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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.

Picture of exceptionally preserved brittle stars from the Lias in Dorset               Exceptionally preserved brittle stars from the Lias of Dorset
   

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.


Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2006-7