I've just found out there is indeed a technical name for fossilised vomit, it is
regurgitalite, so both Um...and Kevin .. were quite close!
Very interesting theory there Clem, could be possible, the thing could have swam and spun as it shat leaving a ring shape. I certainly cannot doubt it, you probably know as much about anomalocaridid butts as I do, and that sounds plausible to me! Kevin, I suppose that is possible though the shape is really quite regular, unfortunately it is less clear in the posted photo than actually looking at it in daylight. The ring is a light brown colour and quite clearly defined. How exactly do you think the fragments would accumulate around algae and stick there? Would one expect fragments to be less ground up? It's quite a mixture in there....I suppose if one had a microscope one could examine the fragments for traces of damage caused by predation and ingestion.
In fact there is a paper available on these anomalocaridid faecal pellets. It was a University of Kansas Paleontological Contributions paper written by R. A. Robison though I am not sure of the exact reference. Hopefully the chap who sold me the fossil will be kindly obtaining a copy for me soon so we can hopefully resolve this.
Personally, I lean towards vomit.
(This thread is turning surreal!
With these various interpretations, do you think we should have a poll? We could call it
Phil's Faecal Pellet Poll).