偶脉羊齿类的始现时间和迁移扩散问题

FIRST APPEARANCE OF PARISPERMAE AND THEIR MIGRATION AND DISPERSION

本文简单回顾了脉羊齿类植物的分类,详细统计了脉羊齿类植物在中国及世界各地开始出现的时间和衰亡时间,大量事实表明,偶脉羊齿类(也许包括全部脉羊齿类)起源于中国并向西欧,北美地区迁移和扩散。

After having counted up at full length the first occurring horizons all over the world of the leading representative genera and species of the plant neuropterids it has been made known that these plants comprising Neuralethospermae, Neurodontospermae and Parispermae, all take the southwest margin (Mt. Qilian) of the crustal blocks of both South and North China as their lowest horizon of first occurrence. Based on the statistics in Text-fig. 1, China might have been the birthplace of neuropterids; with the elapse of time, this kind of plants migrated and dispersed from China westward, taking their route somewhat like this: South and North China Plates→Cimmerian Massif→Southwest Europe→North America. In their general pattern of migration and dispersion from east to west, one group of communities might have started from the North China Massif and migrated into Asia Minor by way of the Tarim Oldland, reaching further to the present Soviet territory in Europe, and E. Europe. As shown in Text-fig. 2, the succession between rise and fall of the plant neuropterids during the geological periods might help prove this conclusion. Text-fig. 3 shows the channels through which the plant neuropterids migrated and dispersed, giving support and verification to the forming process of the Pangaea at its earlier stage. In Late Devonian—Early Carboniferous, the Gondwana land drifted northward, while the West Gondwana land at its part close to the equator was rapidly approaching the European continent. In Namurian—Westphalian, the Gondwana land made a clockwise rotation, while the north margin of the West Gondwana land (North Africa and the northern part of South America) linked up with the European continent and the Pangaea began to take shape in its embryonic form. At that time, the Tethys Ocean, narrow in the west and broad in the east, came into being between the Pangaea and the Gondwana land. Such a palaeogeographical configuration created a favorable condition for the atmospheric circulation from east to west, for this plant to migrate and scatter gradually and smoothly from east to west. During the period from Late Carboniferous to Permian, the Gondwana land kept on drifting northward and the Tethys was increasingly shrinking in scope, bringing about the mixed phenomenon among the Euranerican, Cathaysian and Gondwana floras along the regions of North Africa and Asia Minor in the Permian. In the evolutionary process of the plant Neurospermae, of particular interest was the occurrence in pairs of pinnate veins and reticulated veins, the former making their appearance as often as not somewhat later than the latter. Among the Neurodontospermae (including the Alethopteridae with relatively intimated affiliation) and the Parispermae, the corresponding sister genera of pinnate veins and reticulated veins are Alethopteris and Lonchopteris, Odontopteris and Anastomopteris. Neuropteris and Reticulopteris, as well as Paripteris and Linopteris. The appearance in pairs of pinnate veins and reticulated veins is considered as the evolutionary law of the plant Neuropteris, then it may be supposed that Palaeoweichselia yuani Sze described by Sze (1933) from the Namurian at Hongshuibao in Jingtai, Gansu, would be the sister genus bearing reticulated veins corresponling to those of Neuralethopteris. P. yuani Sze is roughly identical with Neuralethopteris in the period of occurrence; in addition, its small pinnule base also sometimes contracts like a heart, and sometimes protracts downward along the pinna rachis. Its pinnule has a conspicuous mid-vein and its small pinnules are often mixed together into a larger pinnule bearing a marked topmost small pinnule; such characters are quite similar to those of Neuralethopteris. According to Laveine (1989), except for Neuropteris ovata Hoff. in the Stepharian of China, none of the real elements of Neuropteris have ever been found. Judged from the material available on hand, the typical specimens of Neuropteris from the Visean—Westphalian strata in China are actually very few in number; such a fact does not indicate the absence of this genus in the older Carboniferous strata of China, but reflects its rather poor preservation. As revealed in this paper, the specimen of Reticulopteris was discovered from the Hongtuwa Formation at Xiaheyan in Zhongwei, Ningxia (Zone R of Namurian B) demonstrating that even more forms of Neuropteris, a sister genus of Reticulopteris, should have come into existence in the strata of Namurian B or a slightly earlier time in China. As mentioned above, the first emergence time of the entire neuropterid community and its successional process between rise and fall in the strata highlight, the fact that this kind of plants originated in China and then migrated and dispersed from east to west, rather than the fact, as Laveine et al. (1989) put it, that unlike Parispermae, Neuralethospermae and Neurodontospermae came originally from West Europe and then dispersed and migrated eastward into China.