Study Notes for Unit 2: Animals

Unit 2: Animals

Animal Diversity and Evolution 2

  • Date of Lecture: Feb. 18, 2026
  • Instructor: Dr. Ana Longo
  • Course: BSC2011-Spring 2026

Today's Outline

  • Chapter 22: Animal Origins and Diversity
    • 22.5 Deuterostomes: Include Echinoderms, Hemichordates, and Chordates
    • 22.6 Life on Land Contributed to Vertebrate Diversification

Learning Objectives

  • Describe how deuterostomes differ developmentally from protostomes.
  • List the synapomorphies that characterize the three major deuterostome groups.
  • Explain how features of amphibians can make them especially vulnerable to environmental change.
  • Summarize the evidence that places the phylogenetic position of birds among the reptiles.
  • Describe how a mass extinction affected the diversification of mammals.
  • Outline the evolution of homeothermy, hair, and feather across the phylogeny of amniotes.
  • Articulate the relationship between bigger brains, smaller jaws, and neoteny.

Animal Phylogeny

  • Key Components of Animal Evolution:
    • Origin of distinct organ systems
    • Development of bilateral symmetry along an anterior-posterior axis
    • Presence of three embryonic cell layers
    • Centralized nervous system
    • Requirements for the characteristics of deuterostomes, which are illustrated in the phylogenetic tree provided (Figure 22.1).
Phylogenetic Tree Highlights
  • Deuterostomes:
    • Blastopore develops into the anus; mouth develops later.
    • Common traits: bilateral symmetry, three embryonic layers, segmentation.
Major Groups of Deuterostomes

  1. Echinoderms:

    • Examples: Sea stars, sea urchins, sea cucumbers, crinoids
    • Features: Internal skeleton made of calcified plates, water vascular system for locomotion and feeding.

  2. Hemichordates:

    • Examples: Acorn worms and pterobranchs
    • Features: Three body parts (proboscis, collar, trunk); acorn worms live in burrow, capturing prey with a mucus-coated proboscis.

  3. Chordates:

    • Key Features:
      • Dorsal hollow nerve cord: key component in chordate structure.
      • Tail that extends beyond the anus: critical feature in early chordates.
      • Dorsal supporting rod (notochord): present during some stages of development.

Detailed Echinoderm Structure

  • Sea Stars:
    • Larval Stage: Bilateral symmetry.
    • Adult Stage: Pentaradial symmetry, anus on the aboral surface.
    • Anatomical Features(given in diagrams):
    • Tube feet used for locomotion and gas exchange
    • Digestive system includes several organs distributed in each arm
    • Central nervous system with nerve nets.
Diagram Anatomy Highlights
  • Each arm of the sea star contains a full complement of organs, emphasizing the internal structure design.

Hemichordates Details

  • Wormlike marine deuterostomes:
    • Acorn Worms:
    • Three main body parts: proboscis, collar, and trunk
    • These worms are noted for their burrowing lifestyle in marine sediments.

Chordate Information

  • Types of Chordates:
    • Lancelets, tunicates, and vertebrates
    • Shared Derived Structures:
    • Dorsal hollow nerve cord
    • Notochord
    • Tail extending beyond anus
Examples of Chordates

  1. Tunicates:

    • Example: Sea squirt larva retains all chordate features, some of which are lost in the adult form.
    • Diagram labeled with key features like pharyngeal slits, nerve cord.

  2. Lancelets:

    • Example: Branchiostoma lanceolatum retains all chordate features in adult form.

Vertebrates

  • Key Traits:
    • Dorsal vertebral column
    • Anterior skull housing the brain
    • Well-developed circulatory system
    • Specialized structures for locomotion and feeding.
Phylogenetic Tree of Vertebrates
  • Synapomorphies:
    • Important vertebrate traits include vertebrae, jaws, teeth, paired fins, and bony skeletons (with swim bladders/lungs).
    • Evolution from hagfish to bony fish and terrestrial adaptations.
Vertebrate Evolution

  • Jaws and teeth evolved to improve feeding efficiency, derived from modifications of anterior gill arches.

  • Notable groups include

    • Chondrichthyans:

    • Sharks, skates, rays, characterized by cartilage skeletons, flexible skin.

    • Over 1,000 species exist.

    • Ray-Finned Fishes:

    • Gas sacs from the digestive tract evolved into swim bladders.

    • 32,000 living species, some weighing up to 900 kg.

    • Lobe-Limbed Vertebrates:

    • Characterized by jointed appendages, transition from fins to limbs includes coelacanths and lungfishes.

Tetrapods and Their Evolution

  • Overview:
    • Earliest tetrapod limbs allowed for upright positioning in shallow waters, later adapted for terrestrial movement.
Classification of Tetrapods
  • Two major vertebrate groups:
    • Amphibians:
    • Amniotes:
Amphibians Overview
  • Three Taxonomic Orders:
    • Caecilians, anurans (frogs and toads), salamanders
  • Environmental adaptations include reliance on moist habitats to prevent water loss.
Life Cycle of Amphibians
  • The transition from egg to adult involves several stages, including respiration methods during different life stages.

Amniote Egg as a Key Innovation

  • Structure:
    • Composed of extraembryonic membranes: yolk sac, chorion, allantois, amnion.
    • Enables exploitation of terrestrial habitats.

Phylogenetic Relationships of Amniotes

  • Key Clades and Characteristics:
    • Reptiles diverged from other amniotes about 300 million years ago.
    • Characteristics of various subgroups including tuataras, turtles, squamates, and crocodilians.
Specific Clades of Reptiles
  • Archosaurs:
    • Include crocodilians and extinct dinosaurs; provide insight into the evolution of birds from theropod lineage.

Birds and Mammals

  • Bird Characteristics:

    • Derived features of theropods include bipedal stance, hollow bones, and feathers.

  • Mammals:

    • Coexisted with dinosaurs; diversified rapidly post-extinction with key traits like hair, mammary glands, and four-chambered hearts.

Conclusion

  • Next Class:
    • Chapter 28: An Animal’s Energy Needs Can Be Quantified.
    • Discuss responses to temperature concerning homeostasis.