Unit 26 Amniotes

26.1 Amniotes

KEY CONCEPT Reptiles, birds, and mammals are amniotes.

26.1 Amniotes

Amniote embryos develop in a fluid-filled sac.

• The amniotic sac contains everything an embryo needs to grow.– some develop inside mother’s body– some develop inside a tough, semipermeable shell

26.1 Amniotes

• The amniotic egg allowed vertebrates to reproduce on land.

Embryo

AllantoisHolds waste materialsas the embryo grows

Yolk sac Contains the nutrient supply for the growingembryo

AmnionProtects andsurrounds theembryo

Chorion Allows gas exchangewith outside environment

26.1 Amniotes

Anatomy and circulation differ among amniotes.

• Other amniotes, including dinosaurs, evolved a more upright stance.

• The first animals walked in a sprawl.

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• All amniotes have two circuits of blood vessels.

– pulmonary circuit moves blood from the heart to the lungs

– systemic circuit moves blood from the heart to the rest of the body

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• Amniotes have a three- or four-chambered heart.– reptiles hearts have three chambers

THREE-CHAMBERED HEART

– birds and mammals hearts have four chambers

FOUR-CHAMBERED HEART

26.1 Amniotes

Amniotes can be ectothermic or endothermic.

• Amniotes manage body heat in different ways.– Ectotherms have body temperatures determined by

the surrounding environment.– Endotherms use metabolic heat to keep tissues

warm.• Endotherms can live in a wider range of climates than

ectotherms.

26.1 Amniotes

KEY CONCEPT Reptiles were the first amniotes.

26.1 Amniotes

Reptiles are a diverse group of amniotes.

• Reptiles share several characteristics.– ectotherms– covered with dry scales– reproduce by laying or retaining amniotic eggs– three-chambered heart– cloaca

26.1 Amniotes

• Reptiles have two reproductive strategies.– Oviparous reptiles deposit eggs into an external nest.– Viviparous reptiles retain eggs and give birth to live

offspring.

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Reptiles have been evolving for millions of years.

• Over time, amniotes evolved into three different groups. – synapsids– anapsids – diapsids

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• The diversity of ancient reptiles led to the evolution of modern reptiles, birds, and mammals.

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There are four modern groups of reptiles.

• Turtles, tortoises, and terrapins are the remaining anapsids.– bony shell encases body– 200 species

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• Sphenodonts are closely related to lizards.– diapsids– primitive characteristics– two species

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• Snakes and lizards are very closely related and share a number of features.– diapsids– shed skin at regular intervals– flexible skull– Jacobson’s organ

brain

tongue

Jacobson’sorgan

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• Crocodilians are more closely related to birds than other diapsids.– diapsids– semi-aquatic predators– 23 species

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KEY CONCEPT Birds have many adaptations for flight.

26.1 Amniotes

Birds evolved from theropod dinosaurs.

• Birds and many theropods share anatomical features.– hollow bones– fused collarbones that form V-shaped wishbone– rearranged muscles in the hips and legs – “hands” that have lost their fourth and fifth fingers– feathers

26.1 Amniotes

• The oldest undisputed fossilized bird is Archaeopteryx.

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A bird’s body is specialized for flight.

• Birds have several unique features that allow them to fly.– wings to produce flight– strong flight muscles to

move the wings– active metabolism that

provides energy to the muscles

– hollow bone structure to minimize weight

– gonads active during only part of year

small intestine

largeintestine

lung

gizzard

kidney

cloaca

crop

sternum(keel)

heartliver

pectoral muscle

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• Wings are structures that enable birds to fly.– airfoil shape– covered with feathers

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• Air sacs help a bird meet its oxygen demand during flight.

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Birds have spread to many ecological niches.

• The shape of a bird’s wing reflects the way it flies.– short and broad– long and narrow

26.1 Amniotes

small intestine

largeintestine

lung

gizzard

kidney

cloaca

crop

sternum(keel) heart

liver

pectoral muscle

– wide and broad

• The shape of a bird’s wing reflects the way it flies.

– stout and tapered

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• Differences in the shape of a bird’s beak reflects how it eats.– spearlike– hooked

– chisel-shaped

blue-footed booby green woodpeckerBald eagle

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• Birds show great diversity in their foot shape.– webbed

blue-footed boobybald eagle

green woodpecker

– heavy claws

– different toe location

26.1 Amniotes

KEY CONCEPT Evolutionary adaptations allowed mammals to succeed dinosaurs as a dominant terrestrial vertebrate.

26.1 Amniotes

All mammals share several common characteristics.

• Mammals are active, large-brained, endotherms with complex social, feeding, and reproductive behaviors.

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• All mammals share four anatomical characteristics.– hair to retain heat– mammary glands to produce milk

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– a middle ear with three bones to hear higher-pitched sounds

– chewing jaw to break up food quicker

• All mammals share four anatomical characteristics.

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• A set of adaptations in the mammalian jaw makes chewing possible.– secondary palate

closes off air passages

– muscles move jaw side-to-side

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Modern mammals are divided into three main groups.

• Monotremes lay eggs.– duck-billed platypus– echidna

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• Marsupials give birth to live young that grow to maturity inside a pouch.– opossum– kangaroo– wombat– koala

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• Eutherians give birth to live young that have completed fetal development.– most familiar mammals– humans

• Eutherians filled many niches after the extinction of the dinosaurs.