A systematic revision of the Ordovician plectambonitoidean brachiopods Chonetoidea and Sericoidea

The revised Treatise on Invertebrate Paleontology, partH: Brachiopoda regards Chonetoidea Jones and Sericoidea Lindstrom synonymic, on the basis of characters that were considered common to both genera (e. g. ornament type, number of septules). However, some features (e. g. number of septules) discriminate specimens at species level, rather than at generic level as previously thought. Other morphological features, never taken into account or described before, e. g. the position of the ventral diductor scars or the presence of a pair of septules developed anterolaterally to the socket ridges (named praeculmen septules) in Chonetoidea solely, permit a confident separation of the two genera. The ornament is not useful for diagnosing the genera. A review of all species belonging to each genus is made in light of the emended diagnoses of both Chonetoidea and Sericoidea. Based on the internal morphologies of their lophophoral supporting structures and on sedimentological data, Chonetoidea and Sericoidea are interpreted as living in different bathymetric conditions. Chonetoidea was adapted to a more dynamic environment, with higher nutrient levels. Sericoidea needed a wider area for trapping food, in an environment (open water) with three to six times less nutrients than mid to inner shelf environments. The palaeogeographic and stratigraphic distributions of Chonetoidea and Sericoidea indicate the progressive disappearance of Sericoidea and adaptative radiation of Chonetoidea in a palaeoworld where epicontinental seas were shallowing, prior to the end of Ordovician glaciation events which coincided with the extinction of Chonetoidea.