Lecture 13: The Vertebrates Part 2 - Tetrapods

Tetrapods: From Fish to Terrestrial Animals

The Transition from Aquatic to Terrestrial Life

  • The lecture addresses the evolutionary transition from fish to tetrapods (four-limbed vertebrates).
  • A meme featuring Tiktaalik, a transitional fossil, humorously highlights the consequences of this transition, such as needing to work and pay rent.

Characteristics of Early Tetrapods

  • Key Features: Early tetrapods possessed a mix of fish and tetrapod characteristics.
  • Fish Characters:
    • Scales
    • Fins
    • Fin skeleton
  • Tetrapod Characters:
    • Neck
    • Ribs
    • Flat skull
    • Eyes on top of the skull
    • Shoulder bones
    • Humerus, ulna, radius (early limb bones)
    • "Wrist"
    • Elbow

Timeline of Tetrapod Evolution

  • The first tetrapods appeared in the Devonian period, approximately 365 million years ago (mya).
  • Key Fossils Showing the Transition:
    • Eusthenopteron
    • Panderichthys
    • Tiktaalik
    • Acanthostega
    • Tulerpeton
  • Timeline:
    • Silurian: Early evolution
    • Devonian: First tetrapods arise
    • Carboniferous: Continued diversification
    • Permian: Further evolution
    • Amphibians: Emerged during this period
    • Amniotes: Also emerged during this period
  • Key limb bones identified are Humerus, Ulna, and Radius.

Phylogenetic Relationships

  • Deuterostomes: Tetrapods belong to this group.
  • Chordates: Distinguished by a notochord.
  • Vertebrates: Characterized by vertebrae.
  • Gnathostomes: Possess jaws and a mineralized skeleton.
  • Osteichthyans: Have lungs or lung derivatives.
  • Lobe-fins: Exhibit lobed fins.
  • Tetrapods: Defined by limbs with digits.
  • Amniotes: Characterized by the amniotic egg.
  • Mammals: Identified by the production of milk.

Major Tetrapod Groups

  • Amphibians:
    • Order Urodela (salamanders)
    • Order Anura (frogs)
    • Order Apoda (caecilians)
  • Amniotes:
    • Reptiles
    • Mammals

Amphibians

  • Approximately 6,000 species.
  • Exhibit biphasic lifestyles (aquatic larval stage and terrestrial adult stage).
  • Some species are fully terrestrial.
  • Life Cycle:
    • Tadpole (aquatic larva)
    • Metamorphosis
    • Adult returns to water to mate
  • Double Circulation:
    • Pulmocutaneous circuit: Includes lungs and skin capillaries.
      • Atrium (A)
      • Ventricle (V)
    • Systemic circuit: Includes systemic capillaries.
      • Right Atrium (A)
      • Left Atrium (A)

Amniotes and Their Extraembryonic Membranes

  • Amniotes share extraembryonic tissues, including:
    • Amnion: Encloses the amniotic cavity with amniotic fluid.
    • Yolk sac: Contains yolk for nutrients.
    • Chorion: Outer membrane.
    • Allantois: Involved in waste storage and gas exchange.
  • Double Circulation in Amniotes:
    • Pulmonary circuit: Includes lung capillaries.
      • Right Atrium (A)
      • Left Atrium (A)
    • Systemic circuit: Includes systemic capillaries.
      • Ventricle (V)

Amniote Groups Based on Skull Structure

  • Anapsid: No temporal fenestrae (e.g., pre-amniotes, turtles).
  • Synapsid: One temporal fenestra (e.g., mammals).
  • Diapsid: Two temporal fenestrae (e.g., reptiles like Komodo dragons).
    • P: Parietal
    • po: Postorbital
    • sq: Squamosal
    • aj: Jugal

Amniote Phylogeny

  • Key Groups:
    • Synapsids: Led to mammals.
    • Diapsids: Led to reptiles (including birds).
    • Lepidosaurs: Tuataras and squamates (lizards and snakes).
    • Archosaurs: Crocodilians, pterosaurs, and dinosaurs (including birds).
  • Phylogenetic Tree:
    • ANCESTRAL AMNIOTE
      • Synapsids (Mammals)
      • Common ancestor of reptiles
        • Turtles
        • Diapsids
          • Lepidosaurs
            • Tuataras
            • Squamates (lizards and snakes)
          • Archosaurs
            • Crocodilians
            • Pterosaurs
            • Dinosaurs
              • Ornithischian dinosaurs
              • Saurischian dinosaurs (including birds)

Reptilian Characteristics

  • Derived Characteristics:
    • Ectothermic (mostly).
    • Scales made of keratin.
    • Internal fertilization.
    • Produce uric acid.

Avian (Bird) Characteristics

  • Derived Characteristics:
    • Endothermic.
    • Keratin feathers.
    • Lack teeth.
    • Single ovary.
    • Flow-through lungs.
    • Excellent eyesight and large brains.

Flight in Birds

  • Flight is energetically expensive.
  • Archaeopteryx: An early bird species with:
    • Toothed beak
    • Wing claw
    • Airfoil wing with contour feathers
    • Long tail with many vertebrae
  • Emu: Flightless bird for comparison.

Synapsids and Mammalian Characteristics

  • Synapsids are represented by mammals.
  • Derived Characteristics of Mammals:
    • Hair made of keratin.
    • Endothermic.
    • Internal fertilization.
    • Produce milk.
    • Produce urine (urea).
    • Internal development.

Mammalian Diversity

  • Monotremes:
    • Lay eggs (e.g., platypuses, echidnas).
    • No nipples; young suck milk from fur.
  • Marsupials:
    • Complete embryonic development in a pouch (e.g., kangaroos, opossums, koalas).
  • Eutherians:
    • Placental mammals with a longer gestation period.
    • Diverse orders include:
      • Carnivora (dogs, cats, bears, seals)
      • Proboscidea (elephants)
        • Long, muscular trunk; thick, loose skin; upper incisors elongated as tusks.
      • Sirenia (manatees, dugongs)
        • Aquatic; fin-like forelimbs, no hind limbs; herbivorous.
      • Hyracoidea (hyraxes)
        • Short legs, stumpy tail; herbivorous; complex, multi-chambered stomach.
      • Xenarthra (sloths, anteaters, armadillos)
        • Reduced teeth or no teeth; herbivorous (sloths) or carnivorous (anteaters, armadillos).
      • Rodentia (squirrels, beavers, rats, mice)
        • Chisel-like, continuously growing incisors worn down by gnawing; herbivorous.
      • Lagomorpha (rabbits, hares, picas)
        • Chisel-like incisors; hind legs longer than forelegs; herbivorous.
      • Primates (lemurs, monkeys, chimpanzees, gorillas, humans)
        • Opposable thumbs; forward-facing eyes; well-developed cerebral cortex; omnivorous.
      • Perissodactyla (horses, zebras, tapirs, rhinoceroses)
        • Hooves with an odd number of toes; herbivorous.
      • Cetartiodactyla (sheep, pigs, cattle, deer, giraffes, whales, dolphins, porpoises)
        • Hooves with an even number of toes (artiodactyls); herbivorous or aquatic (cetaceans).
      • Chiroptera (bats)
        • Adapted for flight; broad skinfold from fingers to body; carnivorous or herbivorous.

Primate Phylogeny

  • ANCESTRAL PRIMATE
    • Lemurs, lorises, and bush babies
    • Tarsiers
    • Anthropoids
      • New World monkeys
      • Old World monkeys
      • Gibbons
      • Orangutans
      • Gorillas
      • Chimpanzees and bonobos
      • Australopithecus
      • Humans and extinct Homo species