Science

摘要

Tetrapods spread Nine genera of the earliest tetrapods (late Devonian, 370-354 million years old) have been described. All of these come from the Euramerican supercontinent (Europe/North America/Greenland), except for one from Australia (Metaxygnathus). Now, however, Zhu and others (Nature, v.420, p.760, 2002) report the discovery of the first Devonian tetrapod from Asia, thereby substantially extending the known geographical distribution of such animals and raising questions about their dispersal. The new specimen, Sinostega pani, comprises an incomplete left mandible about 355 million years old from the non-marine Zhongning Formation of northwestern China (Fig. 1 a) and was found in association with various plants, land-plant spores, antiarch fishes and lobe-finned fishes. During the late Devonian, northern China lay in tropical latitudes adjacent to northeastern Gondwana (Fig. 1b), but Sinostega resembles the Euramerican genus Acanthostega more closely than it resembles Metaxygnathus of Australia. Zhu and his colleagues explain this puzzle by suggesting that tetrapods, which evidently originated during the Frasnian (370-364 million years on the Gradstein-Ogg time scale of 1996), had by the end of Famennian (364-354 million years) achieved a largely global distribution. This suggests, in turn, that the scarcity of tetrapods in northern Gondwana and China is more apparent than real, a sampling artefact that will be eliminated by future discoveries.