KINGDOM ANIMALIA Phylum Chordata Subphylum Vertebrata Class Amphibia

Vertebrate Classes

History Chordate group to evolve after fish were true tetrapods = Amphibians Ichthyostega 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 cranium Ichthyostega still retained a caudal fin and scales

History Ancient 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 Features Well-muscled appendages, supported by an central and peripheral skeleton Further development of lungs Skin highly vascularized, other site of gas exchange, and maintains water balance Circulatory 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 blood All of these features serve to increase the mobility of amphibians

Common Features Other 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 features Novelty = pedicellate bicuspid teeth –crown and base = dentine –middle = fibrous connective tissue –bicuspid = 2 cusps/points

3 Main Groups Caecilians (165 species) Salamanders (502 species) Anurans (frogs/toads, 4000 species)

Caecilians Tropical, 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

Caecilians Tiny 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

Salamanders Most closely resemble amphibian tetrapod ancestor Long tails, 2 pairs of limbs of approximately the same size Primarily live in Northern Hemisphere (abundant in cool, moist forests, only 1 type tropical)

Salamanders Generally (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

Many deposit eggs in water; get free- swimming tadpoles Others lay terrestrial eggs Some carry their eggs with them