Vertebrate Evolution:
Animals with a vertebral
column made of bone or
cartilage
Vertebrates are part of a larger phylum called Chordata
which includes Urochordata (tunicates),
Cephalochordata (amphioxus), Myxini (hagfish)
and Vertebrata (you)
Phylum Chordata
Synapomorphies present at some point in the development of all
chordates include:
Pharyngeal slits (gill slits in fish and other critters)
Notochord (stiff, fibrous rod running the length of the body)
Dorsal nerve cord (your spinal cord)
 Post-anal tail (we have something of a tail while a fetus, but i
is pretty stumpy in adult humans)
Invertebrate chordates provide clues
to the origin of vertebrates
• Subphylum Urochordata - commonly called sea squirts or
tunicates
• only show the chordate traits in their larval stage
• notochord is restricted to the tail
• adults filter feed
Invertebrate chordates provide clues
to the origin of vertebrates
• Subphylum Cephalochordata - “headless chordates”- lancelets, amphioxus
• show all four chordate traits as adults
• All early chordates before the evolution of jaws and teeth were suspension
feeders
Introduction to the Vertebrates
• Larger, more active lifestyle than other
chordates
• Cephalization
• Brain (anterior end of dorsal nerve chord)
• Sensory equipment in head region
• Axial Skeleton
• Cranium
• Vertebral column (except in hagfishes)
Vertebrate Diversity
• Jawless fishes:
hagfishes & lampreys
• Gnathosomes: Jaws
• Cartilogenous fish
• Bony fish
• Tetrapods: 2 pairs of
appendages
• Amphibians
• Amniotes
• Mammals
• Reptiles (traditional)
• Birds
Class Myxini: Hagfishes
• Most primitive living
vertebrate (?)
• No Vertebrae
• Cartilogenous
cranium and
notochord
• “Degenerate” anatomy
was thought to be
secondary
• It’s now thought that
hagfish are the
earliest branch of the
vertebrates
Class Cephalaspidomorphi: Lampreys
• Used to be classified with hagfishes, but are more
“advanced”
• No jaws or vertebrae but has cartilage “pipe”
surrounding notochord--> early stage of a vertebral
column
Lampreys are parasites
• Lampreys attach their mouths
onto victims such as lake
trout or whitefish, and literally
suck the life out of them!
• Since its introduction into the
Great Lakes in the 1930's,
sea lampreys have caused
many problems for the
commercial and recreational
fishing industries and some
10 million dollars are spent
each year to control them.
Gnathostomes: Jawed Vertebrates
• Jaws evolved by modification of the skeletal rods of
the anterior pharyngeal slits
• Jaws opened up new lifestyles and nutrient sources
• Early Gnathostomes mostly replaced the agnathans
during the Devonian Period (360-400MYA)--> “Age of
the Fishes”
Class Chondrichthyes: Sharks and Rays
• Cartilogenous skeleton ->lost dermal bone
• “nostrils” used only for
smelling, not breathing
• Internal fertilization but
may be:
• Oviparous= lay eggs that
hatch outside mother’s
body
• Ovovivparous= retain
fertilized eggs inside
mom’s body
• Viviparous= young born
aliveafter developing in
uterus
Class Osteichthyes: Bony fish
• Ossified skeleton
• “nostrils” used only for
smelling, not breathing
• 3 extant classes
• Ray-finned fishes
• Lobe-finned fishes
• lungfishes
Figure 34.14 A coelocanth (Latimeria), the only extant lobefinned genus
Tetrapods
• The earliest tetrapods most
likely arose from organisms
similar to today’s lungfishes
• In the Devonian period, plants
and insects had already
invaded land
• Lungfish living in
shallow water were
able to spend
increasingly longer
amounts of time on
land
Phylum Chordata
Class Amphibia
• Salamanders, frogs, caecilians
• Some are strictly aquatic and others terrestrial
• After the Devonian, the Carboniferous period
was “The age of the amphibians”
Most amphibians are still dependent on water
to some extent
• Gas exchange occurs in the lungs or through the
surface of the skin
• The surface must be kept continuously moist and
numerous glands continually secrete fluid onto the
surface of the skin.
Reproduction is also many times dependent on water
• Fertilization is often external with females
releasing eggs into water
• Eggs have no shell and will desiccate quickly
• Many (not all) frogs under go metamorphosis -->
“two lives” = amphibian
What’s happening to our frogs?
• Globally frogs are disappearing at alarming rates and
those that survive are often deformed or infertile.
• It's not clear just what the cause of this
• Depletion of ozone layer has increased UV radiation
reaching these frogs who have little or no protection
against UV damage to their skin or their eggs.
• Pesticides
• Parasites
Amniotes
• Reptiles, birds, and mammals
• An extra-embryonic
membrane, the amnion,
encloses the developing
embryo
• Some amniotes have an
additional outer shell
• The amnion freed us from the
necessity of returning to water
to reproduce, and allowed the
amniotes to conquer the land.
The number of taxonomic categories
of amniotes is under debate
Reptiles (Old-fashioned)
• Dinosaurs, snakes &
lizards, turtles, crocodiles
• Many terrestrial
adaptations:
• Shelled Egg
• Lungs
• Scales
• Limb placement
• Use environment to
regulate temperature =
exothermic
Reptiles
Testudines
Squamata
Squamata
Crocodilia
Birds: Class(?) Aves
• Flight!
• Feathers are modified scales
• Probably evolved from small,bipedal dinosaurs
• Closest living relative today = crocodiles
• Use internal metabolism to regulate body temp = Endothermic
The fossil record provides evidence of the
connection between dinosaurs and birds
Sinoauopteryx and
Caudipteryx:
“feathered”, flightless
dinosaurs
Archaeopteryx,
feathered, poor flyer
Class Mammalia: Mammals
• Mammary Glands
• Hair
• 4-chambered
heart
• Three major
groups:
• Monotremes
• Marsupials
• Eutherians
(placentals)
Class Mammalia
Order Monotremata
• Egg-laying mammals
• Platypus & echidnas
• Milk, but no nipples
• Early birth
Class Mammalia
Order Marsupialia
• Young develops in
external pouch of
mother
• Only 3 families of
marsupials outside
Australia
Class Mammalia
Eutherian mammals
• Placenta: lining of mother’s
uterus and extraembryonic
membranes of embryo
• Several orders:
• Order Proboscidea
• Order Rodentia
• Order Primates
• Order Carnivora
• Order Insectivora
Figure 34.32 Evolutionary convergence of marsupial and eutherian (placental) mammals
Class Mammalia
Order Primates
• Prosimians- lemurs,
tarsiers
• Anthropoidsmonkeys, apes,
humans
• Hominoids- great
apes (gorillas,
chimps) and humans
• Hominids- humans
and human ancestors
The Arboreal Theory of Primate evolution
• Early primates were insectivores (predatory)
• Selection for binocular vision - 60 MBP
• Manual dexterity (opposable thumb), large forehead and
cerebral cortex, short snout - 50 MBP
• Color vision (cone cells in retina) in diurnal primates helped
with depth of field perception
Brachiating Apes
• Swinging in trees
• Straightened spine
• Legs parallel to spine
• Preadaptation to erect posture?
• Apes are knuckle-walkers
• Bipedalism is unique to humans
and our human-like ancestors
A comparison of ape and hominid skeletons
Side effects of bipedalism
• Necessitated curvature of the spine & restructured pelvis
• New pelvis had smaller opening (birth problems)
• Necessitated life history changes
-birth earlier in development
-Extensive parental care
Other problems:
• Shorter jaw: wisdom teeth (no room)
• Back problems (curvature, pressure on lower spine)
• Hemmorhoids (pressure of intestinal system on lower parts, cuts off
blood flow)