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Chapter 34: The Origin and Evolution of Vertebrates

Concept 34.1: Chordates

Chordates are animals with backbones or related structures. They belong to the group Deuterostomia and include all vertebrates (animals with backbones) and invertebrate urochordates and cephalochordates (animals without backbones but related).

Key Features of Chordates

  • Notochord: A flexible rod that supports the body.

  • Dorsal Hollow Nerve Cord: A tube that develops into the brain and spinal cord.

  • Pharyngeal Slits/Clefts: Openings in the throat area that are used for different purposes such as:

    • Suspension feeding in invertebrates (filtering food from water).

    • Gas exchange (breathing) in vertebrates.

    • Development into head and neck parts in tetrapods (four-limbed animals).

  • Muscular Post-Anal Tail: A tail that extends beyond the anus, used for swimming in aquatic species.

Examples of Chordates

  • Lancelets (Cephalochordata): Small, fish-like animals that filter food from the water.

  • Tunicates (Urochordata): Marine animals that are chordates as larvae but become filter feeders as adults. They also have fewer Hox genes (genes that control body development) compared to other chordates.

Craniates

Chordates with a head, skull, brain, eyes, and sensory organs. This group includes all vertebrates and some invertebrates.

Key Features of Craniates

  • They have two clusters of Hox genes.

  • Neural crest cells, which form bones and cartilage in the skull.

Concept 34.2: Vertebrates

Vertebrates are chordates with a backbone, a well-developed skull, and fin rays (in aquatic forms).

Examples of Jawless Vertebrates

  • Hagfishes and Lampreys: These are jawless vertebrates that form the group called cyclostomes.

Origins of Bone and Teeth

The first signs of mineralization (formation of hard tissues) appeared in vertebrate mouthparts.

Concept 34.3: Gnathostomes

Gnathostomes are vertebrates with jaws. This group includes diverse animals such as sharks, ray-finned fishes, and tetrapods.

Key Features of Gnathostomes

  • Jaws evolved from skeletal rods.

  • Genome duplication (more genes).

  • Enlarged forebrain (larger brain size).

  • Lateral line system (sensory organs to detect movement and vibration in water).

Examples of Gnathostomes

  • Chondrichthyans (Chondrichthyes): Animals with a cartilage skeleton, including sharks, rays, and skates.

Sharks

Sharks are carnivores with excellent senses and reproduce through internal fertilization. They can be oviparous (laying eggs), ovoviviparous (eggs hatch inside the mother), or viviparous (live birth).

Ray-Finned Fishes and Lobe-Fins

Osteichthyans are animals with bony skeletons. Aquatic forms are called fishes.

Ray-Finned Fishes (Actinopterygii)

These have fins supported by flexible rays.

Lobe-Fins (Sarcopterygii)

These have muscular fins and include coelacanths, lungfishes, and tetrapods.

Concept 34.4: Tetrapods

Tetrapods are gnathostomes with limbs, adapted for living on land.

Key Features of Tetrapods

  • Four limbs with digits (fingers and toes).

  • A neck, allowing head movement.

  • Pelvic girdle fused to the backbone for support.

  • No gills in some species.

  • Ears for detecting airborne sounds.

Amphibians

Amphibians include:

  • Urodela (salamanders).

  • Anura (frogs).

  • Apoda (caecilians).

Lifestyle and Ecology of Amphibians

Amphibians undergo metamorphosis (transformation from larval to adult form).

  • They require moist skin.

  • External fertilization (eggs are fertilized outside the body).

  • Amphibian populations are declining due to various factors.

Concept 34.5: Amniotes

Amniotes are tetrapods with eggs that are adapted for life on land. This group includes reptiles, birds, and mammals.

Key Features of Amniotes

  • Amniotic egg with extraembryonic membranes (specialized membranes that protect and nourish the developing embryo).

  • Terrestrial adaptations such as impermeable skin (skin that prevents water loss).

Reptiles

Reptiles include tuataras, lizards, snakes, turtles, crocodilians, and birds. They have scales and lay shelled eggs.

Birds

Birds are adapted for flight with wings, feathers, and specialized anatomy.

Concept 34.6: Mammals

Mammals are amniotes with hair and mammary glands (glands that produce milk).

Key Features of Mammals

  • Milk production for feeding young.

  • Hair for insulation.

  • High metabolic rate (energy use).

  • Large brain.

  • Differentiated teeth (different types of teeth for different functions).

Types of Mammals

  • Monotremes: Egg-laying mammals like echidnas and platypuses.

  • Marsupials: Mammals that develop in the uterus with a placenta but complete their development in a pouch (marsupium).

  • Eutherians (Placental Mammals): Mammals with a complex placenta that allows for complete development in the uterus.

Primates

Primates include lemurs, tarsiers, monkeys, apes, and humans. They are adapted for grasping with forward-looking eyes and exhibit social behavior.

Concept 34.7: Humans

Humans are mammals with large brains and the ability to walk on two legs (bipedal locomotion).

Key Features of Humans

  • Upright posture.

  • Large brains.

- Reduced jaws.


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Chapter 34: The Origin and Evolution of Vertebrates

Concept 34.1: Chordates

Chordates are animals with backbones or related structures. They belong to the group Deuterostomia and include all vertebrates (animals with backbones) and invertebrate urochordates and cephalochordates (animals without backbones but related).

Key Features of Chordates

  • Notochord: A flexible rod that supports the body.

  • Dorsal Hollow Nerve Cord: A tube that develops into the brain and spinal cord.

  • Pharyngeal Slits/Clefts: Openings in the throat area that are used for different purposes such as:

    • Suspension feeding in invertebrates (filtering food from water).

    • Gas exchange (breathing) in vertebrates.

    • Development into head and neck parts in tetrapods (four-limbed animals).

  • Muscular Post-Anal Tail: A tail that extends beyond the anus, used for swimming in aquatic species.

Examples of Chordates

  • Lancelets (Cephalochordata): Small, fish-like animals that filter food from the water.

  • Tunicates (Urochordata): Marine animals that are chordates as larvae but become filter feeders as adults. They also have fewer Hox genes (genes that control body development) compared to other chordates.

Craniates

Chordates with a head, skull, brain, eyes, and sensory organs. This group includes all vertebrates and some invertebrates.

Key Features of Craniates

  • They have two clusters of Hox genes.

  • Neural crest cells, which form bones and cartilage in the skull.

Concept 34.2: Vertebrates

Vertebrates are chordates with a backbone, a well-developed skull, and fin rays (in aquatic forms).

Examples of Jawless Vertebrates

  • Hagfishes and Lampreys: These are jawless vertebrates that form the group called cyclostomes.

Origins of Bone and Teeth

The first signs of mineralization (formation of hard tissues) appeared in vertebrate mouthparts.

Concept 34.3: Gnathostomes

Gnathostomes are vertebrates with jaws. This group includes diverse animals such as sharks, ray-finned fishes, and tetrapods.

Key Features of Gnathostomes

  • Jaws evolved from skeletal rods.

  • Genome duplication (more genes).

  • Enlarged forebrain (larger brain size).

  • Lateral line system (sensory organs to detect movement and vibration in water).

Examples of Gnathostomes

  • Chondrichthyans (Chondrichthyes): Animals with a cartilage skeleton, including sharks, rays, and skates.

Sharks

Sharks are carnivores with excellent senses and reproduce through internal fertilization. They can be oviparous (laying eggs), ovoviviparous (eggs hatch inside the mother), or viviparous (live birth).

Ray-Finned Fishes and Lobe-Fins

Osteichthyans are animals with bony skeletons. Aquatic forms are called fishes.

Ray-Finned Fishes (Actinopterygii)

These have fins supported by flexible rays.

Lobe-Fins (Sarcopterygii)

These have muscular fins and include coelacanths, lungfishes, and tetrapods.

Concept 34.4: Tetrapods

Tetrapods are gnathostomes with limbs, adapted for living on land.

Key Features of Tetrapods

  • Four limbs with digits (fingers and toes).

  • A neck, allowing head movement.

  • Pelvic girdle fused to the backbone for support.

  • No gills in some species.

  • Ears for detecting airborne sounds.

Amphibians

Amphibians include:

  • Urodela (salamanders).

  • Anura (frogs).

  • Apoda (caecilians).

Lifestyle and Ecology of Amphibians

Amphibians undergo metamorphosis (transformation from larval to adult form).

  • They require moist skin.

  • External fertilization (eggs are fertilized outside the body).

  • Amphibian populations are declining due to various factors.

Concept 34.5: Amniotes

Amniotes are tetrapods with eggs that are adapted for life on land. This group includes reptiles, birds, and mammals.

Key Features of Amniotes

  • Amniotic egg with extraembryonic membranes (specialized membranes that protect and nourish the developing embryo).

  • Terrestrial adaptations such as impermeable skin (skin that prevents water loss).

Reptiles

Reptiles include tuataras, lizards, snakes, turtles, crocodilians, and birds. They have scales and lay shelled eggs.

Birds

Birds are adapted for flight with wings, feathers, and specialized anatomy.

Concept 34.6: Mammals

Mammals are amniotes with hair and mammary glands (glands that produce milk).

Key Features of Mammals

  • Milk production for feeding young.

  • Hair for insulation.

  • High metabolic rate (energy use).

  • Large brain.

  • Differentiated teeth (different types of teeth for different functions).

Types of Mammals

  • Monotremes: Egg-laying mammals like echidnas and platypuses.

  • Marsupials: Mammals that develop in the uterus with a placenta but complete their development in a pouch (marsupium).

  • Eutherians (Placental Mammals): Mammals with a complex placenta that allows for complete development in the uterus.

Primates

Primates include lemurs, tarsiers, monkeys, apes, and humans. They are adapted for grasping with forward-looking eyes and exhibit social behavior.

Concept 34.7: Humans

Humans are mammals with large brains and the ability to walk on two legs (bipedal locomotion).

Key Features of Humans

  • Upright posture.

  • Large brains.

- Reduced jaws.