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Vertebrates

Chapter 34

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The Chordates

• Distinguished by four principle features some time in their lives:

– nerve cord– notochord– pharyngeal slits– postnatal tail

• Muscles arranged in segmented blocks• Most have internal skeleton

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Principle Chordate Features

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The Nonvertebrate Chordates

• Tunicates– exhibit neither a major body cavity nor

visible segmentation tadpole larva clearly exhibit all basic

characteristics of a chordate adults exist as sessile filter-feeders

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Tunicates

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The Nonvertebrate Chordates

• Lancelets– scaleless, fishlike marine chordates

notochord runs entire length of dorsal nerve cord

feed on microscopic plankton using cilia-generated current

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Characteristics of Vertebrates

• Vertebral column• Distinct, well-differentiated head• Neural crest• Internal organs• Endoskeleton

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Overview of the Evolution of Vertebrates

• First vertebrates evolved in the oceans about 470 mya.

– hinged-jaw– amphibians on land– reptiles take over– split into birds and mammals

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Overview of the Evolution of Vertebrates

• Four classes are land-dwelling tetrapods– Amphibia - amphibians– Reptilia - reptiles– Aves - birds– Mammalia - mammals

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Fishes

• Over half of all vertebrates are fishes.• Characteristics

– vertebral column– jaws and paired appendages– gills– single-loop blood circulation– nutritional deficiencies

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Evolution of the Fishes

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History of the Fishes

• First fishes– members of five Ostracoderm orders

jawless bottom-dwellers• Evolution of the jaw

– jaws developed about 410 mya

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History of the Fishes

• Rise of active swimmers– Sharks and bony fishes replaced primitive

fishes due to a superior swimming design. caudal (tail) fin dorsal (stabilizing) fins pectoral (shoulder - elevator) fins pelvic (hip- elevator) fins

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History of the Fishes

• Sharks become top predators– sharks among first vertebrates to develop

teeth sit on top of jaws programmed tooth loss

teeth are always new and sharp– extremely advanced reproduction

shark eggs fertilized internally

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History of the Fishes

• Bony fishes dominate the water– bony fish evolved at same time as sharks,

but adopted a heavy internal skeleton made of bone

strong base for muscles– evolved in fresh water– highly mobile fins, thin scales, and

completely symmetrical tails

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History of the Fishes

• Important adaptations of bony fishes– swim bladder - regulates buoyancy– lateral line system - assesses rate of

movement through water as pressure waves against the lateral line

– gill cover (operculum) - flexing the operculum permits bony fish to pump water over their gills

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History of the Fishes

• Path to land– Lobe-finned fishes evolved 390 mya.

have paired fins that consist of a long fleshy muscular lobe supported by a central core of bones that form fully articulated joints

amphibians almost certainly evolved from lobe-finned fishes

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Lobe-Finned Fish and Primitive Amphibians

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Amphibians

• First vertebrates to walk on land• Characteristics

– legs– cutaneous respiration– lungs– pulmonary veins– partially divided heart

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History of the Amphibians

• Adaptations for the invasion of land– legs to support body’s weight– lung to extract oxygen from the air– redesigned heart to drive new respiratory

system– reproduction in water to prevent egg

desiccation– system to prevent body desiccation

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History of the Amphibians

• Rise and fall of amphibians– became common during Carboniferous

period 360-280 mya– began to leave marshes for dry uplands

during early Permian period large size and complete body covering

indicate skin was not used as respiratory system

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History of the Amphibians

• By the end of Permian, therapsid (reptile) ousted amphibians from their niche on land

– by the end of the Triassic, there were only 15 families of amphibians left

almost all were aquatic only two groups are known from

Jurassic period (213-144 mya) Anura - frogs and toads Urodela - salamanders and newts

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History of the Amphibians

• Amphibians today– Anura - amphibians without tails– most live in or near water, and return to

water to reproduce eggs fertilized externally and hatch into

tadpolesmetamorphosis

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History of the Amphibians

– Urodela (Caudata) - salamanders have elongated bodies, long tails, and

sooth, moist skin fertilization is usually external

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History of the Amphibians

– Apoda (Gymnophiona) caecilians - highly specialized group of

tropical burrowing amphibians legless, but have jaws and teeth internal fertilization

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Reptiles

• Characteristics– amniotic egg

chorion - outermost membrane amnion - encases embryo yolk sac - surrounds yolk (food) allantois - surrounds waste cavity

– dry skin– thoracic breathing

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Rise and Fall of Dominant Reptiles

• Pelycosaurs: a better predator– first land vertebrates to kill organisms their

own size

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Rise and Fall of Dominant Reptiles

• Therapsids: speeding up metabolism– extremely high food consumption

endotherms?

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Rise and Fall of Dominant Reptiles

• Thecodonts: wasting less energy– warmer climates - ectothermic– first bipedal land vertebrates

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Rise and Fall of Dominant Reptiles

• Dinosaurs: learning to run– body located directly over legs

increased speed and agility

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Today’s Reptiles

• Of the 16 orders of reptiles that have existed, only 4 survive

– turtles– lizards and snakes– tuataras– crocodiles

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Evolutionary Relationships

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Rise and Fall of Dominant Reptiles

• Other important characteristics– internal fertilization– improved circulatory system– ectothermic - heat obtained from external

sources endothermic - generate own heat

homeothermic - constant body temperature

poikilothermic - body temperature fluctuates with ambient temperature

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Living Reptiles

• Order Chelonia: turtles and tortoises– differ from other reptiles because their

bodies are encased within a protective shell

anapsid - lack temporal opening in the skull, characteristic of other living reptiles

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Rise and Fall of Dominant Reptiles

• Order Rhynchocephalia: tuatara– lizardlike animals about half a meter long– contain parietal eye– only found on island off New Zealand coast

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Rise and Fall of Dominant Reptiles

• Order Squamata: lizards and snakes– three suborders

Sauria - lizards Amphisbaenia - worm lizards Serpentes - snakes

– paired copulatory organ in males– lower jaw not joined directly to skull

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Rise and Fall of Dominant Reptiles

• Order Crocodilia: crocodiles and alligators– remained relatively unchanged

only two species of alligatorssouthern US and China

– resemble birds more than other living reptiles (care for young and four-chambered heart)

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Birds

• Class Aves contains 28 orders containing 166 families and about 8,600 species.

– key characteristics feathers

modified reptilian scales flight skeleton

thin, hollow bones

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History of the Birds

• Descended from dinosaurs– Archaeopteryx– Aves listed as separate class because of

key evolutionary novelties of feathers, light bones, and super-efficient lungs

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History of the Birds

• Birds today– adaptations for flight energy demands

efficient respiration efficient circulation endothermy

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Mammals

• Key mammalian characteristics– hair

heat loss camouflage sensory structures defense weapon

– mammary glands about 50% of energy in milk comes from

fat

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Key Mammalian Characteristics

• endothermy– crucial adaptation that allowed activity at

any time of the day to colonize severe environments

• placenta– specialized organ allowing food, water,

and oxygen to pass from mother to child• teeth

– heterodont dentition

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Key Mammalian Characteristics

• digestion of plants– cellulose major source of food for herbivores

mammals do not have necessary digestive enzymes to break apart cellulose

some have evolved four-chambered stomachs

some contain mutualistic bacteria in a cecum

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Key Mammalian Characteristics

• hooves and horns– hooves specialized pads of keratin– horns composed of core of bone surrounded

by keratin sheath• flight

– bats have wing of leathery membrane of skin stretched over the bones of four fingers

second largest order of mammalsecholocation

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Orders of Mammals

• Origin of mammals– first mammals arose about 220 mya– tiny shrewlike creatures with large eye

sockets - nocturnal?• Early divergence

– Subclass Prototheria duckbill platypus

– Subclass Theria marsupials and placental mammals

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History of the Mammals

• Orders of mammals– monotremes: egg-laying mammals

lay shelled eggs– marsupials: pouched mammals

finish development in external pouch– placental mammals

placenta nourishes embryo throughout entire development

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Evolution Among Primates

• Primates– two distinct features allowed them to

succeed in arboreal environment: grasping fingers and toes binocular vision

• Evolution of prosimians– earliest primates split into prosimians and

anthropoids about 40 mya “before monkeys”

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Anthropoids

• Higher primates - includes apes, monkeys, and humans

– one of most contentious issues in primate biology is identity of first anthropoid

• Direct descendents:– New World monkeys – Old World monkeys

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Primate Evolutionary Tree