PATTERNS OF SPICULE PRESERVATION IN CAMBRIAN SPONGES

The gap between potential sponge origin and their spicule preservation in the fossil record has been a longstanding enigma. While molecular clock studies and demosponge specific biomarkers suggest Porifera diverged over one hundred million years before the Cambrian period, solid fossil evidence of sponge spicules did not appear until the latest Ediacaran and early Cambrian. The spicule gap, which refers to the discrepancy between the inferred presence of sponges and their first fossils, has been the subject of numerous hypotheses. The most widely accepted posits that sponges evolved in the Precambrian, but unfavorable environmental conditions impeded the preservation of spicules, leading to a scarcity of fossil evidence. Patterns of body-fossil associated spicule preservation in Cambrian sponges reveal possible impact of environmental factors, especially temperature. Progressive increase in relative frequency of original silica preserved in sponge body fossils throughout the Cambrian period indicates a slight tendency for increased silica preservation when temperatures are coolest. Other environmental factors such as depositional environments and living habitats have impacted sponge fossil occurrences, but not silica preservation. Likewise, the preservation of isolated spicules tends to be higher during intervals inferred to have lower temperatures. Since the Ediacaran is inferred to be on the whole cooler than the intervals with good spicule preservation, we can infer that if sponges had evolved spicules prior to or during the Ediacaran, they would be preserved. Because of the observed lack of spicules during all but the latest Ediacaran, the most likely explanation is that siliceous spicules evolved convergently in demosponges and hexactinellids when they appeared in the fossil record. This conclusion is also supported by differences in the genomic pathways that demosponges and hexactinellids use to produce spicules.