Dehydration: A mechanism for the preservation of fine detail in fossilised soft tissue of ancient terrestrial animals

Early mineralization is widely regarded as an important factor in exceptionally preserved soft tissue in the fossil record. However, there is a race against time between soft tissue destruction and the mineralization process. Dehydration of terrestrial animal carcasses may gain valuable time in that race. The decay of collagen, the dominant protein in most animals, is poorly understood and consequently difficult to assess in fossils because of complex changes following the death of an animal. We analysed the ultrastructure of collagen from decomposed dolphin, python and turtle tissue, which had subsequently been air-dried, using scanning and transmission electron microscopy and small angle X-ray scattering. Despite severe exposure of the dolphin tissue to decay and dehydration, many collagen fibres maintained their structural integrity, showing little degradation. Fibril shrinkage as indicated by the characteristic D-banding of collagen was quite small in all the dehydrated tissue studied compared to native tissue. Thus, with respect to the first stage of preservation the structural identity of the tissue is retained. However, the ephemeral benefits of dehydration must be transcribed to long-term geological survival — usually by mineralization. This would require re-hydration of the tissue so that decay by bacteria and release of minerals occur or the tissue is exposed to water-borne minerals that permeate the tissue. Re-hydrating the dehydrated tissue showed minimal structural loss. In nature seasonal dry periods may have a devastating effect on animals often accounting for mass mortalities and unlikelihood of mineralization at the time (either authigenic mineralization or permineralization). However, with the return of the seasonal summer rains and floodwaters often rich in minerals, re-hydration of the dried animal carcasses or parts may present, as we have shown, a viable second chance for mineralization and long term survival of informative tissue structure. Briefly mentioned in comparison is keratin, which has a higher natural propensity for survival compared to collagen.