Unit 5: Reptiles, Aves and Mammals

Table of Contents

1. Class Reptilia

Reptiles (from Latin: repere = to creep) are the first truly terrestrial vertebrates. Their key innovation is the amniotic egg, which allowed them to break their dependence on water for reproduction.

1.1 General Features and Classification

General Features

Classification (Up to Order)

Order Common Name Key Characteristics Examples
Chelonia (Testudines) Turtles & Tortoises Body enclosed in a bony or leathery shell (carapace and plastron). Jaws are toothless. Chelone (Turtle), Testudo (Tortoise)
Rhynchocephalia Tuatara "Living fossil." Lizard-like, with a parietal (third) eye. Found only in New Zealand. Sphenodon
Squamata Lizards & Snakes Most successful group. Skin is shed periodically. Males have paired copulatory organs (hemipenes). Hemidactylus (Lizard), Naja (Cobra)
Crocodilia Crocodiles & Alligators Large, aquatic reptiles with a fully four-chambered heart and thecodont teeth (set in sockets). Crocodylus (Crocodile)

1.2 Poisonous and Non-poisonous Snakes

This is a critical identification skill. While exceptions exist, the following general rules are helpful:

Feature Poisonous Snakes (e.g., Cobra, Viper) Non-poisonous Snakes (e.g., Rat Snake, Python)
Fangs Have large, specialized fangs (maxillary teeth) at the front of the upper jaw to inject venom. Lack fangs. Have many small, solid teeth of uniform size.
Head Shape Often (but not always) triangular, due to large venom glands (e.g., Vipers). Usually narrow and elongated.
Pupil of Eye Often vertical or elliptical (e.g., Vipers). *Exception: Cobras have round pupils.* Usually round.
Pit Organ Pit Vipers have a heat-sensing loreal pit between the eye and nostril. Absent.
Ventral Scales In many (like vipers), the scales on the belly are small and do not cover the full width. Often have large, transverse scales (ventral plates) that cover the full width of the belly.
Important Exception: The Coral Snake is highly venomous but has a narrow head and round pupils, mimicking non-poisonous snakes.

1.3 Biting Mechanism in Snakes

The biting mechanism is most specialized in Vipers (solenoglyphous fangs), which have long, erectile fangs.

  1. Resting: The long fangs are folded back, lying flat against the roof of the mouth.
  2. Strike: The snake launches its head forward.
  3. Mouth Opening: The lower jaw drops, and a complex system of bones (quadrate, pterygoid) pushes the maxillary bone.
  4. Fang Erection: This pushing action rotates the maxillary bone, causing the folded fangs to swing forward and lock into an erect, stabbing position.
  5. Injection: The fangs penetrate the prey. Compressor muscles squeeze the poison glands, ejecting venom through the hollow fangs like a hypodermic needle.
  6. Release: The snake retracts its fangs, the bones pull back, and the fangs fold back into their resting position. This entire process is incredibly fast.

2. Class Aves (Birds)

Aves are highly specialized vertebrates adapted for aerial life. They are endothermic (warm-blooded) and are characterized by feathers.

2.1 General Features and Classification

General Features

  • Feathers: The unique diagnostic feature of birds. Feathers are modified epidermal scales, essential for flight and insulation.
  • Forelimbs Modified into Wings: The primary adaptation for flight.
  • Bones: Lightweight and strong. Many bones are pneumatic (hollow and filled with air sacs) to reduce weight.
  • Beak: Jaws are modified into a toothless, horny beak (bill).
  • Heart: Four-chambered (two atria, two ventricles), ensuring complete separation of oxygenated and deoxygenated blood.
  • Thermoregulation: Homeothermic (warm-blooded).
  • Respiratory System: Highly efficient. Lungs are connected to a system of air sacs that allows for a one-way flow of air.

Classification (Up to Order)

  • Order Passeriformes: "Perching birds" or songbirds. Largest order. (e.g., Sparrow, Crow, Myna).
  • Order Columbiformes: Pigeons and Doves.
  • Order Anseriformes: Waterfowl. (e.g., Ducks, Geese, Swans).
  • Order Galliformes: "Fowl." (e.g., Chicken, Peacock, Quail).
  • Order Struthioniformes: Flightless "Ratites." (e.g., Ostrich).

2.2 Flight Adaptations in Birds

Birds have numerous morphological and anatomical adaptations to make flight possible:

  • Body Shape: Streamlined, boat-shaped body to reduce air resistance.
  • Feathers: Provide lift (flight feathers on wings) and steering (tail feathers), while contour feathers provide streamlining.
  • Skeletal Adaptations:
    • Pneumatic Bones: Hollow bones are lightweight yet strong.
    • Fused Bones: Many bones (e.g., in the skull, pelvis, and 'hand') are fused for rigidity and strength.
    • Keeled Sternum: A large, projecting breastbone (keel or carina) provides a massive attachment point for the powerful flight muscles.
    • Furcula: The "wishbone" (fused clavicles) acts as a spring to help power the wing strokes.
  • Muscular Adaptations: Enormous pectoralis major (for downstroke) and pectoralis minor (for upstroke) muscles, located ventrally, which lowers the center of gravity.
  • Respiratory System: The air sac system provides a continuous, one-way flow of fresh air, enabling the high metabolic rate required for flight.
  • Excretory System: No urinary bladder. Uric acid (a lightweight semi-solid) is excreted, saving water and weight.

3. Class Mammalia

Mammals are a highly successful class of homeothermic (warm-blooded) vertebrates, known for their intelligence, complex social behavior, and adaptability.

3.1 Classification and Characteristic Features

The defining general features of mammals are mammary glands (producing milk) and the presence of hair. They also have a four-chambered heart, a diaphragm for breathing, and three middle-ear ossicles.

Classification (Up to Order)

Class Mammalia is divided into three major groups based on reproductive strategy:

Group Characteristic Features Order / Examples
Subclass Prototheria (Monotremata)
  • Egg-laying mammals (oviparous).
  • Lack nipples; milk is secreted through pores on the skin.
  • Possess a cloaca (a single opening for urinary, defecatory, and reproductive tracts), similar to reptiles.
 Order Monotremata:
Platypus, Echidna (Spiny Anteater)
Subclass Metatheria (Marsupials)
  • Pouched mammals.
  • Give birth to very immature, altricial young.
  • The newborn crawls into a pouch (marsupium) and attaches to a nipple to complete its development.
 Order Marsupialia:
Kangaroo, Koala, Opossum
Subclass Eutheria (Placentals)
  • Placental mammals (viviparous).
  • Young undergo a long gestation period inside the uterus.
  • Nourished by a complex placenta, which connects the fetus to the mother.
  • Give birth to relatively well-developed young.
 Order Primates (Monkeys, Humans),
Order Rodentia (Rats, Squirrels),
Order Carnivora (Tigers, Dogs),
Order Cetacea (Whales, Dolphins)