lecture notes3 - amphibians

8/2/2019 Lecture Notes3 - Amphibians

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Class: Amphibia

Subclass Labyrithodontia (extinct)

Order Anthracosauria (extinct)

Subclass Lepospondyli (extinct)

Subclass Lissamphibia (living amphibians)

Order Anura (frogs and toads)

Order Urodela (salamanders, newts, sirens, etc.)

Order Gymnophiona (aka Apoda, Caecilians)

From Romer

From Conant and Collins

From Linzey 2001


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Amphibians represent the first truly tetrapod vertebrates

-first to move about and support themselves on land

All amphibians have bony skeletons

Most have 4 limbs, except Caecilians (limbless) and Sirens (forelimbs only)

Many have webbed feetTypically the glandular skin is smooth, moist, and lacks scales

-Caecilians have concealed dermal scales

One advance associated with a more terrestrial existence is the refinement of

the ear

-the amphibian ear, unlike that of fishes, consists of three components

-Inner ear, middle ear, outer ear

-There is increased sensitivity to accommodate airborne sounds

Class: Amphibia


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-Gas exchange occurs through lungs, gills,

and/or the skin

-Some salamanders have no lungs

(secondarily lost)

-Adults have double circulation

-Both a pulmonary and a systemic circuit

-Heart has three chambers

-Two atria and one ventricle

Class: Amphibia

-Some mixing of oxygenated and deoxygenated blood occurs in

ventricle


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Class: Amphibia

Body

The evolution of the tetrapod limb was the

third major hallmark in vertebrate evolutionIn early forms, these limbs represented

modified Sarcopterygian pectoral and pelvic

fins

Development of limbs required only a few minor modifications

-elongation of proximal elements into long-bones of limbs

-Proliferation of more distal elements into bones of feet

-Elaboration of pectoral and pelvic girdles (for anchoring the limbs)

-Early locomotion was probably very crude

-Orientation of limb did not effectively allow body to be raised above

substrate

-Limbs acted as pivot points rather than for propulsion. Modern

salamanders still move about in this way.


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Class: Amphibia

Emergence on to land required adaptations to accommodate desiccation

-amphibians have an external layer of skin, the stratum corneum, thatconsists of a single layer of dead cornified cells

-Reduces water loss through skin to some extent

However, amphibian eggs are very vulnerable to desiccation

Hence, amphibians are reproductively constrained to exist in moistenvironments.

Fertilization is external in anuran and some urodelids

-most salamanders have external fertilization, but no copulation

-Males deposit a spermatophore which is picked up by the female and

placed into a spermatheca (in wall of cloaca)

-Eggs are fertilized as they pass through the cloaca


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Class: Amphibia

Most amphibians are oviparous

-females lay eggs

Some amphibians undergo direct development in the egg

-embryos pass through larval stage in the egg and emerge from the

egg as miniature versions of the adults.In most amphibians, the young must pass through a larval stage

-larvae are aquatic with external gills

In a few forms, the adults will retain larval characteristics

= neoteny

A very few forms are viviparous-the females retain the eggs in their bodies until they are ready to

hatch.


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Amphibian Origins

It is clear that amphibians evolved from some lineage of Sarcopterygian fishes

-However the details of this origin remain obscure

One fossil group, from the Crossopterygian

lineage, that provides a good candidate for

amphibian origins are the Rhipidistians

-shared many characteristics with fossil

amphibians

-Limb/fin structure, jaw structure,

both had labyrinthodont teeth

However, recent mitDNA evidence shows that modern amphibians are more

similar to lungfishes (Dipnoi) than to Coelacanths (Crossopterygii)

-this suggests a different origin for the amphibians

-Critics reject this argument based on certain methodological arguments

From Linzey 2001


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Amphibian Origins

Though actual origins are uncertain, it is clear that there was plenty ofselective pressure for amphibians to emerge onto land

-the Devonian was a period of tremendous climatic fluctuation

-Wet and dry spellsdry spells would have set up intense

competition in aquatic habitats, would favor an ability to move

from one pond to another, oxygen poor waters would favor air-

breathing

-Also, at this time, there was a tremendous radiation of arthropods into

the terrestrial environment

-Centipedes, millipedes, spiders, insects, etc.

-This would create an open niche in a relatively low-competition

terrestrial environment

-Lastly, the ability to temporarily move out of water would allow early

amphibians a means of escaping from aquatic predators.


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Amphibian Origins

The fossil record of amphibians includes two extinct subclasses

Labyrinthodontia and Lepospondyli

The earliest fossil amphibians belonged to the Labyrinthodontia

From Romer


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Amphibian Origins

Labyrinthodonts first appeared in the upper Devonian-were the most abundant and diverse amphibians though the

Carboniferous, Permian, and Triassic

-Some were quite large, reaching a length of 5 feet.

-Most were aquatic or primarily aquatic

-Many retained fish-like tails, some retained internal gills

-Several groups of Labyrinthodonts existed, two were important for

vertebrate evolution

-Anthracosaurs represented a small group but are important becausethey are believed to be ancestral to reptiles

-Temnospodylians were common during Permian and are suggested

to be ancestral to modern Amphibians


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Amphibian Origins

The other fossil subclass was Lepospondyli

-represent a group of small, salamander-like amphibians thatappeared in the Carboniferous and Permian.

-Though superficially similar to salamanders, they are not believed to

be closely related to any living amphibians

-Based on unique structure of the vertebrae

From Romer


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lecture notes3 - amphibians

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