U NIT Q UESTIONS 1. How did the tetrapod leg evolve? 2. Why did early animals move from water to land? 3. How did amphibians transition from life in water to life on land? 4. What are the three major groups of modern amphibians

By: Debra Cunningham E ARLY T ETRAPODS AND M ODERN A MPHIBIANS

E ARLY T ETRAPODS  Terrestrial vertebrates adapted  Tetrapod leg evolved  Modern amphibians transitioned  Caecilians  Salamanders  Frogs and toads

O RIGIN OF T ERRESTRIAL V ERTEBRATES  The Devonian period  400 million years ago  time of mild temperatures  alternating droughts and floods  Lungs and limbs evolved for terrestrial life  Unstable freshwater environment  Pools and streams evaporated  Foul water resulted  Lobe-finned and lungfish evolved into modern amphibians

E VOLUTION OF T ETRAPOD L EG  Lobe-finned fish fins  Paddle-like fins  Paired muscular fins  Anterior fin - humerus  Forearm bones - radius and ulnar  Bony elements - wrist bones

L OBE - FINNED F ISH

A DAPTATIONS FOR L IFE ON L AND S ENSORY D EVELOPMENT S KELETAL S TRUCTURE  Special sense priority shift  Foreshortening skull  Lengthened snout  Advanced ear structure (tympanic membranes)  Olfactory epithelium to detect airborne odors  Skeletal strengthening  Stronger backbone  Strengthened shoulder and hip girdle; tetrapod leg  Protective rib cage  Muscles to support body in air and elevate head

M ODERN A MPHIBIANS C AECILIANS Gymnophiona gymnos + opineos = “naked of a snake” ≈ 160 species South America principal home S ALAMANDERS Caudata caudatus = having a tail ≈ 360 species Found in most northern temperate regions of the world F ROGS AND T OADS Anura an + oura = without a tail ≈ 3450 species Found near water, swamps or on damp forest floors

Amphibian Breeding Habits ▪ Caecilians: Internal fertilization Deposit eggs in moist soil ▪ Salamanders: Internal fertilization Lay eggs in stringy masses, clusters in water ▪ Frogs and Toads: External fertilization Lay eggs in masses anchored to vegetation

C AECILIANS Elongated body, limbless Small scales on skin Many vertebrae (95-285) Small eyes (adults blind) Sensory tentacles on snout Pantropical

S ALAMANDERS

F ROGS AND T OADS Typical tetrapod pattern of bones and muscles Shortened body-only nine (9) trunk vertebrae Three main joints in each limb Light-weight skull with flattened profile Tail and gills resorbed Ectothermic

L IFE C YCLE OF A F ROG 

C ONCLUSION  Frogs traded their tails for legs  Modern amphibians evolved from lobe-finned fish  Modern amphibians transitioned from water to land

R EFERENCES Hickman, Jr. C.P., Roberts, L.S., & Larson, A. (2001). Integrated Principles of Zoology, ed 11. McGraw-Hill: Boston, Color illustrations and pictures from:    