Molecular paleontology of Cenozoic: claims of non-mineralized soft tissues in mammals found in sedimentary deposits little favorable to preservation

Evidence of non-mineralized remains in extinct mammal fossils from different depositional environments has been documented in specialized journals. However, the data that have already been systematized are dispersed in the literature, associating the idea of the discoveries with isolated phenomena. Thus, the aim of this study was to review the literature published in the last six decades, in order to understand the frequency of non-mineralized soft tissue findings in extinct mammalian fossils from cenozoic sedimentary deposits. Therefore, there was a systematic review of the articles that address the descriptors “fossil mammals”, “soft tissues”, “exceptional preservation”, “ancient protein” and “ancient DNA”; in Portuguese and English. The search, with the terms together and, soon after, separated, was carried out in peer-reviewed journals of the platforms PubMed, SciELO and CAPES, in the time interval between 1965 to 2020. In the absence of findings with the theme, it expanded the search spectrum for academic Google. Exclusion criteria were used: articles that identified only cases of mineralized soft tissue (total replacement with autogenous minerals); articles that identified original tissues of hard parts (e.g., endogenous bone minerals); articles that identified non-mineralized soft tissue in non-cenozoic mammals; as well as those articles that, although they identified non-mineralized soft tissues in mammal samples, were from depositional environments more favorable to taphonomic preservation (permafrost, cave, amber and tar pit). The results identified 22 articles describing organic material for extinct representatives of Perissodactyla, Carnivora, Artiodactyla, Primates, Proboscidea, Sirenia, Cetacea, Pilosa, Pholidota, Rodentia and the extinct Notoungulata and Liptoterna orders. Of these, collagen proteins (34.4%) were the most frequent findings, followed by osteocytes (25%) and blood vessels (12.5%). The most frequently used analytical techniques for biochemical detection were chemical based (35%), followed by biological (31%) and optical (28%), ahead of detections that did not use such analytical methods (6%). We identified a predominance of articles that do not report the type of depositional environment of the findings (54%), followed by the river and wind environments (9% each). In general, the findings are comprehensive and with wide geographical distribution (except continents Oceania and Antarctica).