2008/02/09 by Richard A. Brand · 3 citations
Environmental Science · Earth and Planetary Sciences · Medicine · #Ichthyology and Marine Biology #Paleontology and Evolutionary Biology #Fish biology, ecology, and behavior #Anatomy #Appendage #Pectoral girdle #Fish fin #Medicine #Biology #Fish <Actinopterygii>
paper · pdf · doi:10.1007/s11999-007-0102-6
openalex publication_date 2008/02/09 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/30
Origin of Paired Appendages The origin of paired appendages has been the source of considerable controversy among morphologists. The lateral-fin theory has supplanted the gill-arch theory of Gegenbaur and is now accepted as the most plausible explanation of the beginning of these appendages. According to the lateral-fin theory, paired limbs are derived from longitudinal lateral folds of epidermis extending backward along the body from just behind the gills to the anus. By accentuation of the anterior and the posterior and suppression and reduction of the intermediate portions of the folds the pectoral and the pelvic fins were formed (Fig. 1). Into these folds muscle buds migrated from the ventral border of the adjoining myotomes, giving rise to radial muscles which motivated the fins and were the forerunners of the intrinsic muscles of the hand (Bunnell). The muscle buds disclosed a metameric arrangement and derived their nerve supply from ventral roots of the spinal nerves.Fig. 1A-E: Hypothetical evolution of paired fins and their skeletal supports. (A) Primitive stage, characterized by continuous fin folds; the dorsal and ventral fins posterior to the anus are median and unpaired. (B) Elasmobranch stage; paired fin-folds persist only in the region of the pectoral and pelvic fins; median fins have become discontinuous. (C-E) Hypothetical stages in the evolution of the skeleton of the pelvic fins of elasmobranch fishes. The right side of C and E represents a later stage n the phylogenesis than the left. E represent the differentiated skeletons of the girdle and the extremity (after Wiedersheim). (Neal and Rand: Chordate Anatomy, Philadelphia, Blakiston)Peripheral nerve fibers in the base of the fin divide repeatedly, giving rise to a complex plexus. The number of myotomes which comprise the muscular apparatus of the fin is disclosed by the number of spinal nerves which contribute to the plexus. In ontogeny, motor nerves always supply the muscles for which they were designed originally. Muscles exhibiting a nerve supply from more than one spinal nerve denote combining of muscular tissue of several segments. Next in the process of evolution of the appendages was the appearance of radials (cartilage rays) between the muscles buds; these provided more strength and support to the fins (Fig. 2).Fig. 2: Formation of adult radial muscles from embryonic muscle buds, and their motor nerve supply. Above, embryonic stage with a pair of buds to each segment; below, adult stage with radial muscles compounded of material from adjacent buds, 1-4, four spinal nerves; A-D, four myomeres; a-d, muscle buds; r, radial muscle. (Goodrich, E. S.: Studies on the Structure and Development of Vertebrates, London, Macmillan, p. 134)Concentration and fusion of the proximal (basal) ends of the radials in the fin gave rise to the basilia (basal cartilages) which extended inward into the body wall to form the most primitive girdle (Fig. 3). In order to meet the requirements of a freely movable fin an articulation appeared in the basal plates. Further evolution of the girdle includes fusion of the basilia of either side in the midline to form a ventral bar; also included is a dorsal extension of the arch above the level of the articulation to join the axial skeleton. Thus a complete girdle is formed around the body. The above steps in the ontogeny of the girdle have been noted in the Selachia (elasmobranches) and also in Chondrostei and Teleostei.Fig. 3: Diagrams illustrating hypothetical evolution of the extremities of diapnoan (I), ganoid (H) and elasmobranch (G) from a fin fold supported by a series of similar radial cartilages. By fusion of radial cartilages basilia (basal cartilages) are formed. Skeletal supports of the fins eventually differ in relation of the basal elements to the radialia (Redrawn from A. Brazier Howell). (Neal and Rand: Chordate Anatomy, Philadelphia, Blakiston)Evolution of the Pectoral Girdle Fishes In its basic pattern the girdle is an inverted arch spanning the ventral surface of the body and extending dorsally on either side above the level of the articulation. Both the girdle and the limb are free. Each girdle comprises a ventral segment (coracoid) and a dorsal segment (Scapula). These at the point of conjuncture form the glenoid fossa, which articulates with the basal component of the skeleton of the limb. Further segmentation of the scapula gives rise to the suprascapula, which may become attached to the axial skeleton (as in skates). All the above elements have separate centers of chondrification (Fig. 4).Fig. 4: Diagrams illustrating scheme of pectoral appendages of lower and higher vertebrates. (Bottom) Names of corresponding parts of pelvic appendages are shown in parentheses. (Neal and Rand: Chordate Anatomy, Philadelphia, Blakiston)Further in the scale of evolution of the pectoral girdle is the appearance of a girdle of membranous bones derived from the skin. It encircles the head starting from behind the gills. The elements of either half of the girdle join and fuse in the midline on the ventral surface of the body through the medium of the interclavicle. Each half of this membranous circle consists of four membranous bones: (1) post-temporal, which is jointed with the skull, (2) supracleithrum, (3) cleithrum and (4) clavicle. The interclavicle which unites the girdle ventrally is an unpaired bone. Both the basal girdle and membranous girdle eventually became attached to one another. Such is the basic plan of the pectoral girdle as noted in two genera (Eusthenopteron and Sauripterus) of the upper Devonian crossopterygians. These are considered the ancestors of the amphibia whose appendages possessed the pattern which made the evolution of the tetrapod limb possible (Fig. 5).Fig. 5: Diagram of reconstructed pectoral girdle and fin of Sauripterus and upper Devonian crossopterygian fish. It exhibits a close similarity of relations of proximal elements of extremity to those found in the pectoral extremity of tetrapods (redrawn from Brown). (Neal and Rand: Chordate Anatomy, Philadelphia, Blakiston)Amphibia With the attainment of terrestrial habits most of the elements of the membranous girdle (post-temporal and supracleithrum) decreased in size and disappeared, while the cartilaginous girdle began to assume a more significant role. The skull was freed of all attachment to the girdle. In urodels all vestiges of the membranous girdle have disappeared. In the amphibia the tripartite type of pectoral girdle made its first appearance; the coracoid represented by the ventral bar in the fishes became segmented into the anterior procoracoid and posterior coracoid, while the clavicle came in relation to the procoracoid No significant alterations occur in the suprascapula and the scapula. A note worthy observation in the pectoral girdle of large amphibia (Rhachitomi) is the direction of the glenoid fossa. It faces laterally, indicating that the humerus extended away from the trunk in the ground. Its articular surface was “screw-shaped” (Howell), indicative of clumsy arm movement. Reptiles Whereas in the amphibia the pectoral girdle is just behind the head, in the reptile it has migrated a considerable distance from this position. Essentially, the girdle comprises a scapula, a procoracoid and a coracoid. In general, the clavicle replaces the procoracoid, as evidenced by the latter's reduction in size. However, in some reptiles the clavicle is absent (Crocodilia and Chamaeleo). Some reptiles lost their limbs, and the girdles are either greatly reduced or have disappeared (Amphisbaenienes, Ophidia). Birds Elements of the girdle of the reptiles were modified in birds to permit flight. The clavicles exhibit a marked degree of development, their ventral ends fusing to form the wishbone (furcula). The scapula is small, curved and narrow, extending backward. The coracoid is large and strong, one end together with the scapula forming the glenoid fossa, while the other unites with the sternum. The keeled sternum provides attachment for the strong pectoral muscles used in flight. In some cursorial birds the clavicles are greatly (emu), while in others they are absent. Mammals In montremes, the lowest order of mammals, large coracoids are found between the sternum and the glenoid fossa. In all other mammals, however, the coracoid tends to become greatly reduced, forming an insignificant process on the scapula. The only other vestige of the bone, is the coracoid ligament, extending from the coracoid process to the bone, in which may be found isolated masses of cartilage. It has a separate center of ossification. This arrangement frees the scapula from any bone attachment to the skeleton. In mammals without clavicles the scapula has no bony attachments whatsoever. It becomes the sole support for limb and provides attachments for muscles necessary for a freely movable extremity. New functional demands on the girdle resulted in the development of a projection of bone on the dorsal surface of the scapula (spina scapulae) which extends downward and ends in the acromion. Generally, the clavicle articulates with the acromion and the sternum, its only connection to the coracoid process being by the coracoclavicular ligaments (conoid, trapezoid). In mammals which have acquired freedom of the forelimb to a marked degree, such as insectivores, primates and some marsupials and rodents, the clavicle is usually well developed. In others, including ungulates, carnivores, cetaceans and some rodents, edentates and marsupials it is absent or rudimentary. Evolution of the Upper Extremities There has been considerable controversy as to the derivation of the cheiropterygium (tetrapod limb, also called the pentadactyl limb) from the icthyopterygium (paired fins of fishes). It was recorded previously that in the evolution of the free paired appendages the proximal or basal ends of the radials (cartilage rays) fused to form basilia, and later with the demand of greater movability of the fin a joint appeared between the radials and the basilia, several of which in turn articulated with the girdle. Such a scheme is discernible in the paired fins of the elasmobranchs, which possess three basilia (propterygium, mesopterygium and metapterygium) located between the girdle and the radials of the fin (Fig. 4). In the pectoral girdles and the fins of the crossopts, Eusthenopteron and Sauripterus (fossils from upper Devonian), is found an arrangement of the skeletal elements, generally accepted as a link between paired fins of fishes and tetrapod limb (Fig. 5). These two genera of crossopterygian fishes are considered close to the forms from which the amphibia evolved. The basic pattern of their pectoral limb comprised a proximal segment, which in turn articulated with several distal elements. The proximal element was destined to became the humerus, the middle elements the radius and the ulna, the distal elements the carpus and the digits. The change from an aqueous to a terrestrial existence was accompanied by pronounced alteration in the skeletal elements of the pectoral fin which now must be used for support and locomotion. Therefore, in the amphibia, the first animals to adopt terrestrial habits, the pentadactyl limb evolved from the paired fins. From the distal element arose the carpus, the metacarpus and the phalanges. The principal element in the radial side became the thumb, and those on the ulnar side the other four digits. In all stages of evolution up to and including man the basic plan of the pentadactyl limb was maintained.Fig. 6A-F: Phylogenesis of the pectoral girdle. (A) Sauripterus (Devonian crossoterygian lung fish). (B) Eogyrinus (Carboniferous embolomerous amphibian). (C) Eryops (Permian rhachitomous amphibian). (D) Moschops (Permian dinocephalian reptile). (E) Cynognathus (Triassic theriodont reptile). (F) Macaca (an Old-World Recent monkey). (Howell: Speed in Animals, University of Chicago Press, p. 138)Scapula During the evolution of the upper extremity, the scapula, more than any other bone of the shoulder girdle, reflects momentous alterations that have been brought about by increased functional demands of a prehensile limb. Changes in posture provided the stimulus which initiated the numerous morphologic changes. In the cervical region but was freed from the skull. Rhachitomous amphibians possessed massive scapulae with the glenoid cavity pointing laterally. The articulating surface was screw-shaped, and the limbs were held in the coronal plane horizontal to the ground. In the Reptilia the scapula with the entire girdle migrated a great distance from the skull in order to permit a more efficient mode of locomotion. The scapula was still broad and massive in the primitive forms. However, later with increased efficiency in locomotion, there was a trend toward reduction of this bone, the glenoid cavity shifting from a position directed laterally to one directed posteriorly and inferiorly. As a result of the change in posture, the coracoid's function decreased. Hence, a gradual reduction in its size is noted in this group. Up to this stage in evolution of the pectoral girdle no evidence of a spine on the dorsal surface of the scapula is found except in the Therapsida whose posture is not unlike that of the mammals. Posture was responsible for the development of the scapular spine which is found in all mammals except the very primitive forms, the Monotremata. With rearrangement of some and disappearance of other muscles, the need of a procoracoid and coracoid no longer existed. Therefore, the former element disappeared entirely, while the latter was reduced to the coracoid process. The shape of the scapula is dependent upon posture and the functional requirements of the muscles attached to it. It is broad and massive in forms which need large powerful serratus anticus muscles to support heavy bodies in a quadruped position. In mammals which have partially or completely freed the pectoral limbs, the shape of the scapula exhibits a trend towards the pattern found in man. These alterations are brought about by change in posture from the pronograde to the orthograde and highly specialized functional requirements of a prehensile limb. The most significant scapular change is in the relation of length to breadth of a bone. Pronograde forms disclose a scapula, while in the toward man it becomes This morphologic change is most in the of scapula the spine the most pronounced those in the region above the spine being in the scapula be by a scapular a of the breadth along the base of the to the length from the from the The scapular is in the pronograde in which the scapula is and The in the stages of development man This is the result of a gradual in the breadth of the scapula and of the bone the level of the giving rise to a in the (Fig. and in their of the function of the shoulder that of the scapula the spine the relation of the border of the scapula to the glenoid fossa, the of of the muscles attached to this a of great in the of the in scapular in stages from the pronograde to the (Redrawn from and the as one the of the of the muscle as in the of the end of the the acromion process. Whereas in pronograde forms the acromion process is in it is a massive the head (Fig. in spine of the scapula and the acromion process development from the pronograde to the This change reflects the of the muscle. note the in size of the coracoid the of the two of the head of the humerus and the of the in stages of During evolution of a prehensile extremity, morphologic in the In rhachitomous amphibians the humerus was a massive bone at either the distal end being than the proximal to attachment for large In reptiles with free in the forelimb the upper extremity was brought the and the humerus became appeared at the proximal which evolved into the of the The anterior became the and the posterior the in mammals for the articular surface of ends of the humerus function in the plane a through the of the head of the humerus, is directed and one through the distal articular surface of these two a of In as the orthograde form is the some in the exhibit an of and the and the are responsible for the of the articular of the Development of the orthograde forms was accompanied by of the and dorsal of the scapula. The glenoid is now directed laterally (Fig. however, demand that the extremity as a function anterior to the body and that the be in the meet these the while the articular at either end in the (Fig. The acquired by the in the higher primates is by the of the on the humerus to more distal position. This together with in size of the greatly the of the muscles (Fig. Changes in the the scapula and the humerus, in stages from the pronograde to the The in the and the scapula to a dorsal position that the glenoid cavity is directed laterally. The humerus a in the in of the humerus in inward of the The articular at either end of the humerus in to a lower level on the of the humerus, indicating the significant by the in higher significant morphologic alterations were of the and of the Pronograde forms disclose the the center of the head of the humerus and the in the In this position it as a strong of the Both in these forms are the size. A is found in In these forms, the has been by of the humerus that a through the center of the head of the humerus in man an of with one through the plane of the and reduction in the size of the is a in the higher From the above it is that the at a greater increased by the arm in a position of In this position the the wall of the and the now as a Muscles Changes in posture and functional requirements of a prehensile extremity were responsible for alterations in the and the of muscles about the Such were responsible for the skeletal previously The of the change in any muscle becomes its is with the of the in which it the scheme of and the muscles which in shoulder be into three (1) (2) and (3) group. The made on the functional of the shoulder by the is and that one is to from this source of this of their are noted in the The These the scapula to the humerus and of the and with of a free limb, the while that of the of the of this in man is made up by the muscle. that the muscle is in mammals and that it evolved from the to form a separate muscle from the of the scapula to the With of the of the scapula, the of this muscle increased in it up of the it is a morphologic component of the of an in the of the shoulder than of the The muscle is by morphologic alterations from the primitive to the higher It up of the of the group. The only significant alteration is an in number of of This is the result of of the scapula. This skeletal change brought about an in the of attachment of the which of the According to and the three muscles and by of alterations in the and the of the and the of the scapula, function as a are and of the head of the The with the of the comprises (1) serratus (2) (3) scapulae and (4) The first three muscles of this from the or and their of the cervical in the cervical region and into the border of the scapula. In primitive forms the function of this was to the of the border of the scapula. In general, those fibers with dorsal of the scapula became the those with ventral the serratus and those with of the scapula, the and posture were responsible for evolution of the muscles as they in the higher The serratus anterior formed the basal for all three of the proximal and distal fibers and reduction of the intermediate fibers gave origin to two muscles, the scapulae and serratus Further morphologic alterations in the serratus anterior comprise of its proximal and distal reduction in size of is intermediate and of the upper and lower portions of the muscle into the and of the scapula. The the evolved from a muscle from the arch to the membranous girdle. In terrestrial forms it a position from the region to the in tetrapods it from the the of and and into the spine of the scapula, the acromion and the scapula. change has in the in the evolution of the There has however, some of its proximal and distal muscle and reduction in and efficiency of its middle The is made up of the the and the muscles and extends from the trunk to the The pectoral evolved from a primitive muscle which the coracoid with the in posture and increased functional demands made on the limb were responsible in the later and forms for of of this muscle dorsally to attachment to the scapula which later gave rise to the the and the of the All other of the muscle migrated from the procoracoid to the sternum and gave rise to the Further morphologic in the resulted in a of this into a and a of the attachment of the fibers and attachment to the clavicle head of the From the evolved the muscle in higher its attachment in primitive forms to have migrated to the coracoid process. The and muscles from a basic muscles extending from the to the scapula, to the in the higher primates no significant morphologic or alterations except that they are well in forms in and Both these muscles evolved from ventral and dorsal muscle elements which were with in the more distal the and the From the ventral elements arose the muscle by proximal along a plane of to the scapula In mammals other than it is a muscle. forms disclose powerful which together with the as a functional to the exhibit two of one from the and the other from the coracoid process. of the from of the the head at a its efficiency as an of the arm which it in other forms. However, the be made to function as an of the extremity the arm is the to the and the center of the This is not by with of the The from a dorsal muscle the its three migrated The scapular or head attachment on the the head to the upper and surface of the humerus, and the lateral head to the upper and surface of the humerus, and the lateral head to the upper No significant morphologic or alterations have in this muscle. It as a powerful of the