Cleiothyridina obmaxima McChesny的个体发育、生态特征及进化途径

本文通过对Cleiothyridina obmaxima在同一化石点产出的大量个体的统计研究,讨论了该种个体发育的阶段及其种群动力学特征,并做出了相应的解释。同时根据该种个体形态的特征及其在个体发育不同阶段的变化,结合其所赋存的群落及其围岩特征,讨论了其个体生态学及其环境意义,进而探讨了该种的演化途径。

ONTOGENY, AUTECOLOGY AND EVOLUTION OF CLEIOTHYRIDINA OBMAXIMA MCCHESNY (BRACHIOPODA)

Abstract Through the statistical analysis of 103 individuals of C leiothyridina obmaxima, 3 ontogenetic stages, i. e. juvenile, pre-dult and adult stages, are delineated. It is clear that the current species belongs to a typical anisometric group in terms of its growth pattern. Based on the statistical figures of the current species, we obtain its survivorship curve and size-frequency distribution diagram by using the conversion formula, D = S ?In(T + 1) from Levinton( 1970). The survivorship curve is characterized by a rightskewed size-frequency distribution. From an overall view, the population structure of the current species favours a recruitment strategy explanation, rather than the traditional ecological one, which points to the presence of a soft, muddy substratum hindering and hampering the normal growth of brachiopod larvae. In the study of both ancient and modern brachiopod population dynamics, Richards & Bambach(1975), Noble & Logan(1981) have achieved a great deal of success. According to their conclusions, the population structure of the current species with a high juvenile peak, which is very rare in fossil population structures, can be accounted for by the special recruitment mode, i.e. brooding, by which a largs number of attachable juvenile individuals is assured. It is observed that the morphology of the curre.i' species experiences a sharp change from its juvenile through to its adult stage. In addition, there are some individuals which show an extreme overdevelopment of the adult stage morphology. The autecological conclusions deduced from the above features involve mainly the different life-tyles of juvenile and adult stages. A striking phenomenon in the ontogeny of the current species is that in the juvenile and pre-adult stages its morphology is incredibly similar to that of C leiothyridina submabranacea (Grabau). According to Haeckel's Recapitulation Law, the evolutionary pattern of the current species belongs to the peramorphosis type,and it is derived from Cleio?thyridina submabranacea (Grabau). This is also in agreement with the stra-tigraphical distribution of these two species and the ecological changes whichthey have experienced. However, the evolution of the current species, which is a odds with the popular catastrophic explanation, supports phyletic gradualism.