Palaeobiological and phylogenetic implications of preserved muscle scars in Devonian tentaculitids from Armenia

Rare soft body impressions were discovered on phosphatised steinkerns of Devonian tentaculitids from Armenia. The muscle scars occur only in the most apical part of the tentaculitid steinkerns. The morphology of muscle scars varies between different species. There are seven different types of muscle scars in tentaculitids, and six of them are present in the Armenian material. The muscle scars were used for attachment of a well-developed retractor muscle. The muscle attachments in tentaculitids migrated forwards during the growth of the shell like the muscle scars in many brachiopods. The hypothesised architecture of tentaculitid muscle system is most similar to that of bryozoans. Tentaculitids had a defensive mechanism that allowed complete retractability of the animal into the shell. This was achieved by prominent retractor muscles that pulled the soft tissues into the protective body wall. This is opposite the protrusion mechanism that involved body‐wall musculature to increase hydrostatic pressure within the soft body to squeeze out the feeding apparatus of the animal, enabling it to filter‐feed again. This muscle arrangement is strong evidence to confidently place the tentaculitids within the Lophotrochozoa, potentially as ‘lophophorates’.