Exceptional Preservation of Fossil Soft Tissues

Abstract

Exceptionally preserved soft tissues yield much more information about ancient organisms than skeletal remains alone. These fossils are rare, because, generally, the postmortem processes of decay rapidly strip away information from a carcass before preservation can occur. However, if certain environmental conditions are present that slow the normal recycling of organic material, there is a possibility that soft tissues can become geologically stabilised by the processes of mineralisation or maturation. In order to understand the biological, environmental, and geological factors that control soft tissue preservation, palaeobiologists combine the study of fossil material with laboratory experiments that decay organisms in strictly controlled environmental conditions. These experimental investigations have enhanced our understanding of the inherent biases of the fossil record, and allow more accurate reconstructions of ancient organisms and a better understanding of evolution and ecology. New techniques and cutting‐edge analytical equipment are revolutionising the field, opening up exciting new avenues of palaeontological research such as the identification of fossilised pigments and palaeoproteins. Key Concepts Fossils that have soft tissues generally provide more information about ancient organisms than skeletal remains alone. Organisms undergo varying amounts of decay prior to preservation. Conservation‐Lagerstätten are sites where depositional environmental conditions disrupted the normal rapid recycling of organic material (e.g. decay) allowing soft tissues to preserve. Conservation‐Lagerstätten are important not only because they preserve soft‐bodied organisms that are extremely rare in the fossil record but also because they tend to preserve a great diversity of organisms, making them vital windows into the ancient past. Many sedimentary Conservation‐Lagerstätten share characteristics of rapid burial, low oxygen, and the early onset of diagenetic processes such as mineralisation and/or maturation. Another type of Conservation‐Lagerstätten are natural traps, such as amber or permafrost, but comparatively, these are geologically young. The study of burial and preservation is termed taphonomy. Since the 1980s, laboratory experiments have been undertaken to understand the drivers of soft‐tissue preservation. Rather than recreating fossils or ancient environments in the lab, robust taphonomic experiments investigate the effect of discrete variables on taphonomic processes. New avenues in taphonomic investigation are opening up with the advent of new technology and laboratory equipment.

The relationship between microbial community succession, decay, and anatomical character loss in non-biomineralised animals

“…Inocula are not required to initiate decay (e.g. Kear 1993a, Sansom et al 2010;Clements et al 2017Clements et al , 2021, so the addition of a natural inoculum to an experiment investigating anatomical decay contributes nothing more than an uncontrolled and potentially confounding variable. Without detailed microbial community analysis, the use of such inocula creates significant reproducibility issues; especially as bacteria are only a part of any natural inoculum taken from the wild: Fungi and Archaea may also be present.…”

“…Purnell et al 2018;. It is also important to note here that while some organic tissues may be geologically stabilised, all other tissues that are not stabilised by a preservational process will continue to decay until obliteration, including biomineralised tissues (Clements and Gabbott, 2021;Gabbott et al 2021;see Purnell et al 2018 for review).…”

“…Eventually, if gas cannot escape the gut can rupture, often with enough force to breach the integrity of the carcass (e.g. Metcalf et al 2013;Clements et al 2021;Aragonés et al 2022). When this happens, exogenous bacteria can rapidly colonise and replace endogenous populations (e.g.…”

“…It is a common misconception that environmental factors such as low temperature, high salinity, low oxygen content etc. stop decay (Clements and Gabbott, 2021). In fact, these factors only slow the rate of decay through modulation of bacterial activity (Briggs and Kear 1993a;Briggs 2003;Muscente et al 2023).…”

“…Bacterially induced mineralisation reflects a delicate balance between competing processes. While bacterial metabolism is required to generate the geochemical conditions that allow mineralisation to occur, too much metabolism will obliterate soft tissues, removing the anatomical information that defines exceptional preservation of non-biominealised remains (Clements and Gabbott, 2021). Clearly, as soft tissue remains in the fossil record are so rare, minute variations in any of these variables may prevent soft tissue preservation from occurring.…”