Late Ordovician (Caradoc-Ashgill) Brachiopod Faunas with Foliomena Based on Data from China

Abstract

New investigations of mid-Ashgill brachiopod faunas with Foliomena in an onshore to offshore transect in the border region of Zhejiang-Jiangxi provinces, East China, indicate a remarkable increase in numbers of Foliomena folium (Barrande) with significant variations of faunal composition, diversity, and abundance from shallow-water to deep-water environments. Based on data from China and elsewhere, faunas with Foliomena are partitioned into typical and atypical associations. The typical Foliomena fauna is characterized by Foliomena along with some distinctive constituents (such as Dedzetina, Leptestiina, Christiania, and Cyclospira), and is essentially located in deep-water environments (mainly BA 5-6). The atypical Foliomena fauna is represented by those assemblages in which rare Foliomena is associated with various genera such as Kassinella at different localities representing shallower environments (outer BA 3-BA 4). The Foliomena fauna previously was reported as latest Caradoc to early Ashgill in age. New Chinese data demonstrate that it ranges from the early Caradoc (Nemagraptus gracilis Biozone) to the mid-Ashgill. It reached its evolutionary acme in the early Ashgill and became extinct before the Hirnantian. Paleobiogeographically, it is associated with eight paleoplates within the Kosov Province, where the shallow-water Hirnantia fauna was widely distributed later in the Hirnantian. Continuous and gradual changes in the taxonomic composition and ecological niches of the Foliomena associations across a range of depths may indicate that the distribution of faunas with Foliomena may not be controlled always by the position of the thermocline. Distribution of the typical Foliomena fauna in time and space verifies a successful early Paleozoic ecological experiment in the marine benthos: brachiopods expanded their ranges into vacant deep-water benthic niches to exploit soft substrates associated with dark, possibly oligotrophic conditions.