A chemical framework for the preservation of fossil vertebrate cells and soft tissues

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Reports of preserved cells and other soft tissues in ancient vertebrates, including dinosaurs, have been met with controversy within the field of vertebrate palaeontology. To explain such reports, Schweitzer et al. (2014) hypothesized that iron-mediated radical crosslinking preserves ancient soft tissues in a manner somewhat analogous to histological tissue fixation. In 2018, Wiemann et al. proposed a second hypothesis that these soft tissues were preserved as advanced glycation/lipoxidation end products (AGEs/ALEs). The chemistry underlying these hypotheses, however, remains poorly described for fossil vertebrates. This review posits a chemical framework describing the persistence of biological “soft” tissues into deep time. The prior iron-mediated radical crosslinking and AGE/ALE mechanisms are re-described in context of established chemistry from a diversity of scientific fields. Significantly, this framework demonstrates the hypotheses presented by Schweitzer et al. (2014) and Wiemann et al. (2018) are, in many cases, subsequent steps of a single, unified reaction mechanism, and not separate hypotheses. Knowledge of the chemical mechanisms underlying vertebrate soft tissue preservation has direct implications for molecular archaeology and palaeontology, including efforts at molecular sequence recovery within the ancient DNA and palaeoproteomic communities. Such implications that are immediately apparent from examining the chemical framework are discussed.