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.
See also…
The Crato Formation outcrops on the northern flanks of the Chapada
do Araripe, a plateaux on the border between Ceará, Pernambuco andPiauí States
in northern Brazil. In is noted for its exceptionally well preserved fossils,
which include Dinosaurs...
The Arthropods are one of the most abundant and diverse groups of...
Trilobites were a group of Arthropods
that flourished throughout the Palaeozoic, but died out at the end of
the Permian. They are abundant and well studied fossils, but little is
known of their internal anatomies.
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