Scientists from the United States and the United Kingdom have described a fossil species of marine arthropod that lived during the Silurian period.

Named Enalikter aphson, the ancient animal belonged to Megacheira, an extinct group of marine-dwelling ‘short-great-appendage’ arthropods.

“It would have lived on the seabed in water possibly up to about 100 or 200 m deep, at a time known as the Silurian, when invertebrates were just beginning to move onto land. It would have been a very warm, subtropical environment,” said Prof Derek Siveter from the Oxford University’s Museum of Natural History, who is the lead author on a paper published in the journal Proceedings of the Royal Society B.

Enalikter aphson was about 2.4 cm long with a rounded rectangular head, no eyes and a curved, whip-like feature protruding from in front of its mouth that may have been used in feeding – for example to capture smaller marine invertebrates. At the rear end of its stick-like body there were two pincer-like projections that were attached to a primitive tail.

The animal had no hard shell but is nevertheless remarkably well preserved in a hard nodule of minerals comprised mainly of calcite.

“Enalikter aphson had a soft and flexible body so it is incredible that it survived. The nodule acted like a womb and kept the creature free from decay and destruction, which would normally have happened very quickly. It meant it was able to survive all the earth movements and history that have happened since,” Prof Siveter said.

To reconstruct Enalikter aphson in 3D, the team imaged sequential surfaces of the fossil, spaced a mere 20 microns apart. These numerous images were then edited on a computer, in places pixel by pixel, to identify true biological structures from background ‘noise.’

The discovery of Enalikter aphson gives support to the notion that Megacheira came before the last common ancestor of all living arthropods in the tree of life. If correct, this would indicate that megacheirans were distant ancestors of all arthropods alive today.

“It has provided us with exceptional data for the fossil record. Its soft body shell, or cuticle, is less than 10 microns thick. It was organic but has not decayed. That is amazing. Such exceptional preservation represents the jewel in the crown of paleontology and provides so much more information than what does the typical shelly fossil record. It offers a rare window on the marine community back then,” Prof Siveter said.

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Siveter DJ et al. 2014. A Silurian short-great-appendage arthropod. Proc. R. Soc. B, vol. 281, no. 1778; doi: 10.1098/rspb.2013.2986