Skin breathing — Primary or secondary?

Respirution Ph_vsiology (1972) 14, 183-192;

SKIN BREATHING

- PRIMARY OR SECONDARY?

ALFRED

SHERWOOD

Museumof' CompurutireZoology,

Abstract. Lungs. then widespread an adaptation could

rare in fishes today. seasonal

promoting

not become

Skin breathing of a developed rib basket, retained sheathed.

aquatic

land dwellers in modem

gill-system

suggesting

functional

Harwrd

life under because

a more advanced

despite

of suitable

orders

their limbs the earliest

and not improbably

that all early amphibians

is uncertain,

to

likewise

amphibians

type of lung breathing

But all early amphibians

mechanism,

probable

limbs was probably

food on land.

with an inefficient

dioxide elimination. breathing

in early times as an adaptation

of terrestrial

such conditions;

is associated

it is highly

of the modern

very common

The development

of a dearth

amphibians

ROMER

Unirersify, Cumbridge, Mass., U.S.A.

were apparently

conditions.

to aid in carbon

gills. Further.

The pedigree

not primitive

drought

North-Holland Publishing Cornpatty. Amsterdam

but they appear

and lack

possessed many

were completely

to be specialized

a good

early forms scale-

forms and

in any regard. Evolution

Lung

Fossils

Rib cage

respiration

Gill respiration

Skin respiration

In the shift from gill-breathing to lung-breathing in terrestrial vertebrates, the major physiological problem encountered has been not so much the mode of absorption of oxygen (for which even lungs of a primitive nature are more or less adequate), but rather the more difficult matter of the elimination of carbon dioxide. It is natural that the attention of physiologists has been attracted to the members of the living orders of amphibians - Anura, Urodela, Gymnophiona - in which the body is naked and much of respiration - particularly carbon dioxide elimination - takes place through the moist skin. In consequence there has developed the theory that the first amphibian dwellers on land were in great measure skin-breathers, and that only later, in reptiles and higher groups, did the lungs take on the full task of carbon dioxide elimination as well as oxygen absorption. It seems probable, from the fossil record, that this theory is incorrect - that the modern skin-breathing amphibians are a side-branch of the vertebrate family tree, and that the evolution of reptiles and higher forms proceeded directly from fishes to scaled fish descendants in which skin-breathing was of little or no account. 1This paper was presented brates”

at the satellite symposium

of the XXV International

Congress

“Comparative

of Physiological 183

Sciences

Physiology (GBttingen,

of Respiration 24

August

in Verte1971).

184

ALFRED SHERWOOD

ROMER

TELEOSTEI

Fig. 1. A diagrammatic family tree of the bony fishes (Osteichthyes). Forms in which lungs are known are underlined. As indicated by hatching, lungs were presumably present throughout the Sarcopterygii, presumably present in the basal members of the Actinopterygii and hence probably present in the ancestral members of the bony fish stock.

It is but natural to assume that the presence of lungs is primarily associated with the progress of vertebrates toward land life. Few living fishes have functional lungs, these including only the three living tropical dipnoan genera - Neoceratodus, Protopterus and Lepidosiren - and two tropical African forms - Polypterus and its eel-shaped relative, Calamoichthys. Other fishes either lack any sort of lung-like structure cyclostomes, elasmobranchs, chimaeras - or, in the case of most members of the great group of ray-tinned fishes, or Actinopterygii (sturgeons, paddletishes, gar-pikes, Amia and the host of teleosts), there is generally present a pouch off the throat into which air may be introduced. Because of the multiplicity of fishes which possess this single dorsal pouch, it was long assumed that the actinopterygian air bladder was the more primitive structure, and that lungs - ventral and paired - were evolved from the air bladder. It seems probable, mainly as the result of paleontological work in recent decades, that the reverse was the case - that lungs were structures widely present in ancient

SKIN BREATHING

- PRIMARY

OR SECONDARY?

185

fresh water fishes and that if there is any homology between lungs and air bladder, it is the latter which is of secondary origin. The bony tishes (fig. 1) are divided by most (but not all) workers into two major groups, the Actinopterygii, or ray-tinned fishes, and the Sarcopterygii, or fleshytinned fishes, including the dipnoans and the crossopterygians. Known members of the two sarcopterygian groups differ in many ways in cranial and dental structures but appear to be a natural unit (Romer, 1968, pp. 50-54; Denison, 1968). The three living dipnoans all possess lungs and it is hence reasonable to believe that the possession of lungs was a basic dipnoan character. The Crossopterygii include two major subdivisions, the Coelacanthini (or Actinistia), a side branch which took up a marine life and to which the only surviving genus, Latimeria, belongs, and the Rhipidistia, mainly fresh water forms, from which the Amphibia were derived. The modern Latimeria, as a marine, deep-water form, has no functional need of lungs, but rudiments of lung structure are present (Millot, 1954) and, although evidences of lungs are very rarely found in the fossil record, they are known to have been present in a Mesozoic member-of the group (Woodward, 1891). We have no direct evidence of lungs in the extinct rhipidistians, but since they are ancestral to the tetrapods and since lungs were present in their coelacanth and dipnoan relatives, the presence of lungs in this group and hence in the Sarcopterygii as a whole is a reasonable assumption. What of the Actinopterygii, in most members of which an air bladder rather than a lung is present? Here we must consider the systematic position of the African genera Polypterus (and its relative Calamoichthys). Polypterus departs in various regards from the typical actinopterygian structural pattern, and hence it was long regarded as perhaps a crossopterygian (because of the presence of lungs) or as representative of a distinct major group of bony tishes. However, Goodrich pointed out that the basic features of Polyptents were actinopterygian in nature, and that this genus rather surely is a surviving offshoot of the varied paleoniscoids which were, in the Paleozoic, the basal actinopterygian stock. The presence of lungs in a primitive actinopterygian strongly suggests that lungs were a primitive actinopterygian feature, air bladder development secondary. We thus tend to reach the conclusion that the presence oflungs was a characteristic of the ancestral bony fish as a whole. Why? The answer seems to lie, not in any teleological “urge” toward terrestrial life, but in the fact that accessory breathing structures would be of great survival value in fresh waters with a low oxygen content. In some cases this situation may have been due to existence in swampy waters. More important, however, seems to have been widespread conditions of seasonal drought in late Paleozoic times, as first emphasized by Barrel1 (1916). Lungs would be of enormous value to a fresh water fish in drought conditions, and it will be noted that the five fishes which today possess lungs dwell in the more restricted modern tropical regions in which seasonal droughts occur. Lungs, then, were apparently widespread amongst ancient higher fishes, the Osteichthyes. As mentioned above, they are unknown today among living “lower”

186

ALFRED

SHERWOOD

ROMER

fish groups. Were they more widespread in ancient times? It is seldom that we find evidence regarding such soft tissues as lungs in the fossil record. An interesting exception is the case of Bothviolepis. This is a form very common in fresh water deposits of the late Devonian, belonging to a group, the Placodermi, generally regarded as exceedingly primitive gnathostome fishes. In unusually well preserved specimens of ~ur~~~o~e~~s,Denison (1941) has found evidence indicating that in this archaic if aberrant form a pair of lung-like pockets were present at the back of the pharynx. It is quite possible that the development of lungs took place at a very early stage in fish history. Although some fishes exhibit a certain amount of cutaneous respiration, none of these are concerned with amphibian ancestry. Due to the studies of a long series of workers, such as Goodrich and Gregory, over the last half century, it now seems certain that the Amphibia are descended from rhipidistians’. All members of this group are completely enclosed in thick cosmoid bony scales, and hence in pre-tetrapods respiration, apart from that fraction cared for by lungs, was a concern of the gill apparatus. It is sometimes assumed that, with the development of tetrapod limbs in ancestral amphibians, which first appear at the end of the Devonian, a terrestrial mode of life was promptly adopted. On consideration, however, it is seen that this could not have been the case. The rhipidistians were predaceous forms, eaters of animal food. By the late Devonian plants were already ashore in profusion but as far as can be told from the dentition not a single tetrapod capable of subsisting on vegetation is known until late Carboniferous times. Until the time of the Coal Measures, little is known of any sort of terrestrial food. Scorpions may have become terrestrial in the early Carboniferous (but the evidence is none too clear) ; Collembola are known from the Devonian but although it seems reasonable to assume that insects generally were beginning their terrestrial career in the early Carboniferous, almost nothing is known of insects of that age. By the Coal Measures, insects were abundant and here for the first time was a source of animal food on which a terrestrial vertebrate fauna could be based. Before this time there was little to tempt an early amphibian ashore, Its food lay in the water, amongst the smaller fishes and aquatic invertebrates. Why then, limbs? Paradoxical as it seems at first sight, the development of the potentiality of terrestrial locomotion presumably was (as I and others have suggested) an adaptation for improved aquatic existence. In times of drought, when a pool dried up, a fish would become, literally, stuck in the mud, and unless water soon returned, was doomed. But if limbs, even if relatively feeble, were developed, a primitive tetrapod could crawl up or down a stream bed, hopefully reach a pool which still retained water, plunge in, and resume its normal existence. But although a certain fraction of the descendants of the ancestral amphibians ’ tn earlier years the dipnoans were proposed as ancestors of all, or part of the tetrapods, but this thesis has now been generally abandoned. Jarvik (1942, etc.) has long advocated a dual origin for tetrapods, but both fish ancestors (for urodeles on the one hand and for frogs and all other tetrapods on the other) are assumed to have been rhipidistians.

SKIN BREATHING

- PRIMARY

OR SECONDARY?

187

notably reptiles - did attain a fully terrestrial life, the trend toward a retention of a water dwelling life was, and is, strong among amphibians. It seems quite certain that few of the ancient Amphibia progressed farther toward land life than a technically “amphibious” condition, in which the animals lived in and about swamps and pools, and in many cases limbs are seen to be so poorly developed that the animal would be unable to emerge from the water. Essentially the same conditions exist among modern amphibians. If we are to attempt to discover when and in what groups cutaneous respiration developed, we are in paleontologi~lly difficult territory. What we wish to know is the degree of development of lungs and breathing apparatus, the degree of retention of gills, and the nature of the skin. Relatively few data are attainable from the fossil record on any one of these three items. There is no direct evidence in the older fossil amphibians as regards the nature of the lungs. There is, however, indirect evidence of importance. In higher tetrapod ciasses aspiration breathing is the rule, in contrast to cruder breathing methods seen in the modern amphibians, in which methods skin respiration for ridding the blood of carbon dioxide is a necessity (Gans, 1970). In the modern amphibians, ribs are reduced and aspiration breathing is impossible. In all the older amphibian groups, however, there was a well-developed thoracic rib basket, and hence aspiration breathing was possible and not improbably present. It thus seems not unreasonable to believe that the breathing habits, as well as rib reduction, in modern amphibians are a secondary condition - in which skin respiration becomes for the first time important. Except for some cases in which impressions of external gills are preserved, we have no knowledge of the soft anatomy of the gills in fossil amphibians. For knowledge of gill structure we must depend upon preservation of the gill skefeton. Unfortunately the usually slender branchial arch elements are not too commonly preserved in fossil amphibian skeletons, and in addition it seems certain that in many cases such structures, even if present, were cartilaginous in nature and hence incapable of fossilization. Despite such handicaps, however, there are available data for a fair number of forms which suggest that the retention of an internal gill system capable of dealing with carbon dioxide discharge was of widespread occurrence among the older amphibian groups. The potentiality of aspiration breathing and the probable presence of a functional internal gil1 system suggest that there was little need in the ancient amphibians for the development of skin breathing. We should, however, examine the fossil evidence relating to the nature of the skin in the older groups. Such evidence is, as would be expected, scanty, for in general in any vertebrate the soft tissues tend to decay and the skin to separate from the skeleton before burial takes place. We can hope for preservation of the skin only in such instances in which the body has been “mummified” and the skin is adhering to the skeleton at the time ofentombment or in small specimens preserved in “slab”’ form. In no known fossil amphibian is the scaly covering of the body as highly developed

188

ALFRED SHERWOOD

ROMER

as in the ancestral rhipidistian crossopterygians. The ventral surface of the body in ancient amphibians (but not the modern orders) was in general covered with rows of stout scales in much the fish pattern ; these scales, known as gastralia, appear to have served a useful function in protection of the belly in these “low slung” animals in which the abdomen was raised but a short distance off the ground. Over the dorsal surface of the body, however, no such stout scales were present. Relatively seldom is there any record in the literature of discovery of dorsal scales in any ancient type, and one would be tempted to assume that the back and flanks were naked. However, the evidence, although scanty, suggests that in general the bodies of the ancient forms were covered with bony scales, although scales much thinner than those of their piscine ancestors. The evidence for cquamation may be reviewed group by group. Although there are many debatable points in amphibian phylogeny and classification, a reasonable summary of the present evidence suggests that the Amphibia, fossil and recent, may be assembled in three major groups, perhaps to be considered as subclasses - Labyrinthodontia, Lepospondyli, and Lissamphibia. The first two are ancient, mainly Paleozoic groups, the third includes the modern orders (fig. 2). First, to consider briefly the members of the Lepospondyli, I have utilized this term to include a highly varied assemblage of small Carboniferous and early Permian Urodelo

&

Gymnophlona

Stereospondyli REPTILIA

,,?/Seymourlamorpha

w ~~__.....LISSAMPHIBIA _:

I : :’ I

/’ \ 4

/

Rhachitoml

7’ : ,’ I’

Embolomeri ,’ $ Anthracosauria

lchthyostegalia

Midrosauria

?

~emnospondy’i,,,,$/~tridea LEPOSPONDYLI LABYRINTHODONTIA \. I’ ‘l\ /’ ‘\ \ ,’ ‘\ I

’y

Fig. 2. A simplified family tree of the Amphibia. The Lepospondyli include three ancient orders not necessarily closely related to one another but all presumably derived from the base’of the labyrinthodont stock. Apart from a few early forms (Ichthyostegalia) the Labyrinthodontia in general are clearly separable into the highly abundant Temnospondyli and the Anthracosauria, important as including reptile ancestors. The three modem orders constituting the Lissamphibia are of uncertain derivation, but presumably are descended either from temnospondyls or microsaurs.

SKIN BREATHING

- PRIMARY

OR SECONDARY?

189

amphibians, arrayed in three orders - Nectridea, Ai’stopoda, Microsauria. The general structure of the members of these three groups can be seen by reference to texts such as my own 1966 edition (pp. 96-99) or Dechaseaux in Piveteau (1955, pp. 275-304). It is reasonable to believe that all lepospondyls are ultimately derivable from early labyrinthodonts, but there is no convincing evidence that the three groups are at all closely related to one another. Except for the Microsauria (as discussed later) it is improbable that the lepospondyls are in any way related to the modern orders. Little evidence for dorsal squamation is available for the Nectridea. The fact that many specimens of Carboniferous lepospondyls are preserved in “slab” form has yielded knowledge of dorsal scales in ophiderpetondid ai’stopods (Baird, 1964, pp. 8,9) from both the early and late Carboniferous strata. Among the microsaurs rounded overlapping dorsal scales have been reported in a considerable number of the members of this group. For example, Fritsch (1883, 1889) figures scales of various microsaurs ; compare also Romer, 1950, p. 633 ; Carroll, 1966 ; Carroll and Baird, 1968, etc. It seems certain that the microsaurs in general were completely scale covered. It is probable that this was the case for the aistopods. The nectridian condition is less certain. But in any case the “horned” or snake-like members of this last group are obviously end lines, unrelated to any later amphibians. Much more important than the lepospondyls in the phylogenetic picture are the Labyrinthodontia which, beginning at the end of the Devonian, flourished in enormous numbers and great variety in the Carboniferous and Permian and survived well into the Mesozoic. In 1947 I reviewed our knowledge of the labyrinthodonts up to that date ; numerous further descriptions of materials of the group continue to be published. The earliest forms, the Ichthyostegalia, are known only from a few late Devonian and lower Carboniferous genera which are as yet incompletely described (Jarvik, 1952). These appear to form an early side branch of the labyrinthodont group. Most, at least, of all later labyrinthodonts can be placed in two major sub-groups, Temnospondyli and Anthracosauria, readily distinguished by key characteristics of the skull roof and vertebral structure. The temnospondyls are by far the most abundant of the two groups ; well upwards of 100 genera are known. In many forms skin structure (apart from frequent finds of gastralia) is unknown. But in a scattering of cases there is definite evidence that the dorsal surface and flanks of the body were covered by circular to polygonal bony scales, although these scales are much thinner than those of their fish ancestors which were preserved little changed in the gastralia. For example, Broili (1927) found dorsal scales in two families of Permian temnospondyls, and a well-preserved slab of Trimerorhachis material proves the presence of dorsal squamation in this last genus (Colbert, 1955). Typical of the exceptional nature of the discovery of body squamation is the case of Eryops (Romer and Witter, 1941). Eryops is the most common of all early Permian amphibians, known from hundreds of specimens from the Texas redbeds. Gastralia of Eryops have long been known, but not until recently was evidence obtained of further squamation. Considering the amount of known material in which no such

190

ALFRED

SHERWOOD

ROMER

evidence had been discovered, it might have been plausibly argued that the skin was naked except for gastralia. However, in our laboratory a few decades ago, a skeleton was being prepared on which a tough matrix was present. To soften this matrix, it was treated with hydrochloric acid ; following this, it was vigorously brushed with a fine steel wire brush. To our astonishment, there appeared in bold relief a pattern of raised oval areas in the matrix covering the tail vertebrae and a similar pattern was found on other body areas. Treated with methylic stain, these ovals proved to be bony scales. In summary, it would appear reasonable to believe that in temnospondyls generally there existed a body covering of thin bony scales. The second division of the labyrinthodonts, the Anthracosauria, would seem at first sight to be of less importance than the Temnospondyli, for only about two score genera are known, and, in contrast to the long-lived temnospndyls, the anthracosaurs became extinct early in the Permian. They are of importance, however, in including as a side branch the Embolomeri (Watson, 1926) common in late Carboniferous beds, and most especially, as the group from which the reptiles have been derived. Certain forms, such as Seymouria (White, 1939) have long been known to approach closely reptilian structure, and Carroll (1969, 1970) has recently studied Carboniferous genera which seem to actually show a phylogenetic transition. Although the situation is known in only a few genera, it seems certain that the body, apart from the gastralia, was covered by bony but thin scales (Carroll, 1969, pp. 426427) and a strong potentiality for dermal ossification is shown by the fact that the Permian Seymouria relative Kotlassia has highly developed dorsal dermal armor (Bystrow, 1944). Some early reptiles, as Carroll notes, have a dorsal squamation similar to that of anthracosaurs, and it can, I think, be safely concluded that a general body covering of bony scales was present throughout the history of reptile ancestry. The three surviving amphibian orders include the frogs and toads (Anura), the newts and salamanders (Urodela or Caudata), and the little worm-like Apoda (or Caecilia or Gymnophiona). The three groups differ so much in structure that in the past it has been thought that they are but distantly related to one another and have had long-separated phylogenetic backgrounds. Recently, however, Parsons and Williams (1963) have pointed out that despite the wide variety of adaptations seen in the three groups, they possess certain distinctive common characters which strongly suggest that the three are related to one another, and form a natural group. For this group Parsons and Williams have resurrected the Haeckel term Lissamphibia. We have in the Lissamphibia forms in which scales are absent (except for rudiments in the Apoda) and skin breathing highly developed, ribs reduced and lung breathing of an inefficient type. What is their ancestry? Apodans are unknown as fossils; the earliest salamander is Cretaceous ; primitive frogs were present in the Jurassic and a “proto-anuran” is known from the Triassic. We have, however, no knowledge as to the nature of a common ancestor of the three lissamphibian orders, and no clear evidence of the provenance of this common ancestor. Unless we assume (gratuitously) that the lissamphibians trace back through an entirely unknown line of ancestors

SKIN

BREATHING

- PRIMARY

191

OR SECONDARY?

in the later Paleozoic to the base of the amphibian stock (or to the ancestral fish), they must have been derived from one of the two known Paleozoic groups - Lepospondyli or Labyrinthodontia. I argued in 1950 for derivation from microsaurian lepospondyls, such as the gymnarthrids, and Cox (1967) advocates this as well. Parsons and Williams, however, believe the microsaurs are too degenerate and specialized to be proper ancestors, and would favor derivation from temnospondylous labyrinthodonts ; Estes (1965) argues for temnospondyl ancestry (particularly from the dissorophids) for the urodeles at least. The question is thus an open one. But whichever pedigree is advocated, it seems clear that the ancestor was one which differed markedly in its respiratory habits from the modern amphibian orders. Quite surely the ancestor was scale-covered, rather than one with a moist, respiratory skin, and quite surely one with a well-developed rib-cage and hence the potentialities for aspirative breathing rather than the less efficient methods seen in the lissamphibians. It seems fairly certain that the modern amphibians are, in respiratory habits, not primitive tetrapods but a specialized and degenerate side branch of the tetrapod family tree. References Baird, D. (1964). The ai’stopod amphibians surveyed. Breoiora, Mus. Comp. Zool. No. 206: l-17. Barrel], J. (1916). Influence of Silurian-Devonian climates on the rise of air-breathing vertebrates.

Bull.

Geol. Sot. Am. 27 : 387436. Broili, F. (1927). uber Ges. 79 : 315-384.

die Hautbedeckung

der Archegosauridae

A. P. (1944). Kotlassia prima Amalitsky.

Bystrow, Carroll,

R. L. (1966). Microsaurs

Bull. Geol. Sot. Am., 55: 379416.

from the Westphalian

between

microsaurs

Tuditanus [Eosauratus]

amphibian

and the dis-

Am. Mus. No&. No. 2337: l-50.

and reptiles.

R. L. (1969). Problems

Carroll, Colbert.

R. L. (1970). The ancestry of reptiles. Philos. Trans. Roy. Sot. (London), B. 257: 267-308. E. H. (1955). Scales in the Permian amphibian Trimerorhachis. Am. Mus. No&. No. 1740: l-17. respiration

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Carroll,

Cox, C. B. (1967). Cutaneous

of the origin

Nova Scotia. Proc. Linn. Sot. (London)

B ofJoggins,

177 : 63-97. Carroll, R. L. and D. Baird (1968). The Carboniferous tinction

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und Actinodontidae.

and the origin of the modern

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4. Current

Problems

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A. (1883, 1889). Fauna

of Bothriolepis. J. Paleont. 15: 553-561. significance

of Lower

and salamander

der Gaskohle

of the earliest

Vertebrate origins.

vertebrates. Goodrich, Jarvik,

Forma

Uranolophus. Nobel

der Permformation

of the external

Biihmens.

gas exchangers

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of ectothermal

et Functio, 3: 61-104.

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und der Kalksteine

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known

Phylogeny

of the snout

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1: 87-91. and lower gnathostomes

in general.

21: 235-675.

Jarvik, E. (1952). On the fish-like tail in the ichthyostegid stegocephalians. Meddel. om Gtzmland 114: l-90. Millet, J. (1954). New facts about coelacanths. Nature (London), 174: 426. Parsons,

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192

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ROMER

Romer, A. S. and R. V. Witter (1941). The skin of the rhachitomous amphibian Eryops. Am. J. Sci. 239: 822-824. Romer, A. S. (1947). Review of the Labyrinthodontia. Bull. Mm. Comp. Zool. 99 : l-368. Romer, A. S. (1950). The nature and relationships of the Paleozoic microsaurs. Am. J. Sci. 248 : 628-654. Romer, A. S. (1966). Vertebrate Paleontology. 3rd ed. Chicago, Univ. of Chicago Press, 468 pp. Romer, A. S. (1968). Notes and Comments on Vertebrate Paleontology. Chicago, Univ. of Chicago Press, 304 p. Watson, D. M. S. (1926). The evolution and origin of the Amphibia. Philos. Trans. Roy. Sot. (London) 214: 189-257. White, T. E. (1939). Osteology of Seymouria baylorensis Broili. Bull. Mm. Comp. Zool. 85: 325-409. Woodward, A. S. (1891). Catalogue of the fossil fishes in the British Museum. Part II. London, British Museum, 567 p.