试论晚白垩世以来气候、地理等因素的变化对鼍类的进化及地理分布等影响

现存的鼍属只有两个种:即东亚的扬子鳄及北美的密河鳄。虽然这两个种的性质、纬度分布以及生态环境等都十分相似;但两者的水平分布的距离却几有地球的半圈。本文根据有关鼍类的进化、迁徙等的历史资料,对此进行了初步的探讨。

SOME PROBLEMS IN EVOLUTION AND DISTRIBUTION OF ALLIGATOR

Most scientists divide the present-day crocodilians into 3 families, 8 genera, and 21 species. They are in general adapted to a semiaqatic life in a warm environment. There is only one genus (Alligator) which lives in the north temperate zone. It is composed of two species. Both are distributed over the same latitudinal areas under the similar ecological environment. However, A. sinensis is in the lower reaches of the Yangtze River in Eastern Asia; while A. mississippiensis in the southeastern U. S. A. The fact may be related to its evolutionary history. Of the Cretaceous alligators the earliest species (Brachychampsa montana) was recovered from the Hell Creek Beds of Montana, North America. The fossil mammals found in the Eastern Asian and North American Cretaceous rocks are surprisingly similar. Their close relationship indicates that migration of land animals between east Asia and western North America was possible in the Cretaceous. The existence of such a connection is also supported by the distribution of Cretaceous dinosaurs, and by the reconstructed location of continents. Frakes (1979) pointed out that the Cretaceous must be recognized as a time of great warmth over the globe. The climate was characterized by extremely equable temperatures with very little variation from day to night, from summer to winter, and from north to south. Paleocene and Eocene epochs apparently experience cool changes. The trans-Bering Bridge acted as a selective filter, for it located at the high latitudes. It is probable that cool conditions may have excluded some forms of animals or, alternatively, appropriate foods to sustain their passage may have been unavailable in these regions. During these epochs, some new alligators appeared in North America, such as Allognathosuchus. In Eastern Asia, Eoalligator chunyii was discovered in Paleocene fossil beds of Nanxiong, Guangdong Province. This species may bear some resemblance with Allognathosuchus, even with the recent species Alligator sinensis. Allognathosuchus was living at the high latitudes, as well as at the low latitudes in Eocene. Some fragmentary materials of Allognathosuchus were found at 79°N in the Ellesmere Island. On the basis of the most recent paleomagnetic data available, McKenna (1980) calculated a paleolatitude of 77.5° N± 6°. So it seems to us that the cool climate at the Bering Bridge would not prevent the alligators from migrating in late Paleocene and early Eocene. At the end of the Eocene, a major climatic deterioration occurred. The cooling continued until Oligocene. Besides, the Bering Bridge was submerged in late Eocene and Oligocene. It is in the Oligocene that Alligator prenasalis and A. visheri made their first appearance in North America. Mook (1923) and Steel (1973) suggested that the probable alligator morphological series begins in the Eocene with Allognathosuchus. In the Oligocene beds of Eastern Asia, any materials of Alligator have not been found yet. This matter may bear much relation to the major climatic deterioration and the submerging of the bridge. From oxygen isotopes, global trend in Miocene paleotemperatures is significant warming. The trend is supported by a broad range of data from other sources. Just as mentioned above, the land bridge was submerged in late Eocene and Oligocene. However, it emerged again in the Miocene. Approximately 60 species of megafossil plants were discovered in the Miocene beds in Alaska. The flora includes Glyptostrobus and Metasequoia. Therefore, the climate in Alaska was very warm during the Miocene time. It is during the Miocene that Alligator made its first appearance in Eastern Asia. In North America, A. thomsoni, A. megrewi, and A. olseni replaced A. prenasalis and A. visheri. It is probable that Alligator crossed Bering Bridge and arrived at Eastern Asia during this warm epoch. Steel (1973) held that the short-snouted A. thomsoni have been antecedent to an unknown species which spread into Asia and became the progenitor of A. sinensis, while the long-snouted A. olseni might have given rise in situ to A. mississippiensis. At the end of the Miocene, another major climatic deterioration occurred. This cooling continued and was capped in the Pleistocene by a period of major glaciation. The cold climate in Bering Bridge became an insuperable barrier for Alligator and other forms adapted to a warm climate. Recently, Alligator cf. sinensis was found in Middle Pleistocene deposits (at about 240, 000—280, 000 years B. P.) in Hexian, Anhui Province. According to Zhou Benxiong(1982), the distribution of A. sinensis by about 6000 years ago had reached northward the Huang-Huai Plain in the area round 36° N, but at present it is limited to the lower reaches of the Yangtze River. In short, it happened that the common progenitors of Alligator Of both Asia and America of present-day, being no migrating between Eastern Asia and North America after the Miocene, might have given rise to two different species in these two continents under isolaring conditions, i. e. A, sinensis and A. mississippiensis. That is the story told by fossil alligators.