Marcelo G. Carrera


"The sponge fauna is one of the most characteristic features in the Ordovician limestones of the San Juan Formation. From a palaeoecological point of view the sponges represent the dominant group in those associations where they are present. Until now sponges were known to occur up to the top of the San Juan Formation (Early Llanvirnian). Recently collected material from a Late Llanvirnian limestones in Las Chacritas and Las Tunas sections, extends the record of the group in Precordillera and allows to analyze the palaeoecological significance of the sponge association. The association is mainly composed of two species of the genus Archaeoscyphia: A. pulchra Bassler and A. minganensis Billings reaching up to 76 % of the sponge biovolume. Other less represented taxa are: Rhopalocoelia clarkii Raymond and Okulitch, Hudsonospongia sp., Calycocoelia sp., ""root-tufts"" and
domical to discoidal sponges. The prevalence of elongated and very tall individuals of the genus Archaeoscyphia suggests low energy conditions. However, the presence of some broken sponge bases in life position and postmortem fragmentation, implies that sudden high energy events strongly affected the sponge community. The alternation of sponge-bearing wackestones and intrabioc1astic grainstones reinforces the idea that these are periodic storrn-related levels. The genus Archaeoscyphia shows the pattern of opportunistic laxa predominating in areas where physical or extrinsic factors prevail over the biotic or intrinsic ones. The recurrence of almost the same faunal association implies that the substrate was occupied by the surviving organisms or by organisms from adjacent areas. The Late Llanvirnian sponge association grew in a shallow subtidal environrnent below the fair-weather wave base and which was periodically affected by the storm wave base. This relatively shallow environment contrasts with the contemporaneous pelitic dominated lithologies to the west and to the north of the study area."

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