Sunken muscle scars in craniid brachiopods (Craniata); a long‐standing misconception

Abstract

Since the early 19th century, depressions in, and holes piercing through, the lower (ventral) valves of craniid brachiopods have been described as ‘sunken muscle scars’. Examination of extant craniid material has shown that the characteristic surface texture of calcitic muscle scars, myotest, is not present in these so-called sunken muscle scars. Reconstruction of the musculature of a fossil specimen of Crania craniolaris with posteriorly directed, valve-piercing holes suggests that muscles could not have passed through these holes and been functional. It is proposed that recently discovered organic muscle attachment tissue that forms muscle attachment surfaces and organic rostella in some extant craniids was also present in fossil craniids and occupied so-called sunken muscles scars. Thus, in most cases, so-called sunken muscle scars are not muscle scars at all. New terminology is proposed for the organic muscle attachment tissues and so-called sunken muscle scars. The presence of calcitic and/or so-called sunken muscle scars introduces a significant level of morphological variation in some craniid genera and species. The taxonomy of craniid genera and species where the presence of so-called sunken muscle scars are part of the diagnosis or description should be reassessed.