Ch. 23 - Chordates

Chordata: Overview and Classification

  • The Phylum: Chordata

    • Evolutionary relationships (phylogeny) among different groups of chordates include:

    • Primitive amniotes

    • Early tetrapods

    • Mammals

    • Placoderms

    • Birds

    • Modern reptiles

    • Modern amphibians

    • Cartilaginous fishes

    • Hagfishes

    • Ostracoderms

    • Lampreys

    • Modern bony fishes

Classification Levels within Chordata

  • Chordata consists of animals that possess a notochord at some stage in their life cycle.

  • Various classifications within Chordata include:

    • Euchordata: Chordates that retain an axial skeleton throughout life.

    • Craniata (or Vertebrata): Euchordates with a cranium.

    • Gnathostomata: Craniata with jaws.

    • Teleostomi: Includes bony fishes and tetrapods.

    • Tetrapoda: Four-limbed vertebrates.

    • Protochordata: Non-vertebrate chordates.

    • Agnatha: Jawless fishes.

    • Osteichthyes: Bony fishes.

    • Amniota: Tetrapods with embryos that have extraembryonic membranes.

    • Subgroups:

      • Reptilia: Reptiles.

      • Actinopterygii: Ray-finned fishes.

      • Cephalaspidomorphi: Lampreys.

      • Amphibia: Amphibians.

      • Mammalia: Mammals.

      • Lepidosauria: Lizards, snakes.

      • Crocodilia: Crocodiles.

      • Aves: Birds.

      • Testudines: Turtles.

    • Urochordata: Tunicates.

    • Cephalochordata: Lancelets.

    • Chondrichthyes: Cartilaginous fishes (sharks, rays, chimaeras).

Key Characteristics of Chordates

  • Six defining characteristics of Chordates:

    1. Bilateral Symmetry: Symmetric body plan reflecting anatomical structures.

    2. Notochord: A flexible, rod-like structure providing muscle attachment during some part of its life cycle; typically replaced by vertebrae in higher organisms.

    3. Dorsal Nerve Cord: A tubular nerve cord located dorsally to the alimentary canal; the anterior end develops into the brain.

    4. Pharyngeal Pouches/Slits: Develop into structures such as the middle ear in many vertebrates; also function as gill slits and a filter-feeding apparatus in early chordates.

    5. Endostyle: Functions in mucus production to trap prey in primitive chordates; has an endocrine role in higher chordates.

    6. Post-Anal Tail: Used for propulsion; muscular with fins added in evolutionary history, vestigial in humans.

Detailed Features of Chordate Characteristics

  • Notochord:

    • Structure: Flexible, rod-like, acting as a skeletal element.

    • Function: Allows muscle attachment; replaced by vertebrae in higher vertebrates.

  • Dorsal Nerve Cord:

    • Function: dorsal organization is critical for the development of the central nervous system, leading to brain formation at the anterior end.

  • Pharyngeal Pouches/Slits:

    • Description: Series of openings between the pharynx and outside of the body.

    • Development: Adaptations include becoming the middle ear in many vertebrates and internal gills in aquatic species.

  • Endostyle:

    • Function in Primitive Chordates: Produces mucous to trap particles and prey.

    • Function in Higher Chordates: Acts as a thyroid gland with an endocrine role.

  • Post-Anal Tail:

    • Function: Enhances swimming and propulsion mechanisms in aquatic species.

    • Evolution: Fins may develop, and the feature tends to be vestigial in humans.

Sub-phyla of Chordata

  • Urochordata (Tunicates):

    • Characteristics: Adults lose notochord and tail, with dorsal nerve cord reduced to a single ganglion.

  • Cephalochordata (Lancelets):

    • Example: Amphioxus, demonstrating all six basic chordate adaptations.

  • Vertebrata:

    • Characteristics: Build upon the six characteristics with additional traits:

    • Integument: Composed of epidermis and dermis.

    • Endoskeleton: Structured with a vertebral column, girdles, and head skeleton.

    • Circulatory System: Typically a ventral 2-4 chambered heart.

    • Kidneys: More advanced paired kidneys.

    • Brain Development: Advanced brain structures evident in vertebrates.

    • Nervous System: Inclusion of autonomic nervous system.

    • Endocrine System: Development of a functional endocrine system.