THE ORIGIN AND EVOLUTION

OF VERTEBRATES

KINDOM ANIMALIA

PHYLUM CHORDATA

Animal Evolution

Porifera

Cnidaria

Nematoda Platyhelminthes

sponges

jellyfish

flatworms

roundworms mollusks

Mollusca

Echinodermata Arthropoda

Chordata

Annelida

multicellularity

Ancestral Protist

tissues

bilateral symmetry

body cavity

segmentation

coelom

insects starfish

vertebrates

↑ body size endoskeleton

segmented

worms

spiders

backbone

specialization & ↑ body complexity

specialized structure & function, muscle & nerve tissue

distinct body plan; cephalization

^↑ body complexity

↑ digestive & repro sys

↑ digestive sys

redundancy,

specialization, ↑ mobility

^↑ body & brain

size, ↑ mobility

radial

bilateral

ANCESTRAL DEUTEROSTOME

Notochord Common

ancestor of chordates

Head

Vertebral column

Jaws, mineralized skeleton

Lungs or lung derivatives

Lobed fins

Limbs with digits

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Amniotic egg

Milk

Echinodermata

Cephalochordata

Urochordata

Myxini

Petromyzontida

Chondrichthyes

Actinopterygii

Actinistia

Dipnoi

Amphibia

Reptilia

Mammalia

Chordates

Craniates

Vertebrates

Gnathostomes

Osteichthyans

Lobe-fins

Tetrapods

Amniotes

We will explore 11 clades of the phylum Chordata

Introduction to the vertebrates

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1. Neural crest, enhanced cephalization, vertebral column, and a closed circulatory system characterize the subphylum Vertebrata

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• Neural crest

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• Embryonic feature that allows for many unique vertebrate characteristics, e.g. bones and cartilage are formed from the neural crest cells throughout the body.
• Forms along the dorsal side of the embryo.

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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

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

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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.

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• Provides skeletal support.
• In most vertebrates, it’s replaced by a jointed skeleton.
• Remains of the notochord exist as disks between the vertebrae.

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• Dorsal, hollow nerve cord

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• Develops into the brain and spinal cord of the adult.

3. Pharyngeal slits

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

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• Slits function as suspension-feeding devices in many invertebrate chordates
• Slits have been modified in more evolved vertebrates for:
• Gas exchange
• Hearing
• Jaw support

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4. Postanal tail

Provides propulsion for swimming

4 Derived Characteristics of Chordates

Muscle segments

Notochord

Dorsal, hollow nerve cord

Muscular, post-anal tail

Pharyngeal slits or clefts

Anus

Mouth

Chordata

• Vertebrates

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^– fish, amphibians, reptiles, birds, mammals

• internal bony skeleton

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• backbone encasing spinal column

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• skull-encased brain

postanal tail

notochord

pharyngeal pouches

– _deuterostome ^{becomes gills or}

hollow dorsal nerve cord

becomes brain

& spinal cord

becomes vertebrae

Eustachian tube

becomes tail or tailbone

Cirri

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Mouth Pharyngeal slits

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Atrium

Digestive tract Atriopore Segmental

muscles

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Anus

Notochord

Tail

Dorsal, hollow nerve cord

1 cm

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Lancelets (Cephalochordata) are named for their bladelike shape

They are marine suspension

feeders that retain characteristics of the chordate body plan as adults

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Cephalochordata

Notochord

Dorsal, hollow nerve cord

Tail

Muscle segments

Intestine Stomach

Atrium

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Pharynx with slits

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(a) Tunicate larva

Anus Intestine Esophagus

Stomach

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(b) Adult tunicate

(c) Adult tunicate

Excurrent siphon

Incurrent siphon

Water flow

Excurrent siphon

Incurrent siphon to mouth

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Excurrent siphon

Atrium

Tunic

Pharynx with numerous slits

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

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

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Urochordata

Early Chordate Evolution

• Ancestral chordates may have resembled lancelets

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

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• Genes associated with the heart and thyroid are common to all chordates

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• Genes associated with transmission of nerve impulses are unique to vertebrates

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Pharyngeal slits

5 mm

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Segmented muscles

Hagfishes

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

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

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

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• Hagfishes are marine; most are bottom-dwelling scavengers

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Lampreys

Lampreys

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

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

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• They inhabit various marine and freshwater habitats

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

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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

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

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Fishes and amphibians

A. Vertebrate jaws evolved from skeletal supports of pharyngeal slits

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1. Animals that replaced jawless vertebrates, and are Gnathostomes.

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• Members of group have two pairs of fins.

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• Jaws and fins allowed fish to become active in pursuit of food and in biting off chunks of flesh.

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• 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

Pectoral fins

Pelvic fins

Chondrichthyans (Sharks, Rays, and Their Relatives)

Chondrichthyans (Sharks, Rays, and Their Relatives)

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

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

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• 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

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• Oviparous: Eggs hatch outside the mother’s body

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

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

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Ray-Finned Fishes and Lobe-Fins

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

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• Osteichthyans include the bony fish and tetrapods

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^– Aquatic osteichthyans are the vertebrates we informally call fishes

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Nostril

Brain

Spinal cord

Swim bladder

Dorsal fin

Adipose fin

Caudal fin

Cut edge of

operculum

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Gills

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Kidney

Heart

Liver

Stomach

Intestine

Gonad

Pelvic fin

Anus

Anal fin

Lateral line

Urinary bladder

Ray-Finned Fishes

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

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

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

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Yellowfin tuna (Thunnus albacares)

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Red lionfish (Pterois volitans)

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Common sea horse

(Hippocampus ramulosus)

Fine-spotted moray eel (Gymnothorax dovii)

Lower jaw

Scaly covering

Dorsal spine

5 cm

Lobe-Fins

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

Lobe-fins also originated in the Silurian period

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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

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Derived Characters of Tetrapods

• Tetrapods have some specific adaptations

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• Four limbs, and feet with digits

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• A neck, which allows separate movement of the head

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• Fusion of the pelvic girdle to the backbone

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• The absence of gills (except some aquatic species)

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• Ears for detecting airborne sounds

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Flat skull

Eyes on top of skull

Shoulder bones Neck

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Head

Ribs

Scales

Fin

Fin skeleton

Elbow Radius

Humerus Ulna “Wrist”

Fish Characters Scales

Fins Gills and

lungs

Tetrapod Characters Neck

Ribs

Fin skeleton Flat skull

Eyes on top

of skull

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

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

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Amphibian orders.

Order Urodela – Salamanders, retain tails as adults Order Anura – Frogs, lack tails as adults

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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.

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

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

Limbs with digits

Amphibians

Amniotes

Silurian

PALEOZOIC

Carboniferous Permian

Devonian

Lungfishes

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Eusthenopteron Panderichthys

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Tiktaalik

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Acanthostega

415 400 385 370 355 340 325 310 295 280 265 0

Time (millions of years ago)

Key to limb bones

Ulna Radius Humerus

Tulerpeton

Amphibians

(a) Order Urodela (salamanders)

^(b) Order

Anura (frogs)

^(c) Order Apoda (caecilians)

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

Order Urodela includes salamanders, which have tails

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(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

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

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

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ANCESTRAL AMNIOTE

Parareptiles

Turtles Crocodilians

Pterosaurs Ornithischian

dinosaurs

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Saurischian dinosaurs other than birds

Birds

Plesiosaurs

Ichthyosaurs

Tuataras Squamates

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Mammals

Reptiles

Synapsids

Diapsids

Archosaurs

Lepidosaurs

Dinosaurs

Saurischians

Amniotes (includes reptiles, mammals, and birds)

Amniotes are tetrapods that have a terrestrially adapted egg

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

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

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

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1. Membranes develop from tissues derived from the embryo.

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• 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

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• Reptiles have scales that create a waterproof barrier

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• Most reptiles lay shelled eggs on land

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

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

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Reptilian heritage is evident in all amniotes

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1. Scales of keratin, waterproof skin - prevent dehydration.

- Reptiles cannot breathe through skin, so all gas exchange occurs via lungs.

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2. Shelled amniotic eggs require internal fertilization. Shell forms around fertilized egg in the reproductive tract.

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• 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.

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• 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:

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• Testudines – Turtles

- Some species returned to water; all lay eggs on land.

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• Sphenodontia – Tuataras

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• Squamata – Lizards, snakes
• Lizards are the most numerous group.
• Snakes are descendants of lizards and have vestigial pelvic and limb bones.

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• Crocodilia – Crocodiles, alligators
• This is the group most closely related to dinosaurs

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^(a) Tuatara

(Sphenodon punctatus)

(b) ^Australian thorny devil lizard (Moloch horridus)

(d) Eastern box turtle (Terrapene carolina carolina)

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

(e) American alligator (Alligator mississippiensis)

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

Birds

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• Many characters of birds are adaptations that facilitate flight

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• The major adaptation is wings with keratin feathers

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

Derived Characters of Birds

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(a) Wing

Shaft Vane

Forearm

Wrist

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Shaft Barb Barbule Hook

(c) Feather structure

(b) Bone structure Palm

Finger 2

Finger 1

Finger 3

Birds began as feathered reptiles, evolved to fly:

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1. Honeycombed skeletons are light and strong ◊ good for flight. Figure 34.25 (p. 698) – Form fits function: the avian wing and feather.

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• Toothless for weight reduction.

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• Endothermic = use metabolic energy to generate heat.
• Feathers provide insulation.
• Efficient circulatory system supports high rate of metabolism necessary for flying.

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• Acute vision ◊ Large brains that allow complex behavior.

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• 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.

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Archaeopteryx, a Jurassic bird-reptile.

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7. Modern birds include about 8,600 species. Some are flightless = ratites.

Wing claw

(reptilian character)

Toothed beak (reptilian character)

Airfoil wing with contour feathers

(avian character)

Long tail with many vertebrae

(reptilian character)

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Behavior and morphology has adapted to fulfill distinct niches

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• 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

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• A larger brain than other vertebrates of equivalent size
• Differentiated teeth

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Mammals are amniotes that have hair and produce milk

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Mammals diversified extensively in the wake of the Cretaceous extinctions

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1. Radiation of mammals occurred during two events:
1. Extinction of dinosaurs
2. Fragmentation of continents

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• There are about 4,500 species of extant mammals

(a) Reptilian jaw

Middle ear

Stapes

Inner ear

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,,\.. ,_•. "' • "_, c_,._t.. _•

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(c) Reptilian ear bone

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Dimetrodon (reptile)

Jaw joint

(b) Mammalian jaw

Eardrum

Middle ear

Doentary

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0Angular

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O squamosal

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0Articular

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• Quadrate

Inner ear Stapes

JA=--\\->1- Incus (evolved

' from quadrate)

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(d) Mammalian ear bones

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Morganucodon (mammal)

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Malleus (evolved from articular)

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.

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• Major groups of mammals:

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• Monotremes – lay eggs and produce milk, but have no nipples.

- Platypus, echidna

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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

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

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• A marsupial is born very early in its development

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• It completes its embryonic development while nursing in a maternal pouch called a marsupium

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(a) A young brushtail possum

(b) Long-nosed bandicoot

Convergent evolution of marsupials and eutherians (placental mammals).

Plantigale

al ls

Eutherian mammals

Deer mouse

Marsupial mole

Sugar glider

Wombat

Tasmanian devil

Kangaroo

Mole

Flying squirrel

Woodchuck

Wolverine

Patagonian cavy

ANCESTRAL MAMMAL

Monotremes (5 species)

Marsupials (324 species)

Eutherians (5,010 species)

Monotremata

Marsupialia

Proboscidea Sirenia Tubulidentata Hyracoidea Afrosoricida Macroscelidea

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Xenarthra

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Rodentia Lagomorpha Primates Dermoptera Scandentia

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Carnivora Cetartiodactyla Perissodactyla Chiroptera Eulipotyphia Pholidota

Orders and Examples

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Monotremata Platypuses, echidnas

Main Characteristics

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Lay eggs; no nipples; young suck milk from fur of mother

Orders and Examples

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Marsupialia Kangaroos, opossums, koalas

Koala

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Tubulidentata Aardvarks

Main Characteristics

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Completes embryonic development in pouch on mother’s body

Echidna

Proboscidea Elephants

African elephant

Sirenia Manatees, dugongs

Manatee

Long, muscular trunk; thick, loose skin; upper incisors elongated

as tusks

Aquatic; finlike fore- limbs and no hind limbs; herbivorous

Xenarthra Sloths, anteaters, armadillos

Tamandua

Lagomorpha Rabbits, hares, picas

Jackrabbit

Reduced teeth or no teeth; herbivorous (sloths) or carnivorous (anteaters, armadillos)

Chisel-like incisors; hind legs longer than forelegs and adapted for running and jumping; herbivorous

Sharp, pointed canine teeth and molars for shearing; carnivorous

Hooves with an even number of toes on each foot; herbivorous

Aquatic; streamlined body;

Carnivora Dogs, wolves, bears, cats, weasels, otters, seals, walruses

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Cetartiodactyla Artiodactyls Sheep, pigs, cattle, deer,

Coyote

giraffes Cetaceans

Aardvark

Teeth consisting of many thin tubes cemented together; eats ants and termites

Hyracoidea Hyraxes

Rock hyrax

Short legs; stumpy tail; herbivorous; complex, multi- chambered stomach

Rodentia Squirrels, beavers, rats, porcupines, mice

Primates

Red squirrel

Chisel-like, continuously growing incisors worn down by gnawing; herbivorous

Lemurs, monkeys, chimpanzees, gorillas,

humans

Golden lion tamarin

Opposable thumbs; forward-facing eyes; well-developed cerebral cortex; omnivorous

Perissodactyla Horses, zebras, tapirs, rhinoceroses

Indian rhinoceros

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Chiroptera Bats

Hooves with an odd number of toes on each foot; herbivorous

Frog-eating bat

Eulipotyphla

Adapted for flight; broad skinfold that extends from elongated fingers to body and legs; carnivorous or herbivorous

Eat mainly insects

Bighorn sheep

Primates and the evolution of Homo sapiens

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1. Primate evolution provides context for understanding human origins

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• Hands and feet adapted for grasping. Possess opposable thumb.

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• Large brains allow complex social behavior.

Derived Characters of Primates

• Hands, feet for grasping

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• Flat nails

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• A large brain and short jaws

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

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• Complex social behavior and parental care

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• A fully opposable thumb (in monkeys and apes)

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Anthropoids

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• There are three main groups of living primates

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• Lemurs, lorises, and pottos

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• Tarsiers

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• Anthropoids (monkeys and apes)

Living Primates

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2. Hominid lineage diverged from other primates about 7 million years ago. Humans compared to other hominids:

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• Brain size – large size allows development of language and social behavior.

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• Jaw shape – shortened to give a flatter face.

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• Bipedalism = walking on two legs.

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• Frees hands to do other things.

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• Eyes set higher; can see farther.

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• Females smaller than males

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

0

Australop/thecus

boisei

Austra/opl/hecus

robustus

Homo neanderthalensis

Homo

Homo l

ergaster •

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4. Females smaller than males

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

Homo sapiens

African European

Asian

Australasian

Interbreeding -

- - - - -

1-2 million

years ago ---------'

Homo erectus

in Africa

(a) Multiregional hypothesis

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100,000

years ago

Homo sapiens

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European Asian Australasian

African

Homo sapiens

in Africa

Homo erectus

in Africa

(b) "Out of Africa" hypothesis (replacement hypothesis)

ANCESTRAL PRIMATE

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 Chimpanzees

and bonobos

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Humans

Anthropoids

(b) Old World monkey: macaque

New(aW) orld monkey:

pider 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

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Derived Characters of Humans

• A number of characters distinguish humans from other apes

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• Upright posture and bipedal locomotion

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

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• Reduced jawbones and jaw muscles

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• Shorter digestive tract

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• The human and chimpanzee genomes are 99% identical

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

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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

Millions of years ago

Australo- pithecus anamensis

Kenyanthropus platyops

Australopithecus africanus

Paranthropus boisei

Paranthropus robustus

Homo ergaster

Homo Homo

neanderthalensis sapiens

Homo erectus

Homo Homo ^rudolfensis habilis

Hominins originated in Africa about 6–7 million years ago

Australopithecus garhi

?

Australopithec^•us afarensis

Ardipithecus ramidus

Orrorin tugensis

Sahelanthropus tchadensis

• 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

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• Misconception: Human evolution is like a ladder leading directly to Homo sapiens

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^– Correction: Hominin evolution included many branches or coexisting species, though only humans survive today

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Australopiths

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

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• Some species, such as Australopithecus afarensis walked fully erect

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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

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• It was the first hominin to leave Africa

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Neanderthals

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

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

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

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Hypothesis: Neanderthals gave rise to European humans.

Expected phylogeny:

Chimpanzees Neanderthals Living Europeans Other living humans

RESULTS

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Chimpanzees

​

​

Neanderthal 1

Neanderthal 2 European and other

living humans

EXPERIMENT

Figure 34.50

Homo Sapiens

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

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• All living humans are descended from these African ancestors

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

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

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

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

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

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Clade Cephalochordata (lancelets)

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Urochordata (tunicates)

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Myxini (hagfishes and relatives)

Description

Basal chordates; marine suspension feeders that exhibit four key derived characters of chordates

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Marine suspension feeders; larvae display the derived traits of chordates

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

Chordates: notochord; dorsal, hollow nerve cord; pharyngeal slits; post-anal tail

Craniates: two sets of Hox

genes, neural crest

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Vertebrates: Dix genes duplication, backbone of vertebrae

Gnathostomes: hinged jaws, four sets of Hox genes

Osteichthyans: bony skeleton Lobe-fins: muscular fins or limbs

Tetrapods: four limbs, neck, fused pelvic girdle

​

Amniotes: amniotic egg, rib cage ventilation

Petromyzontida (lampreys)

Chondrichthyes (sharks, rays, skates, ratfishes)

Actinopterygii (ray-finned fishes)

Actinistia (coelacanths) Dipnoi

(lungfishes)

​

Amphibia (salamanders, frogs, caecilians)

Reptilia (tuataras, lizards

and snakes, turtles, crocodilians, birds)

Mammalia (monotremes, marsupials, eutherians)

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

Aquatic gnathostomes; have cartilaginous skeleton, a derived trait formed by the reduction of an ancestral mineralized skeleton

Aquatic gnathostomes; have bony skeleton and maneuverable fins supported by rays

​

Ancient lineage of aquatic lobe-fins still surviving in Indian Ocean

Freshwater lobe-fins with both lungs and gills; sister group of tetrapods

​

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

One of two groups of living amniotes; have amniotic eggs and rib cage ventilation, key adaptations for life on land

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

Figure 34.UN10