Evolving preservation and facies distribution of fossil jellyfish: a slowly closing taphonomic window

Although medusoids (medusa-like fossils, putative medusae) are widely reported in the literature, there are only thirteen confirmed medusa-bearing deposits. These have a predictable and evolving distribution through time and across facies - including in coarse clastic sand flats where mouldic preservation predominates, restricted lagoonal facies where anoxia and hypersalinity fostered preservation, and offshore settings where rapid burial and early diagenetic mineralisation catalysed preservation. The medusan preservational window became progressively more restricted through the Phanerozoic. Preservation in sandy beach and sandflat facies is only known from the Cambrian, there are no open shelf medusa-bearing deposits after the Carboniferous, and the latest known lagoonal fossil medusae are from the lower Cenozoic. This restriction in occurrence through time is associated with changes in bioturbation, evolution of scavengers, occurrence of microbial mats, and possibly evolution of microbial communities. There is evidence of jellyfish blooms in five of the thirteen deposits, and medusan fossilisation may have been associated with anoxia caused by the partial decomposition of large masses of dead medusae.