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The Amphibian Ear
By Ernest Glen Wever
PRINCETON UNIVERSITY PRESSCopyright © 1985 Princeton University Press
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NATURE AND ORIGIN OF THE AMPHIBIA
The name Amphibia literally signifies "both lives" and refers to the proclivity of many of these animals for alternating between aquatic and terrestrial habitats. This name was first used broadly by Linnaeus in reference to a wide variety of animals, including such species as seals, turtles, and crocodiles as well as frogs and salamanders, that live sometimes in the water and sometimes on land. Soon thereafter the term was applied more selectively to a particular class of vertebrates, a class including the frogs, salamanders, and caecilians among modern types, together with a number of ancient animals known only as fossils that are commonly considered to be distant relatives of these.
The living amphibians — the anurans, urodeles, and caecilians — form three somewhat disparate orders that constitute a relatively small group among the vertebrates, a group dwarfed in numbers by the varied forms of fishes and exceeded also by the three classes of higher vertebrates, the reptiles, birds, and mammals. The amphibians are located after the fishes and before the other classes in the suborder Vertebrata as an indication of their phylogenetic rank: these animals are considered to have evolved out of certain of the fishes, and some of their earlier forms are regarded as having given rise, through the long and complex process of evolution, to all the higher animals. It is estimated that there are about 2100 species of amphibians now living, out of a total of something like 40,000 vertebrates of all kinds.
On the basis of general body form and habits the three groups of modern amphibians would not seem to belong together in one class, but closer study reveals many characters in common. Parsons and Williams (1963) in a study primarily of tooth structure and then of numerous other features concluded that these three groups form a natural monophyletic assembly, to which they applied the collective name of Lissamphibia, a name first used for this group by Gadow in 1901.
A pedicellate tooth structure is the most distinctive characteristic of this assembly. This tooth, as shown in Fig. 3-9 below, consists of two portions, a basal pedicel and a peripheral crown connected to the base part by a hinge. These two elements consist of dentine covered by a dense enamel layer, and the hinge consists of uncalcified dentine or fibrous tissue to provide flexibility. It is supposed that such a tooth structure may give an increased facility in seizing prey; the prey object readily enters the mouth but tends to catch if pulled away. Somewhat similar teeth occur in teleost fishes, but in these the hinge is between the tooth and the jaw rather than within the tooth itself. The true pedicellate tooth is not found in any other living vertebrates, but has recently been discovered in certain fossil forms belonging to the labyrinthodonts, in two closely related species from the Lower Permian. These are Doleserpeton annectens and Tersomius texensis, and their discovery has led to new speculations about the origin of the Lissamphibia (Bolt, 1969, 1977).
Other features now to be described are not exclusive to the amphibians, but taken as a whole serve to characterize this group.
1. Ectothermy. The amphibians are "cold-blooded"; their body temperature is close to that of their immediate environment. However, this is true of the other lower vertebrates, the fishes and reptiles, as well.
2. Skin Character. The skin is soft; it is smooth in some species and rough in others; its surface lacks the layer of keratin and lipids found in reptiles, and also lacks special coverings such as hair and scales. In the caecilians, however, minute scales may be found deep in the grooves between the body segments, though a dissection is necessary to reveal them. These scale remnants are indicative of a descent from ancient fishes.
3. Skin Glands. The skin is kept moist by numerous mucous glands distributed over its surface. The secretion produced by the glands of many species is at least mildly toxic, and seems to serve in some degree to repel predators. In many species there are also more specialized skin glands, such as the paratoid glands in the shoulder areas of frogs, that produce highly toxic secretions.
4. Body Form. There are marked variations in body form in the three groups. This form in salamanders is elongate, well streamlined, with legs of moderate size and a well-developed tail; Romer (1966) regarded this form as the original, basic one for the class, and considered the types in frogs and caecilians as highly specialized. There are two pairs of legs in all salamanders except the sirens, which have only the anterior pair. The frogs are specialized for leaping or hopping; they have compact bodies, short forelegs, long, well-developed hindlegs, and they have no tails. The caecilians are elongated, wormlike in appearance, without legs, and have a compact skull that in most species is utilized for burrowing.
5. The Egg. The amphibian egg lacks a shell, but is covered by several layers of gelatinous membrane. For the most part the eggs are laid in water or in moist places, or a variety of means are employed to prevent them from drying, and often also to protect them from predators. In some species the female curls around the cluster of eggs and both guards them and keeps them moist; in other species the eggs are carried in vesicles in the skin of the back; in one genus the eggs are retained in the male's vocal pouch, and in another genus in his stomach. In a few species the eggs remain in the oviducts of the female, are fertilized internally, and the young hatch out as small individuals closely approaching the adult form.
6. Metamorphosis. Typically the larval and adult forms are greatly dissimilar; and at the end of a larval stage there is a rapid metamorphosis in which the new adult form emerges, with profound changes of both structure and function. This metamorphosis is most complete in frogs, in which legs appear and the tail dwindles away, the gills are lost and usually replaced by lungs, and the notochord is largely replaced by a jointed vertebral column, or else is incorporated into such a column. There is developed a three-chambered heart, with pulmonary and systemic divisions of the circulatory system in an arrangement that permits only a slight mixing of spent and oxygenated bloodstreams.
7. Respiratory Patterns. Respiration in amphibians takes a variety of forms, and usually varies greatly between larval and adult stages. Gaseous interchange through the skin is a general feature; the skin is moist and well supplied with capillaries, and cutaneous respiration is present in some degree in all species. This form of respiration alone can sustain life under certain conditions, as during hibernation when the body temperature is low and activity is minimal.
A more efficient form of respiration is buccopharyngeal, in which throat movements facilitate an interchange of gases between dense capillary beds in the lining of the mouth and pharynx and either the air or the water in contact with these surfaces. Aquatic larvae when in need of added oxygen will speed up their throat movements, or sometimes will rise to the surface and take in a bubble of air that is then held near the buccopharyngeal surface. Terrestrial animals such as frogs use integumentary respiration much of the time but supplement it by involvement of the lungs as described below.
Gills are present in all larval amphibians and are retained in those salamanders that fail to metamorphose, such as Necturus maculosus. These gills are often obvious as branched plumes arising from the branchial arches.
At the terrestrial stage most of the amphibians are equipped with lungs, which arise from branchial pouches. In some species, however, the lungs are reduced or even absent: in caecilians the left lung is usually rudimentary, and in many salamanders the lungs are small, or as in all species of the plethodontid family are entirely lacking. In both urodeles and frogs buccopharyngeal respiration is regularly combined with the pulmonary type.
Respiratory Patterns in the Frog. — The respiratory process has been extensively studied in frogs, where it is surprisingly complex (De Jongh and Gans, 1969; Gans, 1974). Rhythmic throat pulsations constitute a sustaining phase, in which the mouth is closed and the nostrils are held open, the glottis is closed also, and moderate cyclic variations of pressure are produced within the buccal cavity. Thus an airflow in and out of the nostrils aids olfaction and refreshes the oxygen contents of the buccopharyngeal region. The lungs are not involved in this type of air flow.
The Pulmonary Cycle. — After the sustaining phase, at rather irregular intervals (after something like 50 throat pulsations in one frog species studied), a new type of activity intervenes, known as the pulmonary or ventilatory cycle. In this pattern there are five successive steps: (1) with the nostrils remaining open, the floor of the mouth is depressed more than usual and the posterior part of the buccal cavity is widened, causing an increased inflow of outside air. (2) Then the glottis opens, permitting a rapid escape of the spent air from the lungs and out the nostrils. (3) Next the nostrils are closed, and the floor of the mouth is vigorously elevated, forcing the air out of the buccal cavity into the lungs. (4) Near the peak pressure the glottis is closed and the nostrils are opened, so that the air in the lungs is maintained at a high pressure. (5) Finally, further rhythmic pulsations of the floor of the mouth are resumed.
The Inflation Cycle. — The pulmonary cycles are arranged in inflation cycles. In these each pulmonary cycle produces a stepwise increase in the volume of air contained in the lungs, mainly by limiting step 2 (as described above) and emphasizing step 3. After a peak value of lung pressure is attained, the glottis and nostrils are opened, and the air flows out of both lungs and buccal cavity.
The respiratory pattern becomes still more complex in amphibians during vocalization.
THE ORIGIN OF THE AMPHIBIA
It is generally agreed that sometime in the Devonian period, about 350 million years ago, the Amphibia originated from the lobe-finned fishes, the crossopterygians, which inhabited freshwater streams and pools and had developed sturdy fins that could be used like legs, increasing their agility as bottom feeders and also enabling them to survive when their pools dried up. These appendages eventually were converted into true legs, and so the first land vertebrates were produced.
The fossil record presents two major groups of these early amphibians, the labyrinthodonts and the lepospondyls, and these gave rise to several other distinct lines from some of which existing groups are generally considered to have had their origins. The labyrinthodonts produced two general lines, the temnospondyls and the anthracosaurs, distinguished by the structure of the vertebrae; in the temnospondyls the vertebral centrum was derived mainly as an enlargement of the intercentrum, whereas in the anthracosaurs the centrum developed from another element, the pleurocentrum.
The temnospondyls divided into two groups, the stereospondyls along one line and the rhachitomes and neorhachitomes along another; but all these appear finally to have become extinct without producing any modern representatives.
Among the anthracosaurs there are two groups, the embolomeres, which appeared toward the end of the Mississippian period (about 310 million years ago) and continued well into the Permian (about 200 million years ago), and the seymouriamorphs, which appeared somewhat later toward the middle of the Pennsylvanian and continued into the beginning of the Triassic (from about 280 to about 180 million years ago).
The embolomeres were aquatic fish predators throughout the Pennsylvanian but disappeared in the early Permian. The seymouriamorphs display a mixture of amphibian and reptilian characteristics, with vertebrae like those of primitive reptiles; Romer (1966) pointed to this group as demonstrating that there is no clear-cut distinction between amphibians and reptiles in their skeletal characteristics.
The lepospondyls are distinguished by a vertebral centrum that forms a single spool-like cylinder around the notochord much like the structure in the modern amphibians. There are three divisions of these: the aistopods, microsaurs, and nectrideans. In the aistopods the legs are lost and the body is long and snakelike, with as many as two hundred vertebrae. These forms reached a peak in the Pennsylvanian, and rapidly disappeared thereafter.
The microsaurs attained their highest level of development in the Pennsylvanian, and nearly disappeared in the Permian period about 200 million years ago. They possessed a number of specialized characters, including a single large bone in the temporal region of the skull, only three digits in the hand, an incompletely roofed skull, and a single occipital condyle.
These fossil forms provide a background for consideration of the specific ancestry of existing amphibians.
Theories of Amphibian Ancestry
With three existing orders of amphibians to be accounted for, a theory of ancestry can take five different forms: (1) these three orders can be considered as having completely distinct origins — a polyphyletic theory; (2) all can be regarded as having a common origin — a monophyletic theory; or (3) there can be three types of diphyletic theory according to what two orders are linked and what one is taken as separate: (a) anurans and caudates paired and apodans considered as separate, (b) caudates and apodans paired and anurans left separate, or (c) anurans and apodans paired and caudates left separate. All these five possibilities have had proponents except the last.
The Polyphyletic Theory. — Only Herre (1935) has favored the theory that the three orders have had distinct origins, and he presented no specific evidence to support this position.
Diphyletic Theories. — One form of diphyletic theory was vigorously defended by Jarvik (1942, 1955), in which he proposed that the anurans, along with the labyrinthodonts and the amniotes in general, were derived from one group of crossopterygians, the Osteolepiformes, whereas the urodeles and apodans came from another crossopterygian group, the Porolepiformes. This view was accepted by Romer (1945) and also by Holmgren (1949) and has been generally popular.
Another form of diphyletic theory, in which urodeles and anurans are closely related while the apodans are considered as of separate ancestry, was favored by Eaton (1959). He indicated numerous points of similarity between anurans and urodeles that seemed to favor a common derivation, including the broad form of the skull with its palatal openings, the presence of an operculum and the opercular muscle, and similarities of the vertebrae in which these forms resemble the temnospondyls.
The Monophyletic Theory. — The evidence linking all three of the living orders of amphibians in a single ancestry is treated in detail by Parsons and Williams (1963), with a discussion of 18 features in which resemblances are seen in addition to the presence of pedicellate teeth. These features include several peculiarities of skull and palatal structure: the skull is broad, with large orbital openings; the palate is of the advanced type, with wide separation of the pterygoids; an operculum is present in the oval window; the occipital condyles are paired; and the mentomeckelian bones are present in the mandible.
In the anurans an otic notch is present and closely resembles the one in temnospondyls. In the apodans there is a notch that is at least vaguely similar to that primitive type, but such a notch is absent in urodeles.
To these skeletal features may be added some further ones: the presence of the amphibian papilla in all three orders of modern amphibia, the presence of a peculiar element in the retina known as the green rod (not known elsewhere), fat bodies in association with the gonads, special structure of the skin glands, and the extensive utilization of cutaneous respiration.
Ancestry of the Lissamphibia
Parsons and Williams further considered the ancestry of the Lissamphibia in relation to four groups of fossil forms: the temnospondyls, anthracosaurs, nectrideans, and microsaurs. From the evidence they found themselves unable to propose even a tentative hypothesis concerning amphibian ancestry and concluded that a solution of this problem will have to await the accumulation of further evidence.
Excerpted from The Amphibian Ear by Ernest Glen Wever. Copyright © 1985 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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