Bohaidina(沟鞭藻)古口之新见

NEW KNOWLEDGE OF ARCHEOPYLE OF BOHAIDINA JIABO 1978

在光学显微镜及扫描电镜下对采自辽河断陷西部拗陷的61块岩芯样品中的沟鞭藻Bohaidlna进行了观察,在相当数量的化石囊孢上发现了保存较完好的古口。大量标本的仔细观察和比较研究表明,虽然多数Bohaidina的囊孢上不见古口构造,但确有一定数量的囊孢具前间式古口,2a或3I型。对古口的新认识将有可能导致Bohaidina这类囊孢的重新分类。

Bohaidina Jiabo 1978, a dinocyst genus widely distributed in the coastal region of the Bohai Sea, China, was first studied by several authors and published in an early article 'On the Paleogene Dinoflagellates and Acritarchs from the Coastal Region of Bohai' (Song et al., 1978). It has long been a curious matter since its establishment because of its limited occurrence within China and its lack of a well-preserved archeopyle. Examination on a large number of Bohaidina cysts in 61 core samples from the Liaohe Depression shows the clearly delimited archeopyles in quite a few Bohaidina cysts, despite that these cysts only occupy a small proportion of the whole Bohaidina cysts. All the archeopyles found in Bohaidina cysts are anterior intercalary archeopyles equivalent to either 2a or all the three anterior intercalary plates in motile thecae. Archeopyle 2a of Bohaidina is 'hexa 2a archeopyle' as described by Lentin and Williams (1976), and more definitely, it belongs to 'eurydeltaform' (Bujak and Davies, 1983). However, it often appears as a trapezoid archeopyle due to the mechanical deformation in diagenetic process. Archeopyle 31 is relatively easier to be recognized due to the characteristic shape, but it is also subject to deformation. The important fea ture shared by these two kinds of archeopyles lies in the 'rectilinear' boundary between paraplate 2a and the adjoining plate in the precingular series (Evitt, 1978). This character may have some taxonomic significance according to Evitt (1985). Corresponding to archeopyles 2a and 31, two types of opercula of Bohaidina also may be differentiated: monoplacoid operculum and compound polyplacoid operculum. All of the opercula are free, either fully in place and adherent, or completely isolated from the cyst walls. According to Xu Jin-li and Mao Shao-zhi (1989), Bohaidina has a combination type of archeopyle ((?)a+31a or (?)a+31). A close examination of the type specimens by the present author indicates that the apicals do not take part in the formation of archeopyles, and therefore the archeopyle of Xu ahd Mao's specimens should be an anterior intercalary archeopyle. Although we have not found any equivalent living cyst or theca to that of Bohaidina, the archeopyle of the genus actually shows the corresponding theca with an anterior intercalary area which is composed of 3 intercalary plates. The archeopyle of Bohaidina may indicate that the genus belongs to the Peridinioid cysts. Further studies on the archeopyle of Bohaidina migh lead to the reclassification of this dinocyst group.