Dissecting the global diversity trajectory of an enigmatic group: The paleogeographic history of tentaculitoids

Tentaculitoids have long been recognized as a problematic group that diversified taxonomically and expanded geographically through much of the Ordovician–Devonian, only to become extinct toward the end of the Devonian. Perhaps because of uncertainties about their biological affinities, there has never been a definitive study of the diversity trajectories of tentaculitoids at either global or regional scales, and paleoecological analyses of the group are almost entirely lacking in the literature. Here, we present a much-expanded, paleogeographically-resolved database for tentaculitoids, which we use to present a first-ever assessment of their global history in a paleogeographic context. w data suggest an increase to a standing-diversity peak in the Devonian of 30 genera, preceded by a Silurian radiation that appears to have been focused mainly in Baltoscandia. Because most of the Silurian Baltoscandian data were derived from a single bibliographic source, the possibility cannot be ruled out that this concentration primarily reflects a monographic bias. The continued radiation in the Devonian was geographically broad-based, however, recognizable in several venues worldwide. Furthermore, whereas previous investigations suggested that a temporal transition could be recognized between the major tentaculitoid orders—Tentaculitida—thought to dominate in the Ordovician and Silurian, and Dacryoconarida and Homoctenida—thought to radiate in the Devonian—the new data point to a much broader overlap in the temporal trajectories of the groups. Coupled with information about differences in their life habits, this would appear to rule out any possibility that the three groups were ever in direct competition.