Tetrapods & Amniotes
Amphibians + Reptiles
Tetrapods (gnathostomes that have limbs)=
amphibians, reptiles, birds, and mammals
• Derived from the lung-fish branch of lobe-finned fish
Characteristics of Tetrapods:
• 2 pairs of limbs WITH DIGITS
– Support weight, provide mobility on land
Amphibians:
--Reproduction is tied to water
--Activity is restricted to aqueous or humid/damp/moist environments
3 clades:
• Urodeles = salamanders
• Anurans = frogs and toads
• Apoda = ceacilians
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Legless, almost blind,
Look segmented like earthworm (but with distinct head)
Live in soil (tropical)
From legged ancestor
Amphibian: common characteristics
• damp habitats
• External fertilization in most
• Eggs require moisture (sometimes retained)
– Susceptible to drying out
• Lungs + Skin for gas exchange
– Positive pressure/buccal breathing
• 3 chambered heart with double circulation
Gas Exchange
Lungs:
• Infoldings with many pockets
– Localized w/in body circulatory system
– Thin layer of tissue between blood and air
SKIN
• Significant contribution to gas exchange
• Permeable to gas (good) = permeable to water (bad)
• Very vascular
• Relative importance of skin varies with activity/O2
demand of individual.
Ventilation (i.e., breathing)
Inhalation:
1. Oral Cavity Expansion w/ nares open
– “sucks” air into oral cavity
2. Oral cavity contracts w/ nares closed
– Air “pumped”/pushed into lungs
Exhalation
1. Elastic recoil of lungs + compression of muscles of
body wall
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Air pushed out
Double Circulation
• Two circuits/loops of blood pumped by a
single heart
– Lungs (pulmonary)
• Picks up O2, removes CO2
– Rest of body (systemic)
• Delivers O2, picks up CO2
• Single pump ensures ‘evenness’ of both
circuits
• Repressurizes blood after it has picked up O2
in the pulmonary circuit
Amphibians: Heart and Blood Flow
• 3 Chambered heart = 2 atria + 1 ventricle
• R atrium = venous blood from muscle and
organs/systemic
• L atrium = venous blood from lungs and skin
(pulmocutaneous circuit)
– Ventricle pressurizes blood/pumps out
• Ridge (spiral valve) in ventricles prevents most
mixing of blood.
• Blood flow to lungs “stops” during diving
Osmoregulation:
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Freshwater
Net gain of water
Large quantities of dilute urine
water can be re-absorbed across the urinary
bladder wall.
Amniotic Egg
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Amnion: encloses embryo in fluid that cushions/protects,
Chorion: gas exchange
Yolk sac: stores/transfers nutrients
Allantois—waste storage and some gas exchange
These are extraembryonic (i.e., not part of the embryonic
tissue)
Amniotes
• Amniotic Egg
– Egg can be retained
– Egg can be retained and no shell, but still has 4
layers
• Also ribs that assist in negative pressure
breathing
Reptile Clade:
Tuataras, lizards, snakes, turtles, crocodilians, & birds
• Scales w/ keratin (makes skin relatively
impermeable)
• Shelled, amniotic eggs
• Internal fertilization
• Clawed
• Mostly ectothermic
– Regulate body temp
– 10% of the calories required by endotherms
• Birds are endothermic
3 Clades
TURTLES:
• Shell (carpace + plastron), fused to ribs and clavicles
• Can breath through GI tract
LEPIDOSAURS (tuataras + lizards + snakes, later two are
squamates)
Crocodilians (alligators, crocodiles, and caimen)
• Breath air through upturned nostrils
• Warm regions
Reptile Heart
Double Circulation
Typically
• 2 atria + 1 partially divided ventricle
– Very little mixing of blood
In crocodiles
• ventricle is completely divided
– aorta are joined by a passageway
– allows blood to be shunted/bypass pulmonary circuit
when diving
Ventilation
• Negative pressure
• Expansion of ribcage