EXCEPTIONAL PRESERVATION OF THE SPONGE FISSISPONGIA-TORTACLOACA FROM THE PENNSYLVANIAN HOLDER FORMATION, NEW-MEXICO

Exceptionally preserved aragonitic demosponge material occurs within t he Virgilian Holder Formation, Sacramento Mountains, south-central, Ne w Mexico Fissispongia tortacloaca (King) is identified, a species prev iously known only from completely calcified specimens. Standard optica l microscopy and SEM examination of broken surfaces of the F. tortaclo aca skeleton distinguishes between primary spherulites and a later epi taxial overgrowth. F. tortacloaca biomineralization, is readily compar able to that of the modern. agelasid demosponge Astrosclera willeyana, thus confirming the demosponge affinity of this form. Cathodoluminesc ence microscopy and back scatter SEM examination, identifies selective silicification and calcitization, of the spherulite centers. These di agenetic minerals affect the initial spherulite biomineralization mimi cking the original microstructural fabric. Elemental composition, off. tortacloaca (Sr = 7600 ppm, S = 1640 ppm and Mg = 150 ppm) is close t o that of modern A, willeyana (Sr = 10,000 ppm, S = 1780 ppm and Mg = 320 ppm). Sr concentration in. F. tortacloaca indicates that Late Penn sylvanian seawater had slightly Less Sr (7 ppm) than modern seawater S is interpreted as having been incorporated into sponge aragonite as S O42-. The stable isotope composition off. tortacloaca (delta(13)C = +5 .5%; delta(18)O = -2.1 parts per thousand) is compatible with other Vi rgilian skeletal carbonates from the USA, but is about 1 parts per tho usand heavier than A, willeyana (delta(13)C = +4.4 parts per thousand; delta(18)O = -0.8 parts per thousand). The delta(18)O composition. of f. tortacloaca indicates tropical to subtropical temperatures between 24-30 degrees C using Grossman and Ku's expression T degrees C = 20.6 - 4.34 (delta(18)O(A) - partial derivative(w)): assuming the Pennsylva nian. ocean had a delta(18)O composition similar to modern seawater wi th an 1.2 parts per thousand variation, due to glacial ice volume effe cts.