2. Assembling a Vertebrate

Evolutionary Recap and Systematic Goals

  • The primary goal for organizing vertebrate knowledge is to use phylogenetic systematics to understand evolution.
  • Branching evolutionary hypotheses are formed using synapomorphies (shared derived characteristics).
  • Central questions addressed include: Where did vertebrates come from? What are their closest relatives? What were their ancestors like?
  • Today's focus is on the evolutionary assembly and developmental assembly of vertebrates, leading to the BAUPLAN (basic adult structures and body plan).

Phylogeny and Origins of Vertebrates

  • Metazoa (Multicellular animals): Characterized by collagen, heterotrophy, early embryos forming a hollow ball of cells (blastula), and sex cells formed in special organs where sperm have whiplike tails.
  • Bilateria: Characterized by organs, bilateral symmetry, and movement as adults.
  • Coelom: A body cavity located within the mesoderm.
  • Deuterostomata: A major group containing chordates and their relatives, characterized by the "Fate of the Blastopore."
  • Ambulacraria: Contains Hemichordata (acorn worms, pterobranchs) and Echinodermata (starfish, sea urchins, etc.).
  • Xenoturbellida: A group closely associated with Ambulacraria.
  • Protostomata: The sister group to Deuterostomes, including Annelida, Mollusca, Arthropoda, and many other phyla.

Embryonic Development and the Fate of the Blastopore

  • Developmental Progression:

    • Fertilized egg (Zygote) undergoes cell cleavage and blastulation.
    • Blastula: A hollow ball of cells formed through mitotic cell division.
    • Gastrulation: Rearrangement of cells to form germ layers and the appearance of the blastopore.
    • Gastrula: The result of gastrulation cell migration.

  • FATE OF THE BLASTOPORE:

    • This determines the classification between Protostomes and Deuterostomes.
    • Protostomes: Derived from Greek proto (1st) and stome (mouth). The blastopore becomes the mouth.
    • Deuterostomes: Derived from Greek deutero (2nd) and stome (mouth). The blastopore becomes the anus; the mouth develops secondarily.

  • Differentiation in Eight-Cell Stage:

    • Protostomes: Spiral and determinate cleavage. Solid masses of mesoderm split to form the coelom.
    • Deuterostomes: Radial and indeterminate cleavage. Folds of the archenteron (primitive gut) form the coelom.

Chordata and Chordate Relationships

  • Chordata consists of three major groups:

    • Cephalochordata (lancelets/amphioxus).
    • Urochordata (tunicates/sea squirts).
    • Vertebrata.

  • Olfactores: A clade consisting of Urochordates and Vertebrates, supported by molecular data.

  • Phylogenetic Node Flexibility: Cladograms can spin at nodes; thus, the relationship between Urochordata, Cephalochordata, and Vertebrata can be represented in various branched configurations while maintaining the logic of relationships.

The Five Chordate Synapomorphies

  • All chordates possess these characteristics at some stage of their development:
    1. Notochord: A fibrous rod along the body axis.
    2. Dorsal Hollow Nerve Cord (DHNC).
    3. Post-anal tail.
    4. Endostyle (or iodine binding structure): The precursor to the thyroid gland.
    5. Pharyngeal slits / pouches: Pouches from the gut that may open to the outside as slits in the throat.
  • Note: "All vertebrates are chordates, but not all chordates are vertebrates."

Chordate Subphyla: Cephalochordates and Urochordates

  • Cephalochordates (Lancelets = Amphioxus):

    • Approximately 2222 species.
    • Small, marine, burrowing organisms.
    • Notochord extends the full length of the body.
    • Pharyngeal slits used for filter feeding (not respiration).
    • Myomeres: Segmental muscle blocks derived from somites (shared with vertebrates), essential for locomotion.
    • Oldest confirmed fossil: Pikaia (Middle Cambrian, >500>500 Million YrBP, Burgess Shale, Canada). Length is 50 mm50\,mm; it indicates chordates are at least that old.

  • Urochordates (Tunicates):

    • Approximately 2,0002,000 species.
    • Marine filter feeders with sac-like bodies.
    • Most adults are sedentary (sessile) and attach to substrates.
    • Tadpole Larva: Exhibits all five chordate characteristics (DHNC, Notochord, Post-anal tail, etc.).
    • Adults are highly modified; difficult to see chordate traits besides the pharynx.

Vertebrate Synapomorphies

  • Cranium: A braincase made of cartilage or bone surrounding the brain.
  • Prominent Head: With complex sense organs.
  • Large, Tripartite Brain: Consists of the forebrain, midbrain, and hindbrain.
  • Duplication of Hox Genes: Enables the structural complexity of the vertebrate body plan.
  • Neural Crest Cells: A brand new embryonic tissue that migrates to form various structures.
  • Other traits: Heart, gills, hemoglobin, multi-layer epidermis, and vertebrae.

Developmental Assembly: Germ Layers

  • Vertebrate Body Plan: Described as a "tube within a tube."
  • Ectoderm: The outer layer. Forms the skin's surface, the nervous system, and neural crest cells.
  • Endoderm: The inner layer. Forms the gut lining and derived organs (liver, pancreas, lungs).
  • Mesoderm: The middle layer. It differentiates into three parts:
    • Somites: Segmented blocks next to the nerve tube. Form myomeres, vertebrae, and the dermis.
    • Intermediate Mesoderm (Nephrotomes): Segmented buds linking somites to the lateral plate. Form kidneys and gonads.
    • Lateral Plate Mesoderm: Not segmented. Forms blood vessels, the heart, the coelom (body cavity), and mesenteries (membranes suspending the gut/organs).

Neurulation and the Neural Crest

  • DHNC Formation: Ectoderm overlying the notochord rolls up to form the neural tube.
  • Neural Crest (Vertebrate Synapomorphy):
    • Cells at the edge of the neural plate that migrate throughout the body.
    • Form: pigment cells, tooth dentine, parts of the skull, nerves, and brain.

Integumentary System

  • Functions: Boundary between environments, protection, temperature regulation, water regulation, gas exchange, vitamin D synthesis, and pheromone production.
  • Two Layers:
    • Epidermis: Superficial layer (Ectoderm). Primary protection and glands.
    • Dermis: Deep layer (Somite-Mesoderm). Unique to vertebrates. Contains blood vessels and melanocytes.
  • Aquatic vs. Terrestrial:
    • Aquatic: Living cells with mucus-secreting glands (drag reduction, anti-parasite).
    • Terrestrial: Two epidermal layers: Stratum germinativum (living) and Stratum corneum (dead, keratinized) to protect against abrasion and water loss.

Skeletal System

  • Framework:
    • Non-mineralized: Notochord and cartilage.
    • Mineralized: Bone, enamel, and dentine.
  • Types of Bone:
    • Endochondral: Cartilage precursor; makes up deep structures (limbs, deep skull).
    • Dermal: Grows in the dermis; no cartilage precursor. Flat, superficial bones (skull roof, armor plates).
  • Biological Matrix: Bone is alive, composed of collagen (protein fibers) and hydroxyapatite (calcium phosphate crystals).
  • Cell Types:
    • Osteoclasts: Destroy old/damaged bone.
    • Osteoblasts: Deposit new bone.

Regions of the Skeleton and Skull

  • Skeletal Regions: Skull, Axial (vertebrae, ribs, sternum), and Appendicular (limbs/girdles).
  • Skull Regions:
    • Chondrocranium (Braincase): Deep bones; cartilage precursor.
    • Splanchnocranium: Arches supporting gills and jaws; neural crest derived; cartilage precursor.
    • Dermatocranium: Superficial dermal bone; skull roof, palate, lower jaw; no cartilage precursor.

Teeth and Dentition

  • Development: From the integument (dermal papilla) and neural crest.
  • Structure: Enamel and dentine over a pulp cavity (blood/nerves).
  • Types:
    • Homodont: Numerous, similar shaped teeth (conical/bladelike); initial use for grasping/holding.
    • Heterodont: Different shapes for different functions; good for prey processing; less frequent replacement.
  • Evolutionary Origin: Possibly evolved from scales.

Muscular System and Locomotion

  • Mechanism: Can only shorten (contraction); require antagonistic pairs or elastic rebounding (tendons) to lengthen.
  • Skeletal Muscle: Striated due to the organization of actin and myosin filaments.
  • Sliding Filament Theory: Contraction occurs when myosin heads bind to actin, forming cross-bridges.
  • Muscular Trends:
    • Fish: Segmental Myomeres separated by Myosepta. A Horizontal Septum divides muscles into Epaxial (top) and Hypaxial (bottom).
    • Terrestrial: Decrease in axial muscle mass; increase in appendicular muscles for limb movement.

Digestive and Respiratory Systems

  • Digestive System:
    • Vertebrates are heterotrophs.
    • Gut tube is "divided" into mouth (intake), stomach (break down), and intestine (absorption).
    • Cloaca: A common exit for digestive, excretory, and reproductive systems.
  • Respiratory System:
    • Cutaneous Respiration: Gas exchange through wet skin (e.g., Cephalochordates, some amphibians).
    • Gills: Earliest specialized structures; utilize countercurrent exchange (water and blood flow in opposite directions to maximize diffusion).
    • Lungs: Internal sacs for air extraction. Modified into swim bladders in most fish for buoyancy.

Circulatory System

  • Closed System: All blood is contained within vessels (arteries, veins, capillaries).
  • Structure: Muscular pump (heart), conduit (vessels), and transport medium (blood with plasma, RBCs/hemoglobin, WBCs).
  • Design Evolution:
    • Ancestral/Fish: Tubular heart pumps blood to gills (ventral aorta), picks up O2O_2, then to body (dorsal aorta).
    • Lungs/Side-by-side pumps:
      • Amphibians/Reptiles: 3-chambered heart; incomplete partition; mixing of oxygenated and deoxygenated blood.
      • Mammals: 4-chambered heart; complete partition; NO mixing of blood.

Excretory and Reproductive Systems

  • Excretory System:
    • Developed from intermediate mesoderm.
    • Nephrons in kidneys filter nitrogenous waste and manage water/salts.
    • Kidney Types:
      • Pronephros: Embryonic, head-located.
      • Opisthonephros: Fishes and amphibians.
      • Metanephros: Reptiles, birds, mammals.
  • Reproductive System:
    • Dioecious: Separate male/female individuals (most common).
    • Hermaphrodite: Both gonads in one individual.
    • Parthenogenic: Females produce young without males.
    • r-strategists: Many young, little care. K-strategists: Few young, high care.
    • Precocial: Born ready. Altricial: Born needing time to develop.

Coordination and Integration: Endocrine and Nervous Systems

  • Endocrine System: Ductless glands secreting hormones. Slow, diffuse distribution; controls long-term processes (growth, metabolism).
  • Nervous System: Brain, spinal cord, and neurons (cell body, dendrites, axon). Fast, targeted response (electrical impulses).
  • Brain Regions:
    • Forebrain (Prosencephalon): Smell.
    • Midbrain (Mesencephalon): Vision.
    • Hindbrain (Rhombencephalon): Hearing and balance. Includes the lateral line in aquatic vertebrates to perceive water disturbances.