CHAPTER 25 Early Tetrapods and Modern Amphibians

Transcription

1 CHAPTER 25 Early Tetrapods and 25-1

2 Physical Adaptations: Oxygen content Movement Onto Land Oxygen is 20 times more abundant in air so terrestrial animals can obtain oxygen much more easily once they possess lungs and other respiratory structures. Density Air is 1000 times less dense Provides less buoyancy than water Limbs and skeleton must support more weight Temperature Regulation Air fluctuates in temperature more rapidly than water Animals must adjust to these extremes via behavior or physiological adaptations. Habitat Diveristy Variety of terrestrial habitats allows greater opportunities for adaptation 25-2

3 Early Evolution of Terrestrial Vertebrates What structural characteristics evolved in aquatic habitats, that made it possible to explore terrestrial habitats? Two structures connected to pharynx Air-filled cavity functioned as a swim bladder Paired internal nares functioned in chemoreception On land, combination would be used to draw in oxygenrich air through nares into the air-filled cavity Bony elements of paired fins Modified for support and movement underwater On land, would provide same function 25-3

4 Early Evolution of Terrestrial Vertebrates Adaptations for life on land included the skull, teeth, pectoral girdle and jointed limbs Tetrapods MUST HAVE: Have stronger backbone Muscles to support the body in air Muscles to elevate the head Stronger shoulder and hip girdles More protective rib cage Modified ear structure to detect airborne sounds Forward shortening of skull Longer snout 25-4

5 Diversity Over 6000 living species are known in the three amphibian orders. Metamorphosed adults use a redesigned olfactory epithelium to sense airborne odors and the ear detects sounds But, they remain tied to water. WHY? Eggs deposited in water or must be kept moist Larvae depend on gills for respiration. Thin skin loses water rapidly They are ectothermic where their body temperature depends on the environment and this can restricts range Eggs easily dessicate and must be shed into water or kept moist 25-5

6 Caecilians: Order Gymnophiona Approximately 173 living species Elongate, limbless, burrowing animals with long ribs, and a terminal anus. Inhabit tropical forests in South America, Africa, India, and Southeast Asia Feed primarily on worms and small underground invertebrates Fertilization is internal with the male using a protrusible copulatory organ Some deposit eggs in the moist ground near water while others will guard them by having them develop in the folds of the body. In other species, viviparity allows embryos to obtain nourishment by eating wall of oviduct Larva are typically aquatic 25-6

7 Salamanders & Newts: Order Caudata (Urodela) Approximately 553 living species are found primarily in northern temperate regions Most are small, under 15 cm long while some like the Japanese giant salamander are over a meter long Limbs are usually about equal length and at right angles to trunk Burrowing species and some aquatic forms may have lost their limbs These animals are carnivorous as both larvae and adults and feed on worms, small arthropods and molluscs, or their own eggs They are ectotherms with a low metabolic rate 25-7

8 Salamanders have some very unique breeding behaviors. Internal fertilization After courtship, the female will recover the spermatophore, that has been deposited on a leaf or stick, in her cloaca by passing over it. Aquatic species lay eggs in clusters or stringy masses Completely terrestrial species deposit eggs in small, grape-like clusters under logs or in soft earth Some are aquatic throughout their life cycle but most have aquatic larvae and terrestrial adults Entirely terrestrial species will undergo direct development and hatch as miniature adults 25-8

9 Some North American newts have aquatic larvae that metamorphose into terrestrial juveniles that again metamorphose into secondarily aquatic, breeding adults Some newt populations skip the terrestrial red eft stage and remain entirely aquatic 25-9

10 Respiration varies among salamanders & newts. Extensive vascular nets in skin that exchange both oxygen and carbon dioxide (cutaneous respiration) At various stages, may also have external gills, lungs, both gills and lungs, or neither Salamanders with an aquatic stage hatch with gills, which are lost at metamorphosis Several diverse lineages fail to undergo metamorphosis and retain gills and a fin-like tail In species with lungs, lungs are present from birth and become functional following metamorphosis 25-10

11 Frogs and Toads: Order Anura Approximately 5283 species Must live near water source Reproduction mode requires water Skin is water-permeable skin Ectothermy prevents anurans from inhabiting polar and subarctic habitats All have a tailed larval stage and tail-less, jumping adults (except for 1 species) 25-11

12 44 families of frogs and toads Family Ranidae Contains the common larger frogs in North America Family Hylidae Includes the tree frogs Family Bufonidae (true toads) Contains amphibians with thicker skins and prominent wart like structures

13 Habitats and Distribution Genus Rana are common in temperate and tropical regions Anuran are declining worldwide and becoming geographically fragmented Malformed limbs are often associated with infection by trematodes or DNA errors

14 Many are easy prey Defend themselves by aggression, concealment, and poison glands Many species have suffered from changes in the environment and climate brought about by humans Climatic changes that reduce water depth at reproductive sites Increases ultraviolet exposure of embryos Also, makes them more susceptible to fungal infection Decline of some amphibians may be caused by other amphibians such as Bufo marinus 25-14

15 Reproduction and Development In spring, males call to attract females When eggs are mature, females enter the water and the males clasp them in amplexus After fertilization, jelly layers of egg absorb water and swell Eggs usually laid in large masses Development begins immediately Tadpole may hatch in 6 9 days and have a long, finned tail, no legs, and internal and external gills Herbivorous tadpoles feed with horny jaws and use a ventral adhesive disc for clinging to objects 25-15

16 3 pairs of external gills develop into internal gills covered with a flap of skin On right side of a tadpole, operculum fuses with body wall On left side, a spiracle remains Water enters mouth, flows past gills, and then out spiracle Metamorphosis Hindlegs are first to appear Forelegs temporarily hidden in folds of operculum Tail is reabsorbed Intestine becomes shorter Mouth transforms to the adult condition Lungs develop and gills are resorbed 25-16

17 25-17 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

18 Males migrate back to breeding ponds or streams Tropical anurans have different reproductive strategies Some lay eggs in foam masses that float on surface of water Some deposit eggs on leaves overhanging ponds and streams into which tadpoles drop Other place eggs in water trapped in tree cavities or water-filled chambers of bromeliads Poison-dart frog tend to their eggs Tadpoles hatch on their back and can be carried for varying lengths of time Marsupial frogs carry eggs in a pouch on the back These mate on land and eggs hatch directly into froglets