MODERN BIOLOGY
Chapter 19
The Evolution of Vertebrate Diversity
Introduction: What Am I?
The duck-billed platypus is a strange animal and hard to classify

It has a furry body, bill, and webbed feet, and it lays eggs

It has mammary glands that produce milk for its young

The platypus bill is not similar to that of a duck, but instead is a sensory organ for locating food
underwater
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A large portion of the platypus brain is devoted to processing sensory information from its bill

The duck-billed platypus is a monotreme, a small group of egg-laying mammals
VERTEBRATE EVOLUTION AND DIVERSITY
19.1 Derived characters define the major clades of chordates

A phylogenetic tree for chordates is based on a sequence of derived characters
19.2 Hagfishes and lampreys lack hinged jaws
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Hagfishes and lampreys are craniates but lack hinged jaws and paired fins

In hagfishes, the notochord is the body’s main support in the adult

Lampreys have a supportive notochord but also have rudimentary vertebral structures, making them
vertebrates

Hagfishes are deep-sea scavengers that produce slime as an antipredator defense

Lampreys are parasites that penetrate the sides of fishes with their rasping tongues

Larval lampreys resemble lancelets

They are suspension feeders that live in freshwater streams, where they feed buried in sediment
19.3 Jawed vertebrates with gills and paired fins include sharks, ray-finned fishes, and lobe-finned
fishes
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Novel vertebrate features arose 470 million years ago
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Paired fins and tail
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Hinged jaws
Where did jaws come from?
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They arose as modifications of skeletal supports of the anterior pharyngeal gill slits (originally used
for trapping suspended food particles)

The remaining gill slits remained as sites of gas exchange
There are three lineages of jawed fishes

Class Chondrichthyes includes sharks and rays

Ray-finned fishes and lobe-fins have lungs (or lung derivatives)

Lobe-finned fishes have muscular fins supported by stout bones
Chondrichthyans

Sharks and rays have a flexible skeleton made of cartilage

Most sharks are fast-swimming predators, with sharp vision and a keen sense of smell

Electrosensors on their heads and a lateral line system aid them in locating prey

Most rays are adapted for life on the bottom, with dorsoventrally flattened bodies and eyes on the
top of their heads
Ray-finned fishes have

Internal skeleton reinforced with a hard matrix of CaPO3

Flattened scales covered with mucus
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Operculum to move water over the gills

Buoyant swim bladder (derived from an ancestral lung)

The diverse group of ray-finned fishes includes 27,000 species
Lobe-fins

Lobe-fins have muscular pelvic and pectoral fins, supported by rod-shaped bones
Three lineages of lobe-fins survive
o Coelacanths
o Lungfishes
o Tetrapods
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cloyd@waukee.k12.ia.us
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19.4 New fossil discoveries are filling in the gaps of tetrapod evolution

During the late Devonian, a line of lobe-fin fishes gave rise to tetrapods, jawed vertebrates with
limbs and feet
19.5 Amphibians are tetrapods—vertebrates with two pairs of limbs

Amphibians were the first tetrapods able to move on land

Most amphibians have tadpole larvae

This group includes frogs, salamanders, and caecilians

Salamanders walk on land with a side-to-side bending

Frogs hop with powerful hind legs

Caecilians are blind and legless, burrowing in moist tropical soil
19.6 Reptiles are amniotes—tetrapods with a terrestrially adapted egg

Reptiles (including birds) and mammals are amniotes

The major derived character of this clade is an amniotic egg with an amnion, a private pond in which
the embryo develops

Amniotic reptiles include lizards, snakes, turtles, crocodilians, and birds
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Terrestrial adaptations of reptiles include scales, waterproofed with keratin

Nonbird reptiles are ectothermic, but regulate their temperature by basking or seeking shade
19.7 Birds are feathered reptiles with adaptations for flight

Birds evolved from a lineage of small, two-legged dinosaurs called theropods

Archaeopteryx is the oldest bird (150 million years old), with feathered wings

It resembled a small bipedal dinosaur, with teeth, wing claws, and a long tail with many vertebrae

Living birds evolved from a lineage of birds that survived the Cretaceous extinctions

Birds are reptiles with feathered wings, endothermic metabolism, and a number of adaptations for
flight
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Loss of teeth

Tail supported by only a few small vertebrae
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Feathers with hollow shafts
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Strong but light honeycombed bones
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Flight is very costly, and birds are endotherms with a high rate of metabolism
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Birds have relatively large brains and display complex behaviors
19.8 Mammals are amniotes that have hair and produce milk
Characteristics

Mammals are endothermic amniotes with hair, which insulates their bodies, and mammary glands,
which produce milk

Mammalians generally have larger relative brain size than other vertebrates and a relatively long
period of parental care
First Mammals

The first true mammals arose 200 million years ago as small, nocturnal insectivores
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Marsupials diverged from eutherians (placental mammals) 180 million years ago

They underwent an adaptive radiation following the Cretaceous extinction, giving rise to large
terrestrial carnivores and herbivores, bats, and aquatic whales and porpoises
Monotremes

Monotremes are egg-laying mammals
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Living monotremes include the duck-billed platypus
Marsupials

Unlike monotremes, the embryos of marsupials and eutherians are nurtured by a placenta within the
uterus

The placenta allows nutrients from the mother’s blood to diffuse into the embryo’s blood

Marsupials have a brief gestation

They give birth to tiny, embryonic offspring

The offspring complete development attached to the mother’s nipples, usually inside a pouch or
marsupium
Placental Mammals or Eutherians
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bear fully developed live young
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They are commonly called placental mammals, because their placentas are more complex than
those of marsupials
Mrs. Loyd 
cloyd@waukee.k12.ia.us
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