Chapter 42: Amphibians 42-1 Origin and Evolution of Amphibians 42-2 Characteristics of Amphibians 42-3 Reproduction of Amphibians
42-1 Origin and Evolution of Amphibians I. Adaptation to Land (~ from LOBE-finned fishes, 370 m.y.a.) • Ancestors LEFT water to escape PREDATION and COMPETITION, AND to access to new TERRESTRIAL resources (i.e., FOOD).
(A) Characteristics of Early Amphibians (~ Crossopterygian ANCESTOR) • Forelimbs ~ homologous to PECTORAL fins, while hindlimbs ~ homologous to PELVIC limbs. NOTE: Ichthyostega (BEST evidence of an EARLY amphibian) had a LONG tail FIN, lateral lines on HEAD, sharp TEETH for FISH, and developed senses for AIRBORNE scents and sounds.
(B) Diversification of Amphibians (~ 360-286 m.y.a.) • DIVERGENT evolution split amphibians into 3 evolutionary lines, (Orders Anura, Urodela, and Apoda).
II. Modern Amphibians • In all THREE orders, traits SHARED include… (1) Metamorphosis between aquatic larval stage (GILLS) to terrestrial adult stage (LUNGS). (2) Moist, thin skin WITHOUT scales. (3) Feet WITHOUT claws, and are often WEBBED. (4) Gills, lungs, and skin are all involved in GAS EXCHANGE (respiration). (5) Shell-less eggs laid in water (or moist habitats) and are usually fertilized EXTERNALLY.
(A) Order Anura “tail-less” (e.g., frogs (smooth skin), toads (bumpy skin)) • Body adapted for JUMPING; short, COMPACT muscular body with RIGID spine and STRONG forelimbs (carnivore adults).
(1) Tadpole • Swimming, TAILED larvae (adults ~ tailless) results from EXTERNALLY fertilized eggs LAID in water.
Critical Thinking (1) There are usually many ecological differences between a frog and its tadpoles. Describe two of these differences and explain why such differences may have been favored by natural selection?
(B) Order Urodela (e.g., salamanders and newts) • ELONGATED body and tail with four equally sized limbs; MOST urodelans are capable of INTERNAL fertilization.
(C) Order Apoda (e.g., mysterious caecilians, BURROWING amphibians) • Legless amphibians (like snakes?), small eyes (nearly BLIND), and rely on a CHEMOSENSORY TENTACLE located on head to FIND prey.