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The
presence in the fossil record of an articulated crinoid is very rare. This
is because of its morphology. The numerous calcareous plates of the crinoid
are held together by soft tissues, such as muscles and ligaments, which decay
within a few days of death of the creatures (Liddell, 1975). To allow the
preservation of the plates as they would be in life mode, burial must be rapid
and deep enough to prevent disruption by scavengers, burrowers or currents.
Tempestites and turbidites (i.e. obrution deposits) are often mechanisms for
rapid burial and have proved in the fossil record to have aided in the preservation
of a number of whole crinoid skeletons.
Stereom is easily transformed into solid calcite during fossilisation. It may be may masked during diagenesis by overgrowing of secondary calcite which becomes interlocked with the primary calcite.
Crinoids
are common throughout much of the fossil record, since their first appearence
in the middle Cambrian and were especially important rock-building constituents
during the Palaeozoic (Ausich, 1997). They are most commonly found in rocks
of Palaeozoic age, with decreasing importance through the Mesozoic and Cenozoic.
The main reason for their greater abundance in Palaeozoic rocks is probably
because the numbers of crinoids themselves at that time was much greater than
today. They were also more common in shallower waters (modern forms are generally
confined to deep water assemblages), leading to a greater likelihood of preservation.
Crinoids are not generally recognised as index fossils. A few crinoids have distinct columnals which allow them to be identified to the family level. However, dislocated brachials in most crinoids are unidentifiable, although some arbitrary schemes have been set up to identify dislocated ossicles on the basis of their morphology.
Where
articulated crinoids do occur, they tend to be confined to particular bedding
planes or localised lenses. The Posidonia Shale or similar deposits also contain
articulated crinoids. In these cases, the crinoids were planktonic and floated
in the upper water layers over a deep anoxic basin. Once they died and sank
to the bottom, the anoxic conditions prevented disarticulation (a result of
lack of scavenging, reworking and decomposition).
Other occurences of articulated skeletons may include those of obrution deposits or cases where species of crinoids evolved which more rigidly sutured calyces, for example the camerate crinoids of the Silurian Reefs of Gotland.
The explanation for the large number of articulated crinoids in the Middle Ordivician Trenton Group (amongst others, e.g. Middle Devonian Windom Shale), is that during a period of non-deposition, crinoids colonised the sea floor. A pulse of muddy sediment, possibly the result of a storm deposit, flushed into the area and killed the benthos by burial. However, because of the fine grained nature of the sediment, little disarticulation occurred.
A result of disarticulation of crinoid skeletons is that the ossicles become sedimentary particles and are incorporated into the natural sorting system of ocean currents. Consequently, under the right conditions, the ossicles can become concentrated enough to form a major proportion of the composition of a sedimentary rock.
Crinoidal
limestones vary in size and shape, from relatively thin localised deposits
to thick laterally extensive beds, possibly with statigraphic thicknesses
up to many metres. The larger units are particularly common during the Palaeozoic,
but have decreasing importance through the Mesozoic and no regional crinoidal
limestones are known from the Cenozoic.
An example of a regionally extensive encrinite is the Burlington Limestone of the Mississippi Valley. According to a paper by Macurda and Meyer (1983), the average cubic metre of crinoidal limestone may contain the remains of 15,000 crinoids. In most argillaceous limestones, 20-30% of the total rock volume is thought to be crinoidal debris. However, due to aforementioned likelihood of dissarticulation, very few skeletal remains can be attributed to species or even family level.
Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2003-4