Taphonomy is the study of how organisms enter the fossil record, the processes that occur following death, including tissue decomposition and mineralization, which result in the fossils we find in the rock record. Many groups of organisms show consistent patterns of preservation across a wide range of fossil sites. One example of this is the preservation of internal organs in Arthropods and other similar small invertebrates; preservation of the digestive tract is known in Arthropods from Lagerstätten (exceptional fossil deposits) from the Cambrian onwards, but preservation of other internal tissues is all but unknown, and where it does occur the preservation is usually too poor to allow determination of exactly what tissue has been preserved.
In a paper published in The Proceedings of the Royal Society, SeriesB: Biological Sciences on 13 May 2015, Aodhán Butler of the School of Earth Sciences at the University of Bristol and the Palaeobiology Programme at the Departmentof Earth Sciences at Uppsala University, John Cunningham of the School of Earth Sciences at the University of Bristol and the Department of Palaeobiology and Nordic Centre for Earth Evolution at the Swedish Museum of Natural History, Graham Budd, also of the Palaeobiology Programme at the Department of Earth Sciences at Uppsala University and Philip Donoghue, also of the School of Earth Sciences at the University of Bristol describe the results of a series of experiments with the Brine Shrimp, Artemia, intended to determine how the bodies of small Arthropods decay under different conditions, and how this affects what we find in the fossil record.
Butler et al. found that while different conditions affected the speed at which Artemia remains broke down, the pattern of breakdown was essentially always the same. At the time of death the Shrimps had micro-organisms within their gut, but other regions of their body cavities lacked such microbes. After death cells in many tissues began to break down by autolysis (spontaneous cell breakdown), while micro-organisms began to escape from the hindgut into the body cavity. Eventually the microbes spread through the internal cavity of the Shrimps and into the limbs, turning the body opaque. During this time internal tissues broke down, and bacterial microfilms began to form within the body cavity. After this the cuticle began to shrink and eventually break down, while all tissues present except the basement membrane of the gut were replaced by layers of biofilms. Finally the cuticle failed completely, releasing the microbes into the surrounding medium, though the unsupported remains for some time.
Generalized decay sequence in Artemia. (a) Undecayed specimen. (b) Thoracopods become matted. (c) Body and limbs (arrow) becomes opaque due to microbial activity. (d) Cuticle shrinks; some distal podomeres are disarticulated, cloudy appearance oflimbs indicates internal biofilm.(e) Cuticle fails, internal biofilm is lost. (f) Cuticle disintegrates into fragments. (g) Only the unsupported gut remains. Butler et al. (2015).
In all instances the breakdown of the Shrimp’s body was largely accomplished by autolysis and the spread of internal Bacteria from the gut, with external micro-organisms playing a much more limited role, typically only involved in the breakdown of the cuticle during late decay; in all experiments gut Bacteria had filled the internal cavity prior to the failure of the external cuticle, pre-empting any role for external microbes in the breakdown of the internal tissues.
Butler et al. therefore suggest that the mineralization of tissues in small Arthropods is entirely dependent on the actions of gut microbes rather than environmental Bacteria. In circumstances where these Bacteria begin to mineralize tissues the first tissue likely to be affected is likely to be the gut, where the microbes are present at the time of death, while other tissues tend to break down due to autolysis to a greater or lesser extent prior to invasion by these Bacteria. In circumstances where decay is slowed down by environmental conditions, the failure of the gut is also likely to be delayed, so that tissues still have time to break down.
Opabinia specimen from the Middle Cambrian Burgess Shale of British Columbia (Smithsonian National Museum ofNatural History) showing the gut and associated microbial fabrics. Butler et al. (2015).
Butler et al. further suggest that the evolution of a through gut may have directly influenced the nature of the fossil record. Animals which lack through guts, such as Cnidarians, must periodically evert their guts to empty their contents, preventing the establishment of a permanent gut flora. In small animals with through guts, such as Arthropods, Annelids, Brachiopods etc., preservation of gut tissue is fairly common, with preservation of other tissues, such as muscle or even neural tissue extremely rare but not completely unknown, while in Cnidarians soft tissue is essentially only ever preserved as an external mould. This suggests that the development of a through gut, at or close to the beginning of the Cambrian, may have significantly altered the way in which fossils were preserved from that point onwards.
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