macroevolution and palaeobiology.

3. Biodiversity and macroevolution

Biodiversity is an astonishing phenomenon. Ever since Darwin [14], and even long before, scientists and philosophers have been amazed at just how diverse life is. Current discussions among biologists have focused on estimating global biodiversity, and yet it is debated how high the figure should go above the total of the 1.7 million named species: whether 5 million, 10 million or 100 million species. Seeking to discover the actual number of living species, even to within an order of magnitude, has fascinated evolutionary biologists [15,16], and it is a theme that has profound scientific consequences, as well as socio-economic implications in terms of designing global conservation policy. In a simple world, one might expect a few thousand species the fact that there are millions of species, and some with remarkably restricted habits and geographical distributions raises wonder among scientists and the general public: why is life so diverse?

These considerations have led to wider discussions of practical concerns, such as what is the current rate of loss of biodiversity, which kinds of species are most at risk, are certain regions or habitats more species-rich than others (and why), are there particular characters or ecologies that ensure high biodiversity, how does biodiversity recover at various scales, and how can humans mitigate the losses? Taking account of the seemingly huge scale of modern biodiversity, there are a large number of questions about causes: why is life so diverse, are some groups more species-rich than others, what are the correlates and causes of such success (in terms of species numbers), and what are the correct scales to understand origins of modern biodiversity?

The key question [17] is ‘How is biodiversity generated and maintained?’ Much work in ecology focuses on the second half of this question my focus is on the first half, how is biodiversity generated?—or specifically—what are the triggers or drivers of success [= high biodiversity] in evolution?

Diversification, the balance between speciation and extinction, is core to half of current evolutionary theory. Darwin [14] explored two great themes in the Origin, evolution by natural selection (=microevolution) and descent with modification (=macroevolution). In arguing that all life can be traced back to a single common ancestor, he was first to show that life diversified according to a branching phylogenetic tree. Modern biodiversity then reflects that long-term pattern of branching (‘descent with modification’) and this underlies all modern studies of biogeography, ecology, behaviour, palaeobiology and physiology.

As Morlon [17, p. 508] notes, ‘Diversification is a key to understanding how biodiversity varies over geological time scales and how it is distributed across the Earth's surface, the tree of life and ecological communities … Diversification is also a primary predictor of three fundamental patterns in macroecology: the species abundance distribution, which describes how individuals are partitioned among species, the species-area relationship, which describes how species richness increases with geographical area, and the distance-decay relationship, which describes how community similarity declines with geographical distance.’

4. Methodological advances and new opportunities

Scaling issues, and the differences in data between living and extinct organisms left palaeontologists and evolutionary biologists in a conundrum because they lacked the tools to cross the living–fossil divide. Now, however, remarkable improvements in data and methods in the past 20 years are converging on a tool kit that should allow the application of acceptable analytical approaches to the study of macroevolution incorporating all data, not just living taxa and genomic trees. These methods also not only explore patterns but also allow testing of models, so addressing the process aspect.

(a) The fossil record

Knowledge of fossils worldwide has improved substantially in the past 30 years, with special focus on aspects of quality (e.g. completeness of fossil documentation, accuracy of rock dating, accuracy of recovered phylogenies). An example of a major effort to document the data in a unified manner is the Paleobiology Database (, a community-based resource that was established in 1998, and has grown substantially since. Whereas evolutionary palaeobiologists used to use rather broad-based temporal and geographical constraints, they exercise more care now in clarifying the provenance of their fossil taxa and their nomenclature, allowing for errors in taxonomy.

(b) Time scales

At the same time, geological time determination has improved by orders of magnitude [18]. Whereas at one time, the precision of radiometric dates might have been qualified by error bars of plus or minus 5%, errors are now frequently in fractions of 1%. This, combined with close attention to the rigours of global stratigraphic correlation through the International Commission on Stratigraphy [19], has allowed questions to be answered in new ways. For example, 20 years ago, the greatest mass extinction of all time, around the Permo–Triassic boundary, was deemed to have lasted for any time up to 10 Myr [20], but it can now be refined to a duration of 180 000 years [21] or even 12 000–108 000 years [22]. The improved precision of geological dating provides a basis for more reasonable calibration of phylogenies [23] and for calculation of realistic rates of evolution.

(c) Phylogeny

Phylogenies of ever-larger size are being compiled by various means, some based on single studies of genomic data from hundreds or thousands of species, and others compiled as supertrees based on numbers of component source trees. While there is considerable debate and development of numerical methods in determining trees, using parsimony, Bayesian methods, and the like, and the mechanics of constructing supertrees are also in development, and debated, the end result of intense phylogenetic study of some clades, especially among vertebrates, since the 1970s has been a stabilization of many portions of the tree. Whereas, for example, the fundamental relationships of the mammalian orders were obscure in the 1980s [24], the discovery of Afrotheria, Laurasiatheria, Boreoeutheria, Whippomorpha and some other unexpected clades has led to increasing stability and agreement [25].

(d) Disparity and morphometrics

Whereas species diversity can readily be quantified, morphology has been harder to document. Either discrete or continuous characters may be used as a basis for measurement of disparity. Continuous characters are often derived from landmark measurements taken from drawings or photographs of whole organisms or parts of organisms (e.g. vertebrate skulls in lateral view leaf shapes in dorsal view). Discrete characters may be presence/absence characters or cladistic characters that record the acquisition of novelties. Statistical protocols for recording and analysing such data are well established [26–30]. The initial measurements are processed to establish intertaxon distances, which can then be subjected to multivariate treatment (e.g. principal coordinates analysis). The multivariate analyses permit visualization of taxa in a morphospace, so the position and range of morphological variation may be compared between pre-determined clusters of species, representing different subclades, time bins or geographical areas. Disparity is summarized by a variety of indices that capture the range and variance of shape variation, and so allow the analyst to track changing disparity through time.

(e) Phylogenetic comparative methods

Phylogenetic comparative methods (PCM) were proposed [31,32] as a means to correct for phylogenetic bias in comparative work in biology, and have since been developed as tools that explore the evolution of characters across trees to identify diversification shifts, evolutionary rates, and models of evolution [33–38]. Most of the currently available methods work through the coding environment R, and so can integrate with each other to explore particular datasets.

5. Key questions in macroevolution

Questions about origins and nature are commonplace concerns of all citizens. Macroevolution incorporates questions that fall into the research domains of biodiversity and global change, two themes of key scientific and socio-economic significance. It would perhaps surprise many enquiring non-scientists that we cannot say how diverse life is today, how diverse it was in the past, what key principles determine patterns of biodiversity, why some groups are more diverse than others, and how successful groups become successful. A number of key current and future questions and research themes may be identified.

(i) Why are some groups successful and others are not? Classic comparisons compare sister clades because both originated at the same time and both have been through the same vicissitudes of Earth history. For example, birds and crocodiles are sister clades that diverged 240–250 Ma: today there are 10 000 species of birds, and only 23 species of crocodiles. Other such sister-clade comparisons with such orders-of-magnitude differences in current biodiversity include Cyclostomata–Gnathostomata (jawless versus jawed fishes), Rhynchocephalia–Squamata (sphenodontians versus lizards), Holostei versus Neopterygii (sturgeons and paddle fish versus derived bony fishes) or Amiidae versus Teleostei (bowfins versus teleosts) and Monotremata versus Theria (monotremes versus marsupials + placentals). There are so many examples of such imbalance that these are not unusual cases from the end of a spectrum in which most splits are equal it truly seems that sister clades of ‘successful’ clades may survive and yet barely expand beyond a handful of species.

(ii) What drives large-scale evolution? Is the driver for the diversification of life internal (e.g. biological adaptation) or external (e.g. environmental change)? Does evolutionary success depend on innovation or chance, or both? This is part of the broader debate about whether evolution is dominated by external drivers (e.g. climate, temperature, atmospheric composition, sea level, topography) or internal drivers (e.g. competition, predation), sometimes characterized as the Court Jester versus Red Queen models [2,10]. The question can be approached in two ways regarding external environmental changes. First, time series of external drivers and patterns of diversity and disparity can be modelled for specific cases, and goodness of fit (explanatory power) assessed. Second, the effects of particular major crises, such as mass extinctions and associated environmental shocks, as well as smaller events, such as the Palaeocene-Eocene Thermal Maximum and the Neogene ice ages can be explored for their one-off effects on particular clades.

(iii) How do clades diversify when triggered by key adaptations or by extinction events? How do clades expand—early burst or gradual? Some clades diversified in the aftermath of mass extinctions (e.g. placental mammals and neognath birds after the Cretaceous-Palaeogene (K-Pg) event 66 Ma archosaurs, neopterygians and bivalves after the Permo–Triassic mass extinction 252 Ma). Others diversified in ‘normal’ times, when there had been no such crisis (e.g. lizards, snakes, passerine birds). Are there differences in evolutionary mode? The dominant evolutionary models can be explored across large phylogenies that include clades which diversified under both kinds of regimes [39], as well as evidence for rates of change, the scale and timing of diversification shifts and the importance of perceived key adaptations.

(iv) Are diversity and disparity decoupled? In other words, does evolution drive species to explore the outer limits of available morphospace early, and then morphospace is filled by ever more specialized species, or do morphospace occupation and diversity expand in tandem (figure 2)? This kind of study has been meta-analytical so far [8,34,41], meaning a summation of ‘random’ case studies, but it would be worth exploring instances across a single large phylogeny to determine the proportion of clades characterized by one or other model. If it turns out that the disparity-first model is general, this would require some re-thinking of common assumptions about adaptive radiations.

(v) Are diversifications and extinctions selective? Much has been written about the selectivity of extinctions and of recovery times [2,7,28,39,42]. Most work up to now has been based on the compilation of case studies, and these have suggested some general ‘rules’, such as that large animals are most vulnerable to extinction restricted diets and narrow geographical ranges are strong contributors to risk small insect-eaters are the most probable animals to lead the recovery after a mass extinction. On the other hand, clade geometry seems to play little role, with no evidence that long-lived clades are more or less likely to survive than newly emerged clades, except in the sense that long-lived clades often have wider geographical distribution than short-lived clades [43].

(vi) Is diversification diversity-dependent, at clade level and at global scale? In other words, do clades (or all of life) reach limits of space or food that slow down their rate of expansion (by elevating extinction rates, or reducing speciation rates)? Models of evolution commonly incorporate density-dependent factors [44–48], and yet it is not clear that these provide the best models in macroevolution [10,49–51]. Certainly, small-scale studies involve density-dependent processes because there is usually a limiting factor (e.g. food or shelter), but such assumptions may not apply at regional or global scales where species may originate into empty ecospace (i.e. not by supplanting a pre-existing species) and where increased diversity itself provides opportunities for further increases in diversity [52,53]. In the end, most evidence supports some diversity-dependence, especially in damping down origination rates, but diversity rarely, if ever, reaches the asymptote, so there may be truth in both viewpoints [54]. Nonetheless, this has been a debate that has rumbled along since the days of Darwin and Wallace, and it has implications at all hierarchical scales from local ecology to deep-time global history of biodiversity.

(vii) Is evolution hierarchical, having once been faster than it is today (developmental model) or are opportunities and innovations as buoyant now as they were deep in time (ecological model)? This debate is fundamental to our understanding of the trajectory of biodiversity expansion, and to our understanding of genomic/developmental flexibility. The developmental model [6–8], as noted earlier, implies stagnation or an exhaustion of potential to adapt and evolve, and so would seem to be at odds with the apparently continuous diversification of life since the Palaeozoic. This, and the debate about the role of diversity-dependent processes (question vi), also speaks to the hierarchical structure of biodiversity, and whether there are real differences between processes acting at different levels in the hierarchy, or whether evolution occurs simply at the level of individuals or species, and these have passive consequences at higher levels.

Figure 2. The null expectation is that diversity and disparity are coupled (a), but most palaeontological examples suggest they are decoupled (b,c) and that the disparity-first model is most common (c). Based on [40]. (Online version in colour.)

6. Examples of palaeontological, macroevolutionary studies

In terms of the origins of modern biodiversity, there are many case studies in phylogeography that focus on clade origins and dispersals in the past 10 000 years, since the end of the last Ice Age [53,55]. However, a deeper-time focus is appropriate in accounting for large-scale, worldwide patterns.

In one example, Alfaro et al. [33] found that the modern biodiversity of all 60 000 species of vertebrates could be reduced to a manageable analytical question: what is special about the six hyper-diverse clades (Ostariophysi, Euteleostei, Percomorpha, non-gekkonid Squamata, Neoaves and Boreoeutheria) that comprise 85% of those 60 000 living species (figure 3)? In the end, this might resolve into some clear answers, because the origin and main diversification burst of each of these clades could well be tied to a particular key innovation (i.e. a morphological or physiological character that allowed all members of the clade to occupy new ecospace) or to a particular external trigger, such as the extinction of a precursor competitor group or the opening of a new habitat or mode of life mediated by climatic or other physical environmental change.

Figure 3. The evolution of key vertebrate clades, showing diversification shifts. Clades are collapsed to 47 representative stem lineages and coloured by extant species diversity. Clades with unusual diversification rates are denoted with numbers yellow and blue squares denote diverse and impoverished clades, respectively, compared with background rates of evolution. The impoverished clades (numbers 3, 6, 5 sarcopterygian fishes, rhynchocephalians, crocodiles) are classic ‘living fossils’, whereas the speciose clades (numbers 8, 2, 1, 7, 9, 4) shows expansions of diversity clustered in the time from 150 to 100 Ma. From [33]. (Online version in colour.)

Several recent studies address the macroevolution of the origin of birds [56–59], and they all come to the same conclusion, that there were continued, elevated rates of evolution for 60 Myr along the stem of theropod evolution towards Archaeopteryx, and that all the classic ‘bird’ characters were acquired piecemeal over that long span of time. Before the Chinese Jurassic–Cretaceous feathered dinosaurs and birds had been reported, Archaeopteryx seemed to emerge fully fledged as a bird 150 Ma. Therefore, it was easy to imagine that birds had evolved fast, and that perhaps the unique assemblage of adaptations for flight could only have emerged as a functioning package. Such considerations encouraged some evolutionists to speculate about sudden genetic revolutions, and of course provided creationists with material for their mockery. Now, the facts speak against a sudden revolution in the origin of birds. Most of the enumerated characters had emerged in stepwise fashion from the Late Triassic onwards. Indeed, the fossils show a remarkable phase of experimentation in flight styles, with some gliding dinosaurs such as Microraptor even experimenting with four-winged flight.

These studies of bird origins [56–59] used different datasets, different phylogenies, and different analytical techniques, and yet they converged on the same result. As an example, Puttick et al. [56] showed that miniaturization and wing expansion, critical anatomical requirements to be a bird, arose some 10 Myr before Archaeopteryx among the wider clade Paraves (figure 4), and that the rate of change was 160 times the normal evolutionary rate, suggesting a rapid, adaptive switch that enabled the diversification and success of this clade of tiny, possibly tree-climbing and gliding dinosaurs. Their analysis, like two others [58,59] was conducted on a phylogenetic tree that had been dated independently of the phylogenetic tree search, whereas Lee et al. [57] ran a tip-dating method that established the favoured tree and its time calibration as a single calculation. Other differences were in terms of the characters used for macroevolutionary analysis, whether body size alone [57,58], body size and wing size together [56], or a broad suite of cladistic characters [59]. The agreement on the main results in these studies, despite their different materials and methods, suggests that the conclusions are robust to the choice of data and methods.

Figure 4. Rates of femur and forelimb evolution in theropod dinosaurs and early birds. Branch lengths are scaled, with the red branch leading from the centre to the Paraves indicating a 200-fold, and the yellow branches (upper left) to Microraptorinae indicating an eightfold, increase in rate of evolution relative to the background rate. The time-calibrated phylogeny is shown in dark grey, and circular rings indicate 5 Myr time intervals from the K–Pg boundary. Silhouettes drawn by Scott Hartman, Matt Martyniuk, Emily Willoughby, Jaime Headon and Craig Dylke, or modified by T. Michael Keesey, were downloaded from From [56]. (Online version in colour.)

In another series of studies, Ruta et al. explored the nature of clade expansion following the Permo–Triassic mass extinction (figure 5), including the radiations of archosaurs [60], anomodonts [41] and cynodonts [61]. These studies all used discrete characters from cladistic datasets to establish the extent of morphospace occupation through time and between clades. A distinction was made between diversity (species richness) and disparity (morphological variance), and the aim was to determine whether diversity and disparity are coupled or not (figure 2). In all cases, the clades diversified into ecospace emptied by the crisis in a disparity-first mode. These three studies represent clades with very different histories—for example, archosaurs (including dinosaurs) and cynodonts had been small clades before the extinction, whereas anomodonts had been a large and diverse clade, and yet the recovery patterns are similar in each case, and not distinguishable from diversifications that did not follow mass extinctions. These disparity methods have also been used in studies of Cambrian animal radiation [26,27], temnospondyl evolution [62], pterosaur subclade evolution [63] and ichthyosaur radiations during recovery from mass extinction [64].

Figure 5. Diversity and disparity (morphological variation) in three clades that recovered after the Permo–Triassic (P–Tr) mass extinction. In each case, disparity (red, with confidence envelope marked) expands before diversity (black), although in anomodonts, this happened before the mass extinction, which generated a macroevolutionary bottleneck from which diversity recovered, but diversity never did. Ans, Anisian Chx, Changhsingian Crn, Carnian ENo, early Norian Het, Hettangian I-O, Induan-Olenekian Lad, Ladinian LNo, Late Norian MNo, middle Norian Plb, Pilensbachian R-W, Roadian-Wordian Sin, Sinemurian Wuc, Wuchaipingian. Based on data from [41,60,61]. (Online version in colour.)

The effects of mass extinctions can be studied through PCM also. These methods test whether a pattern of change differs sufficiently from random (Brownian motion model) to be identified as a directional trend, a stabilizing or constrained pattern (Ornstein–Uhlenbeck model), or some other selective model. In a study of the effect of the K–Pg mass extinction 66 Ma on the evolution of mammals, Slater [39] found strongest support for his ‘K–Pg release and radiate’ model (figure 6). In cases such as these, the macroevolutionist is not simply describing a pattern of explosive expansion in diversity and disparity, but also determining an evolutionary process. Here, the conclusion is that Cretaceous mammal evolution was held back by the dinosaurs, but after their extinction, mammals radiated rapidly, having been released from those constraints.

Figure 6. The phylogeny of modern mammals, and testing for the role of the K–Pg mass extinction. The evolutionary tree shows the main clades (1, Mammalia 2, Theria, 3, crown Monotremata 4, crown Metatheria 5, crown Eutheria), and the dashed grey horizontal line corresponds to the K–Pg boundary. The inset histogram shows the likelihood of different models to explain variants of the tree for living and fossil Mammalia. Based on [39]. (Online version in colour.)

There is a third component used to describe a diversifying clade, after diversity and disparity (=morphology), namely adaptation or function. In describing morphospaces, it is easy to equate form and function, but the many-to-one mapping dilemma [65] is prevalent: one structure might have multiple functions, and one function might be performed by multiple structures. In exploring the macroevolution of early fishes, Anderson et al. [66] distinguished so-called ‘functional’ characters, especially those metrics of feeding efficiency and diet, from other morphological characters, and found different patterns of change through time. This may not be a complete solution of how to explore function through time—after all, the ‘functional characters’ are simply declared to be so, and it is not clear whether these are the crucial functions that might have triggered an adaptive radiation—but it is a start, and is a means to test particular hypotheses.

The need for an integrated programme of macroecological studies was highlighted by Schluter [67, p. 181], ‘Most tests of key innovation hypotheses attempt to correlate appearance of a novelty with change in the net rate of speciation rather than with adaptive radiation, of which speciation is only part. The lack of attention to effects of novel traits on ecological and phenotypic expansion is an outstanding gap in the study of key innovations.’ In other words, little has been done until recently [17,68,69] to identify patterns of timing, shapes of clades and subclades over millions of years, and the distribution of anatomical characters, including key innovations. Now, however, many researchers have the intention to carry out such studies in an appropriate manner, and the data and tools have converged in a way that makes the work possible.

7. Future advances

The themes of clade dynamics and origins are both fascinating problems couched in sometimes elusive data, but fundamental to our understanding of the world and life. They are also currently important challenges to humanity, because of intense interest in biodiversity drivers and challenges [10,17,70,71]. All these issues have a profound human and political dimension in view of concerns about climate change and biodiversity. Understanding the origins of biodiversity can be framed as a macroevolutionary question: why are some clades more successful than others? The answers will lead to rethinking of much current evolutionary research, and a re-focus of smaller-scale, phylogeographic, genomic and conservation biological approaches to comparing species-rich and species-poor subclades. There are strong opportunities to make major theoretical advances in questions of evolutionary success, hierarchy of processes and the relative significance of biotic and abiotic factors in driving evolution.

27.4C: Post-Cambrian Evolution and Mass Extinctions - Biology

The Cambrian Period marks an important point in the history of life on Earth it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion," because of the relatively short time over which this diversity of forms appears. It was once thought that Cambrian rocks contained the first and oldest fossil animals, but these are now found in the earlier Ediacaran (Vendian) strata.

Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Ediacaran Period immediately preceding the Cambrian. Some of the odd fossils of the biota from the Ediacaran may also have been animals representative of living phyla, although this remains a somewhat controversial topic. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years. Trace fossils made by animals also show increased diversity in Cambrian rocks, showing that the animals of the Cambrian were developing new ecological niches and strategies — such as active hunting, burrowing deeply into sediment, and making complex branching burrows. Finally, the Cambrian saw the appearance and/or diversification of mineralized algae of various types, such as the coralline red algae and the dasyclad green algae.

This does not mean that life in the Cambrian seas would have been perfectly familiar to a modern-day SCUBA diver! Although almost all of the living marine phyla were present, most were represented by classes that have since gone extinct or faded in importance. For example, the Brachiopoda was present, but greatest diversity was shown by inarticulate brachiopods (like the one pictured below, left). The articulate brachiopods, which would dominate the marine environment in the later Paleozoic, were still relatively rare and not especially diverse. Cambrian echinoderms were predominantly unfamiliar and strange-looking types such as early edrioasteroids, eocrinoids, and helicoplacoids. The more familiar starfish, brittle stars, and sea urchins had not yet evolved, and there is some controversy over whether crinoids (sea lilies) were present or not. Even if present, crinoids were rare in the Cambrian, although they became numerous and diverse through the later Paleozoic. And while jawless vertebrates were present in the Cambrian, it was not until the Ordovician that armored fish became common enough to leave a rich fossil record.

Left: Acrothele, a fairly common inarticulate brachiopod from the the Wheeler Shale of western Utah. Middle: Olenellus fremontii from the Latham Shale of southern California. Right: A hyolith, also from the Latham Shale.

Other dominant Cambrian invertebrates with hard parts were trilobites (like the one pictured above), archaeocyathids (relatives of sponges that were restricted to the Lower Cambrian), and problematic conical fossils known as hyolithids (like the one pictured above, right). Many Early Cambrian invertebrates are known only from "small shelly fossils" — tiny plates, scales, spines, tubes, and so on. Many of these were probably pieces of the skeletons of larger animals.

A few localities around the world that preserve soft-bodied fossils of the Cambrian show that the "Cambrian radiation" generated many unusual forms not easily comparable with anything today. The best-known of these sites is the legendary Burgess Shale (middle Cambrian) in the British Columbian Rocky Mountains. Sites in Utah, southern China, Siberia, and north Greenland are also noted for their unusually good preservation of non-mineralized fossils from the Cambrian. One of these "weird wonders", first documented from the Burgess Shale, is Wiwaxia, depicted at lower left. Wiwaxia was an inch-long, creeping, scaly and spiny bottom dweller that may have been a relative of the molluscs, the annelids, or possibly an extinct animal group that combined features of both phyla.

A lot can happen in 40 million years, the approximate length of the Cambrian Period. Animals showed dramatic diversification during this period of Earth's history. This has been called the "Cambrian Explosion". When the fossil record is scrutinized closely, it turns out that the fastest growth in the number of major new animal groups took place during the as-yet-unnamed second and third stages (generally known as the Tommotian and Atdabanian stages) of the early Cambrian, a period of about 13 million years. In that time, the first undoubted fossil annelids, arthropods, brachiopods, echinoderms, molluscs, onychophorans, poriferans, and priapulids show up in rocks all over the world.

Stratigraphic boundaries are generally determined by the occurences of fossils. For instance, the trace fossil Treptichnus pedum marks the base of the Cambrian. This boundary is an unusual case, since stratigraphic boundaries are normally defined by the presence or absence of groups of fossils, called assemblages. In fact, much paleontological work is concerned with questions surrounding when and where stratigraphic boundaries should be defined. At first glance, this may not seem like important work, but consider this: if you wanted to know about the evolution of life on Earth, you would need a fairly accurate timeline. Questions such as: "how long did something stay the same?" or,"how fast did it change?" can only be assessed in the context of time.

Tectonics and paleoclimate

The Cambrian follows the Ediacaran Period, during which time the continents had been joined in a single supercontinent called Rodinia (from the Russian word for "homeland", rodina). As the Cambrian began, Rodinia began to fragment into smaller continents, which did not always correspond to the ones we see today. The reconstruction below shows the rifting of Rodinia during the second stage (Tommotian) of the Cambrian . Green represents land above water at this time, red indicates mountains, light blue indicates shallow seas of the continental shelves, and dark blue denotes the deep ocean basins. (For clarity, the outlines of present-day continents have been superimposed on the map.)

World climates were mild there was no glaciation. Landmasses were scattered as a result of the fragmentation of the supercontinent Rodinia that had existed in the late Proterozoic. Most of North America lay in warm southern tropical and temperate latitudes, which supported the growth of extensive shallow-water archaeocyathid reefs all through the early Cambrian. Siberia, which also supported abundant reefs, was a separate continent due east of North America. Baltica — what is now Scandinavia, eastern Europe, and European Russia — lay to the south. Most of the rest of the continents were joined together in the supercontinent Gondwana, depicted on the right side of the map South America, Africa, Antarctica, India, and Australia are all visible. What is now China and east Asia was fragmented at the time, with the fragments visible north and west of Australia. Western Europe was also in pieces, with most of them lying northwest of what is now the north African coastline. The present-day southeastern United States are visible wedged between South America and Africa they did not become part of North America for another 300 million years. Tectonism affected regions of Gondwana, primarily in what are now Australia, Antarctica, and Argentina. The continental plate movement and collisions during this period generated pressure and heat, resulting in the folding, faulting, and crumpling of rock and the formation of large mountain ranges.

The Cambrian world was bracketed between two ice ages, one during the late Proterozoic and the other during the Ordovician. During these ice ages, the decrease in global temperature led to mass extinctions. Cooler conditions eliminated many warm water species, and glaciation lowered global sea level. However, during the Cambrian there was no significant ice formation. None of the continents were located at the poles so land temperatures remained mild. In fact, global climate was probably warmer and more uniform than it is today. With the retreat of Proterozoic ice, the sea level rose significantly. Lowland areas such as Baltica were flooded and much of the world was covered by epeiric seas. This event opened up new habitats where marine invertebrates, such as trilobites, radiated and flourished.

Plants had not yet evolved, and the terrestrial world was devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production were the monopoly of bacteria and algal protists that populated the world's shallow seas.

Also during the Cambrian, the oceans became oxygenated. Although there was plentiful atmospheric oxygen by the beginning of the period, it wasn't until the Cambrian that there was a sufficient reduction in the number of oxygen-depleting bacteria to permit higher oxygen levels in the waters. This dissolved oxygen may have triggered the "Cambrian Explosion" — when most of the major groups of animals, especially those with hard shells, first appeared in the fossil record.

Aldan River, Siberia,: This early Cambrian fauna tells us about the early evolution of animals with skeletons.

Burgess Shale, British Columbia: Thousands of soft-bodied animal fossils paint us a picture of early marine life.

House Range, Utah: An array of Cambrian critters has been found in the Wheeler Shale and the Marjum Formation.

Marble Mountains, California: Olenellid trilobites and more are found in this Mojave Desert locality.

White-Inyo Mountains, California: Visit ancient Cambrian reefs in these mountains of eastern California.

  • Find out more about the Cambrian paleontology and geology of North America at the Paleontology Portal.
  • See the Wikipedia page on the Cambrian.

* Dates from the International Commission on Stratigraphy's International Stratigraphic Chart, 2009.

Page content written and completed by Ben M. Waggoner and Allen G. Collins, 11/22/1994 tectonics and paleoclimate material added by Karen Hsu, Myun Kang, Amy Lavarias, Kavitha Prabaker, and Cody Skaggs as part of a Biology 1B project for Section 112 under Brian R. Speer, 5/1/2000 Ben M. Waggoner revised the Life content, 9/2001 Sarah Rieboldt updated the pages to reflect the Geological Society of America (GSA) 1999 Geologic Timescale, 11/2002 Dave Smith recombined the content into a single page, adapted it to the new site format and made some content updates, 7/6/2011 Acrothele and hyolith photos by Ben M. Waggoner Olenellus photo by Dave Smith source of Tommotian map unknown