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
for:
- 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 of
extant 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 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
Figure 34.11 (p. 688) – Cartilaginous fishes.
Osteichthyes: Extant classes of bony fishes
Lake trout
Whitefish
Sturgeon
Great Lakes
Walleye
Some of the natives
Rainbow trout/brown trout
Alewife
Smelt
Great Lakes
Some Exotics
Ruffe
Bluegill
Yellow perch
Great Lakes
Some successors
Largemouth bass
D. Tetrapods evolved from specialized fishes that inhabited
shallow water  Figure 34.15 (p. 690) – The origin of
tetrapods.
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 of
vertebrates 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. 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
Figure 34.24 (p. 697) – Extant reptiles.
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 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.
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. Features of mammals:
a. Defined by Linnaeus as having mammary glands, which
produce milk rich in fats, sugars, proteins, minerals, and
vitamins.
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 to
learn.
e. Differentiation of teeth for efficient eating.
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 to
mother 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. 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.