unit 26 amniotes
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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
26.1 Amniotes
• 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
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• 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
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• 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
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KEY CONCEPT Evolutionary adaptations allowed mammals to succeed dinosaurs as a dominant terrestrial vertebrate.
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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.