紡錘虫の形態進化とその古生物学的意味
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In this paper, the writer discussed the significance of the growth and form of fusulinids, especially of the relations between such shell forms as cylindrical or fusiform and the spiral growth of the shell. Most of the materials, used in this study, are chosen from the specimens of Pseudoschwagerina miharanoensis AKAGI (MS), collected from the limestone of Miharano, Tojo-machi, Hiba-gun, Hiroshima Prefecture. It is the most abundant fusulinids in the Miharano Limestone (Sakmarian). The fossils occur as free specimens detouched from matrix, so that the external characteristics are well observed. First, the meanings of the shell form were discussed. Geometrically, the mode of growth is regular and rather simple, in spite of the apparent complexity of the internal structures exhibited in thin sections. An increase of protplasm results the growth of body and the coiling chambers. Additions of chambers along the preceding ones make up the volutions, which are performed regularly, because their floors and backside walls are substituted by the corresponding roofs and frontal walls of the preceding ones. Organization has not been studied in fusulinids. They have been included in the "Anaxonia" together with such higher invertebrates as gastropods and others, as was the case of the other members of Amoebae. But, it should be better to remove them from the Anaxonia, because they have a distinct axis in their bodies, around which the shell is formed in a certain regular and definite way of coiling, whereas the members of true Anaxonia do not show any regularity in their organization. So, the writer proposes a new group "Spiralia" for those animals characterised by the spiral organization of body. Second, the spiral curves of coiling were studied in some detail, and were compared with curves drawn geometrically. In this case, the writer found that the spiral curves of fusulinids were nearly identical with logarithmic spiral, which was characterized by a constant angle of tangent against the corresponding radius at their contact. Such spirals as those are called equiangular spirals. The angles of contact in Pseudoschwagerina miharanoensis are about 86 degrees throughout the most stages of growth, except the earliest and the gerontic stages. A deployed figure is drawn in such a way as follows : First, an ideal cross section is deduced from an axial section. In this operation, the law of constant angle of contact is used. The ideal cross section is better than a real cross section for the prepareation of a deployed figure, because it represents a cross section of the same individual from which the axial section is obtained. Further, an ideal cross section confirms the specific identity of a real cross section and the axial section, from which the ideal cross section is deduced. The spiral curve of the ideal cross section, is, then, unrolled and straightened in a straight line. Then, the chambers of the axial section are figured serially and successively on the straight line, where the positions of each chamber are located so as to equal to the lengths of corresponding half a volutions. Next, both ends of each sections were connected successively by lines on both sides of the figure. The figure, thus obtained, represents an unrolled sheet of a fusulinid indvidual, and it, really, shows the mode of growth very clearly. The deployed figure of Pseudoschwagerina miharanoensis, deduced from the holotype specimen, shows that the rate of growth changes rapidly in several stages. Also, it shows the relation between growth and form, the increase of tunnel angles and the relative growth of several parts of shell. The growth of the shell in spiral direction is larger in the center, and becomes smaller toward the polar regions. From such deployed figures of fusulinids, followings are noticed. 1. In fusnlinids, four factors of growth are important in the study of shell form. These are ; a) height of volution including the thickness of spirotheca, b) growth in directions parallel to the axis of coiling, c) growth in directions perpendicular to the axis, and d) the differential growth of the shell between the upper (roof) and the lower (floor) sides, between the polar and the central regions. 2. The growth of fnsulinids is shown as gnomon increase, and the shell form is due to the coiling of the added gnomon around the proloculus in the center. 3. The life history of fusulinids can be clarified very well by the use of a deployed figure. The figure shows that the various shells of fusulinids ranging from discoidal, sphaerical to cylindrical, have a common type of deployed figures, i. e. triangular outline. The apical angle between the both sides of the triangle is acute and small in the earlier, discoidal shells, and is more obtuse and large in the later, cylindrical shells. It is well known that the shells of fusulinids, generally, change from the ealier discoidal form to the later fusiform. It means that the growth of the shell was more rapid in the axial directions than in directions perpendicular to it. The rapid growth in the axial directions produces cylindrical shells, such as of Sumatrina and Parafusulina. And the rapid growth in directions perpendicular to the axis results discoidal shells such as of Millerella and Nankinella. When the growth is well proportioned in both directions, such sphaerical shells as of Verbeekina and Pseudoschwagerina, are produced. Irregular shells of fusulinids, known in such genera as Schubertella and Codonofusiella are due to the irregularities of growth or to the rotatory changes of the coiling axis itself.
- 地学団体研究会の論文
- 1958-06-28