Evolution of Tetrapods

Vertebrata II: Evolution of Tetrapods

Tetrapods

  • Includes descendants of all terrestrial vertebrates with four limbs.
  • Overcoming terrestrial challenges:
    • Large-bodied organisms move via legs → Four limbs each with five digits.
    • Gills require water, lungs primarily used for respiration.
    • Pulmonary veins: Veins return oxygenated blood to the heart.
    • Partially divided heart: Blood circulation is divided into pulmonary and systemic circuits.

Limbs

  • Large-bodied organisms move via legs → Four limbs each with five digits.
  • Some early tetrapod-like sarcopterygians had more than five digits!
  • All living tetrapods descended from a 5-digit ancestor.

Lungs

  • Gills require water; lungs are primarily used for respiration.
  • Need to pump air into internal sacs.
  • Amphibians squeeze their throat to pull air into their lungs.
  • Gills are delicate external structures that require buoyancy for support, collapsing outside of water.

Heart and Pulmonary Circuit

  • Systemic circuit → Blood pumped to the body.
  • Pulmonary circuit → Blood pumped to the lungs.
  • Animals with gills have two-chambered hearts; tetrapods have three or four-chambered hearts.

Tetrapods: Three Main Living Groups

  1. Amphibians
  2. Reptiles → Includes birds
  3. Mammals

Amphibians

  • Basal tetrapods, utilize cutaneous respiration to supplement lungs → Must keep skin moist!
  • Three living groups:
    • Anura: Frogs and toads
    • Caudata: Salamanders and newts
    • Apoda: Caecilians

Anura: Frogs and Toads

  • Live in many environments, move via jumping.
  • Requires water for reproduction → Tadpoles!
  • Most species undergo metamorphosis.
Anuran Reproduction
  • Eggs are laid in water → Lack watertight membranes, so would dry out and die.
  • Eggs fertilized externally!

Caudata: Salamanders and Newts

  • Elongated bodies with long tails.
  • Mostly live in moist environments.
  • Eggs are laid in water or moist environments and are fertilized internally.
  • Do not undergo metamorphosis!
    • Axlotl → Retains gills! Lives entirely in water.
    • California newt → Incredibly poisonous. Do not lick!

Apoda: Caecilians

  • Tropical burrowing amphibians, lack legs (secondarily).
  • Wormlike vertebrates with a jaw and teeth.
  • Internal fertilization and do not undergo metamorphosis!

Amniota

  • Terrestrial vertebrates with watertight eggs, typically enclosed in a hard or leathery shell. Contains a food source and four membranes:
    1. Chorion: Outermost membrane that facilitates gas exchange, but is waterproof.
    2. Amnion: Encases embryo within fluid-filled sac.
    3. Yolk sac: Provides nutrients for the embryo via blood vessels.
    4. Allantois: Surrounds sac containing waste products from the embryo.

Amniote Skulls

  • Amniotes can be separated by the number of temporal fenestrae (holes) within their skulls. Temporal fenestrae provides additional attachment for jaw musculature.
    • Anapsid: No temporal fenestrae, found in living turtles, may potentially be a reversal in the diapsid condition.
    • Diapsid: Two temporal fenestrae, separated by the postorbital bone, found in living reptiles.
    • Euryapsid: One temporal fenestrae, above the postorbital bone, found in extinct marine reptiles. Derived from the diapsid condition, loss of the second temporal fenestra.
    • Synapsid: One temporal fenestrae, below the postorbital bone, found in living mammals.

Reptilia

  • Diapsids, including euryapsids and anapsids. Some shared characteristics:
    1. Dry skin → Skin is watertight to reduce water loss and covered in scales.
    2. Thoracic breathing → Increased expansion of the thoracic cavity by expanding the rib cage.
  • Living lizards, snakes, turtles, crocodiles, and birds!

Chelonia

  • Turtles and tortoises, both aquatic and terrestrial. Some shared characteristics:
    1. Lack of teeth → Utilize a sharp beak to eat.
    2. Shell → Carapace and plastron derived skeletal elements.

Rhyncocephalia

  • Tuataras are terrestrial → Only living species found in New Zealand
  • Currently only found on offshore islands due to invasive species on main islands

Squamata

  • Lizards and snakes (secondarily loss of limbs), mostly carnivores.
  • Along with tuatara, forms the clade Lepidosauria
Marine Reptiles
  • Mosasaurs likely derived from varanids (monitor lizards)

Archosauria

  • Living lineages include crocodiles and birds. Extinct lineages include dinosaurs and pterosaurs.

Crocodylia

  • Crocodiles and alligators, all are carnivores. Types of living crocodilians include crocodiles, alligators, caimans, gharials.

Pterosauria

  • First flying vertebrate! May belong to Dinosauria. Utilized 5th digit for powered flight.
  • First evolved flight by 210 MYA

Dinosauria

  • Lineage that leads to birds, mostly endothermic (warm-blooded).
  • Two major lineages:
    • Saurischia (lizard-hipped)
    • Ornithischia (bird-hipped)

Aves

  • Birds, descendant of non-avian saurischian dinosaurs. Second group of vertebrates to evolve flight → Reduced hand for powered flight.
    1. Feathers → Modified scales to provide lift.
    2. Light, hollow skeleton for flight → Reduces weight!
    3. Efficient respiration → One-way flow of air (mostly). Air sacs throughout the body allow for an increased amount of oxygen to be utilized.
Evolution of Birds
  • The vast majority of dinosaurs had some sort of feather covering. Reduction in digits and hand, and flight feathers!
  • Archaeopteryx lithographica 150 MYA

Pelycosaurs

  • Synapsids, first evolved ~300 MYA. Often referred to as mammal-like reptiles → This is wrong! They are NOT reptiles, but descendants of mammals!
  • Temporal fenestra = Synapsid

Mammalia

  • This is us! First evolved by 220 MYA. Some shared characteristics:
    1. Hair → Keratinous covering for body temperature regulation.
    2. Endothermic.
    3. Mammary glands → Secretes oily substance to feed infants.
    4. Specialized teeth → Undergo precise occlusion for chewing!
Hair and Endothermy
  • Keratinous covering for body temperature regulation.
  • Other keratinous structures for protection: Nails, horns, antlers, spines, etc.
Mammary Glands
  • Secretes oily substance to feed infants → Milk!
Specialized Teeth
  • Undergo precise occlusion for chewing!

Monotremata

  • Echidnas and platypus. Egg-laying mammals! Also lack teeth (secondarily). Found only in Australia and New Guinea

Marsupialia

  • Pouched mammals, includes opossums and kangaroos. Live birth occurs very early during development → Constricts limb development!
  • Only four species living today. No nipples, but still have mammary glands!
Marsupial Biogeography
  • Originally evolved in North America, but today mainly found in South America and Australia

Placentalia

  • Most mammals today, includes elephants, dogs, and us! Live birth occurs much later in development via a placenta → Specialized organ to connect blood stream of fetus and mother
Placental Locomotion
  • Little developmental constraint on limbs allowed for placentals to expand into nearly every environment
    • Chiroptera: Flight, Uses multiple fingers for powered flight
    • Cetacea: Marine, Hands modified into flippers, and legs are lost!