First soft body morphological data on the tracemaker of the endolithic bryozoan trace fossil Terebripora ramosa d'Orbigny, 1842

Abstract

Terebriporidae is one of the four extant endolithic ctenostome bryozoan families, with colonies immersed into carbonate substrates like molluscan shells. This monogeneric family comprises 17 species, with 11 extant and 6 fossil species. It is currently considered closely related to vesicularioid ctenostomes, a group characterized by colonies interconnected by polymorphic stolons and a distinct gizzard as part of their digestive systems. However, confusion persists regarding the correct species identities and affiliations of many terebriporid species, and even the description of the entire family is based solely on a few external features of their boring traces, rendering the family an ichnotaxon (trace fossil). Our molecular analysis does not support a vesicularioid affinity, but corroborate a close relationship to Immergentia, another genus of boring bryozoans. Consequently, this study aims to untangle the systematic confusion surrounding Terebriporidae by examining the tracemaker of the type species of the family, Terebripora ramosa from Chile, and investigating its morphology and histology using modern techniques. The current analysis could not confirm typical vesicularioid characters such as a gizzard or true polymorphic stolons. Instead, all characters point towards a closer relationship to Immergentiidae as suggested by a recent molecular phylogeny. In fact, these two taxa share several characters such as cystid appendages and duplicature bands, and appear closely related, with the only difference being a characteristic vane with tubulets present in the tracemaker of T. ramosa. The sister-group relationship of the tracemaker and the genus Immergentia infers that these borers share a common boring ancestor, but also emphasizes that additional species from the ichnogenus Terebripora need to be studied for more clarity.