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Anisian (Middle Triassic) ammonoids from North America: quantitative biochronology and biodiversity

2005; Volume: 2; Issue: 4 Linguagem: Inglês

10.29041/strat.02.4.02

1547-139X

Claude Monnet, Hugo Bucher,

Geology and Paleoclimatology Research

This study synthesizes and revises the ammonoid zonations as well as their correlationwith each other for westernNevada (USA), British Columbia (Canada), and the Sverdrup Basin (Canada) by utilizing the unitary association method. Based on a standardized taxonomy, the Anisian in the studied areas contains 13, 10, and 3 zones and a total of 174, 90, and 7 species, respectively. The zonations are correlated bymeans of a ‘common taxa zonation’, which includes all taxa common to the studied basins. This leads to new andmore precise correlations,which are at slight variancewith those of the literature.Hence, the Buddhaites hagei Zone (Canada) correlates only with the Intornites mctaggarti Subzone (Nevada) and not with the entire Acrochordiceras hyatti Zone (Nevada). The Tetsaoceras hayesi Zone (Canada) appears to correlate with the Unionvillites hadleyi Subzone (Nevada) of the hyatti Zone and not with the Nevadisculites taylori Zone. The Hollandites minor Zone (Canada) more than likely correlates with the taylori Zone (Nevada) rather than the Balatonites shoshonensis Zone as is usually acknowledged. The unitary association method enables us to quantify the diachronism of the studied taxa,which affects about 67% of the genera and 18% of the species common toNevada and British Columbia. Therefore, this diachronism is significant and its value for correlation should not be overlooked. Finally, a diversity analysis based on the revised zonations is performed. This analysis reveals that the major event occurred during the Nevada hadleyi Subzone (early Middle Anisian), which in Nevada and British Columbia, records the highest species richness of the Anisian as well as enhanced exchanges between usually latitudinally restricted faunas. This event may reflect significant changes in climate or oceanic circulation at that time.