ИСТИНА

It is well known that jelly organisms decay faster than other multicellular organisms. Nevertheless, medusoid fossils have been found in Lagerstätten, from the early Cambrian or even to the late Ediacaran (Grazhdankin, 2016). Clearly, the fossilization of jelly organisms requires an exceptionally effective mechanism of conservation and/or molding. The hypotheses based on actualistic experiments and paleoreconstructions have suggested that jellyfish fossils might be relief moulds, filled with sediment throughout or after the decay of dead jellyfish. However, the formation of certain jellyfish fossils (for example, those from the middle Cambrian layers of the Marjum Formation, Utah, or jellyfish in siderite concretions from the Carboniferous Mazon Creek) cannot be interpreted in this way. In our taphonomic experiments on the burying of medusae (ephyrae of Aurelia aurita), we demonstrated that in clay sediment, (kaolinite) the medusae carcasses preserved significantly longer than in the sediment-free seawater. In sea water, the ephyrae completely dissolved within 6-7 days, whereas in the clay sediment the ephyrae remained morphologically recognizable after 30 days. They retained their radial symmetry, 8 contracted lappets and a manumbrium. Interestingly, the micro-globular structure of the ectoderm can also be seen on the exhumed specimens. The ephyrae remains dramatically decreased in size: they became 2-3 times smaller compared with the original size. The postmortem shrinkage has been noted in previous experiments on jellyfish burials in sand sediment and plaster (Norris, 1989), and also confirmed in our auxiliary tests. This shrinkage affected not only the overall size of the body, but also the microstructure of the ectoderm - the constituent micro-globules of the ectoderm decreased in size by a factor of 3. This observation indicates that a decrease in size is characteristic for decaying medusa carcasses in both water and sediment, and not only through desiccation of the carcasses during subaerial exposure. In the unconsolidated clay sediment, the ephyrae were preserved as relief casts. The clay particles adhered so tightly to the surface, that five repeats of rinsing did not destroy these casts. Inside the casts, the organic remains were visibly preserved. By this mode of preservation, there is the possibility of both relief moulds and organic compressions forming. It is therefore confirmed that not only the recalcitrant chitinous tissues of arthropods, but also the readily decaying jellyfish persisted substantially longer in the clay sediment, increasing the taphonomic time frame to generate a fossil. In the clay sediment, robust relief casts of jelly organisms can be formed that preserve both the gross topology of the body and the microstructure of the ectoderm. The postmortem allometric contraction of the carcass can produce characteristic concentric signs in the sediment during any type of burial. This study gives perspective to the meaningful interpretation of problematic cnidarian remains.