层孔虫的发生、发展和消亡

RISE, DEVELOPMENT AND EXTINCTION OF STROMATOPOROIDS

最古老的层孔虫究竟是从何时开始出现的?苏联学者亚沃尔斯基(Yavorsky)在1932年描述了西伯利亚西部中寒武世的Clathrodictyon和Anostylostroma属的几个分子。此后,哈尔菲娜 (Khalfina,1960)又收集了一些资料,把Anostylostroma vologdini改为Praeactinostroma vologdini,而Clathrodictyon的一些分子定名为Korovinella属。它们是不是层孔虫?采集的层位是否有问题,曾为弗吕格尔(Flugel,1957)讨论过,指出它们不是层孔虫而可能是古杯。盖洛威(Galloway,1957)更为明确的提出它们就是古杯。

When did the oldest stromatoporoids made their first appearance? Yavorsky, a Soviet geologist, described in 1932 the Middle Cambrian stromatoporoids Clathrodictyon and Anostylostroma from western Siberia. Later, based on some new materials he had acquired, Khalfina (1960) renamed Anostylostroma vologdini as Praeactinostroma vologdini and designated some species of Clathrodictyon to Korovinella. Are they really stromatoporoids? Are the fossil-bearing beds ocrrectly defined? After discussion, Flugel (1957) pointed out that they probably belong to Archaeocyathida but not to Stromatoporoidea. Galloway (1957) thought that the oldest stromatoporoids began to appear in the Middle Ordovician and the simplest stromatoporoid Cystostroma vermontense in the ordovician. In China, the stromatoporoids are known to have been first discovered from the lower member of the Majiagou Formation (late Early Ordovician) in northern Anhui, including Labechia, Rosenella, Aulacera and Cryptophragmus (the latter being simple in structure), but with no Cystostroma. Whether they are the oldest stromatoporoids still remains a pro- blem to be settled. Nevertheless, the above-mentioned facts indicate the rise of stromatoporoids from the late Early Ordovician. In the Silurian, stromatoporoids gradually diversified; up to the Late Silurian, they were further flourishing. In their evolutionary history, the period frorn late Early Devonian to Middle Devonian is an important stage, in which they were rapidly increasing both in number and in taxon, and widely distributed in the world. There were not many of them in the Late Devonian Famennian, but in many places of China they were found from the horizons equivalent to the 'Etroeungt?' bed. Now it seems that these stromatopotold-bearing beds are probably of the Late Famennian stage. However, in Yecheng of Xinjiang a stromatoporoid species, Pseudolabechia kunlunensis Dong et Wang, has been found from the fossil-bearing sediments of the Carboniferous Visean age. This stromatoporoid is the youngest one from the Palaeozoic so far discovered in the world. From Middle Carboniferous to Permian or even to Early and Middle Triassic, no stromatoporoids have been reported as yet. But in the Late Triassic, they reappeared though not many both in number and in taxon. Strange to say, in Early and Middle Jurassic, rare stromatoporoids have been found indicating their evolutionary decline in this period. From the Late Jurassic on, they became further developed not only in number but also in taxon, mainly distributed in the Tethyan Realm, northern Xizang (Tibet) and southern Xinjiang. Although still existing in Early Cretaceous, they became entirely , oxtinct from then on. This paper makes a brief account on the rise and development of st romatovoroids together with their main characters and their interrelationship in different geological times. Why did they suddenly disappear from Early Carboniferous to Middle Triassic and reappear in Late Triassic? These problems are still unsettled, and therefore it is necessary to make a further study especially on their evolution and interrelationship in the coming days in order to sovlve all these problems.