15.6

Animal

An organism that is a consumer and does not perform photosynthesis.

Animal feeding strategies

Animals capture food using many methods including filter feeding, stingers, strength, or specialized hunting adaptations.

Vertebrates

A subgroup of chordates characterized by the presence of a vertebral column (backbone).

Phylum Chordata

The phylum containing all chordates, including invertebrate chordates and vertebrates.

Chordate characteristics

The four defining features are a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

Notochord

A flexible support structure that provides strength for movement and serves as a place for muscles to push against.

Function of the notochord

Provides support and aids locomotion.

Notochord in vertebrates

Present during embryonic development and later replaced by the vertebral column in most vertebrates.

Vertebral column

A backbone made of vertebrae that replaces the notochord as the main support structure.

Dorsal hollow nerve cord

A hollow nerve cord located along the dorsal (back) side of the body.

Human dorsal hollow nerve cord

The spinal cord.

Spinal cord

Runs through the vertebral column and enlarges into the brain.

Pharyngeal slits

Openings in the throat region that allow water to pass through for filter feeding.

Function of pharyngeal slits

Used for filter feeding and gas exchange.

Gas exchange in pharyngeal slits

Large surface area allows oxygen to enter and carbon dioxide to leave.

Post-anal tail

A tail that extends beyond the digestive tract.

Function of the post-anal tail

Contributes to movement and is a defining chordate feature.

Subphylum Urochordata

The tunicates; invertebrate chordates.

Tunicates

Invertebrate chordates that use pharyngeal slits to filter food from water.

Subphylum Cephalochordata

The lancelets; invertebrate chordates.

Subphylum Vertebrata

Chordates with a vertebral column including fishes, amphibians, reptiles, birds, and mammals.

Backbone

May be made of bone or cartilage depending on the vertebrate group.

Evolution of vertebrates

Progresses from jawless fishes to jawed fishes, amphibians, reptiles, birds, and mammals.

Primitive vertebrates

The earliest vertebrates had simple vertebral columns and lacked jaws.

Major vertebrate evolutionary innovations

Vertebral column, jaws, paired fins, bony skeleton, lobed fins, tetrapod limbs, amniotic egg, mammary glands, and hair.

Jaws

Evolutionary innovation that allowed vertebrates to consume a wider variety of foods.

Paired fins

Improve swimming efficiency and maneuverability.

Bony skeleton

Skeleton composed primarily of bone rather than cartilage.

Lobed fins

Fleshy fins containing bones that resemble the limb bones of tetrapods.

Tetrapods

Vertebrates with four limbs.

Amniotic egg

An egg that can develop on land without drying out.

Importance of the amniotic egg

Allowed vertebrates to reproduce entirely on land.

Mammary glands

Glands that produce milk for offspring and define mammals.

Hair

A defining mammalian characteristic that helps maintain body temperature.

Jawless fishes

The earliest vertebrates, lacking movable jaws.

Lampreys

Jawless parasitic fishes with sucker-like mouths.

Lamprey feeding

Attach to fish and feed on blood and tissue fluids.

Lamprey mouth

Round sucker-like mouth without movable jaws.

Hagfish

Jawless scavengers that feed on dead organisms.

Scavenger

An organism that feeds on dead animals.

Hagfish tongue

Contains teeth used to scrape and bore into food.

Hagfish slime

A large amount of mucus released as a defense mechanism.

Hagfish flexibility

Have reduced vertebrae that allow them to tie themselves into knots.

Benefit of hagfish knotting

Helps generate torque for boring into food and assists with slime removal.

Jaw evolution

Jaws likely evolved from supportive arches around the gill slits.

Gill slits

Another name for pharyngeal slits in fishes.

Cartilaginous fishes

Fishes with skeletons made of cartilage instead of bone.

Examples of cartilaginous fishes

Sharks, skates, and rays.

Most cartilaginous fishes

Are carnivores.

Whale shark

A cartilaginous fish that feeds by filtering plankton.

Cartilage

Flexible connective tissue that forms the skeleton of cartilaginous fishes.

Advantages of jaws

Allow vertebrates to bite, grasp, and consume a much wider variety of prey.

Shark teeth

Attached to tissue and continuously replaced throughout life.

Bony fishes

Fishes with skeletons made primarily of bone (calcium phosphate).

Two major groups of bony fishes

Ray-finned fishes and lobed-finned fishes.

Ray-finned fishes

Fishes with delicate, thin rays supporting their fins.

Examples of ray-finned fishes

Most aquarium fish, food fish, seahorses, and true eels.

Lobed-finned fishes

Fishes with fleshy, muscular fins supported by bones.

Importance of lobed-finned fishes

They represent the evolutionary transition to four-limbed vertebrates.

Homologous structures

Bone structures shared among vertebrates because of common ancestry.

Lobed-fin bones

The fins contain bones similar to the arms and legs of tetrapods.

Advantage of lobed fins

Strong enough to support movement across land between ponds.

Natural selection and land movement

Individuals spending more time on land were favored, leading to larger limbs over generations.

Amphibians

The first vertebrates adapted for life on land while still reproducing in water.

Amphibian life cycle

Egg → Tadpole (larva) → Adult frog or salamander.

Amphibian reproduction

Eggs are laid in water.

Tadpole

The aquatic larval stage of frogs with gills and a tail.

Metamorphosis

The transformation from larval stage to adult form.

Changes during metamorphosis

Gills disappear, legs develop, and the tail is reabsorbed.

Adult amphibians

Typically live on land but return to water to reproduce.

Examples of amphibians

Frogs and salamanders.

Axolotl

An amphibian that retains its larval appearance throughout adulthood.

Axolotl characteristics

Retains external gills and remains aquatic throughout life.

Larval stage

A non-reproductive stage of development.

Adult stage

The reproductive stage that produces sex cells.

Water-to-land transition

A major evolutionary step in vertebrate history.

Requirement before animals moved onto land

Plants first had to colonize land to provide food.

Problem of living on land

Gills collapse without water and cannot function properly.

Solution to breathing on land

Lungs evolved for gas exchange.

Lungs

Internal respiratory organs that prevent drying out while allowing gas exchange.

Gravity on land

Required stronger skeletons to support body weight.

Natural selection and stronger skeletons

Individuals with stronger skeletons survived and reproduced more successfully.

Problem with reproduction on land

Unshelled eggs dry out easily.

Amphibian solution to egg drying

Return to water to lay eggs.

Amniotic egg

A waterproof egg that protects the developing embryo from drying out.

Importance of the amniotic egg

Made complete life on land possible.

Amniote

An animal that lays an amniotic egg.

Amnion

A membrane surrounding the embryo inside the amniotic egg.

Desiccation

Drying out.

Amniotic egg adaptation

Prevents desiccation of the embryo.

Tetrapods

Four-limbed vertebrates that evolved from lobed-finned fish.

Front limbs and hind limbs

A defining characteristic of tetrapods.

Scales in reptiles

Reduce water loss and prevent drying out.

Amphibian skin

Typically moist or slimy to reduce water loss.

Difference between amphibians and reptiles

Amphibians require water for reproduction; reptiles reproduce entirely on land.

Evolutionary importance of amphibians

Represent the transition between aquatic fishes and fully terrestrial vertebrates.

Gas exchange in amphibians

Adults use lungs while larvae use gills.

Evolutionary trend

Vertebrates gradually became better adapted for life on land through natural selection.

Natural selection

Individuals with beneficial adaptations survived and passed those traits to offspring.

Adaptation

A characteristic that improves survival or reproduction.