Saturday, 26 June 2010

Return of the Soft-Bodied Ediacaran Trilobite!

I previously wrote about a mention of a soft-bodied Ediacaran trilobite in a couple of textbooks which perplexed me, see here. Whenever I come across this organism being mentioned somewhere it is likely that I am going to mention it here and will repeat the information I found about it. In this case I was reading The Garden of Ediacara by Mark McMenamin and found "Soft-Bodied Trilobite" discussed on pages 35 and 36. This unnamed organism was found in the early '90s by Jenkins and Gehling and looks to be one of those forms which will always be controversial in interpretation. There are about 7 specimens of about 9mm in length.

It does indeed resemble a trilobite as it has an apparent cephalon (the head) which appears to have eye ridges and a glabella between them. The body is a broad thorax with 21 segments and the initial report mentioned a pygidium (the tail bit). There are also many differences. The individual segments get larger away from the midline and reportedly have faint, regularly spaced lines perpendicular to the width, a trait which is typical for Ediacaran organisms which are seen as quilted in structure. 

No legs are preserved on this soft-bodied fossil, so it resembles a discarded trilobite shell, however, it cannot simply be a shell as there is no evidence that it was shed. It is even mentioned by McMenamin that it resembles a trilobite trying to expand its surface area to increase absorption, perhaps even for light.  It may be (especially under the Vendobiont view of Seilacher) a shortened frond fossil, perhaps juvenile, however, the purported cephalon is an issue for this view. If it were a Vendobiont (also referred to as Vendozoa) then it could potentially still be a head, but with the implication that these non-animals also achieved cephalisation independently of animals. They converged upon the concentration of sense organs at the anterior end. 

"Soft trilobite" is one of those fascinating mystery fossils which gets thrown about from one kingdom to another, seemingly resembling members of each and at the same time nothing is like it. If it is an animal then it is very weird and mysterious, with potential arthropod affinities; if it is a Vendobiont, then it seems they were converging on heads and concentrated sense organs. Then again, it could be something completely different. 

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