MACROBORERS IN A LOWER DANIAN CORAL REEF OF NORTHERN PATAGONIA, ARGENTINA

Records of hermatypic coral reefs after the Cretaceous-Paleogene boundary are very scarce and their evolution during this seminal time interval is poorly understood. Likewise studies of the sclerobiotic communities associated with coral reefs of this age are sparse and mostly comprise references to the presence of bioerosion. Therefore, the finding in Patagonia, southern Argentina of a lower Danian coral reef is particularly significant. The reef is formed primarily by Siderastrea adkinsi (Wells, 1934) with massive-foliose or encrusting colonies ranging between a few centimeters to over 1 m in diameter. Other members of the coral reef community are Haimesastraea conferta Vaughan, 1900, sponges and coralline algae, all playing important roles in the reef framework. Facies analysis suggests that the reef grew in a highly wave-agitated environment of 0-10 m depth. The Patagonian reef opens a temporal window allowing the study of the sclerobiotic community associated with coral reefs. From three reef sites 24 in-situ colonies of Siderastrea adkinsi were collected. One or two parallel slabbed sections were cut from each colony, with the cut surfaces then traced to record the position and the outline of each borer. A total of 32 slabbed sections were scanned and mapped using Arcview 3.1 to determine: (1) the percentage of each sample affected by boring; and (2) the relative percentage abundance of each macroborer at different sites. Macroboring organisms, which include sponges, bivalves and polychaetes, are ubiquitous within the coral reef framework and their relative abundances show similar rates. No distributional trend across the reef was detected. Results show a lower biodiversity of boring organisms and a lower rate of bioerosion than in Recent reefs.