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Lecture VI.The Origin of Tetrapods, and the Amphibians
- Origin of Tetrapods
- Common Characteristics of Amphibians
- Amphibian Diversity
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The Origin of Tetrapods
When and how did vertebrates invade the land?
Land vertebrates (tetrapods) probably originated in the mid Devonian, ~360mya, from sarcopterygian ancestors.
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Invasion of the land required several morphological andphysiological changes:
- Skeletal System: - an axial skeleton that can support the weight of the body- limbs for support- a strong, flexible neck, including an atlas, and a lighter skull
- Digestive System: a muscular tongue, salivary glands, and a largeintestine (colon)
- Excretory System: N waste excreted as urea; abladder to hold both ureaand water
- Nervous System: eyelids and tear ducts - more sensitive hearing: amiddle ear, with atympanum, columella(stapes) and oval windo
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Tetrapods underwent a major radiationin the early Carboniferous (~330 mya),leading to two main lineages:
- (1) amphibians
- (2) amniotes (reptiles, birds andmammals).
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Common characteristics of amphibians
Are amphibians just “primitive” tetrapods?
No - modern amphibians have the basic characteristics oftetrapods, plus many specializations:
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Integument
- extremely important for protection,osmoregulation, temperature regulation, and respiration
- highly vascularized, has numerous mucous glands, and lacks scales
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Digestive System
most species arecarnivorous, with aprotrusible tongue
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Respiratory System
- external gills in aquaticforms
- buccal-pump (forcedpump or positive pressurepump) ventilation of lungs
- may be supplemented orentirely replaced bycutaneous respiration
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Circulatory System
- a three chamberedheart and spiralvalve enabledifferentialcontrol of bloodflow to differentbody regions
- lymphatic systemwith lymphatic'hearts'
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Reproductive System
- development is usuallyindirect, involving metamorphosis under direction ofpituitary and thyroid hormones
- some species have direct development
- some are paedomorphic
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Nervous System
- the brain, esp. the cerebrum, is involved in more sensoryprocessing and control than in fishes
- a special sensory patch in the inner ear detects high frequency sounds transmitted bythe columella
- the pectoral girdle isconnected to the oval window todetect low frequency sounds
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Amphibian Diversity
There are about 4200 species of amphibians,of three main types:
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Order Gymnophiona
- caecilians
- worm-like, burrowing; sensory tentacles and reduced eyes, maybe ovoviviparous
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Order Caudata
- salamanders and newts
- generalized body with four legs of ~equal length; tail presentthroughout life, internal fertilization via a spermatophore;elaborate courtship involving pheromones
- many paedomorphic species
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Order Anura
- frogs and toads
- specialized jumpers, with long hind legs,fused tail bones, fused pelvis, short body;bifocal vision; courtship involvingvocalizations; herbivorous larvae(tadpoles)
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