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Chapter 34

Vertebrate Evolution and Diversity

Four anatomical features that characterize the phylum Chordata

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

- Gas exchange

- Hearing- Jaw support

4. Postanal tail

Provides propulsion for swimming

B. Invertebrate chordates provide clues to the origin of

vertebrates

1. Subphylum Urochordata

Adult is sessile and feeds via pharyngeal slits.

Subphylum Urochordata: a tunicate

2. Subphylum Cephalochordata

a. Adult form shows chordate features.

b. Adults feed and swim.

Subphylum

Cephalochordata:

the lancelet

Branchiostoma

II. 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.

Figure 34.6 (p. 683) – The neural crest, embryonic source of

many unique vertebrate characters.

2. Skeletal elements, such as the cranium (braincase), allow

for the big evolutionary feature of vertebrates, cephalization.This gives us the term “Craniates”

3. Vertebral column is the main support for the body axis. It

allows for large size, fast movement, and protection of the

nerve cord.

4. The closed circulatory system pumps oxygenated blood to

cells and allows rapid metabolism, rapid movement to search

for food, escape predators.

B. Overview of vertebrate diversity

Figure 34.7 (p. 684) – Phylogeny of the major groups ofextant vertebrates.

**Note the three super groups:

Gnathostomes,Tetrapods,

Amniotes**

III. Jawless vertebrates

A. These are the most primitive vertebrates.

B. Groups include hagfish (no skeleton, no

notochord in adult); lamprey (early version of a vertebral

column).

Figure 34.8 (p. 685) – A hagfish.

Figure 34.9 (p. 685) – A sea lamprey.

Lamprey on trout

IV. 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 ofanterior 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

Figure 34.11 (p. 688) – Cartilaginous fishes.

Osteichthyes: Extant classes of bony fishes

Lake trout

Whitefish

Sturgeon

WalleyeGreat LakesSome of the natives

Great LakesSome Exotics

Rainbow trout/brown trout

Alewife

Great LakesSome successors

Largemouth bass

Yellow perch

Bluegill

D. Tetrapods evolved from specialized fishes that inhabited

shallow water Figure 34.15 (p. 690) – The origin oftetrapods.

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

amphibians.

Figure 34.17 (p. 691) – 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.

Figure 34.18 (p. 692) – The “dual life” of a frog.

V. Amniotes (includes reptiles, mammals, and birds)

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

Figure 34.19 (p. 693) – The amniotic egg.

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.

B R tili h it i id t i ll i t

B. Reptilian heritage is evident in all amniotes

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

- Reptiles cannot breathe through skin, so all gas exchangeoccurs 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 M d til i l d 6 500 i th t i f

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

Figure 34.24 (p. 697) – Extant reptiles.

C Bi d b f th d til l d t fl

C. 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 wingand 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 ofbirds. Example: Velociraptor Archeopteryx is an example

of a Mesozoic bird that shows reptilian features.

Figure 34.27 (p. 699) – Archaeopteryx , a Jurassic bird-reptile.

7. Modern birds include about 8,600 species.

Some are flightless = ratites.

Figure 34.29 (p. 701) – A small sample of birds.

D. 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

3 F t f l

3. Features of mammals:

a. Defined by Linnaeus as having mammary glands, which

produce milk rich in fats, sugars, proteins, minerals, andvitamins.

b. Hair and subcutaneous fat help retain metabolic heat.

c. Most embryos develop in a uterus. In placental mammals,

the lining of the uterus and extraembryonic membranes form

the placenta.

d. Large brains and long period of parental care. Ability tolearn.

e. Differentiation of teeth for efficient eating.

4 Th li t l l d f til b t 220

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

Figure 34.31 (p. 703) – Australian monotremes and

marsupials.

c. Eutherians – long pregnancy with embryonic attachment tomother in uterus via placenta.

- Human, Wolf

Figure 34.32 (p. 704) – Evolutionary convergence of marsupial

and eutherian (placental) mammals.

Table 34.1 (p. 705) – Major Orders of Mammals

VI. 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.

Figure 34.35 (p. 708) – A phylogenetic tree of primates.

B H i id li di d f th i t b t 7

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 andsocial 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 andenhances learning and social behavior.

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