From Sea to Slime: Evolution of Amphibians Late Devonian

Late Devonian: Rhipidistians
From Sea to Slime: Evolution of Amphibians
Lungs were developed in two groups of lobe-finned fishes - Rhipidistians
and lungfishes .
The Rhipidistians are considered to be the ultimate ancestors of later land
animals.
Rhipidistians such as Eusthenopteron had evolved "land animal-like
features":
Eusthenopteron
An Important Link: Tooth Structure
Labyrinthodont tooth
of Rhipidistian fish
(Eusthenopteron)
Skeletal Modifications
Labyrinthodont tooth
of early amphibian
(Archegosaurus)
Labyrinthodont tooth structure (with complexly infolded enamel) is
shared between Rhipidistian fishes and the earliest amphibians.
This strongly supports a close relationship between the two groups.
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Late Devonian: Ichthyostega and Acanthostega
-Ichthyostega was a cross between a fish and an amphibian
-Ichthyostega had legs and walked and was a true tetrapod.
-With true legs, it could live on land for extended periods.
-The primitive amphibians like Ichthyostega had a special kind of skin that
helped them retain bodily fluids and deter desiccation.
-Stronger skeletons allowed the primitive amphibians to live more
comfortably with the increased gravity on land.
-Animals like Ichthyostega used their limbs for locomotion and their tails
for balance.
Ichthyostega
Carboniferous to Permian
Evolution of neck and ear
· Amphibian nostrils became increasingly functional for breathing air.
· Amphibians evolved "hands" and "feet" with five digits.
· Amphibian tails became reduced in size.
· Amphibian backbones grew stronger (this enabled amphibian
bodies to grow bigger).
· Amphibians obtained eardrums.
-Fishes need limbs to support bodies and ears to hear sounds
in the air.
-Fins changed to limbs
-Several bones of the skull changed to the shoulder bones
-Tongue cartilage (part of the jaw in fish) became an ear bone.
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Amphibian Diversification
Advantages for amphibians living on land:
Diplocaulus
(1 m long)
Less competition for food
Avoidance of large predatory fishes.
Disadvantages for amphibians living on land:
Eryops
(1.5 m long)
Amphibians have gas-permeable skin to aid their
inefficient lungs. This skin must be kept moist !
They must have water to reproduce, Water is needed for
the external fertilization that is characteristic of
amphibians.
By the Permian Period, amphibians had become
quite diverse (and some were very large).
Modern Amphibians: 1.Anura (frogs and toads)
2.Caudata(salamanders and newts)
3.Apoda (caecilians)
Amphibian jelly-like eggs can not survive out of water.
END OF LECTURE
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