KINGDOM ANIMALIAPhylum Chordata

Subphylum VertebrataClass Amphibia

Vertebrate Classes

HistoryChordate group to evolve after fish were true tetrapods = AmphibiansIchthyostega presents rudimentary amphibian features (different from fish)– Girdles = skeletal bones connecting the central

skeleton to the bones of the appendages– Other skeletal strengthening: rib cage and craniumIchthyostega still retained a caudal fin and scales

HistoryAncient and modern

amphibians have features that enhance their survival on land but also limit this existence.Most amphibian

evolution took place when Earth was warm, humid, and swampy (350 mya). Insects were abundant.No pressure to develop

into a truly terrestrial animal.

Common FeaturesWell-muscled appendages, supported

by an central and peripheral skeletonFurther development of lungsSkin highly vascularized, other site of

gas exchange, and maintains water balanceCirculatory system now includes a 3-

chamber heart– Increased pressure to peripheral arteries– Atrium separated by septum, but still

one ventricle– More efficient but still mixing of

oxygenated/deoxygenated bloodAll of these features serve to increase

the mobility of amphibians

Common FeaturesOther features illustrate the

amphibian’s primitive nature:– ectothermic-sluggish when

cold, hibernation or death– respiration through skin

requires it to be thin and moist

– lose lots of water through skin and must keep it continually moist to prevent lethal desiccation

– must reproduce in the water since eggs would dry up on land

– aquatic larval stage more closely related to fish than terrestrial animals

Common featuresNovelty = pedicellate

bicuspid teeth– crown and base = dentine– middle = fibrous

connective tissue– bicuspid = 2 cusps/points

3 Main GroupsCaecilians (165 species)

Salamanders (502 species)

Anurans (frogs/toads, 4000 species)

CaeciliansTropical, limbless amphibians

Resemble giant earthworms and burrow in the ground

– unlike other tetrapods, skin is bound to body wall musculature which bestows great burrowing efficiency

– powerful body with well-developed endoskeleton

– skull used as battering ram as it burrows

Carnivorous: eat earthworms if terrestrial, fish/inverts if aquatic

CaeciliansTiny eyes if present

(most are vestigial, covered by skin)

Chemosensory tentacles on head in front of eyes used to locate food

Internal fertilization; some species bear live young while others lay eggs

SalamandersMost closely resemble

amphibian tetrapod ancestorLong tails, 2 pairs of limbs

of approximately the same size

Primarily live in Northern Hemisphere (abundant in cool, moist forests, only 1 type tropical)

SalamandersGenerally

(semi)terrestrial as adults

Most pass through larval stage – few days to a

few years– some species

never metamorphose (axolotl)

Anurans“without tail”Most successful, diverse,

evolutionarily divergent of the living amphibians

Jumping locomotion allowed exploitation of new terrestrial niches

Live in almost any climate (except high latitudes in Arctic, Antarctic, some oceanic islands, some extremely dry deserts)

Adaptations for jumping locomotion– Hind limbs much

longer than forelimbs

– Short trunk– Tail lost– Flattened head– Large eyes

Anurans

AnuransMany deposit eggs in water; get free-swimming

tadpolesOthers lay terrestrial eggsSome carry their eggs with

them