Abstract

We present the first record of siliceous hexactinellid and demosponge (?) spicules from lower Tommotian chert and phosphorite of the Lower Tal Formation of the Mussoorie and Garhwal synclines in the Lesser Himalaya. Spicules include cross-shaped as well as single-rayed forms, and occur sporadically in a chert or phosphatic groundmass. Scanty records of siliceous sponge spicules in the Proterozoic sedimentary rocks and their abundance and rapid proliferation during the Early Cambrian are interpreted as a result of major change in seawater chemistry due to enhanced availability of nutrients, leading to progressive evolution of siliceous biological forms and phosphatic shelly faunas.

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