A dark stain is often associated with

fossils of the Burgess Shale The fossils of the Burgess Shale, like the Burgess Shale itself, formed around 505 million years ago in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 65,000 specimens in a seri ...

, representing decay fluids that were squashed out of the organism during the taphonomic process.

Occurrence

It is most famously known in (and most commonly associated with) ''

Marrella ''Marrella'' is an extinct genus of marrellomorph arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale, with tens of thousands of specimens collected. Much rar ...

'', and is also known in other organisms: for example, ''

Aysheaia ''Aysheaia'' is an extinct genus of soft-bodied lobopod, known from the middle Cambrian of North America, with an average body length of 1–6 cm. Anatomy ''Aysheaia'' has ten body segments, each of which has a pair of spiked, annulate le ...

'', where it protrudes from the anterior (and in one case posterior) end of the animal; ''

Hallucigenia ''Hallucigenia'' is a genus of Cambrian animal resembling worms, known from articulated fossils in Burgess Shale-type deposits in Canada and China, and from isolated spines around the world. The generic name reflects the type species' unusual ...

'', ''

Naraoia ''Naraoia'' is a genus of small to average size (about 2-4½ cm long) marine arthropods within the family Naraoiidae, that lived from the early Cambrian to the late Silurian period. The species are characterized by a large alimentary system and s ...

'', and ''

Alalcomeneaus ''Alalcomenaeus'' is one of the most widespread and longest-surviving arthropod genera of the Early and Middle Cambrian. Known from over 300 specimens in the Burgess Shale and the Chengjiang biota. it is a member of the group Megacheira. Morph ...

''. It is also present in other

Lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

n, for example in

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

crustacea from Ireland.

Formation

The formation of this stain is the subject of ongoing research. It was originally speculated that it represented body contents forced out by the pressure of overlying rocks, although decay contents would form more quickly and thus be a more likely interpretation. This is supported by their presence around organisms whose external wall apparently ruptured; dark stains are also found in Carboniferous fish, and oily liquids have been seen to ooze out of the mouths and anuses of decaying seals. A high abundance of elemental copper has been taken to suggest that it represents "blood" (i.e. the

hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

of arthropods and molluscs). Although copper was not analyzed in the canonical work on Elemental Mapping of Burgess Shale organisms, the dark stain was higher in silicon and lower in aluminum and potassium than elsewhere.

References

{{reflist Burgess Shale