A hyolithid with preserved soft parts from the Ordovician Fezouata Konservat-Lagerstatte of Morocco

Hyolithids are an extinct group of early lophotrochozoans with an external skeleton consisting of a conical conch, an externally fitting operculum, and a pair of long, lateral spines named helens. Their phylogenetic position within the lophotrochozoans is unclear, but traditionally they have been regarded as molluscs. Although hyolithids occur in Konservat-Lagerstatten, their soft-part preservation had so far been restricted to the gut. The few known examples suggested that a simple U-shaped gut was characteristic of the group, but new occurrences show a diversity of gut configurations suggesting certain ecological diversity within the group. Here is reported an exceptionally preserved hyolithid specimen from the Ordovician Fezouata Konservat-Lagerstatte of Morocco with a relatively complete gut possibly including an oesophagus, a mouth and a faecal string. Uniquely for hyoliths, other soft parts, likely those associated with the inner surface of the operculum, appear to be preserved as well protruding from the aperture. The anatomy of the preserved soft parts is in accordance with inferences from skeletal anatomy suggesting that hyolithid soft parts did not extend much beyond the aperture, and that only the internal portion of the helens was associated with soft tissues. It is also compatible with lophotrochozoan affinities, but molluscan apomorphies, such as a foot or a radula are not present in the specimen. Taken together, the newly available information suggests that hyolithids may have had a relatively generalized feeding organ, which could adapt to various feeding modes in the suspension-detritus-deposit feeding spectrum. A specialized deposit feeding strategy would be at odds with what is known about hyolithid functional morphology, therefore the presence of a sediment-like infill in the posterior part of the gut of the presently described specimen is puzzling and, as it has been suggested for some Chengjiang taxa, may represent a taphonomic artefact. (C) 2016 Elsevier B.V. All rights reserved.