Amphibians

The double life

Origin and Evolution of Amphibians

• Lobe – finned fish– Bone structure within their fins allowed for them

to function as “legs”– Modified pouches in digestive tract evolved into

lungs of the lungfish and swim bladder of modern fish

– Amphibians also share skull and vertebral column

Lobed – finned fish: Coelocanth

• Oldest amphibian fossils were dated to 360 m.y.a

• Between 359 – 299 m.y.a amphibians split into two main evolutionary lines– Modern amphibians– Ancestors of reptiles

Classification• Domain: Eukarya • Kingdom: Animalia • Phylum: Chordata • Subphylum: Vertebrata• Class: Amphibia, 4500 species– Order Anura, largest order contains the frogs and

toads– Order Caudata, contains salamanders– Order Gymnophiona, contains caecilians

Order Anura• Frogs and toads (squat bodies, shorter legs, skin

not as smooth as frog)

• Found in a variety of moist habitats• Adults are carnivorous• Reproduction– Amplexus

• Metamorphosis

Life Cycle

• Fertilization takes place• Eggs hatch, tadpole lives off yolk stored in its

body• Tadpole grows larger, three gills develop• Mouth opens, begins to feed on algae• Tadpole grows slowing changing from an

aquatic larva into an adult• May vary depending on species

Metamorphosis• Process of change from an egg to larva to adult

• Triggered by the hormone thyroxine that is produced by the thyroid gland

Order Caudata

• Salamanders, newts, mudpuppies• Elongated tails, and smooth, moist skin• Lay eggs in water or moist places• Both young and adult are carnivorous• Hatchlings are small versions of parent except

young have gills

Order Gymnophiona

• Caecilians• Legless amphibians• Live in tropical areas, burrow in the soil, also

aquatic• Lay eggs or young are born alive

Characteristics of Amphibians

• Five key characteristics; legs, lungs, double – loop circulation, 3 - chambered heart, cutaneous respiration

• Live in moist habitats• Mostly external fertilization

Movement• Skeleton– Strong endoskeleton– Vertebrae of the spine is interlocked to form a

rigid structure– Limbs support body• Forelimbs attach to the pectoral girdle (shoulder)• Hind limbs attach to the pelvic girdle• Transfers body weight to the limbs• Frogs, lower limbs are fused into a single bone

– Forelimb (radio – ulna) and hind limb (tibiofibula)

Response• Sense Organs– Lateral line as larvae– Sight, sound, smell

– Nictitating membrane• Transparent membrane that covers eyes

– Tympanic membrane• Eardrum

Respiration

• Gills, most loose as adults• Skin serves two important functions –

respiration and protection– Cutaneous respiration– Diffusion– Mucous glands

• Pulmonary respiration

Pulmonary Respiration

• Pump air into lungs in a process called positive – pressure breathing

• Frog breathes by changing the volume and pressure of air in its mouth while either opening or closing its nostrils

Circulation

• Three chambered heart and double – looped circulation

• Increase pressure and speed of delivery for oxygen rich blood to organs

• Divided into two separate loops

Two separate loops

• Pulmonary circulation– Carries deoxygenated blood from the heart to the

lungs and back to the heart

• Systemic circulation– Carries oxygenated blood from the heart to the

body and back to the heart

Three chambered heart

• Right and Left Atrium (septum divides)• Ventricle (spongy surface and contraction)– Deoxygenated blood enters from the body, right atrium– Oxygenated blood enters from the lungs, left atrium– Contraction of the atria forces blood into a single

ventricle– Ventricle contracts pushes all of the blood into the

conus arteriosus, which directs the blood to the lungs or the body