Chordate Evolution

• Chordates are divided into two main groups: invertebrate chordates and vertebrates.
• Chordates have four key features: a tail, notochord, hollow nerve cord, and pharyngeal slits.
• The tail extends posterior to the anus, becoming the tailbone in adults.
• The notochord is a flexible supporting rod, which develops into part of the spinal column in adults.
• The hollow nerve cord runs along the dorsal surface and becomes part of the spinal cord in adults.
• Pharyngeal slits are grooves behind the mouth, the remnants form structures like the eustachian tubes.

Invertebrate Chordates

• Invertebrate chordates lack a skull and backbone, unlike vertebrates.
• Lancelets are small, sword-shaped animals that burrow into sand and filter water for food.
• Tunicates are sessile organisms that adhere to surfaces and filter seawater.

Early Vertebrates

• Vertebrates evolved approximately 500 million years ago during the Cambrian period.
• The Devonian period (419-359 million years ago) is the 'Age of Fish', marked by diverse fish species.
• Jawless fishes, like hagfishes and lampreys, were among the earliest vertebrates, possessing skulls but no jaws.

Types of Vertebrates

• Cartilaginous fishes, including sharks and rays, evolved around 470 million years ago and have flexible cartilage skeletons.
• Bony fishes are the most diverse vertebrate group, with skeletons reinforced with calcium, adapted for buoyancy and respiration.
• Lobe-finned fishes, ancestors of amphibians, have muscular fins supported by rod-shaped bones.

Tetrapod Evolution

• Tetrapods are terrestrial vertebrates with four limbs, evolving from lobe-finned fishes.
• The first tetrapods were amphibians, showing both aquatic and terrestrial adaptations.
• Modern tetrapods, including amphibians and reptiles, evolved from these early forms.

Amphibians

• Amphibians reproduce in water, with aquatic and terrestrial adaptations.
• Aquatic adaptations include eggs without shells, gills in larvae, and a lateral line system.
• Terrestrial adaptations include lungs, moist skin for respiration, and strong limbs.

Reptiles

• Reptiles are adapted to land, with amniotic eggs for embryo life support.
• Reptiles are in four groups: turtles, crocodilians, dinosaurs & birds, and lizards & snakes.
• Birds evolved from dinosaurs around 160 million years ago, adapted for flight.

Mammalian Features

• Mammals have mammary glands for milk to nourish young and have extended parental care.
• Hair is a defining feature, insulating and maintaining constant internal temperature (endothermy).
• Mammals regulate their body temperature through metabolic processes, unlike ectothermic animals.

Types of Mammals

• Monotremes are egg-laying mammals, like the platypus, secreting milk onto fur for feeding young.
• Marsupials give birth to underdeveloped young that develop in a pouch attached to the mother.
• Marsupials include kangaroos and koalas, primarily in Australia.