Biology 202 - Lecture 6
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Biology 202 - Lecture 6
Lecture VI.The Origin of Tetrapods, and the Amphibians
Origin of Tetrapods
Common Characteristics of Amphibians
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.
Invasion of the land required several morphological andphysiological changes:
: - 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
: a muscular tongue, salivary glands, and a largeintestine (colon)
: N waste excreted as urea; abladder to hold both ureaand water
: eyelids and tear ducts - more sensitive hearing: amiddle ear, with atympanum, columella(stapes) and oval windo
Tetrapods underwent a major radiationin the early Carboniferous (~330 mya),leading to two main lineages:
(2) amniotes (reptiles, birds andmammals).
Common characteristics of amphibians
Are amphibians just “primitive” tetrapods?
No - modern amphibians have the basic characteristics oftetrapods, plus many specializations:
extremely important for protection,osmoregulation, temperature regulation, and respiration
highly vascularized, has numerous mucous glands, and lacks scales
most species arecarnivorous, with aprotrusible tongue
external gills in aquaticforms
buccal-pump (forcedpump or positive pressurepump) ventilation of lungs
may be supplemented orentirely replaced bycutaneous respiration
a three chamberedheart and spiralvalve enabledifferentialcontrol of bloodflow to differentbody regions
lymphatic systemwith lymphatic'hearts'
development is usuallyindirect, involving metamorphosis under direction ofpituitary and thyroid hormones
some species have direct development
some are paedomorphic
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
There are about 4200 species of amphibians,of three main types:
worm-like, burrowing; sensory tentacles and reduced eyes, maybe ovoviviparous
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
frogs and toads
specialized jumpers, with long hind legs,fused tail bones, fused pelvis, short body;bifocal vision; courtship involvingvocalizations; herbivorous larvae(tadpoles)