chordates

Learning Outcomes for Deuterostomes II: Chordates and Vertebrates

  • Chordate Identification: By the end of the session, students should be able to identify the chordates (Chordata) and specifically the vertebrates (Vertebrata), including their basic characteristics.

  • Evolutionary Innovations: Identify the key evolutionary milestones that allowed vertebrates to thrive across all of Earth's ecosystems.

Phylogenetic Context and Species Diversity

  • Ancestral Lineage: All animals originate from an ancestral "protist."

  • The Parazoa vs. Eumetazoa Split:     

    • * Parazoa: Sponges (no true tissues).     

    • * Eumetazoa: Tissues present (endoderm and ectoderm). This group includes Cnidarians (radial symmetry) and Bilateria (bilateral symmetry, mesoderm).

  • The Protostomia vs. Deuterostomia Split:     

    • * Protostomia: Includes Flatworms, Mollusks, Annelids, Roundworms, and Arthropods.     

    • * Deuterostomia: Includes Echinoderms and Chordates.

  • Comparative Species Counts:     * Arthropods: >1,000,0001,000,000 species.     * Mollusks: ~110,000110,000 species.     * Chordates: ~65,00065,000 species (most of which are vertebrates). Chordates represent approximately 6%6\% of all described animals.     * Annelids: ~20,00020,000 species.     * Roundworms: ~20,00020,000 species.     * Cnidarians: ~12,00012,000 species.     * Flatworms: ~10,00010,000 species.     * Sponges: ~6,0006,000 species.     * Echinoderms: ~7,0007,000 species.

Fundamental Characteristics of Chordata

At some point in their lives, all chordates possess the following four features:

  • Hollow Nerve Cord: Located along the back; in most species, this develops into the brain and spinal cord.

  • Notochord: A flexible, strong rod that runs along the back of the body to provide support.     * In vertebrates, it serves as the developmental template for the skull and vertebral column.     * In adult humans, the remains of the notochord are the discs between the vertebrae.

  • Pharyngeal Slits/Pouches: In aquatic chordates like fishes, these form the gills.

  • Post-anal Tail: Used for balance and movement.

  • Segmentation: Chordates are segmented animals. This is visible in:     * Vertebrae     * Ribs     * Muscles (e.g., visible in a 4-day-old chicken embryo stained in blue or a fish embryo stained in red).

The Vertebrate Endoskeleton

  • Composition: Made of cartilage or bone.     

    • * Cartilage: Primarily collagen; it is lighter than bone but less strong.     

    • * Bone: Primarily calcium carbonate (CaCO3CaCO_3); it is stronger and heavier than cartilage and contains live cells.

  • Advantages of the Endoskeleton:     

    • * Body Support: Allows for much larger body sizes compared to invertebrates.     

    • * Dynamic Growth: Bone is a live tissue that grows with the body, can mend itself, and can be reshaped based on usage.     

    • * Muscle Attachment: Does not constrain muscles; allows for stronger muscles and extraordinary powers of locomotion via jointed appendages.     

    • * Protection: Shields internal organs, the brain, and the spinal cord.

Evolutionary History of Vertebrates

  • The First Vertebrates (Fishes): Evolved in the ocean approximately 545545 million years ago (mya) during the Cambrian period of the Paleozoic Era.     

    • * The earliest fishes were small with no jaws or teeth (e.g., Haikouichthys from China).

  • Fishes Overview: They comprise more than 50%50\% of all vertebrate species.     

    • * Size Variance: Ranges from Paedocypris progenetica (7.9 mm7.9\,mm) to Whale sharks (up to 18 m18\,m long).     

    • * Shape Variance: Includes abyssal anglerfish, eels, and sea dragons (sea horses).

    • * Habitat Variance: Includes land-transitioning species like the Mudskipper.

Major Vertebrate Groups and Transitions

Jawless Fishes

  • Hagfishes ("Slime Eels"): Among the last remaining jawless fishes. They are deep-ocean floor scavengers with a cartilaginous skeleton mostly restricted to a skull.

Evolution of Jaws and Teeth

  • Jaws: Evolved from gill arches (the cartilage supporting the gills).

  • Teeth: Modified scales. Shark skin is covered in microscopic "toothlike" scales.

  • Benefits of Jaws: Allow for feeding on hard foods, taking chunks out of large prey, better manipulation of objects/mates/offspring, and better mechanical processing for digestion.

Cartilaginous Fishes (Chondrichthyes)

  • Includes sharks, rays, skates, and chimaeras.

  • Possess a cartilaginous skeleton (lighter than bone) and a streamlined body with paired fins.

Bony Fishes (Osteichthyes)

  • Contains ~30,00030,000 species (the majority of vertebrates).

  • Features a stronger, heavier skeleton made of live bone tissue.

  • Ray-finned fishes (Actinopterygii): No muscles or bones within the fin; includes most fish species.

  • Lobe-finned fishes (Sarcopterygii): Muscular fins with bones. Only 8 species survive today (e.g., Coelacanth). These are the ancestors of all terrestrial vertebrates.

The Move to Land: Tetrapods

  • Tetrapods: 4-legged vertebrates.

  • Tiktaalik: Lived ~385385 mya; represents the evolution of lungs and muscular fins for crawling onto land.

  • Amphibians (Amphibia): ~5,2005,200 species. First vertebrates on land but remain tied to water.     * Challenges to Terrestrial Life: Supporting body weight, desiccation (drying out), and the need for new breathing/reproductive strategies.    

    •  * Amphibian Solutions: Legs/toes for movement, moist skin with mucus glands, cutaneous respiration (breathing through skin) to supplement small lungs.     

    • * Subgroups: Frogs and Toads (Anura, 5,4005,400 spp.), Salamanders (Urodela, 619619 spp.), and Caecilians (Gymnophiona, 173173 spp.).

Specialized Terrestrial Vertebrates

Reptiles (Reptilia)

  • Over 10,00010,000 species. Better adapted to land than amphibians.

  • Key Adataptions: Efficient lungs and heart, keratinous scales to prevent moisture loss, and the Amniotic Egg.

  • The Amniotic Egg: A watertight egg with membranes that provide food, remove waste, and promote gas exchange. Analogous to the pollen grain in plants as it broke dependency on water for reproduction.

  • Groups: Turtles/Tortoises (Testudines), Lizards/Snakes (Lepidosaurs), and Crocodilians.

Birds (Aves)

  • 10,00010,000 species. Known for flight and endothermy.

  • Feathers: Provide insulation and enable flight.

  • Flight Skeleton: Pneumatic (air-filled) bones; some bones are lost or fused to reduce weight.

Mammals (Mammalia)

  • ~4,5004,500 species. Defined by Hair (keratin-filled dead cells) and Mammary glands (secretion of milk).

  • Groups:     

    • * Monotremes: Egg-laying mammals (e.g., Platypus, Echidna). Only 3 species exist in Australia and New Guinea.     

    • * Marsupials: Give birth to under-developed young (e.g., Kangaroos, Opossums).    

    • * Placental Mammals: Most mammals. The placenta nourishes the embryo in the uterus (carrying water, oxygen, and food) and is homologous with the amniotic egg.

Questions & Discussion

  • Q1: A fish embryo with the notochord stained in red was shown to illustrate segmentation.

  • Question 1: Which is EXCEPT regarding the notochord?     * A. Strong flexible rod.     * B. Supports the body.     * C. Develops into brain and spinal cord. (CORRECT - this is false; the nerve cord does this).     * D. Beginning of vertebral column.

  • Question 2: Accurate comparison of skeletons (Arthropod vs. Echinoderm vs. Vertebrate).     * Discussion: Arthropods have chitinous exoskeletons; echinoderms and vertebrates have endoskeletons.

  • Question 3: Advantages of the vertebrate endoskeleton.     * Discussion: It allows for bigger body size, consists of live tissue, and protects the central nervous system.

  • Question 4: True statements on vertebrate evolution.     * Discussion: Mammals are more closely related to reptiles than to amphibians.

  • Question 5: Advantages of jaws and teeth.     * Correction: Jaws do NOT specifically prevent the mixing of food and waste (that is a function of a complete digestive system and distinct openings).

  • Question 6: Challenges of terrestrial life.     * Correction: Respiration is typically considered easier in air because oxygen concentration is higher in air than in water; therefore, lower oxygen is NOT a challenge of land.

  • Question 7: The amniotic egg analogy.     * Answer: C. It broke the dependency of water for reproduction (similar to pollen/seeds).

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