Approximately 4,400 species are included in the three orders of amphibians. About 3% of the species are caecilians (Gymnophiona), 12% are salamanders (Caudata) and 85% are frogs (Anura). Amphibians are the only vertebrates to have a free-living aquatic developmental stage and a terrestrial juvenile/adult stage. Amphibians rely on cutaneous respiration and two types of skin glands, have a double auditory system, and specialized visual cells in the retina.
Amphibians differ greatly in morphology between and within orders. Caecilians resemble earthworms while the salamanders resemble lizards; frogs are characterized by large hindlimbs built for jumping.
Caecilians are limbless, earthworm-like vertebrates that are almost pan-tropical (excepting Madagascar and Papua-Australia; Zug 1993). Their habitat includes moist soil, abandoned termite mounds, decaying material, and underneath rocks (Obst 1988). The skull is compact and tightly knit to withstand the pressures of tunneling. The eyes are regressed under skin and sometimes skull. They have no ear openings, and a pair of retractable sensory tentacles and an under-hung jaw to identify and capture prey in the tunnels. Caecilian bodies are grooved and segmented; their tails are short or non-existent, and their skin bare (some species have scales below the surface). Movement in tunnels is swift in either a forward or backward direction. Caecilians surface after dark to feed on arthropods, earthworms, cockroaches and other invertebrates and possibly to mate. Some cannibalism also occurs (Obst 1988).
Fertilization is internal for all caecilians. Males have a large, reversible copulatory organ. Development can be either internal or external (Zug 1993). In egg laying species, the clutch of about 30 large eggs are attached to one another; the female curls up around the eggs, and those which are separated from the mother do not seem to develop normally (Obst 1988).
CAUDATA (Salamanders and newts)
There are about 390 extant species of salamanders and newts in eight or more families. Caudates are distributed almost exclusively in Northern Hemisphere temperate forests. Lifestyles range from exclusively aquatic to entirely terrestrial (Zug 1993). All have long tails, a cylindrical body and distinct heads. Protective adaptations include nocturnal behavior, poisonous secretions (usually accompanied by bright warning coloration), and breakable tails.
Fertilization is generally internal (except in Cryptobranchoidea), though the male lacks a copulatory organ. The male releases a spermophore (sperm filled gelatinous capsule) which the female then picks up in her cloaca. The female stores the sperm for days to years. Typically the eggs are deposited in water. Larvae are predominately predatory, eating little or no plant matter. Terrestrial species tend to lay larger eggs with young that hatch in miniature adult form (Obst 1988).
ANURA (frogs and toads)
Anurans are pandemic except for some oceanic islands and the Polar Regions and are most diverse in the tropics. Frogs are found in rainforests, deserts, lowlands, mountains, in water and far from water (Obst 1988). All extant frogs share compact frame, strongly developed hind limbs, no tail and, in most species, no ribs. They have flat, broad heads and large mouths. A large, anteriorly attached tongue is used to capture prey. Most males have vocal sacs used in territory maintenance and courting.
Fertilization is typically external through amplexus. The male grasps the female from above and sperm are deposited as eggs are shed. Most commonly, the egg and sperm are released into open water where fertilization occurs. Anuran larvae (tadpoles) differ from those of Caudata in having a respiratory tube (spiracle). External gills are only present immediately after hatching. Larvae of most species are herbivorous (Obst 1988).
Over 5500 species in four orders comprise the reptiles. Snakes and lizards (Squamata) make up over 5000 of these species. There are approximately 200 species of turtles (Testudinata). Characteristics shared by all reptiles include pulmonary respiration, a single occipital condyle, two sacral vertebrae, epidermal scales, internal fertilization, and shelled, amniotic eggs. There are two ancient lines of reptiles, the diapsids (the lizards, snakes, tuataras, crocodilians, and sometimes birds) and the anapsids (turtles).
CROCODYLIA (crocodiles, alligators, caimans)
There are three extant families of Crocodylians; about 14 known fossil families extend back to the Triassic. They occur in the tropics and subtropics, in or near freshwater. Crocodylians have powerful tails same length or longer than their bodies, and are excellent swimmers using an undulating motion of the tail to propel themselves. Crocodylians rely on stealth and ambush to capture invertebrates, small fish, birds, and mammals. Ranging in size from 2 to 7 m or more, the larger Crocodylians have no enemies except humans (Obst 1988; Zug 1993).
Crocodylians are the only heterothermic reptiles with a fully developed secondary palate and a four chambered heart (Zug 1993). A large lung capacity and low metabolism allows them to stay submerged for more than an hour. Webbed hind limbs aid in swimming; however, Crocodylians are also highly adept at walking or running on land. Crocodylian skin is thick with large armor-like scales. They have strong jaws and angled teeth that are ideal for tightly gripping prey (Obst 1988).
Crocodylians lay clutches of 15 to 100 eggs and incubation lasts 64 to 115 days at constant temperatures of 29 to 34 degrees Celsius and humidity near 100%. Females guard the nest and aid the young in hatching from the eggs (Obst 1988). With low predation, crocodylians may attain natural ages of up to 100 years.
TESTUDINES (turtles and tortoises)
Modern turtles first appeared in the Upper Cretaceous and have remained basically unchanged since that time (Obst 1988). Distributed throughout all warm and most temperate areas, they can be found in varied habitats, including damp terrestrial, dry steppes and deserts, rivers and lakes, swamps, and marine habitats. The carapace (dorsal shell) contains dermal bones spread over and fused to the trunk vertebrate and ribs. The plastron (ventral shell) is an expressed plate of dermal plates with a few elements of the pectoral girdle and sternum. Bony ridges or ligaments connect the carapace and the plastron. In most extant turtles, the limbs, head and neck can be drawn inside the shell (Zug 1993, Obst 1988).
Turtles lack teeth but have a keratinous jaw-sheath that grows continuously. Various species of turtles may be herbivores, carnivores, or omnivores. Their necks have eight cervical vertebrae allowing high mobility and retractability. Limbs are highly developed for the environment; fin or flipper-like limb structures for aquatic species and stump-like limbs in terrestrial species. The rigidity of the carapace results in ribs that are not able to move; instead, muscles push air in and out of the lungs. Some aquatic species have also developed cutaneous respiration and gas exchange (Obst 1988).
All turtles are oviparous. There are no live-bearing forms, and all presumably lay eggs at a fairly early stage of development. Females can store sperm, delaying fertilization for up to four years. Nesting is uniform in turtles. Regardless of hindfoot morphology, they dig an egg pit with an alternating scooping motion of the hindfeet (Zug 1993).
Today there are only two extant species of tuatara (family Sphenodontida). Tuataras are native to roughly 30 small islands off the New Zealand coast where they have few predators but are able to prey on nesting seabirds. Although tuataras are most active at night, they are not entirely nocturnal, taking a wide variety of prey including insects, skinks, geckos, and hatchling seabirds (Zug 1993). They have a stout, lizard-like body, a large head, thick tail, and free abdominal ribs (Obst 1988). Mating of tuataras occurs in January, but egg laying is delayed until October-December. Females dig a nest cavity and deposit 8-15 eggs which will not hatch for 11 to 16 months (Zug 1993).
SQUAMATA (lizards, snakes, amphisbaenians)
Squamata are scaled reptiles such as lizards and snakes. They are distributed worldwide (Obst 1988). A recent study identified more than 70 shared traits of squamates in support of their monophyly including the paired hemipenes, which is unique among vertebrates. Also distinct for this order are the more or less regularly arranged horny scales that have to be shed periodically by molting (Obst 1988). Most squamates lay soft, parchment-like eggs that harden secondarily only in the Gekkonidae. A few squamates are ovoviviparous or viviparous (Obst 1988).
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