Exceptional preservation of the sponge Fissispongia tortacloaca from the Pennsylvanian Holder Formation, New Mexico

Exceptionally preserved aragonitic demosponge material occurs within the Virgilian Holder Formation, Sacramento Mountains, south-central, New Mexico Fissispongia tortacloaca (King) is identified, a species previously known only from completely calcified specimens. Standard optical microscopy and SEM examination of broken surfaces of the F. tortacloaca skeleton distinguishes between primary spherulites and a later epitaxial overgrowth. F. tortacloaca biomineralization, is readily comparable to that of the modern. agelasid demosponge Astrosclera willeyana, thus confirming the demosponge affinity of this form. Cathodoluminescence microscopy and back scatter SEM examination, identifies selective silicification and calcitization, of the spherulite centers. These diagenetic minerals affect the initial spherulite biomineralization mimicking the original microstructural fabric. Elemental composition, off. tortacloaca (Sr = 7600 ppm, S = 1640 ppm and Mg = 150 ppm) is close to that of modern A, willeyana (Sr = 10,000 ppm, S = 1780 ppm and Mg = 320 ppm). Sr concentration in. F. tortacloaca indicates that Late Pennsylvanian seawater had slightly Less Sr (7 ppm) than modern seawater S is interpreted as having been incorporated into sponge aragonite as SO42-. The stable isotope composition off. tortacloaca (delta(13)C = +5.5%; delta(18)O = -2.1 parts per thousand) is compatible with other Virgilian skeletal carbonates from the USA, but is about 1 parts per thousand 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. off. 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 Pennsylvanian. ocean had a delta(18)O composition similar to modern seawater with an 1.2 parts per thousand variation, due to glacial ice volume effects.