Class Amphibia
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First chordates/vertebrates on land
About 375mya
Evolved from Dipnoian ancestors
Swim AND walk sinusoidally
Not totally adapted for life on land
Must return to water or very moist place, at
least for reproduction
• Skin not well-waterproofed
Class Amphibia
• Dioecious
• Three-chambered heart
• Lungs often aided by considerable
cutaneous gas exchange
• Most oviparous
• Some ovoviviparous or viviparous
• Cloaca present
• 3 Orders: Urodella, Anura, Apoda
Tetrapod Limb Origin
Order Urodella
• Salamanders and newts (400 species)
• Some totally aquatic
• Aquatic species usually have gills in the
immature stage
• Some aquatic species retain gills as adults
(Paedogenesis)
• Some found in damp places on land
• Adults can regenerate limbs!
Redbacked Salamander
Necturus (mudpuppy)
Order Anura
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Frogs and toads (~3,500 species)
Tadpoles aquatic (gills and tails)
Adults lose gills and tail
Enlarged hindlegs for jumping
Tongue is attached anteriorly
Some have camouflage coloration
Others are brightly colored and often
produce a toxic or distasteful mucus
Tadpole
Bull Frog
Order Apoda
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The caecilians
Legless
Almost blind
Wormlike and appear to be segmented
Some burrow in moist tropical soils
Others are aquatic (freshwater)
Order Apoda (caecilians)
Class Reptilia
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8,000+ species
300+ million year history
Dioecious
Most are oviparous
Cloaca present
Adapted to live on land
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Keratinized scales
Gas exchange via lungs
Internal fertilization
Amniotic egg (cleidoic)
Amniotic Egg
Reptilian Diversity
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Many extinct groups
Gave rise to mammals
Gave rise to birds
Should birds be in the same class as
reptiles?
• Three main groups of reptiles:
– Lizards and snakes
– Turtles and tortoises
– Crocodiles and alligators
Order Squamata
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Lizards and snakes (~7,900 species)
Three-chambered heart
Snakes evolved from burrowing lizards
Some snakes (boas) have vestigial pelvic
bones
• Good sensory structures (heat, odor, ground
vibrations)
• Snakes have jaws that are very loosely
articulated
Carolina Anole
Python Eating a Wallaby
Order Chelonia
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Turtles and tortoises (~300 species)
Three-chambered heart
Protected by hard shell
Aquatic turtles lay eggs on land
Long-lived
Eastern Box Turtle
Order Crocodilia
• Crocodiles, alligators, caimens (20 species)
• Four-chambered heart!!!!
• Confined to warm regions of the globe
Class (???) Aves
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8,600+ species in 28 orders
Dioecious
Sexual reproduction, oviparous
Reptilian traits
– Amniote egg
– Keratinized scales on legs
– Feathers are modified scales
• Birds fly – flightless birds came from
ancestors that could fly
Modifications for Flight
• Honeycombed bones
• Reduced systems (no teeth, no bladder, one ovary
in females)
• Endothermic!!!
• Efficient circulatory system
• Four-chambered heart!!!
• Efficient lungs connecting to air sacs
• Acute vision
• Large pectoral muscles anchored to a keeled
sternum
Additional Information
• Cloaca present
• Parental care
– Incubating eggs
– Feeding young
• Often have complex mating rituals
• Originated at least 130 mya
• Archaeopteryx not the ancestor of modern
birds (reptilian side branch?)
• Flying birds (carinates) (includes penguins)
• Flightless birds (ratites)
Archaeopteryx lithographica
Class Mammalia
• Hair composed of keratin (did NOT evolve
from reptilian scales
• Endothermic
• 4-chambered heart
• Mammary glands produce milk for young
• Internal fertilization
• MOST are viviparous with a placenta that
delivers nourishment to the developing
embryo
Class Mammalia
• Large brains; capable of considerable
learning
• Parental care of young
• Evolved from a reptiliam ancestor about
200 mya
• 1st mammals small and insectivorous
• Efficient respiratory system (diaphragm
aids ventilation of lungs)
Mammalian Evolutionary “Tree”
Monotremata
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Platypus and Echidnas (5 species)
Oviparous (lay reptilian type eggs)
Cloaca
Found in Australia & New Guinea
Marsupalia
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~325 species
Marsupials are therians
Rudimentary placenta
Young born very premature and must
complete development in the marsupium
(pouch)
• Convergent evolution of marsupials and
Eutherians
• First evolved in what is now North America
Eutheria
• ‘True’ placental mammals
• ~5,025 species
• Embryonic development completed within
the uterus
• Diverged from common ancestor with
marsupials about 100mya
• We are a Primate
• What other two groups are in our clade?
(see p. 724)
Evolutionary Convergence
(Marsupials & Eutherians)
Convergence (Concluded)
Primates