Name: Orsten
Location: Sweden
Age: Upper Cambrian
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Taphonomy


Taphonomy describes how the fossil is buried and preserved. When the Orsten organisms died, they slid to the anoxic seafloor. The fossils are not flattened or distorted as the nodules may have been formed by the replacement of pore waters with calcium carbonate, which prevented great compaction (Müller, 1990).

The fossils are phosphatised, giving 3D preservation of original tissue. The mechanism of phosphatisation has not yet been determined, but it is unlikely to have occurred in the open sea. Many of the fossils would have been phosphatised prior to decay and as fine structures survived the animals are assumed to have been buried alive or immediately after death, otherwise delicate soft integument would not be present (Müller, 1985). There was fine preservation of minor details due to secondary phosphatisation of the body wall, which in most cases was chitinous. This may explain the restriction of preservation to arthropods or arthropod-like organisms (Müller, 1990).

The phosphatic fossils fall into two distinct groups (Tang, 2001):

  1. Organisms that originally had primarily phosphatic hard parts (eg inarticulate brachiopods).
  2. Arthropods possessing cuticles that were replaced with phosphate or secondarily coated with fine to fairly coarse grained phosphatic matter.

Only fragments less than 2 mm in dimension were secondarily phosphatised, so this type of preservation is limited to small growth stages and isolated appendages. The best preservation was of organisms of approximately 100-500 micrometres in size (Müller, 1990).

Replacement generally produces better preservation of details than does coating of the cuticle. The fossils are generally preserved as hollow moulds, though some are solid. It appears that arthropod cuticles are suitable catalysts for precipitation of phosphates. Preservation is variable, probably because of differences in organic content and/or physical weathering (Müller, 1990).

The source of the phosphate is unknown. One hypothesis is that it comes from weathering of granite rocks. Another is that the sediment deposited below the flocculent layer provides the significant amounts of phosphate needed (Tang, 2001).

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