Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXXVIII The Evolution Of The Amphibia The First Land Chordates

That the amphibia arose from fish there is no doubt, and their ancestor must have been one of the primitive Osteichthyes, related to the stock which also gave rise to Osteolepis and Dipterus. For purposes of comparison Osteolepis may be taken as approaching the structure of this ancestor.

The resemblances between Osteolepis, on the one hand, and one of the earliest Stegocephalian amphibia such as Loxomma on the other extend to the following features. In both, the skull is a complete bony box, the dermal bones of which can in most cases be identified with certainty because the amphibia also had lateral-line canals which occupied grooves in the bones. The bones of the palate are similar, and both had nostrils which lead through into the cavity of the mouth. The amphibian Eogyrinus had a shoulder girdle the dermal bones of which were attached to the post-temporal bone of the skull by the supra- cleithrum, as in the fish. Also, these early amphibia had no sacrum, for the ilium was not attached to the ribs. The walls of the teeth were folded, in the Labyrinthodont pattern. The amphibia are autostylic, as are the Dipnoi including Dipterus. The otic process in Osteolepis did not reach the auditory capsule, however, and it is a question as to whether the common ancestor of Osteolepids, Dipnoi, and Tetrapods was autostylic or not. A lung was almost certainly present in Osteolepis.

The ancestor of the Tetrapods must, however, have had pectoral and pelvic fins equally developed and similar in structure, and this condition has not yet been found in any Osteolepid (or other) fish. The really distinctive feature of all the Tetrapods is the possession of limbs ending in five digits, and it has already (see p. 315) been shown that the skeleton of the pectoral fin of the Osteolepid fish Sauripterus is such as to render it easy to suppose that the pentadactyl limb arose from a fin like that of the Osteolepids. Osteolepis is Devonian, and the earliest known amphibia are from the Lower Carboniferous. It is fairly certain, therefore, that at some time in the Devonian, fish living in the estuaries and fresh-water basins became subjected to the desiccation which characterised this period. They were able to breathe atmospheric oxygen by means of their nostrils and lungs, and as they floundered about in the mud, the number of rows of radials in their fins became reduced to five, separate from one another instead of being united by the web of a fin. The persistence of the lateral-line canals shows that these animals still spent much of their time in the water, and their excursions on land probably took the form of wandering from pond to pond. In fact, the amphibia never succeeded in making themselves completely independent of water, and for three reasons. In the first place the eggs had to be laid in water, and the larval stages which breathed by gills required then, as they do now, a watery medium. Next, fertilisation was external, and for the sperms to be able to find the eggs, there must be a liquid medium for them to swim in. Lastly, amphibia breathe largely through their skins, and these must be moist to enable the gaseous exchange to take place.

Fig. 176. — A few examples of different types of Amphibia. (Not drawn to scale.) a, restoration of Stegocephalian ; b, male newt in breeding season (Urodele) ; c, Amblystoma (larval form or Axolotl showing the external gills) ; d, frog (Anuran) ; e, Ichthyophis (Gymnophiona) . (e after Sarasin).

The transition from water to air necessitated a development of the olfactory organs to greater sensitiveness, for the concentration of substances in water is very much greater than that which can be obtained in air. The result was an increase in development of the olfactory organs and of the corresponding centres in the forebrain. The latter development accompanied and perhaps assisted the formation of the cerebral hemispheres, which are regarded as connected with an adaptation to the poor oxygen- content of the water in which the amphibia and their ancestors evolved.

It appears, therefore, that the transition from aquatic to terrestrial life was accomplished without any very striking changes or modification of organs, but it must be remembered that the function of these organs is controlled by the pattern of nerve-fibres in the central nervous system, and it becomes necessary to inquire whether the transition necessitated any great neurological rearrangement. Two aspects of the transition will be considered, regarding breathing and locomotion. In connexion with respiration, it will be remembered that the amphibia breathe by means of respiratory movements performed by the visceral muscles in the floor of the mouth, in a manner very similar to that of the fish . The only real difference is that whereas the fish take in water and pass it back and out through the gill-slits, the amphibia take in air and pass it back and into the lungs. The mechanism is the same, and it is obvious that the transition from water to air involved no functional rearrangement of importance as regards respiration.

The same holds true with regard to locomotion. The amphibia were clumsy sluggish beasts with bodies disproportionately large in comparison with their limbs. As a consequence, the body was not supported by the limbs but its ventral surface dragged along the ground. The limbs stuck out at right angles to the body, and as the body performed the same undulatory movements by means of the myotomes as does a fish when swimming, the limbs were moved forwards and backwards. In other words, the limbs were used as oars to row the animal along on land, and the same muscles and nervous connexions came into play as in the aquatic ancestor. In the very earliest Amphibia, the sacrum was absent {e.g. Eogyrinus). In the others it was present, and by anchoring the pelvic girdle on to the vertebral column, it strengthened the hind limbs.

One consequence must be mentioned of the possession of an autostylic method of suspension of the jaws, and of the abolition of branchial respiration and the closure of the visceral clefts, in particular the spiracle. The hyomandibula being no longer required to suspend the quadrate from the auditory capsule, its function became converted into that of conveying vibrations from the skin covering the spiracular cleft to the auditory capsule. In this way the hyomandibula became the columella auris ; the covering of the spiracular cleft became the tympanic membrane, and the cavity of the spiracular cleft became the middle-ear and Eustachian tube ; all quite simply and without involving any great rearrangement. So the ear became an organ for the delicate appreciation of sound as well as balance.

The early amphibia had a covering of dermal bones more or less all over the body. The vertebral column of the earliest forms or Embolomeri is remarkable in that each vertebra possessed two centra. There was an anterior hypocentrum and a posterior pleurocentrum. The later amphibia preserved the hypocentrum at the expense of the pleurocentrum, which disappeared. It will be seen that in the reptiles the opposite occurred.

Collectively, the early amphibia are known as the Stegocephalia or Labyrinthodonts, the former term referring to the complete bony covering of the skull. They flourished in the Carboniferous, and persisted until the Triassic period, when they were extinguished by the competition of their more successful descendants the reptiles, leaving only the frogs (Anura), newts (Urodela) and Gymnophiona alive to-day. An important feature in the amphibia is the outgrowth from the gut to form a bladder. It is homologous with the allantois of the Amniotes.

With regard to the living amphibia, it is most important to realise that they have departed far from the primitive type of their Stegocephalian ancestors. This is shown by the great reduction in the bones of the skull and other parts of the skeleton. The Gymnophiona have evolved a worm-like burrowing habit ; the frogs have become modified in connexion with the habit of leaping with the hind legs, and the newts have become secondarily readapted to living in water, even going so far as to develop median fins which differ entirely from those of fish in not possessing radials or dermal fin-rays. Now, in several points, the newts of to-day resemble living Dipnoi (such as Ceratodus) very closely. It is of the utmost importance, however, to realise that these resemblances are due to parallel evolution and convergence, and not to genetic affinity. It is only necessary to look at the list of specialised characters of Ceratodus to see that forms like it could not have given rise to the Tetrapods : they are cousins and not ancestors. Similarly, an examination of the specialised characters of Triton and a comparison between it and the Stegocephalia show that all the points in which Triton resembles Ceratodus have been evolved within the amphibia, and a long time after the amphibia came on land.

Literature

Watson, D. M. S. The Structure, Evolution and Origin of the Amphibia. Philosophical Transactions of the Royal Society, Ser. B, vol. 209, 1919.

The Evolution and Origin of the Amphibia. Philosophical Transactions of the Royal Society, Ser. B, vol. 214, 1926.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Cite this page: Hill, M.A. (2024, April 26) Embryology Book - Vertebrate Zoology (1928) 38. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Zoology_(1928)_38

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