Amphibians

Amphibians A primitive amphibian was essentially a

crossopterygian lobe-finned fish in which limbs capable of progression on land had developed. They inhabited fresh-water pools and streams in Carboniferous and Devonian times. Lived alongside ancestral crossopterygians who were similar to them in food habits and in many structural features, except limb development.

What caused the limb development and potential for land dwelling? To take advantage of oxygen supply in air?

Lungs in fishes Driven out in search of food? They were fish

eaters so little food was to be found on land. To escape enemies? Among largest animals

of the streams and pools of the time. Limbs developed as adaptations for

remaining in the water! Paradox that true land life was probably an accident.

If water supply adequate, crossopterygians have the advantage because can swim better (legs in the way).

Devonian (when amphibians originated) = time of seasonal droughts. Pools crowded, foul and stagnant. If remained this way for a period of time, local food supplies would become exhausted and water might dry up completely. Amphibians could move to another water source while crossopterygians died.

Limbs were developed to reach the water, not to leave it.

Terrestriality followed once limbs developed. Instead of immediately taking to the water again, the

amphibian might have learned to linger about the drying pools and devour stranded fishes.

Insects were present and would have afforded the beginnings of a diet for a terrestrial form.

Later, plants were taken up as a source of food supply, while (as is usually the case) the larger forms on land probably took to eating their smaller or more harmless relatives.

d.  Finally, through these various developments, a terrestrial fauna would have been established.

Preadaptations for terrestriality Crossopterygian preadaptations for

terrestriality = lungs, bony skeleton and stout fleshy and bony lobe in the paired fins.

Contribution to vertebrate evolution - Skeletal developments. Amphibians developed limbs that lifted the

body off the ground. Developed the arrangement of limb bones found in all terrestrial vertebrates. 

Forelimb with one large proximal element (humerus) and two distal (radius and ulna), carpals (wrist bones), metacarpals (hand bones). 

In hindlimb, get femur, tibia and fibula, tarsals and metatarsals. 

Contribution to vertebrate evolution - Skeletal developments. Gradual trend toward shifting the limbs under the

body and changing the plane of movement of the humerus and femur from horizontal to vertical.

Strengthened vertebral column and interlocking of vertebrae by means of overlapping articular processes.

No persistent notochord. No exoskeleton (but first amphibians had skulls

completely covered by an armor of bone like the crossopterygians).

Contribution to vertebrate evolution - Sensory changes. Lateral line system in larvae, but gone in

adults. Shift in emphasis to smell and hearing.

Smell was enhanced by development of sensory epithelium lining nasal cavity which is capable of detecting air- born olfactants.

Contribution to vertebrate evolution - Sensory changes.  Hearing on land is quite a different problem

from hearing in the water. Had to amplify sounds. Developed eardrum and stapes bone (from hyomandibular bone of fish used to prop up jaws).

Vision on land presented probs. Must keep eye moist and clean, so developed eyelids and lachrymal (tear) glands. Rods and cones in retina, so get color vision.

Contribution to vertebrate evolution -Respiratory changes Pulmonary circulation developed and get

heart with three chambers. Result of not sending through gills is tripling of blood pressure.

Lymphatic system Nostrils open into anterior portion of oral

cavity so act as air passages as well as olfactory receptors.

Bony scales lost. Skin smooth, moist and contains many glands. Used as respiratory surface in living amphibians

Contribution to vertebrate evolution Digestive system - muscular and movable

tongue Urinary bladder

 Purely amphibian characteristics  From water they come and unto water they

must return. Could solve many of the problems of

terrestrial life, but have essentially fish eggs which desiccate on land. Various devices developed to avoid double set of adaptations of tadpole and adult.

Devices to avoid aquatic part of lifecycle Eggs laid in moist burrows rather than in water. Some toads carry eggs in pockets on their back or

twined in strings about their legs. Larval stage shortened by an initial development

away from the water, larva entering the pond or stream only when partially grown (increase amt. of yolk in egg to do this).

With extra yolk can eliminate tadpole stage. Neoteny in salamanders. There are other neotenic

vertebrates, but none carry it to  the extremes of the amphibians.

Failure of the amphibians "The amphibians are a defeated group." First to

emerge from waters onto land, but didn't finish the job. Once abundant, but have shrunken into insignificance among four-footed vertebrates. Mode of development is the problem. Chains amphibians to water. Devices to free them of this constraint have been only partially successful. The amphibian is conservative in its basic developmental processes. It is, in many respects, little more than a peculiar type of fish which is capable of walking on land.  (From Romer)

Evolution of the Vertebrate Heart http://www.biointeractive.org/