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Sinemurian ammonite succession from the Sierra Madre Oriental (Mexico)

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Blau J. , Meister C. , Dommergues J.-L. , Schlatter R. , Schmidt-Effing R.

tom Vol. 4, no. 1

147-148

In a series of studies (Blau et al. 2001, 2002, 2003; Meister et al. 2002, 2005) dedicated to Liassic (Sinemurian) ammonites of the Sierra Madre Oriental (Mexico), we have described 42 taxa, amongst them one new genus and five new species. After a critical analysis of Erben’s collection and with our own collections, we propose a synthetic succession of 13 correlable units “biohorizons”. Even though the Sinemurian sediments of the Huayacocotla Basin are very thick only two periods are well represented: the Bucklandi and Semicostatum zones for the Lower Sinemurian and the upper Obtusum and Raricostatum zones for the Upper Sinemurian. These two time intervals can be well correlated with South and North American biostratigraphic schemes. The Lower Sinemurian is characterized by the acme of Arnioceras a genus well distributed on the eastern Panthalassian rim. The Upper Sinemurian allows a much more refined biostratigraphic subdivision and more acute correlations. In the upper Obtusum Zone Euerbenites is an index fossil and can be correlated from Sonora to Peru. In the Raricostatum Zone several horizons allow acute correlation with South America (O. incaguasiense and P. tardecrescens horizons) and with North America (P. harbledownense and P. rothpletzi horizons). Mainly in the Upper Sinemurian there is quite strong endemism of the ammonite fauna in the Huayacocotla Basin (nearly 40%) which may be due to a quite isolated palaeogeographical position of the basin. On the other hand the affinities to Tethyan faunas are obvious. There is little palaeontologic evidence for a Sinemurian connection between Eastern Pacific (Panthalassa) and Western Tethys through the Hispanic Corridor. First palaeontological evidence indicates a possible connection not before the Pliensbachian. If this connection was not established in the Sinemurian, we must imagine other migration ways to explain the Tethyan affinities. There are at least four possibilities or combinations of migration routes: southern peri-Pangean, peri-Asiatic, latudinal trans-Panthalassian, and Boreal (Viking Corridor, Arctic seas). All these migration routes remain hypothetical and the establishment of the Hispanic Corridor already in the Sinemurian cannot be ruled out completely.