| Large densely-packed cylindrical chert concretions were analyzed in detail in the Rex and Tosi Chert Members of the Phosphoria Formation in the Gros Ventre and Teton Ranges, Wyoming. Analysis of outcrops, hand samples, and thin sections leads to a new interpretation of the cylindrical chert concretions as body fossils of siliceous sponges preserved in life position, rather than large burrows. These are the first sponge body fossils reported in large numbers in the Phosphoria Basin, and record the glass factory of an extensive glass ramp depositional system. In situ production of siliceous sponge spicules outpaced and buried carbonate producers, and accumulated to form the thick, spiculitic Rex and Tosi Chert Members in a shallow-water lower ramp environment. The thick bedded cherts of the Phosphoria Formation represent the southernmost extent of the Permian spiculite belt, one of the largest accumulations of chert in earth history, which records the Permian Chert Event. Keywords: Phosphoria Formation, Permian, sponges, chert, spiculite, glass ramp, Permian spiculite belt, Permian Chert Event |