Lecture 9 - Chordata

Introduction to Organisms and Ecology

• Presenter: Bethany Williams in Otterbein Teaching Seminar.

Learning Objectives

• Defining characteristics of chordates.

• Differentiation between nonvertebrate and vertebrate chordates.

• Evolutionary significance of physiological innovations in chordates.

Phylogenetic Position of Chordates

• Overview of the relationships among different animal phyla:

  • Choanoflagellates: Belong to the Protista kingdom.

  • Metazoa: An umbrella term for multicellular animals. Includes subcategories:

  • Parazoa: Acoelomates such as Porifera (sponges).

    • More details on major groups:

    • Acoelomorpha

    • Micrognathozoa

    • Chaetognatha

    • Platyhelminthes: Acoelomate and exhibit spiral cleavage.

  • Eumetazoa: Higher animals with tissue organization.

  • Bilateria: Animals with bilateral symmetry, divided into:

  • Protostomes: Include Pseudocoelomates and coelomates.

    • Example: Ecdysozoa, which undergo molting.

    • Highlighted classes: Onychophora, Echinodermata.

  • Deuterostomes: This includes Chordata.

Characteristics of Chordates

• Unique features that define chordates include:

  • Notochord: Also referred to as "back cord."

    • A flexible rod-like structure that serves as a skeletal axis for muscle attachment.

    • It persists throughout life in urochordates, cephalochordates, and jawless fishes.

    • It is typically replaced by cartilage or bone during vertebrate development.

  • Dorsal Nerve Cord: A single, hollow tube located dorsally.

    • In contrast to protostomes, which often have a double, solid, ventral nerve cord.

    • In vertebrates, it develops into the brain and spinal cord.

  • Pharyngeal Slits: Openings from the pharyngeal cavity to the outside environment.

    • Serve different functions in various groups:

      • Filter-feeding in nonvertebrate chordates.

      • Respiratory gills in aquatic chordates.

      • Modified into pouches in tetrapod vertebrates.

  • Post-Anal Tail: An extension of the body beyond the anal opening.

    • Evolved primarily for swimming; enhanced by the development of fins in fish and lobes in amphibians.

    • Present in some forms as a vestigial structure (e.g., coccyx in humans).

Phylogenetic Position of Chordates

• Chordates compared to Echinoderms:

  • Echinoderms: Have an internal skeleton and exhibit pentaradial symmetry.

  • Chordates: Possess a vertebral column, a distinct head, neural crest cells, internal organs, and an endoskeleton.

Review and Definitions

• Key questions for understanding chordate evolution:

  • Deuterostome vs. Protostome Development:

    • Protostome: Blastopore develops into mouth.

    • Deuterostome: Blastopore develops into anus.

  • Ancestral Traits in Chordates:

    • Plesiomorphy: Shared ancestral trait (e.g., notochord).

    • Synapomorphy: Shared derived trait distinguishing groups.

    • Autapomorphy: Unique derived trait found only in a specific lineage.

Chordate Characteristics

• All chordates exhibit certain traits during either embryonic or adult stages:

  • The notochord is crucial in providing structure.

  • The dorsal nerve cord is critical for nervous system functioning.

  • Pharyngeal slits, depending on the group, facilitate different functions (respiratory vs. feeding).

  • The post-anal tail aids in movement and locomotion.

Discussion of Notochord's Advantages/Disadvantages

• Potential advantages of having a notochord include structural support during early development, allowing for muscle attachment and flexibility.

• Disadvantages may include limitations in size and rigidity compared to a fully formed vertebral column.

Subphyla of Chordates

• Urochordata: Features include:

  • Notochord present only in the tail region of larvae.

  • Adults are mostly sessile, exemplified by tunicates and sea squirts.

• Cephalochordata: Traits include:

  • Notochord extends into the head region, exemplified by the lancelet (Genus Branchiostoma).

• Vertebrata:

  • Most diverse subphylum with significant innovations such as:

  • Cranium protecting the brain, vertebral column, jaws, and endoskeleton.

  • Jaws evolved from anterior gill arches, aiding in developing a predatory lifestyle.

Physiological Innovations in Vertebrate Evolution

• Physiology upgrades: The evolution of a closed circulatory system with a heart significantly enhances metabolic rates.

• Cephalization and sensory systems: Development of new head features and a centralized nervous system allows for more complex behaviors and interactions with the environment.

• Musculoskeletal Modifications: Living endoskeleton allows continual growth and muscle attachment, leading to larger body sizes.

• Adaptations in Limbs: Evolved paired limbs for improved locomotion, including stabilizer fins in aquatic species and jointed limbs in terrestrial species.

Exceptions in Vertebrate Traits: Hagfish and Lampreys

• Hagfish: Jawless and with rudimentary vertebrae that only appear during embryonic stages.

• Lampreys: Also jawless with a notochord that persists throughout life, showcasing unique evolutionary traits.

Evolutionary Challenges of Terrestrial Life

• Challenges for organisms transitioning from aquatic to terrestrial environments echo fundamental issues:

  • Weight support against gravity.

  • Obtaining and delivering oxygen, as gills are ineffective in air.

  • Preventing dehydration in adults and eggs.

• Benefits include access to new resources and habitat types.

Interactive Learning Activities

• Various activities encourage self-assessment about understanding subphylum distinctions based on chordate characteristics and traits.

Summary Questions

• Recap on how many traits and characteristics of chordates were identified and associated with each subphylum. Students reflect on what specific clues indicate each subphylum.