AMPHIBIANS

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

1. 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

2. 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

3. 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