Revisiting Umbichnus inopinatus, an unassigned perforation in heteroconchia bivalve hinges

Umbichnus inopinatus is an ichnospecies named by Martinell et al. (1999). The sac-shaped perforation appears in the hinge of numerous species of heteroconch bivalves from the Paleocene to the present day. However, what produced it is unknown and is attributed to some group of commensal worms. Two basic morphologies have been distinguished, one with a linear profile and the other with a bunched profile. The initial diagnosis has therefore been amended. This paper reports its presence in 85 species of bivalves, based on the review of several malacological collections and the published bibliography. The geographical distribution of U. inopinatus, and therefore of its producer, has increased since the Paleocene (present in 2 bivalve species). The greatest diversity in host species has been observed in the Miocene with 28 species, while in the Pliocene 21 species have been identified. At present, it is thought that hosts belong to 10 different species. Colonization routes from the Paleocene coasts of northern France and expansion through the European and North American Eocene basins are proposed. During the Miocene it reached the coasts of South America and, in the Pliocene, those of California. In the Quaternary it would have been limited to areas of cold seas, but nowadays is found in a large part of the oceans