Chordata: Vertebrates and Key Traits

• Chapter 34 introduces the phylum Chordata, focusing on vertebrates and their characteristics.

Vertebrates

• Named for the vertebrae, the bones that form the backbone.
• Around 52,000 known vertebrate species exist.

Phylum Chordata

• Four key traits shared by all chordates include a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a postanal tail.

Notochord

• A flexible rod providing skeletal support.
• Develops into part of the backbone in vertebrates.

Dorsal Hollow Nerve Cord

• Originates from the ectoderm plate that rolls, developing into the central nervous system (brain and spinal cord).

Pharyngeal Gill Slits

• Used for suspension feeding and gas exchange or develops into parts of the ear, head, and neck.

Postanal Tail

• A tail that extends past the anus, present at least during one life stage.

Subphylum Cephalochordata

• Contains lancelets, known for their bladelike shape.
• Adults exhibit chordate characteristics.
• Commonly found in tropical and temperate marine environments with soft, sandy substrates.
• Suspension feeders.

Subphylum Urochordata

• Includes tunicates, also known as sea-squirts.
• Lacks a vertebral column.
• Sessile marine filter feeders.
• The larval stage displays chordate traits.

Clade Vertebrata

• Chordates characterized by vertebrae.
• Vertebrae can range from small cartilage prongs alongside the notochord to hardened structures enclosing the spinal cord.
• Have two or more sets of Hox genes.
• Have vertebrae enclosing a spinal cord.
• Possess an elaborate skull.
• Exhibit cephalization, concentrating sensory organs in the head.
• Have a closed circulatory system.

Class Myxini

• Includes hagfish.
• Jawless marine vertebrates.
• Possess a head with a cartilaginous skull, brain, eyes, and sensory organs.
• Lack a true backbone, having only rudimentary vertebrae.

Class Petromyzontida

• Includes lampreys.
• Jawless vertebrates.
• Display cartilaginous segments that surround the notochord and arch over the nerve cord.
• Inhabit temperate marine and freshwater areas.

Clade Gnathostoma

• Vertebrates with jaws.
• Jaws evolved from skeletal supports of the pharyngeal slits.
• Have paired appendages.
• Have a mineralized endoskeleton.
• Possess enhanced sensory systems, including sight and a sensory lateral line system.
• Three surviving lineages exist which include cartilaginous fish (Chondrichthyes), ray-finned fish (Actinopterygii), and lobe-finned fish (Sarcopterygii).

Class Chondrichthyes

• Includes sharks, rays, skates, and related species.
• Skeleton is primarily cartilage.
• Evolved from an ancestor with a mineralized skeleton.
• Use internal fertilization and display development occurs in the form of being oviparous, ovoviviparous, and viviparous.
• Need to keep swimming to breathe and avoid sinking.

Superclass Osteichthyes

• Largest amount of species, which includes Tetrapods,.
• Have a bony endoskeleton.

Class Actinopterygii

• Ray-finned fishes.
• Fins are supported by long, flexible rays.
• Fins are modified for maneuvering and defense.
• Breathe by drawing water over gills protected by an operculum.
• Control buoyancy with a swim bladder (air sac).
• Possess a lateral line system.

Class Sarcopterygii

• Includes lobe-finned fishes.
• Have four stubby, muscular fins with bony support.
• Use their fins to swim and move on substrates.
• Freshwater lungfish can use a swim bladder to help with buoyancy and survive when ponds dry up

Superclass Tetrapoda

• Defined by having 4 limbs and feet.
• Neck
• Possess a fused pelvic girdle to the backbone.
• Lack gills.
• Have ears.

Class Amphibia

• Includes frogs, toads, salamanders, and caecilians.
• Have appendages adapted for terrestrial movement.
• Exhibit an aquatic larval stage.
• Perform gas exchange through the skin.
• Use external fertilization.

Clade Amniota

• Defined by having tetrapods with a terrestrially adapted egg.
• Includes reptiles, birds, and mammals.
• Major derived character: Amniotic egg.
• Possess relatively impermeable skin.
• Use their rib cage to ventilate the lungs.

Class Reptilia

• Reptiles includes tuataras, lizards, snakes, turtles, crocodilians, birds and extinct dinosaurs.
• Scales contain waterproof keratin.
• Gas exchange occurs through the lungs.
• Internal fertilization.
• Most are ectothermic; birds are endothermic.

Class Aves

• Are part of Reptiles
• Contain wings with keratin feathers.
• Possess a modified forelimbs for flying.
• Gas exchange occurs through lungs (parabronchi).
• Examples include ostriches, penguins, ducks, songbirds, raptors, and loons.
• Lack a urinary bladder.
• Only one ovary for females.
• Possess small gonads in both sexes.
• Lack teeth.
• Likely descend from small theropods (carnivorous dinosaurs).

Class Mammalia

• Have hair and produce milk.
• Have mammary glands to produce milk.
• Possess a differentiated teeth.
• Have Amniotes
• Are generally have a larger brain than other vertebrates of equivalent size
• Endothermic
• Possess mammary glands.
• Exhibit differentiation of teeth.
• Evolved from synapsids.
• Have two bones that formerly made up the jaw joint incorporated into the mammalian middle ear.
• Monotremes are egg-laying mammals consisting of echidnas and the platypus.
• Marsupials include opossums, kangaroos, and koalas.
• Marsupials are born born very early in development with embryonic development is completed within their maternal pouch called a marsupium.
• Eutherians (placental mammals) have a longer period of pregnancy than marsupials, where young eutherians complete embryonic development within a uterus.
• All share a common need for certain processes early in embryogenesis.
• Primates includes includes lemurs, tarsiers, monkeys and apes.
• All share common grasping hands and feet adapted for grasping and flat nails.
• See opposable thumbs