A neck is advantageous, because head can turn to facilitate feeding or vision without affecting the mechanics of locomotion

With paired fins -> limbs, need:

strong appendages

strong, well-attached girdles

vertebral column that can resist bending

When a terrestrial vertebrate stands, its body hangs from the vertebral column which supports the weight of the trunk like the arch of a suspension bridge and transmits it to the ground by two sets of vertical supports, the girdles and legs.

The vertebral column must be rigid & the girdles and legs sturdy and firmly connected with the vertebral column. Without these features, a terrestrial vertebrate cannot stand.

Lungs and a pulmonary circulation replace gills.; no gill cover necessary.

Cornification of skin to resist abrasion and drying

Oral glands are necessary to moisten dry food.

Eye, ear and nose must be modified to function in air.

Not how to acquire oxygen using air (solved by use of lungs)

Rhipidistians and Dipnoi used air long before it was necessary for terrestrial life.

Amphibians switched priority from gills to lungs.

Later in vertebrate evolution, the lung greatly increased in complexity,

especially with evolution of endothermy

Increased metabolic rate with increased oxygen dependence

Lungs of early amphibian were not very different from those of fish.

Problem was how to get the air into the lungs.

Fish could gulp air at surface and force air into lungs, using buccal pump or by gulping air and diving head first, letting air rise and pass into the lungs.

Land animals couldn’t use passive method (diving)

Buccal pump would have to be very strong to pump air into lungs and lift the body off the ground. (Limbs were not very strong.)

Buccal pump is force pump:

lower floor of mouth, close nostrils, raise floor, push air into lungs

Structural changes evolved that assisted with the ventilation of the lungs:

i. Presence of limbs assisted in raising body off the ground, making ventilation easier.

ii. Early "labyrinthodonts" had long stout ribs.

Ribs were fairly broad, with sites for muscle attachment in thoracic region.

This allowed contraction and relaxation of muscles of body wall, changing volume of thoracic cavity and allowing amphibian to suck

air in and expel it.

This is a reasonably effective respiratory mechanism, still seen in modern vertebrates (though not in most modern amphibians who are generally rib-less). Respiration is primarily by the buccal pump in modern amphibians.

iii. One evolutionary trend in early amphibians was flattening and broadening of the head. This may be related to the development of strong buccal pump.

Modern amphibians have further adaptations to increase respiration:

i. use of cutaneous respiration

Gas exchange supplemented through vascular system in thin skin.

Lungless salamanders may rely solely on cutaneous respiration.

eg clouded salamanders on Vancouver Island

Early amphibians had bony scales that would have reduced respiration through the skin; perhaps some cutaneous respiration occurred.

ii. Development of limbs also helped breathing.

If the body was raised, then the weight of the body and viscera would not press on the lungs. Breathing would be easier because less weight would have to be displaced as lung’s volume expanded.

Fish body supported by water is weightless, especially if fish has a well-developed gas bladder.

Very different stresses on aquatic vs terrestrial vertebrate:

Aquatic vertebrate needs a strong axial skeletal support to prevent body from buckling as it moves through the water.

On land, gravity pulls vertebrate body down and puts weight on ventral surface.

In fishes, the spine must resist stresses imposed by strong axial muscles.

With move onto land, spine had to support limbs, resist bending in some places and increase mobility elsewhere.

Requirements were for:

*vertebrae with firm centra

*intervertebral joints that could move or resist motion, depending on location, more intimate relationship between vertebral column and girdles

In amphibians, a number of evolutionary trends represent adaptations to gravity and the need for support.

i. Pectoral and pelvic girdles became larger and stronger.

Rhipidistians had fleshy fins with axial skeleton.

In amphibians, the skeleton has been modified, with the radial bones positioned near base of limbs.

The appendages developed an attachment to the pelvic and pectoral girdles.

In fishes, the girdles don’t support body but rather act as pivot/anchor points.

In amphibians, the pectoral girdle becomes fused to the sternum which provides the thoracic region with ventral support, functioning like a sling that body rests on.

In fish, the pelvic girdle is not attached to the vertebral column.

As a result, any weight put on girdle would push it into soft

tissues.

In amphibians, girdles and limbs become attached to vertebral column, transferring the support of the body to the vertebral column.

Originally girdle was attached to one vertebra.

In later amphibians, more vertebrae became involved.

The result was the transfer of gravitational forces to the vertebral column.

ii. Limbs became oriented in a partially vertical direction.

The attachment to the vertebral column and the vertical orientation of the limbs allows the body to be lifted off the ground.

Limbs are oriented out and down.

Forelimb of Ichthyostega was a strong prop, permanently bent at

the elbow.

This reduces contact with ground, resulting in less wear on body & less friction when moving on ground. It also facilitates respiration, making it easier to ventilate the lungs.

iii. In Osteichthyes, the pectoral girdle is attached indirectly to the vertebral column, and the head and body move as a unit.

In amphibians, head movement becomes independent of body movement.

In fish, the articular surface at the back of the skull where it meets the vertebral column is a single flattened surface.

In amphibians, the occipital condyle (articulation between skull and vertebral column) is divided into a rounded pair, with one articular surface on each side.

This change in skull attachment allows the head to move up and down independent of the body.

iv. Interlocking vertebrae

The simple discs or rings that constituted the center of vertebrae in the Rhipidistia evolved into interlocking vertebrae that, with the aid of muscles and ligaments, form a strong horizontal column for body support.

The vertebral column provides the main structural support for the body, replacing the notochord as the main longitudinal girder of the vertebrate body.

Principal part of a tetrapod vertebra is the spool-like body or centrum, which lies just below the spinal cord.

Neural arch encloses spinal cord.

Neural spine may extend from summit of neural arch.

Hemal arch may extend ventrally from the centrum to enclose blood vessels (especially posteriorly).

Adjacent vertebra articulate by their centra and may articulate by processes carried by the neural arches, the zygapophyses.

Shapes of articulating surfaces at the ends of centra are of evolutionary and functional importance (handout).

If both surfaces are concave: amphicoelus

permits limited movement in any direction

Centra concave anteriorly and convex posteriorly, the bulge in one vertebra fitting into the hollow of the next: procoelus

Centra concave posteriorly and convex anteriorly, the bulge in one

vertebra fitting into the hollow of the next: opisthocoelus

Procoelus and opisthocoelus vertebrae permit motion in any direction (except as modified by the zygapophyses) and they resist dislocation.

Some centra have flat ends and are acoelus.

These centra withstand compression and limit motion.

Heterocoelus centra have saddle-shaped ends. They allow vertical and lateral flexion but prevent rotation around the axis of the spine.

Amphibians may have centra that are opisthocoelus, procoelus or amphicoelus.

Living on land poses problems of water balance, as moisture is continously lost from the lung’s moist membranes and from the skin.

Early labyrinthodont amphibians had bony scales; some even had dermal plates.

These structures likely reduced water loss as well as providing some protection.

In modern amphibians, no scales or dermal plates are found, and the skin is permeable and moist.

Skin is thin and covered by a thin layer of dead epidermis.

Mucus glands in the dermis secrete mucus over the epidermis, keeping it moist to facilitate subcutaneous respiration

Excessive water loss is prevented by a number of adaptations.

i. High concentrations of salts can be tolerated in the body fluids. Amphibians have much greater tolerance for variation in salt concentration than do other vertebrates.

ii. Behavioral adaptations: shade seeking, selection of cool, moist habitats

iii. Permeable skin allows absorption of water from damp ground.

Amphibians are still dependent on water for reproduction

Amphibian eggs will readily dry out and die if not in water.

Some forms have made partial adaptations and are able to lay eggs in moist locations on land or within gelatinous froth.

However, in general, the amphibians are still highly dependent on aquatic environments.

The next vertebrate group to arise - the reptiles - finally broke that dependence.