蜂巢珊瑚类横板珊瑚的体壁构成及有关问题

ON FORMATION OF THE WALL OF FAVOSITIDS (TABULATA) AND OTHER RELATED PROBLEMS

一、前言本文所指蜂巢珊瑚类,包括所有具联接构造的块状复体横板珊瑚,即蜂巢珊瑚亚目(含蜂巢珊瑚超科、厚孔珊瑚超科)和槽珊瑚亚目(含槽珊瑚超科、小槽珊瑚超科)。多年来,在研究珊瑚体显微构造时,最先为人们注意的是体壁的构成。本文以“构成”一词代表构成体壁的部分,而以“微细构造”一词代表每一部分的构造细节。微细构造又分显微构造和超微构造两类,分别代表光学显微镜下和电子显微镜下所见的微细构造。由于光学显微镜放大倍数超过40倍后,视野昏暗,形象模糊,难以分辨。

The favositid of tabulate corals as called in this paper is a liberal interpretative concept including all the ceroid tabulate corals with connecting structures.The optical microscope only possesses a lower discerptibility. During the growth process of the wall, especially in the early stage, the polyps might secrete organic materials and stain some parts of the wall with a dark colour, resulting in the de ailed structures of the wall which cannot be clearly observed under an optical microscope, so hat some conclusions on the microstructures of the wall previously made based on observation by means of an optical microscope are not in conformity with the reality.The present paper is to clarify some doubtful traditional concepts based on the study by means of SEM.In fundamental microstructure, the skeletal units of the wall of tabulate corals are basically classified into two kinds, i. e., crysatl needle and erystal flake. These two kinds of fundamental units combined with each other in various ways to form seven types of microstructure in two groups, as given in Table Ⅰ and illustrated in Textfigure 1.The following are the main conclusions on the formation of the common wall studied under SEM.1. The median suture. This is the free space between two neighboring corallites filled with calcite spars during diagenetic and katagenetic stages. There are no any organic structures secreted by the polyps. Two types of the filling mode can be recognized according to the structures, i.e., granule type (Pl. Ⅰ, figs. 1c, 2b; P. 4, fig. 1d, along S-S direction) and plate type (Pl. Ⅰ, figs. lb; Pl. Ⅱ, fig. 3).2. The so-called median suture in Alveolitidae. The common wall of Alveolitidae (except those with columnar corallum which had been established as a new family by the author in 1982) is composed of crystal fascicles and belongs to the smelted type. But the real median suture is lacking (Pl. Ⅴ, figs. 1a, 1b). Sometimes a broad bright zone (the so-called 'median suture') can be observed in the middle part of the common wall under an optical microscope. The central part of the crystal fascicles is mostly distributed in this zone where the section of the sample passes through the crystal fascicles perpendicularly. As a result, many transverse sections of the crystal needle appearing like dots can be seen clearly under SEM (Pl. V, figs. 1b, 2a, 2b.). But they appear as a bright zone and cannot be well distinguished under an optical microscope.3. The so-called 'epitheca'. Under an optical microscope, two dark-coloured layers can be seen on both sides of the median suture (Pl. Ⅰ, fig. 1a), which have been called the 'epitheca' or 'primary wall' by some paleontologists. However, under the SEM magnifying over 500 diameters, the dark colour disappears entirely, without any differentiation which can be recognized between the so-called 'epitheca' and the stereolayer, either in structure or in texture. This indicates that it is merely a single struotural layer on both sides of a suture. The author calls the so-called 'epitheca' the 'primary stained layer', so as to distinguish it from the stromatolitic stained layer in the peripheral zone of the columnar corallum, and regards the so-called 'epitheca' as only the exterior part of the stereolayer stained by organic materials secreted by the polyps. Therefore, it cannot be regarded as an independent structural layer of the common wall.4. The so-called 'stromatolitic structure'. In the peripheral zone of the corallum with a columnar outline especially that of the genus Thamnopora, sometimes many light lamellae can be seen intercalated with dark lamellae (Pl. Ⅳ, figs. 1a, 1b), which are called the 'stromatolitic structure' by some paleontologists who even consider them as the lamellar type microstructure. Under SEM, the microstructure is the same in both the light and dark lamellae; when the SEM magnifies the object enough diameters, the dark colour will disappear gradually (Pl. Ⅳ, fig. 1a). The dark colour lamellae were originally simalar tO the primary sttained layer; they proceeded from the stain caused by the organic materials secreted periodically from the polyps, so that it cannot be considered as a structural layer.5. The so-called 'secondary thickening'. In the peripheral zone of the columnar corallum, the thickness of common wall and tabulae increases gradually or sharply. Many paleontologists have regarded this as 'secondary thickening'. In fact, the microstructural types of the common wall in the peripheral zone under SEM are evidently different from that of the axial zone. In the peripheral zone there is no any trace which can subdivide the common wall into primary and secondary structural layers (Pl. Ⅱ, figs. 1b; Pl. Ⅲ, fig. 2; Pl. Ⅳ, fig. 1e; Pl. Ⅴ, fig. 3b). This is why the author considers the whole peripheral zone as of primary structure. Both the microstructure and thickness of the common wall in the peripheral zone differ from that in the axial zone; this can be considered as a measure of physiological function taken by the coral in order to strengthen and prevent the skeletons from breaking off under the action of gravity. Such a measure includes the absence of the median suture, the thickening of the common wall and a more solid type of microstructure of the common wall, as an increase in thickness and density of the tabulae.6. Formation of the common wall. Four styles of the common wall belonging to two types are recognized.(1) Separated type The common walls are separated by median sutures. Although the latter are not the original organic structures, they occupied a definite space in the common walls, and therefore should be accounted as a member in the formation of the common wall. On the contrary, the primary stained layers are the products resulting from secretion of the polyps, representing only the exterior part of the stereolayer in space, without any structure differentiated from the stereolayer but the colour, and therefore they cannot be considered as an independent structural member of the common wall. On this account, the separated type common wall can be subdivided into two styles.A. Three-layer style This is the most common style in the formation of the common wall. The common wall is composed of a median suture and two stereolayers beside the median suture. This style can be observed in the whole corallum of most Favositicae (Pl. Ⅰ, figs. 1b, 1c, 2b, 3b; Pl. Ⅳ, fig. 3) and in the axial zone of the coralla of Pachyporicae (Pl. Ⅳ, fig. 1d).B. Five-layer style This is a style rarely seen. In some genera such as Coenites, by the margins of its cavities in the corallites, there occurred a lamellar type microstructural layer formed by crystal flakes (Pl. Ⅲ, fig. 2b). This is an unstable layer, with its thickness varying from place to place. This layer together with a median suture and two stereolayers forms the common wall in five-layer style.(2) Melted type This type of common wall lacking the median suture occurs in the common wall of Alveolitidae, espacially in the nonfavositids (except Syringoporida). The common walls in the peripheral zone of the favositids with columnar corallum also belong to this type when the median suture vanishes from sight. This type can be subdivided into two styles.A. Single-layer style This is the most common style. The common walls with rabecular type microstructure are composed of crystal tuff in Alveolitidae (Pl. Ⅴ, figs. 1a, 1b). This style also can be observed in the peripheral zones in the coralla of Thamnoporidae (Pl. Ⅳ, figs. 1a, 1a, 1e, 2), Alveolitellidae (Pl. Ⅴ, figs. 3a, 3b) and Coenitidae(Pl. Ⅲ, figs. 1a, 1b, 2a).B. Three-layer style A lamellar type microstructural layer formed by crystal flakes appears on the margins of the cavities in the corallites of the genus Alveolites (illustrated by Lafuste, 1971). The lamellar type structural layer together with the trabecular microstructural layer in the middle part of tile common wall forms a common wall in this style.The common wall of the genus Pseudoroemeripora also belongs to the mehed type common wall of the three-layer style. Its middle part consists of a lamellar type microstructural layer formed by crystal flakes which appears as a series of conics ipside down with one piling upon another and rising upwards. By its both sides there occur a comb-type microstructural layer, and a lot of crystal needles arranged in the direction perpendicular to the margins of the cavities, together with the middle part to form a three-style common wall with a compound double comb-like microstructural type common wall (Pl. Ⅱ, figs. 2a-d).