Form And Function Of Amphibian
General features
The form and structure of the skeletal, nervous, digestive, and urogenital systems of amphibians are intermediate between fishes and reptiles, but those of other systems are specifically modified for amphibians. The integument (skin) and circulatory and respiratory systems exemplify these adaptations and act together to provide cutaneous respiration. Amphibians employ various combinations of branchial, pulmonary, and cutaneous respiration. The buccal pump mechanism is a unique feature of the pulmonary respiration system. Water and ion diffusion as well as gas exchange are facilitated by a myriad of cutaneous capillaries. The integument is involved in respiration and maintaining water balance. It also contains poison glands that release toxins found only in amphibians: these toxic substances provide defense against predators.
Adaptations
The amphibian eye has a lid, associated glands and ducts, and muscles that allow accommodation as well as depth perception and true colour vision. These adaptations are regarded as the first evolutionary improvements in vertebrate terrestrial vision. Green rods in the retina, which permit perception of a wide range of wavelengths, are found only in amphibians. The amphibian auditory system is also specially adapted. One modification is the papilla amphibiorum, which is a patch of neuroepithelium that can receive external stimuli. Also unique to amphibians is the columella-opercular complex, a pair of elements associated with the auditory capsule that transmit airborne (columella) or seismic (operculum) signals.
The environment helps to mold the morphology of an organism. The markedly different morphologies of the three living orders of amphibians suggest that each group has had a long, separate evolutionary history. Salamanders have less-specialized morphologies than do the other two orders. They have small heads, long slender bodies, four limbs, a tail, and a sprawling gait. Although the skulls of most terrestrial salamanders consist of more individual pieces than do those of either caecilians or anurans, they are arched and narrow and not well-roofed. Their skulls have an extra set of articulations with the vertebral column, a characteristic that may have been an evolutionary strategy for stabilizing the head on the axial skeleton in terrestrial salamanders; other amphibians developed specialized trunk musculature to meet this challenge.
The hyoid apparatus in the floor of the mouth enables salamanders to capture prey by projecting their fleshy tongues from the buccal cavity, although most are only able to roll their tongues forward over their lower jaws to snare their dinner. Food is held and manipulated in the buccal cavity by the teeth and tongue. This mechanism does not require adaptations of mandibular and jaw muscles or sturdy, specialized teeth features that most salamanders lack. Well-developed eyes and nasal organs, however, are needed to locate prey. Because salamanders do not depend on their vocal abilities, their auditory apparatus is less specialized than that of anurans. Most salamander species have a generalized mode of locomotion, which is reflected by a lack of specialization in the musculoskeletal system. The salamander cuts a deliberate and diagonal path, moving each limb alternately and forcing the trunk to curve, thereby advancing the forelimb. Aquatic salamanders show the greatest divergence from this generalized morphological pattern. Because they are kept afloat by their aquatic environment, they are often larger, devoid of limbs, and able to undulate through the water.
All but a few aquatic species of caecilians lead subterranean existences and so have similar specialized morphologies; of the three living amphibian orders, they show the least divergence in structure and form. They have a wormlike appearance, with compact and bony heads in which the centres of ossification have fused to provide a strong, spadelike braincase. While useful in tunneling through the soil, this compact cranium does not allow much room for the jaw muscles to develop. Thus, the lower jaw is attached to the main adductor muscle of the jaw by a retroarticular process outside the cranium. The caecilian cannot extend its tongue from the buccal cavity. Visual sense, of little importance in the caecilian environment, is not acute, but nasal organs are well developed and chemosensory perception is greatly enhanced by the existence of a tentacle. Caecilians’ sense of hearing is probably less sensitive than that of salamanders or anurans. If the operculum is present, it is incorporated into the columella. Subterranean movement and feeding are aided by alterations of the axial musculoskeletal system. The overlying skin is firmly attached to the axial muscles, and this creates a tough sheath that encases the long, slender body and covers the posterior part of the skull. This allows caecilians to propel themselves through soil by a process called concertina locomotion, in which the body alternately folds and extends itself along its entire length.
Anurans are more widespread, diverse, and numerous than either salamanders or caecilians. Greater anuran specialization in locomotion, feeding, and reproduction has allowed frogs and toads to inhabit many different environments. In general, anurans have a broad, flat head, almost as wide as the body, and a short trunk that, aside from the sacral area, is inflexible. Long, powerful hindlimbs propel the fused head and trunk in a forward trajectory (see video). These leaping movements require more complex pectoral and pelvic girdles than salamanders have. The pectoral girdle is designed to absorb the shock of the anuran as it lands on its forelimbs; an elastic, muscular suspension connecting the pectoral girdle to the skull and vertebral column provides this ability. The pelvic girdle horizontally flanks the coccyx, the bony rod at the posterior end of the vertebral column, and differs from the pelvic girdle of the salamander. Muscles and ligaments attach the pelvic girdle to the coccyx, sacrum, presacral vertebrae, and proximal part of the hindlimb. Thus, when the animal jumps, the pelvic girdle lies in the same plane as the axial column, and, when the animal sits, the posterior end of the girdle is deflected ventrally.
In addition to the specializations for leaping, many anurans have developed structures that allow them to burrow or climb trees, primarily involving modifications in limb proportions and iliosacral articulation. Arboreal (i.e., tree-dwelling) anurans have long limbs and digits with large, terminal, adhesive pads; anurans that burrow have short, sturdy limbs and large, keratinous, spatulate tubercles on their feet. The pipids, specialized for their aquatic environment, have little flexibility in their axial skeletons and instead propel their flat, fused bodies through the water with powerful hindlimbs and large, fully webbed feet. Anurans depend on their visual acumen for feeding and locomotion, and hence their eyes are large and well developed. Because vocalizing is part of their mating and territorial behaviour, their ears are highly specialized. Most species have an external tympanum (eardrum) that salamanders and caecilians lack.
The many morphological adaptations of anurans have permitted them to explore a variety of lifestyles, which undergirds the success of this order and may account for their greater numbers as compared with salamanders and caecilians. All modern amphibians, however, have exhibited amazing evolutionary diversity. This has occurred in spite of the physiological limitations, such as dependence on water, that shackle the amphibian to its environment.
Evolution
Amphibians are generally thought to be intermediate between aquatic fishes, whose eggs lack a shell and protective membranes, and terrestrial amniotes (reptiles, birds, and mammals), which have cleidoic (self-contained) eggs with membranes that function in respiration, feeding, and waste disposal. Amniotes and amphibians form the group of four-legged vertebrates called the tetrapods. The first tetrapods evolved from their aquatic ancestors, undergoing many structural and functional modifications. Movement to land necessitated aerial respiration a feature requiring nares, nasal tubes, and lungs. Bony fishes had already developed these structures early in their evolution, but the ventilation mechanism was modified from a passive pump in fishes to a force pump in tetrapods. Limbs developed from lobe fins presumably to allow terrestrial locomotion, and this involved bone lengthening and musculature modification.
The pelvic and pectoral girdles, to which the limbs were attached, were strengthened to support the weight of the body with connections to the axial skeleton. Some species developed mucous and granular glands in their skin, while others developed bony plates. Tetrapods developed snouts, long jaws, tongues, and a movable skull in response to the need for a different method of foraging. The sense organs were modified for aerial perception, which involved the formation of a middle ear and accommodation in the eye lens. The lack of an impervious skin in primitive tetrapods, however, probably limited their range to humid or aquatic environments. Reproduction remained similar to that of fishes, with external fertilization, aquatic eggs encased in gelatinous capsules, and free-living larvae with gills. The adaptive strategy of metamorphosis developed uniquely in primitive amphibians.
During the latter part of the Paleozoic (i.e., the Carboniferous (360 to 286 million years ago) and the Permian (286 to 245 million years ago)) and the early Mesozoic (the Triassic (245 to 208 million years ago)), primitive amphibians spread into terrestrial habitats hospitable to their physiology. Moisture was then, as now, a necessary element for amphibian life. While inhabiting an aquatic environment, amphibians vied with a wide variety of freshwater and marine fishes for food; as amphibians moved away from their aquatic lifestyle, reptiles, which appeared midway into the Carboniferous, most likely become their primary terrestrial competitors. All primitive amphibians are believed to have preyed on various animals such as fishes (both freshwater and marine), other amphibians, and aquatic and terrestrial invertebrates.
Two groups of bony fishes, the crossopterygians and dipnoans, have been identified as the possible closest relatives of tetrapods. Crossopterygians, or lobe-fin fishes of which Eusthenopteran is best known, inhabited marine and freshwater environments during the Devonian Period (408 to 360 million years ago) and Carboniferous; no record of them exists beyond the Early Permian (286 to 258 million years ago). Morphological features included one pair of narial openings that perforated the palate, a pair of external nares on the dorsal aspect of the snout, and two pairs of short lateral fins with a robust, fleshy base. Crossopterygians also are believed to have had a spiracle (accessory opening for the intake of water), lungs, and gills.
Dipnoans, or lungfishes, inhabited marine environments in the Early Devonian (408 to 387 million years ago). Five living species survive from the diverse array that exists in the fossil record. These slender fishes are round in cross section and have two pairs of lateral fins. In two genera these fins are thin, long, fleshy structures with slightly webbed margins, but in the genus Neoceratodus they are broader and fleshier. Two pairs of narial openings (one through which water entered, the other through which it exited) are associated with the interior of the mouth, and external nares are not present. Dipnoans have lungs and gills but lack a spiracle.
Ichthyostega, the earliest known tetrapod, was found in the Upper Devonian (i.e., in rocks deposited from about 374 to 360 million years ago) of Greenland. Measuring 65 centimetres in length, this primitive tetrapod had broad ribs, well-developed, pentadactyl limbs, and a massive pectoral girdle that was independent of the skull. A bony rod was formed by the fusion of the most posterior vertebrae. Unlike other labyrinthodonts, Ichthyostega had dermal fin rays and a dorsomedial tail fin supported by endoskeletal elements. Many features of its skull are more rudimentary than those of other primitive tetrapods, which supports evidence that Ichthyostega is the earliest labyrinthodont. Additional factors, however, such as proportional differences between skull sizes of Ichthyostega and crossopterygian fishes, lack of data about soft anatomy, and anomalous characters have raised questions about the phylogenetic relationships among primitive tetrapods. A reinvestigation of the matter suggests that lungfishes are the closest tetrapod relatives, but controversy still exists.
The earliest tetrapods, the labyrinthodont amphibians, are from the Upper Devonian to the Triassic; these fossils include Ichthyostega. Two subclasses Labyrinthodontia and Lepospondyli are traditionally distinguished based on the arch vertebrae in the former and the husk vertebrae in the latter. The Labyrinthodontia are further broken down into the orders Anthracosauria and Temnospondyli. The dominant amphibians into the Permian were the anthracosaurs, which became extinct after this period. Various types of anthracosaur existed, such as the small, slender, limbless aistopod, which was no longer than 70 centimetres, and the large, aquatic embolomere, which had a skull like a crocodile and could reach a length of 4 metres.
Living amphibians belong to the subclass Lissamphibia. Lissamphibians are thought to be most closely related to the temnospondyls of the family Branchiosauridae and less closely related to the Micromelerpetonidae, Dissorophidae, and Trematopidae. These families existed between 305 and 240 million years ago (from the Late Carboniferous, or Middle Pennsylvanian, to the Early Triassic) and were all small, primarily quadrupedal, terrestrial animals with a primitive amphibian biphasic life cycle.
Lissamphibians are descended from one ancestor (monophyly) and are defined by 14 shared-derived characters that are based on osteological data. Seven of these features are unique to this group and involve the lack of elements. The retention of many cranial elements that other orders have lost characterizes the primitive status of the superorder Gymnophiona (caecilians). Many derived features are associated with the semifossorial or fossorial (adapted to digging) way of life that distinguishes all caecilians except the secondarily aquatic typhlonectids. Six of the seven shared-derived features that define this clade include cranial modifications of fossil and living species. Only living caecilians (order Apoda) and not the entire Gymnophiona are marked by the seventh character, loss of limbs a fossil caecilian from the Jurassic Period (208 to 144 million years ago) had appendages.
The morphotype of most adult salamanders attenuate bodies, tails, and (generally) forelimbs and hindlimbs of equal size is not specialized, unlike that of caecilians and anurans. This permits a generalized terrestrial existence. Salamanders (order Caudata) are categorized in the superorder Urodela with the Jurassic salamander Karaurus. The features diagnostic of this group are a four-faceted articulation between the skull and vertebral column and an incomplete maxillary arcade that lacks bony connections with the neurocranium and palatoquadrate.
The superorder Salienta consists of the frogs and toads (order Anura) and the primitive froglike Triadobatrachus. Fused centres of ossification of the frontal and parietal bones, an elongated ilium that is anteriorly orientated, and a lack of a lacrimal bone are distinguishing characteristics. The structures involved in saltatorial locomotion are more specialized in anurans than in Triadobatrachus; the hindlimbs are elongated, discrete caudal vertebrae are fused to form a urostyle, the radius and ulna are partially or completely fused, fewer presacral vertebrae are present, and the sacrum is modified.
