CH.25 L.1

25.1 Vertebrate Origins

KEY CONCEPT

All vertebrates share common characteristics.


The phylum Chordata contains all vertebrates and some invertebrates.

• Chordates share four features at some stage of development.

– notochord

– hollow nerve cord tail

– pharyngeal slits

– tail hollow nerve cord notochord pharyngeal slits


• Most chordates lose some or all of these characteristics in adulthood. hollow nerve cord tail notochord pharyngeal slits


All vertebrates share common features.

• An endoskeleton allows vertebrates to grow to large sizes.

– internal

– made of bone or cartilage


• An endoskeleton can be divided into four parts.

– braincase (cranium)

– vertebrae

– bones

– gill arches (in fish and some amphibians) braincase vertebrae bones


• There are seven classes of vertebrates.

– Agnatha are jawless fish.

– Cartilaginous and bony fish are characterized by the presence of jaws.

– Amphibians are characterized by the presence of four limbs.

– Reptiles, birds, and mammals are characterized by the presence of an amnion.

– Birds are characterized by the presence of feathers.

– Mammals are characterized by the presence of hair.


Agnatha Chondrichthyes Osteichthyes Amphibia Aves Mammalia

VERTEBRAE

Vertebrates have a segmented backbone.

JAWS

Jaws helped vertebrates to become successful predators.

FOUR LIMBS

Four limbs let animals move from the water to life on land.

FEATHERS

Feathers insulate birds from the cold and allow for flight.

HAIR

Hair helps mammals to maintain constant body temperatures by roviding insulation from the cold.


Fossil evidence sheds light on the origins of vertebrates.

• Tunicates may be the closest relatives to vertebrates.

• The first recognizable vertebrates were jawless fish.

• Two groups of jawless fish still exist today.

– lampreys

– hagfish


KEY CONCEPT 25.2

The dominant aquatic vertebrates are fish.


Fish are vertebrates with gills and paired fins.

• Fish use specialized organs called gills to breathe underwater.

– sheets of thick, frilly tissue filled with capillaries

– take in dissolved oxygen from water, release carbon dioxide water flow


• Countercurrent flow is the opposite movement of water against the flow of blood in the fish’s gills.


• Fins are surfaces that project from a fish’s body.

– keep fish stable

– redirect water around fish as it swims

– help fish maneuver in water dorsal fin caudal fin anal fin pectoral fin pelvic fin


Jaws evolved from gill supports.

• Jaws developed from gill arches located around the pharynx.

cranium cranium cranium mouth gill arches mouth mouth

• Jaws gave vertebrates a huge advantage as predators.


Only two groups of jawed fish still exist.

• Cartilaginous fish and bony fish are still in existence.

• Cartilaginous fish have skeletons made of cartilage.


• Cartilaginous fish include the Holocephali and

Elasmobranchs.

– Holocephali include ratfish, a small group of deep-sea fish.

– Elasmobranchs include sharks, rays, and skates.


• All fish have a lateral line system.

– sensory system

– sensitive to small changes in water movement lateral line


• Bony fish have skeletons made of bone.

– operculum protects a bony fish’s gills

– movements of operculum help bony fish move water over gills



Bony fish include ray-finned and lobe-finned fish.


Ray-finned fish have a fan of bones in their fins.

• Ray-finned fish have fins supported by a fan-shaped array of bones.

– embedded in a thin layer of skin and connective tissue

– light, collapsible, and easy to move


• Ray-finned fish have a variety of body plans.

– long torpedo-shaped bodies (barracuda)



– flattened bodies (plaice)



– elaborate camouflage (sea dragon)


• A swim bladder helps a fish float higher or lower in the water.

swim bladder


• Some ray-finned fish have both lungs and gills.

– can breathe air and survive out of water for several hours at a time

– example: bichir found in West Africa


Lobe-finned fish have paired rounded fins supported by a single bone.

• Lobe-fins are paired pectoral and pelvic fins that are round in shape.

– not as maneuverable as ray-fins

– able to support weight lobe fin


• Only seven species of lobe-finned fish exist today.

– coelacanths

– lungfish



Amphibians evolved from lobe-finned fish.


Amphibians were the first animals with four limbs.

• Tetrapods are vertebrates that have four limbs.

The fossilized remains of Tiktaalik roseae indicate it was a transitional species between fish and tetrapods.


• Vertebrates that lack four limbs evolved from limbed ancestors.

• Amphibians are animals that can live both on land and in water.


• A number of adaptations allow amphibians to live on land.

– large shoulder and hip bones

– mobile, muscular tongue

– middle ear

– breathe through skin or with gills or lungs


Amphibians return to the water to reproduce.

• Amphibians use many strategies to keep their eggs wet.

– lay eggs directly in water

– lay eggs on moist ground

– wrap eggs in leaves

– brood eggs in pockets on the female’s back


• Tadpoles are aquatic larvae of frogs.

• During metamorphosis, tadpoles develop into their adult form.

adult frog fertilized eggs tadpoles young frog

• Not all amphibians undergo metamorphosis.


Modern amphibians can be divided into three groups.

• Salamanders have a long body, four walking limbs, and a tail.

• There are over 300 species of salamanders.


• Frogs are the largest amphibian group and include toads.

• There are over 3000 species of frogs.

• Glands in the skin of frogs and toads contain poisons that help to protect them from predators.


• Caecilians are legless, burrowing, tropical amphibians.

• There are 160 species of caecilians.



Reptiles, birds, and mammals are adapted for life on land.


Amniotes can retain moisture.

• An amniote develops inside a thin, tough, membranous sac as an embryo or fetus.

Amnion Protects and surrounds the embryo


• Several characteristics help amniotes prevent water loss.

– keratin forms a hydrophobic layer

– larger size of kidneys and intestines increases water absorption


Amniotes do not need to return to water to reproduce.

• The amniotic egg is an almost completely waterproof container.

– prevents embryo from drying out as it develops

– each egg represents a large investment of energy


• Not all amniotes lay eggs.

– reptiles such as garter snakes retain their eggs

– most mammals develop inside the mother’s reproductive tract


• The placenta is a membranous organ that develops in female mammals during pregnancy.

– lines the uterine wall and partially envelops the fetus

– carries nutrients from mother to embryo and removes wastes placenta umbilical cord uterus amniotic sac

CH.25 L.1

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