evolution of vertebrates

THE ORIGIN AND EVOLUTION

OF VERTEBRATES

KINDOM ANIMALIA

PHYLUM CHORDATA

Porifera

Cnidaria

Platyhelminthes

sponges jellyfish flatworms roundworms

Nematoda

Mollusca Arthropoda Chordata

Annelida Echinodermata

mollusks

multicellularity

Ancestral Protist

tissues

bilateral symmetry

body cavity

segmentation

Animal Evolution

coelom

starfish vertebrates

endoskeleton

segmentedworms

insectsspiders

backbone

specialization & ↑ body complexity

specialized structure & function,muscle & nerve tissue

distinct body plan; cephalization

↑ body complexity↑ digestive & repro sys

↑ digestive sys

↑ body size

redundancy,specialization, ↑ mobility

↑ body & brain size, ↑ mobility

radial

bilateral

ANCESTRALDEUTEROSTOME

Notochord

Commonancestor ofchordates

Head

Vertebral column

Jaws, mineralized skeleton

Lungs or lung derivatives

Lobed fins

Limbs with digits

Amniotic egg

Milk

Echinodermata

Cephalochordata

Urochordata

Myxini

Petromyzontida

Chondrichthyes

Actinopterygii

Actinistia

Dipnoi

Amphibia

Reptilia

Mammalia

Ch

ord

ate s

Cran

iates

Verteb

rat esG

nath

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Osteich

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Lo

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We will explore 11 clades of the phylum Chordata

Introduction to the vertebrates

A. Neural crest, enhanced cephalization, vertebral column, and a closed circulatory system characterize the subphylum Vertebrata 1. Neural crest a. Embryonic feature that allows for many unique vertebrate characteristics, e.g. bones and cartilage are formed from the neural crest cells throughout the body.b. Forms along the dorsal side of the embryo. The neural crest, embryonic source of many unique vertebrate characters.

Chordates have a notochord and a dorsal, hollow nerve cord

• Chordates (phylum Chordata) are bilaterian animals that belong to the clade of animals known as Deuterostomia

• Chordates comprise all vertebrates and two groups of invertebrates, the urochordates and cephalochordates

© 2011 Pearson Education, Inc.

1. Embryos all have a common skeletal structure called a notochord. The notochord is a flexible rod located between the digestive tube and nerve chord. a. Provides skeletal support.b. In most vertebrates, it’s replaced by a jointed skeleton.c. Remains of the notochord exist as disks between the vertebrae. 2. Dorsal, hollow nerve cord a. Develops into the brain and spinal cord of the adult.

3. Pharyngeal slits Water enters through the mouth and passes out through the slits in the pharynx, without going through the digestive system.

i. Slits function as suspension-feeding devices in many invertebrate chordates ii. Slits have been modified in more evolved vertebrates for: - Gas exchange - Hearing - Jaw support 4. Postanal tail Provides propulsion for swimming

Musclesegments

Notochord

Dorsal,hollow

nerve cord

Muscular,post-anal tail

Pharyngealslits or clefts

Anus

Mouth

4 Derived Characteristics of Chordates

Vertebrates– fish, amphibians, reptiles, birds, mammals

– internal bony skeleton• backbone encasing

spinal column

• skull-encased brain

– deuterostome

postanaltail notochord

hollow dorsalnerve cord

pharyngealpouches

Chordata

becomes brain & spinal cord

becomes vertebrae

becomes gills or Eustachian tube

becomes tail or tailbone

Mouth

Cirri

Pharyngeal slits

Atrium

Digestive tract

Atriopore

Segmentalmuscles

Anus

Notochord

Tail

Dorsal,hollow

nerve cord

1 cm

• Lancelets (Cephalochordata) are named for their bladelike shape

• They are marine suspension feeders that retain characteristics of the chordate body plan as adults

Cephalochordata

Notochord

Dorsal, hollownerve cord

Tail

Musclesegments

Intestine

Stomach

Atrium

Pharynx with slits

(a) Tunicate larva (b) Adult tunicate (c) Adult tunicate

Excurrentsiphon

Incurrentsiphon

Water flow

Excurrent siphon

Anus

Intestine

EsophagusStomach

Incurrentsiphonto mouth

Excurrentsiphon

Atrium

Tunic

Pharynxwith

numerousslits

• Tunicates (Urochordata) are more closely related to other chordates than are lancelets

• When attacked, tunicates, or “sea squirts,” shoot water through their excurrent siphon

Urochordata

Early Chordate Evolution

• Ancestral chordates may have resembled lancelets

• The same Hox genes that organize the vertebrate brain are expressed in the lancelet’s simple nerve cord tip

• Genome sequencing suggests that– Genes associated with the heart and thyroid are

common to all chordates– Genes associated with transmission of nerve

impulses are unique to vertebrates


Pharyngeal slits

Segmented muscles

5 mm

Hagfishes

• The most basal group of craniates is Myxini, the hagfishes

• Hagfishes have a cartilaginous skull and axial rod of cartilage derived from the notochord, but lack jaws and vertebrae

• They have a small brain, eyes, ears, and tooth-like formations

• Hagfishes are marine; most are bottom-dwelling scavengers


Lampreys

Lampreys

• Lampreys (Petromyzontida) represent the oldest living lineage of vertebrates

• They are jawless vertebrates that feed by clamping their mouth onto a live fish, suck blood

• They inhabit various marine and freshwater habitats

• They have cartilaginous segments surrounding the notochord and arching partly over the nerve cord


Dental elements(within head)

Conodonts were the first vertebrates with mineralized skeletal elements

Gnathostomes are vertebrates that have jaws

• Today, jawed vertebrates, or gnathostomes, outnumber jawless vertebrates

• Gnathostomes include sharks and their relatives, ray-finned fishes, lobe-finned fishes, amphibians, reptiles (including birds), and mammals


Fishes and amphibians A. Vertebrate jaws evolved from skeletal supports of pharyngeal slits

1. Animals that replaced jawless vertebrates, and are Gnathostomes. 2. Members of group have two pairs of fins. 3. Jaws and fins allowed fish to become active in pursuit of food and in biting off chunks of flesh.

4. Jaws evolved from modifications of skeletal elements of anterior pharyngeal gill slits.

Hypothesis for the evolution of vertebrate jaws

5. Fishes were prevalent about 360 to 400 million years ago- the “Age of Fishes” 6. Two groups are alive today: a. Class Chondricthyes: Sharks and rays have cartilaginous skeletons

Fossil of an early Gnathostome.

0.5 m

(a) Blacktip reef shark (Carcharhinus melanopterus)

(b) Southern stingray (Dasyatis americana)

(c) Spotted ratfish (Hydrolagus colliei)

Dorsal fins

Pectoralfins Pelvic fins

Chondrichthyans (Sharks, Rays, and Their Relatives)

Chondrichthyans (Sharks, Rays, and Their Relatives)

• Chondrichthyans (Chondrichthyes) have a skeleton composed primarily of cartilage

• The largest and most diverse group of chondrichthyans includes the sharks, rays, and skates


• The reproductive tract, excretory system, and digestive tract empty into a common cloaca

• Shark eggs are fertilized internally but embryos can develop in different ways

– Oviparous: Eggs hatch outside the mother’s body

– Ovoviviparous: The embryo develops within the uterus and is nourished by the egg yolk

– Viviparous: The embryo develops within the uterus and is nourished through a yolk sac placenta from the mother’s blood


Ray-Finned Fishes and Lobe-Fins

• The vast majority of vertebrates belong to a clade of gnathostomes called Osteichthyes

• Osteichthyans include the bony fish and tetrapods– Aquatic osteichthyans are the vertebrates we informally

call fishes


Nostril

BrainSpinal cord

Swimbladder

Dorsal fin

Adipose finCaudalfin

Cutedge ofoperculum

Gills

Kidney

Heart

Liver

Stomach

Intestine

Gonad

Pelvicfin

Anus

Anal fin

Lateralline

Urinarybladder

Ray-Finned Fishes

• Actinopterygii, the ray-finned fishes, include nearly all the familiar aquatic osteichthyans

• Ray-finned fishes originated during the Silurian period (444 to 416 million years ago)

• The fins, supported mainly by long, flexible rays, are modified for maneuvering, defense, and other functions


Yellowfin tuna (Thunnus albacares)

Redlionfish(Pteroisvolitans)

Commonsea horse(Hippocampusramulosus)

Fine-spotted moray eel(Gymnothorax dovii)

Lowerjaw

Scalycovering

Dorsal spine

5 cm

Lobe-Fins

• The lobe-fins (Sarcopterygii) have muscular pelvic and pectoral fins

• Lobe-fins also originated in the Silurian period

Tetrapods are gnathostomes that have limbs

• One of the most significant events in vertebrate history was when the fins of some lobe-fins evolved into the limbs and feet of tetrapods


Derived Characters of Tetrapods

• Tetrapods have some specific adaptations– Four limbs, and feet with digits

– A neck, which allows separate movement of the head

– Fusion of the pelvic girdle to the backbone– The absence of gills (except some aquatic

species)

– Ears for detecting airborne sounds


Flat skull

Eyes on top of skull

Head

Neck

Shoulder bonesRibs

Scales

Fin Fin skeleton

Elbow

Radius

Humerus

Ulna

“Wrist”

ScalesFinsGills andlungs

NeckRibsFin skeletonFlat skullEyes on topof skull

TetrapodCharacters

FishCharacters

Tetrapods evolved from specialized fishes that inhabited shallow water The origin of tetrapods.

1. The first tetrapods to spend much time on land were amphibians. Amphibian orders.

Order Urodela – Salamanders, retain tails as adults

Order Anura – Frogs, lack tails as adults

Order Apoda – Caecilians, lack legs

2. There were earlier tetrapods. These were specialized fish that

occupied shallow ponds, breathed air by gulping, and developed lobed walking fins for moving from one pond

to another. 3. Why go on dry land? There were no other competitors for plants and insects that serve as food.

4. Amphibians need to return to water to lay eggs and for development of larvae.

Lungfishes

Limbswith digits

Amphibians

Amniotes

Silurian PALEOZOICDevonian Carboniferous Permian

415 400 385 370 355 340 325 310 295 280 265 0Time (millions of years ago)

Key tolimb bones

UlnaRadiusHumerus

Eusthenopteron

Panderichthys

Tiktaalik

Acanthostega

Tulerpeton

(a) Order Urodela (salamanders)

(b) OrderAnura(frogs)

Order Apoda(caecilians)

(c)

• Amphibians (class Amphibia) are represented by about 6,150 species

• Order Urodela includes salamanders, which have tails

Amphibians

(a) Tadpole

(b) During metamorphosis

(c) Mating adults

• Amphibian means “both ways of life,” referring to the metamorphosis of an aquatic larva into a terrestrial adult

• Most amphibians have moist skin that complements the lungs in gas exchange


• Fertilization is external in most species, and the eggs require a moist environment

• In some species, males or females care for the eggs on their back, in their mouth, or in their stomach

ANCESTRALAMNIOTE

Parareptiles

Turtles

Crocodilians

Pterosaurs

Ornithischiandinosaurs

Saurischian dinosaursother than birds

Birds

Plesiosaurs

Ichthyosaurs

Tuataras

Squamates

Mammals

Rep

tilesS

ynap

sids

Diap

sids

Arch

osau

rs

Lep

ido

saurs

Din

osau

rs

Sau

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Amniotes (includes reptiles, mammals, and birds)

Amniotes are tetrapods that have a terrestrially adapted egg

A. Evolution of the amniotic egg expanded the success of vertebrates on land

1. Amniotic eggs allowed vertebrates to sever the link with water and live their whole lives on land.

2. Specialized membranes, called extra-embryonic membranes that function in gas exchange, waste storage, and transfer of nutrients.

a. Membranes develop from tissues derived from the embryo.

b. One membrane, the amnion, gives the name for the amniotic egg.

Reptiles• The reptile clade

includes the tuataras, lizards, snakes, turtles, crocodilians, birds, and some extinct groups

• Reptiles have scales that create a waterproof barrier

• Most reptiles lay shelled eggs on land


• Most reptiles are ectothermic, absorbing external heat as the main source of body heat

• Birds are endothermic, capable of keeping the body warm through metabolism


Reptilian heritage is evident in all amniotes 1. Scales of keratin, waterproof skin - prevent dehydration.- Reptiles cannot breathe through skin, so all gas exchange occurs via lungs. 2. Shelled amniotic eggs require internal fertilization. Shell forms around fertilized egg in the reproductive tract. 3. Reptiles don’t use metabolism to regulate body temperature; they are ectotherms. Ectotherms absorb external heat (i.e. sunlight) Reptiles are able to survive on about 10% of calories required by mammals. 4. Oldest reptiles are from the late Carboniferous (about 300 million years ago) dinosaurs and pterosaurs.

5. Modern reptiles include 6,500 species that are in four groups: a. Testudines – Turtles - Some species returned to water; all lay eggs on land. b. Sphenodontia – Tuataras c. Squamata – Lizards, snakes - Lizards are the most numerous group. - Snakes are descendants of lizards and have vestigial pelvic and limb bones. d. Crocodilia – Crocodiles, alligators - This is the group most closely related to dinosaurs .

(a) Tuatara(Sphenodonpunctatus)

Australianthorny devillizard (Molochhorridus)

(b)

(d) Eastern box turtle(Terrapene carolinacarolina)

(c) Wagler’s pit viper(Tropidolaemus wagleri)

(e) American alligator (Alligator mississippiensis)

Birds

• Birds are archosaurs, but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight


• Many characters of birds are adaptations that facilitate flight

• The major adaptation is wings with keratin feathers

• Other adaptations include lack of a urinary bladder, females with only one ovary, small gonads, and loss of teeth

Derived Characters of Birds


(a) Wing

Vane

Shaft

ForearmWrist

ShaftBarbBarbuleHook(c) Feather structure

(b) Bone structure

Finger 1

Finger 2

Finger 3

Palm

Birds began as feathered reptiles, evolved to fly: 1. Honeycombed skeletons are light and strong good for flight. Figure 34.25 (p. 698) – Form fits function: the avian wing and feather.

2. Toothless for weight reduction. 3. Endothermic = use metabolic energy to generate heat. - Feathers provide insulation. - Efficient circulatory system supports high rate of metabolism necessary for flying. 4. Acute vision Large brains that allow complex behavior. 5. Wings - Flight enhanced the ability to hunt and scavenge, escape predators, and move with changing seasons.

6. Theropods were the closest dinosaur relative of birds. Example: Velociraptor Archeopteryx is an example of a Mesozoic bird that shows reptilian features.

Archaeopteryx, a Jurassic bird-reptile.

7. Modern birds include about 8,600 species.

Some are flightless = ratites.

Behavior and morphology has adapted to fulfill distinct niches

Mammals are amniotes that have hair and produce milk

• Mammals, class Mammalia, are represented by more than 5,300 species

• Derived characters of mammals:– Mammary glands, which produce milk– Hair – A high metabolic rate, due to endothermy– A larger brain than other vertebrates of equivalent size

– Differentiated teeth

•


Mammals diversified extensively in the wake of the Cretaceous extinctions 1. Radiation of mammals occurred during two events: a. Extinction of dinosaurs b. Fragmentation of continents 2. There are about 4,500 species of extant mammals

4. The earliest mammals evolved from reptiles about 220 million years ago. Therapsids gave rise to mammals. Early example is the Morganucodon in previous figure. 5. Major groups of mammals: a. Monotremes – lay eggs and produce milk, but have no nipples.- Platypus, echidna

b. Marsupials – born early in embryonic development; climb to mother’s pouch and attach to a nipple.- Opossum, kangaroo

• Monotremes are a small group of egg-laying mammals consisting of echidnas and the platypus

Monotremes

Marsupials

• Marsupials include opossums, kangaroos, and koalas

• The embryo develops within a placenta in the mother’s uterus

• A marsupial is born very early in its development• It completes its embryonic development while nursing in a maternal pouch called a marsupium


(a) A young brushtail possum

(b) Long-nosed bandicoot

Convergent evolution of marsupials and eutherians (placental mammals).

Plantigale

Marsupialmammals

Eutherianmammals

Marsupial mole

Sugar glider

Wombat

Tasmanian devil

Kangaroo

Deer mouse

Mole

Flyingsquirrel

Woodchuck

Wolverine

Patagonian cavy

ANCESTRALMAMMAL

Mo

no

tremes

(5 specie s)

Marsu

pia ls

(324 spec ies)

Eu

therian

s(5,010 sp

ecies)

Monotremata

Marsupialia

ProboscideaSireniaTubulidentataHyracoideaAfrosoricidaMacroscelidea

Xenarthra

RodentiaLagomorphaPrimatesDermopteraScandentia

CarnivoraCetartiodactylaPerissodactylaChiropteraEulipotyphiaPholidota

Monotremata

Orders and Examples Main Characteristics Orders and Examples Main Characteristics

Platypuses,echidnas

Echidna

ProboscideaElephants

African elephant

SireniaManatees,dugongs

Manatee

Lay eggs; no nipples;young suck milk fromfur of mother

Long, muscular trunk;thick, loose skin; upperincisors elongatedas tusks

Aquatic; finlike fore-limbs and no hindlimbs; herbivorous

XenarthraSloths,anteaters,armadillos

Tamandua

LagomorphaRabbits, hares,picas

Jackrabbit

Reduced teeth or noteeth; herbivorous(sloths) or carnivorous(anteaters, armadillos)

Chisel-like incisors;hind legs longer thanforelegs and adaptedfor running and jumping;herbivorous

Sharp, pointed canineteeth and molars forshearing; carnivorous

Hooves with an evennumber of toes on eachfoot; herbivorous

Aquatic; streamlined body; paddle-like fore-limbs and no hind limbs; thick layer of insulating blubber;carnivorous

CarnivoraDogs, wolves,bears, cats,weasels, otters,seals, walruses Coyote

CetartiodactylaArtiodactylsSheep, pigs,cattle, deer,giraffes

CetaceansWhales,dolphins,porpoises

MarsupialiaKangaroos,opossums,koalas

Koala

Completes embryonicdevelopment in pouchon mother’s body

TubulidentataAardvarks

Aardvark

Teeth consisting ofmany thin tubescemented together;eats ants and termites

HyracoideaHyraxes

Rock hyrax

Short legs; stumpytail; herbivorous;complex, multi-chambered stomach

RodentiaSquirrels,beavers, rats,porcupines,mice Red squirrel

Chisel-like, continuouslygrowing incisors worndown by gnawing;herbivorous

PrimatesLemurs, monkeys,chimpanzees,gorillas, humans Golden lion

tamarin

Opposable thumbs;forward-facing eyes;well-developed cerebralcortex; omnivorous

PerissodactylaHorses, zebras,tapirs,rhinoceroses

Indian rhinoceros

Hooves with an oddnumber of toes oneach foot; herbivorous

ChiropteraBats

Frog-eating bat

Eulipotyphla“Coreinsectivores”:some moles,some shrews

Star-nosedmole

Eat mainly insectsand other smallinvertebrates

Adapted for flight;broad skinfold thatextends from elongatedfingers to body andlegs; carnivorous orherbivorousBighorn sheep

Pacific white-sided porpoise

Primates and the evolution of Homo sapiens A. Primate evolution provides context for understanding human origins 1. Hands and feet adapted for grasping. Possess opposable thumb. 2. Large brains allow complex social behavior.

– Hands, feet for grasping– Flat nails

– A large brain and short jaws

– Forward-looking eyes close together on the face, providing depth perception

– Complex social behavior and parental care

– A fully opposable thumb (in monkeys and apes)


Derived Characters of Primates

• There are three main groups of living primates– Lemurs, lorises, and pottos

– Tarsiers

– Anthropoids (monkeys and apes)

Living Primates


B. Hominid lineage diverged from other primates about 7 million years ago. Humans compared to other hominids: a. Brain size – large size allows development of language and social behavior. b. Jaw shape – shortened to give a flatter face.

c. Bipedalism = walking on two legs.

- Frees hands to do other things.

- Eyes set higher; can see farther.

d. Females smaller than males

e. Extended parental care changes family structure and enhances learning and social behavior.

d. Females smaller than males

e. Extended parental care changes family structure and enhances learning and social behavior.

ANCESTRALPRIMATE

Time (millions of years ago)

60 50 40 30 20 10 0

Lemurs, lorises,and bush babies

Tarsiers

New World monkeys

Old World monkeys

Gibbons

Orangutans

Gorillas

Chimpanzeesand bonobos

Humans

An

thro

po

ids

(b) Old World monkey: macaque(a)New World monkey:spider monkey with prehensile tail

(a) Gibbon(b) Orangutan

(c) Gorilla

(d) Chimpanzees

(e) Bonobos

Humans are mammals that have a large brain and bipedal locomotion

• The species Homo sapiens is about 200,000 years old, which is very young, considering that life has existed on Earth for at least 3.5 billion years


Derived Characters of Humans

• A number of characters distinguish humans from other apes– Upright posture and bipedal locomotion

– Larger brains capable of language, symbolic thought, artistic expression, the manufacture and use of complex tools

– Reduced jawbones and jaw muscles

– Shorter digestive tract


• The human and chimpanzee genomes are 99% identical

• How can we be this close, yet so different?


7.0

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

Mil

lio

ns

of

yea

rs a

go

Australo-pithecusanamensis

Kenyanthropusplatyops

Australopithecusafricanus

Paranthropusboisei

Paranthropusrobustus

Homoergaster

Homoneanderthalensis

Homosapiens

Homo erectus

Homo rudolfensisHomo

habilis

Australopithecusgarhi

?

Australopithecusafarensis

Ardipithecus ramidus

Orrorin tugensis

Sahelanthropustchadensis

• Hominins originated in Africa about 6–7 million years ago

• Early hominins show evidence of small brains and increasing bipedalism

Ardi, 4.4 million years old

• Misconception: Early hominins were chimpanzees– Correction: Hominins and chimpanzees shared a common ancestor

• Misconception: Human evolution is like a ladder leading directly to Homo sapiens– Correction: Hominin evolution included many branches or coexisting species, though only humans survive today


Australopiths

• Australopiths are a paraphyletic assemblage of hominins living between 4 and 2 million years ago

• Some species, such as Australopithecus afarensis walked fully erect


Evidence that Hominins walked upright 3.5 million years ago.

(a) The Laetoli footprints (b) Artist’s reconstruction of A. afarensis

• Homo erectus originated in Africa by 1.8 million years ago

• It was the first hominin to leave Africa


Neanderthals

• Neanderthals, Homo neanderthalensis, lived in Europe and the Near East from 350,000 to 28,000 years ago

• They were thick-boned with a larger brain, they buried their dead, and they made hunting tools

• Debate is ongoing about the extent to which genetic material was exchanged between neanderthals and Homo sapiens


Hypothesis: Neanderthals gave rise to European humans.

Expectedphylogeny:

Chimpanzees

Neanderthals

Living Europeans

Other living humans

Chimpanzees

Neanderthal 1

Neanderthal 2

European and otherliving humans

EXPERIMENT

RESULTS

Figure 34.50

Homo Sapiens

• Homo sapiens appeared in Africa by 195,000 years ago

• All living humans are descended from these African ancestors


• The oldest fossils of Homo sapiens outside Africa date back about 115,000 years and are from the Middle East

• Humans first arrived in the New World sometime before 15,000 years ago

• In 2004, 18,000-year-old fossils were found in Indonesia, and a new small hominin was named: Homo floresiensis


• Homo sapiens were the first group to show evidence of symbolic and sophisticated thought

• In 2002, a 77,000-year-old artistic carving was found in South Africa


Clade DescriptionC

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Cephalochordata(lancelets)

Urochordata(tunicates)

Myxini(hagfishes andrelatives)

Petromyzontida(lampreys)

Chondrichthyes(sharks, rays,skates, ratfishes)

Actinopterygii(ray-finned fishes)

Actinistia(coelacanths)Dipnoi(lungfishes)

Amphibia(salamanders,frogs, caecilians)

Reptilia(tuataras, lizardsand snakes, turtles,crocodilians, birds)

Mammalia(monotremes,marsupials,eutherians)

Basal chordates; marine suspension feeders thatexhibit four key derived characters of chordates

Marine suspension feeders; larvae display thederived traits of chordates

Jawless marine organisms; have head that includesa skull and brain, eyes, and other sensory organs

Jawless vertebrates; typically feed by attaching to alive fish and ingesting its blood

Aquatic gnathostomes; have cartilaginous skeleton,a derived trait formed by the reduction of anancestral mineralized skeletonAquatic gnathostomes; have bony skeleton andmaneuverable fins supported by rays

Ancient lineage of aquatic lobe-fins still survivingin Indian OceanFreshwater lobe-fins with both lungs and gills; sistergroup of tetrapods

Have four limbs descended from modified fins; mosthave moist skin that functions in gas exchange; manylive both in water (as larvae) and on land (as adults)

One of two groups of living amniotes; have amnioticeggs and rib cage ventilation, key adaptations for lifeon land

Evolved from synapsid ancestors; include egg-layingmonotremes (echidnas, platypus); pouched marsupials(such as kangaroos, opossums); and eutherians(placental mammals, such as rodents, primates)

Figure 34.UN10

evolution of vertebrates

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