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» Origin of Paired Fins. ee, The paired appendages of vertebrates are considered nanenemeined, , paired fins of fishes. However clues to the origin of fins themselves explain the, , obscured and unresolved. Several theories have been advanced to exp, , source and manner of origin of fins., , a Gill Arch Theory., , Gagenbaur suggested that the paired fins are the modified aN oe, The gill arch represents the girdle, branchial rays the fin-rays and t - . - ie, represents the fin fold. The gill arch of a gill, supports the gill septum an ea i, branchial rays. Of these the median ray is assumed to have been drawn out, while, the others arranged bilaterally on either side of the extended ray. Such an, arrangement is assumed to have given rise to a biserial archipterygium. From such, 2 simple type the fins of fishes are said to have developed by reduction or, Tearrangement of one or more gill rays on the median ray. The presence of pectoral, girdle immediately behind the last gill in some fishes and its resemblance to gill, cartilage prompted this theory., , The posterior position of pelvic fins is explained by assuming a backward, , migration of the gill arch on the body. The theory. However, could not gain much, support and a number of objections are made., , 1. The gills are internal in position while the fins are external., 2. The branchial arches are ecto-mesodermal in origin., , 3. The pectoral girdle is connected to the head by the dermal elements and not by, the chondral elements as in fins., The posterior position of pelvic fins is the greatest difficulty. They are, to have been formed by backward migration of gill arches, but th, tins should have been found close to the pectoral fins in the, fossil fishes. In some teleosts, they have become closely, and pectoral girdle, but this seems only, 5. Thus, the resemblanc, and misleading., 6. The posterior position of pelvic line is however explained by assuming a series, of gill arches in ancient gnathostomes., 7. The vestigial muscle buds in front of pee fin Would also not support the, backward migration of pelvic fins, for — occur behind the gills,, Thus, the theory proposed by ee aur remains silent to explain a, . origin of essentially comparable median and the Paired fing whi ‘, — ’ heir structure and development. whic, , assumed, en the pelvic, primitive and the, , associated with head, a secondary modification,, , e between the girdle and the gill arch is only superficial, , resemble closely int
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—_—, , a Fin fold Hypothesis., , The Fin Fold [Hypothesis proposed by Balfour, Thacher and Mivart has, received support of a number of workers, The theory supyests that, both the paired, and the median fins are derived from a continuous fold of a hypothetical ancestor., By considering continuous fold of Amphioxus, the theory can be easily explained., The fold of this animal courses along the dorsal, caudal and then upto the atriopore, on the ventral side, At this point, the fold diverge into a pair metapleural folds that, extends anteriorly upto a short distance behind the mouth, [Lis proposed that both, the median and paired fins are derived by interruption of this fold at many places., The fold disappeared at interruptions, but enlarged further where it was retained, to differentiate into the paddle shaped fins, The theory has received support on the, following, grounds,, , 1. It suggests a common origin for median and paired fins. The basic supporting, skeleton in both is derived by stiffening, of dermal elements to form the radials., , 2. The presence of a row of spines between the pectoral and the pelvic fins of, , suggestive of their origin from a continuous fold. These spines or, , , , Climatius, finlets are regarded as the remnant of the lost fin-fold, , 3. The theory agrees with the embryological derivation of fins; arising, by the, concentration of a series of myotomic buds. The muscle buds found both in, front and behind the paired tins are indicative of their origin from a continuous, fold., , 4. The broad bases of paired fins in some fishes like Cladoselache and Xenacauthus, and the parallel arrangement of their fin rays are suggestive of primitive, condition of fin arising from a continuous fold. From this type several lines of, modifications have possibly emerged., , The primitivity of the broad based fins of Cladoselache, however has been, seriously argued, because in the earlier known fishes like Ostracoderms and, Placoderms, fins had narrowly constricted bases. The idea of their origin, therefore, goes back to the earliest known fishes, Westoll (1958) studied the fossil records of, Ostracoderms and gave a conclusive sequence to the oripin and evolution of the, fins. He documented three separate lines: the forms which were heavily armoured, had long spines ‘but no fins, the others that had reduced spines and smaller fins, and finally the fishes that were devoid of spines but had large fins. With this, emerged a more recent fin-spine hypothesis explaining, quite well the origin of, paired fins., , a Fin-Spine Hypothesis., According to this theory, the idea of origin of paired fins goes back to, Ostracoderms which are regarded as the ancestors of the axisting evclostowmes:
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These forms were heavily armoured and possessed fins that were often no more, than spines. Initially such spines were defensive or were the bottom support, devices, but as the protective function became less important than the locomotion, the fins developed in association with the spines. ., , It is assumed that the fold of skin raised and extended between the spines., These spines later elaborated into the endoskeletal framework to convert the fins, into the locomoter devices. Placoderms, arising in sequence were the numerous, experimental models, for, while many of them possessed fins entirely as bony, structures, several others had them more like a fin supported by fixed spines., , In many acanthodians, the pectoral and pelvic fins were members of a series, of spiny appendages that extended the length of the trunk and were provided with, the fleshy webs. Loss of all the appendages in the series but there persistence in the, pectoral and the pelvic region might well account for the origin of paired fins from, some Ostracoderm like ancestor. Thus, if the concept holds true for the median and, the paired fins of the Ostracoderms, the Placoderms strongly suggest thy same to, be true of paired fins., , With regards to the muscles of the fins, the metameric myotomes are, proposed to have grown later into the bases of spines and spread outwards in a, fan-like manner to accomplish their movement. As the muscles required place for, insertion, the radial skeletal elements accompanied the metameric muscles while, their basal pieces pushed deep into the body to differentiate into girdles., , Thus, conclusively, both the median and paired spines appeared in, Ostracoderms and the movable fins with basal skeletal elements arose later in, connection with the spines. Much palaeontological evidences have been found to, support the fin spine hypothesis.
