Bio153: lecture 19
the amniotes
4 major groups of amniotes
Amniota: all vertebrates that possess an
amniotic membrane (and other extraembryonic membranes surrounding the egg)
4 major groups: defined by # of
fenestrae in the skull that allow for the
passage of the jaw muscles
extinct
a) synapsids
- mammals
b) anapsids:
- turtles
c) diapsids
- lizards
- snakes
- crocodiles
- birds
- dinosaurs
d) euryapsids
- plesiosaurs
- icthyosaurs
primitive condition: anapsid
• turtles and other primitive reptiles
• anapsid = no fenestra
turtles:
• dorsal carapace & ventral
plastron
• vertebrae fused to shell;
can’t expand ribs (limits
intake of O2)
• no teeth
synapsids: 1 fenestra
• extinct mammal-like reptiles; mammals
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• early dominant group were synapsids
• most now extinct
• one lineage → mammals
Pelycosaurs (Dimetrodon)
• sail probably for thermoreg.
• backbone stiff
• partially solved
“Carrier constraint”
euryapsids: 1 fenestra
Carrier constraint
• lateral movement compresses
lungs
• new: legs move forward by
rotation
• change from sprawling to
upright stance
• separation of axial and appendicular
muscle groups: different muscles for moving
and breathing
result: faster movement, more endurance
• marine reptiles
• flippers
• plesiosaurs may have
bred on land
• all extinct: plesiosaurs and ichthyosaurs
• derived from diapsids (lower temporal
fenestra is lost )
• euryapsid skull evolved independently in
these 2 groups
diapsids: 2 fenestrae
lepidosaurs:
1. lizards:
• ecologically diverse
• lower jaw fused;
movable eyelids;
homodont teeth
• sprawling stance
2 lineages:
• lepidosaurs: lizards, snakes, tuatara
• archeosaurs: crocodiles, pterosaurs,
dinosaurs, birds
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2. snakes:
• no eyelids; no pelvic or pectoral
girdle
• deaf; poor vision; all carnivores
• special sense organs:
Jacobson’s organs: olfactory epithelium
pit organs: detect ± 0.003 ºC!
• all have toxic saliva; some specialized as
venom (hemolytic or neurotoxic)
3. tuatara
• relictual (most species
extinct) – living fossil!
• not a lizard:
• no copulatory organs
• no external ear openings
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“third eye”: gland that absorbs UV radiation
extremely slow growth – lives for ~100
years!
archeosaurs: crocodiles, pterosaurs,
dinosaurs, birds
defined by:
• antorbital fenestra (opening in skull
before the eye)
• hinged ankle joint
2. pterosaurs
• 1 of only 3 vertebrates to
evolve flight
• leathery wings supported by
an elongated fourth finger
• first 3 fingers present as a
claw
• largest fossil: 63 ft wingspan
1. crocodiles:
• crocodiles & birds are the surviving
archeosaurs
• have limbs splayed out to side (like lizards)
• are able to move with legs under the body
3. dinosaurs
• extremely successful group
• diversified at end of Triassic (many
synapsids went extinct)
• 225 mya – 65 mya
• “not everything big and dead is a
dinosaur!”
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2 major lineages of dinosaurs:
• ornithischians: pubis
points backward, parallel
with the ischium
• saurischian: pubis
points down and forward
• birds have backwardpointing pubis
(convergent evolution!)
2. Ornithischians
later in Jurassic
• stegosaurs
• hadrosaurs
• ankylosaurs
• ceratopsians
• arose in early Jurassic
e.g. Mononychus (discovered 1994)
1. saurischians
• sauropods (herbivores)
• theropods (carnivores)
theropods gave rise to the
birds
4. birds: with crocodiles, only surviving
archeosaurs
all birds:
1. feathers
2. lack teeth
3. beaks
4. lay eggs
birds arose from dromeosaurs
• tree-dwellers
fused wrist bones
fused clavicle
these adaptations were the first step to flight
• feathers evolved
first for insulation?
courtship?
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adaptations for flight:
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keeled sternum
hollow bones
fused clavicle
many fused bones
reduced # of bones
4-chambered heart
feathers (modified scales)
advanced respiratory system
respiratory system of birds:
lung volume ½ of similar
sized mammal
air sacs, air space in
bone, large trachea
total volume of
respiratory system 3X
mammal
birds lack diaphragm : flow-through ventilation
• flow-through parabronchi
of bird lung
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