EARLY ORDOVICIAN REEFS FROM ARGENTINA - STROMATOPOROID VS STROMATOLITE ORIGIN

Late Arenigian biohermal reef mounds and biostromes within the shallow -marine platform facies of the upper San Juan Formation of the Precord illera (Western Argentina) represent a new Early Ordovician reef type. The meter-sized reefs are dominated by Zondarella communis n.g. n. sp . The new taxon is characterized by domical, bulbous and laminar morph otypes exhibiting growth layers and thin horizontal and vertical as we ll as intermingled skeletal elements included within different sets. T he fossil may be compared with stromatolites and stromatoporoids but a n interpretation as primitive stromatoporoids is favoured.