Newly-found fossil shows gills and legs
16 March 2010, by Sara Coelho
Meet Nasunaris flata, the 425 million year old ostracod from Herefordshire. Ostracods are microscopic crustaceans that resemble tiny shrimps covered by a mussel-like hinged shell. Nasunaris is only the third ostracod fossil found with a complete set of soft parts preserved inside the shell.
Modern freshwater Cypris ostracod
Modern ostracods, sometimes known as seed-shrimps, live in all kinds of aquatic environments, from ponds and lakes to the bottom of the sea. They first appeared in the beginning of the Ordovician era, about 500 million years ago. Their tiny shells, normally only 1 to 3mm long, are very common in the fossil record.
Ostracod fossils are an important tool for geologists because 'they can be used to reconstruct past environments,' says Professor David Siveter, a palaeontologist based at the University of Leicester. Particular species are typical of a given aquatic environment so, if scientists can identify which ostracods exist in a rock sample, they are able to gauge ancient conditions such as water depth and salinity.
Most fossil ostracod species are known only from their shells: 'you need exceptional conditions to preserve the soft body,' explains Siveter. There are only two other known examples of ancient fossil ostracods where the complete soft parts of the animal are preserved along with the shell.
Siveter and his colleagues have now unearthed a third remarkably preserved ostracod fossil, found in 425 million year old rocks from Herefordshire.
Nasunaris flata, the ostracod
The 5mm-long fossil was officially named as Nasunaris flata in a paper published last week in the Proceedings of the Royal Society B. Siveter and colleagues were able to identify the ostracod's body and appendages inside the shell, including the antennae and also a set of paired eyes.
Virtual reconstruction of the Nasunaris flata fossil, seen from the back. Grey, ostracod's shell. Other colours, soft parts
The ostracod was so well preserved that the team managed to spot the Bellonci organ, a sensory structure observed in modern species which protrudes out of the middle eye located at the front of the head. 'This is the first time the Bellonci organ is observed in fossil ostracods,' says Siveter.
'This ostracod probably lived at the bottom of the sea and the front limbs suggest that it could swim,' he adds. Nasunaris flata was probably a scavenger feeder.
The species is extinct nowadays, but its relatives in the ostracod myodocopid group still inhabit the planet's oceans. Nasunaris flata's modern cousins are very similar, which 'demonstrates that the group's morphology is very efficient for its ecological niche,' Siveter says.
The team was able to identify Nasunaris flata as a myodocopid thanks to the shape of its seventh limb, lateral eyes and Bellonci organ. The presence of gills assigns the species to the - prepare yourself - Cylindroleberididae family.
Unlike other fossil ostracods, which are known only from their shells, Nasunaris flata was classified according to its soft body parts. And just as well, because if scientists had looked at its shell alone, they would have placed the species in a different group.
The mismatch, Siveter suggests, shows that shell features alone may not always be a reliable way to classify fossil ostracods.
David J. Siveter, Derek E. G. Briggs, Derek J. Siveter and Mark D. Sutton. An exceptionally preserved myodocopid ostracod from the Silurian of Herefordshire, UK. Proc. R. Soc. B published online 27 January 2010 doi: 10.1098/rspb.2009.2122
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