GLASS RAMPS AND CERAMIC SPONGES: SCULPTURALLY RECONSTRUCTING TAPHONOMIC PATTERNS OF PERMIAN DEMOSPONGES IN THE PHOSPHORIA ROCK COMPLEX, IDAHO

The Permian Phosphoria Rock Complex (PRC) of Idaho is a widespread sedimentary deposit that accumulated in a shallow sea along the western margin of Pangea. The Rex Chert Member of the PRC records an interval of atypical ocean conditions that favored biosiliceous sedimentation, a glass ramp populated by sponge meadows and starved of siliciclastics. The Rex Chert is up to 30 m thick, and is composed of the disaggregated spicules of siliceous demosponges. Few intact specimens of PRC sponges are preserved, presenting challenges in reconstructing their forms in life.

This study utilizes an interdisciplinary approach to model assemblages and morphologies of PRC demosponges, combining limited fossil data with modern demosponge data to inform artistic fabrication of models of whole PRC sponges. Complete PRC sponge fossils are mostly exposed in two dimensions on bedding planes, due to pervasive silicification of the sponges and surrounding matrix. Based on field data and samples, PRC sponge fossils were 1.5-8 cm in diameter, typically ovoid in shape, and had body wall thicknesses of 0.2-4 cm. Thin sections of the varied spiculites reveal densely packed monaxon spicules (commonly up to 0.3 mm in length) that are horizontally oriented to chaotic (disturbed by bioturbation), and associated with and infilled by glauconite, phosphate, and iron oxides.

The silicified preservational style necessitates a conceptual reversal of the taphonomic processes that fossilized these sponges to reconstruct the paleoenvironments from their remains. Terra cotta clay has a natural smoothness and plasticity, allowing clay to be sculpted from the interior, creating volume within the form. Using a lowfire body of clay permits a wider range of colors and textures of glazes. The glaze application strategies and layerings in this study were based on test tiles, resulting in a realistic suite of reflective and absorbent properties similar to living sponge textures.

Fabrication of sponge sculptures is key to reconstructing this non-analogue paleoenvironment, including filling in features that are lost to time, such as color. The silicified matrix and sponge fossils within obscures many morphological details; however, combining macro- and microscale taphonomic data with experimentation of form allows for the development of a more complete model.