“Fauna cámbrica” versus “fauna paleozoica” en el Ordovícico temprano del Oeste de Argentina: Interacción entre provincialismo y ambiente

Abstract

The limestones of the San Juan Formation (Upper Tremadoc-Lower Llanvirn) in the Precordillera basin bear a group of communities dominated by sessile-suspension feeder organisms, including mainly brachiopods and sponges. The broadly contemporaneous siliciclastic Acoite (Lower-Middle Arenig) Formation in the Northwestern basin yield a succession of trilobite dominated communities. According to the global model of the three evolutionary faunas the former associations fit in the “Palaeozoic Fauna” and the latter could be referred to the “Cambrian Fauna”. Comparative analysis of structure and function of both types of communities, taking in account tiering strategies and the development of different guilds, demonstrates similar values of dominance diversity and richness as well as number of occupied guilds and tiers. Besides, trilobite dominated communities extend along different bathymetric ranges in the shelf environment -from offshore to the shoreface and even are represented in estuarine settings- without significant shifts in higher taxa proportions, showing a pattern of ecological stasis through time. Possible explanations for this pattern are derived from the ecological properties underlain by individualistic community histories, governed by the paleogeographic position of both areas during the Arenig times. We conclude that the three evolutionary faunas concept could be applied to equatorial and subequatorial areas. Cold water Gondwanan and peri-Gondwanan associations could be included in a new model of evolutionary faunas. For this purpose further tests in other Ordovician Gondwanan communities would be necessary.