Ophiuroidea Fossil Record
The brittle stars (Ophiurida) are first seen in the Ordovician (500 million years ago) and are still extant today, as are the baskets stars (Euryalae) who have been seen in the fossil record from the Carboniferous (350 million years ago) to present day. They have passed though every extinction event relatively untouched since they evolved.
The calcareous skeleton of the brittle stars may
allow preservation but overall the fossilisation potential of brittle
stars and basket stars is very poor, due to their brittle nature.
During times of stress they drop their arms which lessens their
potential of being preserved in the fossil record. The arms are also
first to be lost if they are decomposing on the sea bed as the
connective muscle from the arms to the disc breaks down easily. It is
difficult to quantify the abundance of ophiuroids in the past, they are
not well represented, due to their poor preservation potential.
Articulated ophiuroids are generally only found in
exceptionally preserved beds. These are event beds where an influx of
sediment has caused a covering of the entire bed. Modern species cannot escape silty sediment any thicker than 5 cm.
A large influx of sediment causes smothering and rapid burial in an
anoxic environment. As a result these specimens are seen in their life
positions. An excellent example of this is in the Lias of Dorset, it
preserves an entire bed of brittle stars all in their life positions.
They can be useful in reconstructing palaeoenvironments, all specimens are found in marine sediments. They
are only found in areas of normal salinity and generally thrive in warm
tropical waters. If they are found with their arms attached they may
show palaeocurrent direction as the arms will be dragged in the
direction the water was moving.
Author: Lorna O'Brien
Last updated: 19/11/2006
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The two images above are taken from www.charmouthfossils.co.uk/catalogue_stolen/fossils_stolen.htm with permission from the copyright holder.
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