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Doklady Biological Sciences, Vol. 375. 2000, pp. 633-635. Translated from Doklady Akademii Nauk, Vol. 375, No. 3, 2000. pp. 427-429. Original Russian Tea Copyright © 2000 by Fornin, Lobachev.

GENERAL BIOLOGY

Specific Structural Features of the Pectoral Arch and Phylogenetic Relations in Some Mammalian Taxa
S. V. Fomin and V. S. Lobachev
Presented by Academician I.A. Shilov March 6, 2000 Received March 6, 2000

Reorganization of the pectoral arch has played an important role in mammalian evolution. The mere emergence of class Mammalia is associated with the reorganization of this group of bones and with acquiring the parasagittal orientation of extremities. Drastic changes in the pectoral arch have also occurred during the formation of large mammalian taxa. In the study of the postcranial skeleton of small mammals, we succeeded in finding some specific structural features of the pectoral arch, which can add to the knowledge of the evolution and phylogenetic relations within three orders: Lagomorpha, Chiroptera, and Rodentia. The total number of species and individuals studied was the following: Lagomorpha, 9 species (44 individuals); Chiroptera, 9 species (23 individuals); and Rodentia, 89 species (903 individuals). We used the material from our own fresh collections, the collection of the Zoological Museum (Moscow State University), the private collection of one of the authors (V.S.L.), and -the collection of the Department of Vertebrate Zoology and General Ecology (Faculty of Biology, Moscow State University). ORDER LAGOMORPHA It is accepted that both modern and fossil forms of this mammalian order are very similar in the structure of skull and dental system, as well as postcranial skeleton [9, 10]. However, our data indicate that the pectoral arch evolved in two different directions in the modern families Ochotonidae and Leporidae. In Leporidae, the clavicle is reduced, but the common ancestral (normal for mammals) position of the rudimentary clavicle or the chord settled in the region of the former clavicle is preserved (between the prestemum and the acromial process of the scapula; Fig. 1 A). Conversely, a full clavicle is preserved in Ochotonidae, but its spatial position has been radically changed. The lateral ending of the clavicle is articulated with the humerus rather than with the acromial process of the
Department of Vertebrate Zoology and General Ecology, Faculty of Biology, Moscow State University, Vorob'evy gory, Moscow, 119899 Russia

scapula. Probably, th is change in the clavicle occurred by its slipping down along the acromial part of the acromiohumeral muscle. Thus, the formation of two types of pectoral arch in the modern families of Lagomorpha could only have occurred through divergence. This assumption is also confirmed by other characteristics of the postcranial skeleton. The skeletons of Leporidae have certain features that demonstrate a much higher specialization in rash run compared to Ochotonidae [1, 2]. On the other hand, Ochotonidae deviate from the ancestral type significantly more than Leporidae, which is confirmed by a set of indices (a large anterior incisure of the scapular crest, the presence of the posterior incisure, general weakening of the crest, the hypertrophy of the iliac tuber, a sharp decrease in the postacetabular pelvic bones, the crest of the shin bone with a distally directed spike, etc.). ORDER CHIROPTERA Because of the absence of the secondary articulation of the humerus with the scapula (between the large tubercle of the humerus and the outer edge of the glenoid fossa of the scapula), the structure of the flight apparatus of Rhinolophidae is usually considered imperfect or primitive, compared to Vespertilionidae [3,4]. The analysis of specific features of the articulation between the clavicle and the scapula does not confirm these views. Pteropodidae and Vespertilionidae preserve the ancestral type of the articulation between the clavicle and the scapula, which is common in Mammal i a (Fig. I B ) . Adaptation to flight has additionally strengthened this articulation: the clavicle is fixed on the scapula by means of a very short chord at the base of the coracoid process. In Rhinolophidae, the claviclescapula articulation has been significantly changed. The primary articulation with the acromial process has disappeared, and the clavicle is directly articulated with the base of the coracoid process; similar to birds, the clavicle-scapula articulation maximally approaches the humeral articulation. Thus, the structure of the pectoral arch of Rhinolophidae is a derivative (or more

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FOMIN, LOBACHEV

Fig. 1. Structure of the pectoral arch in (A) Lagomorpha and (B) Chiroptera: (a) the common ancestral structural type of placental mammals, (b) Vespertilionidae, (c) Rhinolophidae, (d) Leporidae, and (e) Ochotonidae. Designations: /, scapular crest; 2, glenoid fossa; 3. coracoid process: 4, clavicle: 5. presternum; 6, humerus.

advanced) state, compared to the structural type of Vespertilionidae. It is interesting that, in Rhinolophidae, though the position of the clavicle has been changed, its association with the acromial process is preserved, but the process is extremely t h in (literally filiform; Fig. IB). The family Rhinolophidae is considered advanced, which agrees well with the structure of other formations related to the flight apparatus: the complication of the structure of the scapula and the formation of the pectoral ring through fusion of the presternum, the first rib pair, and the first pectoral vertebra. In addition, Rhinolophidae, compared to other bats, have the most specialized suspensor apparatus (right up to the total loss of the capability of terrestrial locomotion and penetration into refuges with narrow entries [6]).

ORDER RODENTIA The isolation of basic phyletic lines in the order Rodentia remains a great problem for taxonomists. They suggested no fewer than 11 systems of the order, which often disagree w i t h or even contradict one another [7-9]. The systems are based on the structure of mastication muscles, the shape of the suborbital foramen, the structure of the angular process of the lower jaw, the number of lophs on molar teeth, and specific structural features of enamel on incisors. Our data on the structure of bones of extremities in 20 families of simple-toothed rodents revealed that, among all the lines of specialization of t h e bones of locomotion organs, changes in the pectoral arch are the most significant. In total, three basic phyletic lin e s can
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be distinguished, which fit Brandt's system of division of Rodentia into suborders [8]. The first line, which is represented by squirrel-like rodents (suborder Sciuromorpha), is characterized by a considerable strengthening of the pectoral arch and by developing additional structures on the scapula: a strengthened clavicle and scapular crest, a short anterior incisure of the scapular crest, a massive scapular cervix, crests on the medial scapular surface, etc. These features could have appeared due to the climbing habits. In such a way of life, the foreleg acts in the lateral plane (to adapt for climbing thick trees). When climbing trees vertically, the foreleg experiences loads that are fundamentally different from those connected with terrestrial movement. The great taxonomic importance of these changes is indicated by the fact that numerous terrestrial and burrowing forms of Sciuromorpha preserve the type of structure similar to that of the climbing forms. The second line, which is represented by hystricomorphs (suborder Hystricomorpha), is characterized by the trend toward reducing the clavicular connection of the foreleg with the axial skeleton. This increases the freedom of movement of the articular ending of the scapula, which enables both an increase in the amplitude of movements of the foreleg in the parasagittal plane and an increase in the functional length of the foreleg by an extra bone segment [5]. Moreover, this line is characterized by considerable reduction of the small and third trochanters of the femur. In the third line, which is represented by various taxa of myomorphs (suborder Myomorpha), neither the pectoral arch's bone structure, nor the way it is joined to the axial skeleton have experienced fundamental changes. There emerges, however, a specific structural type of the proximal part of the humerus with a welldeveloped deltoid crest, which is not characteristic of other simple-toothed rodents. It is possible that the directions of morphological evolution of the pectoral arch in Rodentia are closely related to the specific locomotion types suggested by Gambaryan [1]. Myomorphs preserve a pectoral arch whose structure is similar to the common ancestral type of Rodentia, and use the most primitive type of locomotion among Rodentia (ricochet). The two remaining taxa have mastered much more advanced asymmetric types of running and significantly modified the structure of bones of the foreleg, but each of them used specific ways of modification.

CONCLUSION The studied features of the pectoral arch suggest that modern families Ochotonidae and Leporidae (Lagomorpha) have been formed independently from each other. In spite of a considerably higher specialization of Leporidae in rash run, the locomotion apparatus of Ochotonidae is not a more primitive precursor to that of Leporidae. The pectoral arch of Rhinolophidae represents an advanced stage of evolution, compared to other bats. In simple-toothed rodents, three basic lines of specialization of the pectoral arch were found, which are also correlated with other features of the bones of locomotion organs. These types agree with the system of suborders suggested by Brandt. We can suggest that, compared to the common ancestral type of the structure of the pectoral arch of placental mammals, the most significant changes have occurred in Hystricomorpha; in Myomorpha and Sciuromorpha, the modifications of the foreleg skeleton were less pronounced. In Myomorpha, they mainly occurred in the humerus, and Sciuromorpha experienced a structural complication of the scapula and strengthening of the clavicle. REFERENCES
1. Gambaryan, P.P., Beg mlekopitayushchlkh (Running in Mammals), Leningrad: Nauka, 1972. 2. Klehanova, E.A., Polyakova, R.S., and Sokolov, A.S.. Tr. Zoo/, lust. Akad. Nauk SSSR, 1971, vol. 48, pp. 58-120. 3. Kovtun, M.F., Apparat lokomotsii rukokrylykh (Locomotion Apparatus in Chiropterans), Kiev: Naukova Dumka, 1978. 4. Kovtun, M.F., Stroenie i evolyutsiya organov lokomotsii rukokiylykh (Structure and Evolution of Locomotion Organs in Chiropterans), Kiev: Naukova Dumka, 1984. 5. Kuznetsov, A.N., Plaiiy stroeniya konechnostei i evolyutsiya tekhniki bega u tetrapod (Schemes of the Structure of Extremities and the Evolution of Running Technique in Tetrapoda), Moscow: Mosk. Cos. Univ., 1999. 6. Kuzyakin, A.P., Letuchie myshi (Bats), Moscow: Sovetskaya Nauka, 1950. 7. Pavlinov. I.Ya. and Yakhontov, E.L., in Filogenetika mlekopitayushchikh (Phylogenetics of Mammals), Moscow: Mosk.'Gos. Univ., 1992. pp. 5-43. 8. Sokolov, V.E., Sistematika mlekopitayushchlkh (Taxonomy of Mammals), Moscow: Vysshaya Shkola, 1977, vol. 3. 9. Andcrson, S. and Jones, J.K., Recent Mammals of the World. A Synopsis of Families, New York: Ronald, 1967. 10. Carrol, R.L., Vertabmte Paleontology and Evolution, New York: Freeman, 1988.

DOKLADY BIOLOGICAL SCIENCES

Vol. 375

2000