Llanvirn-early Caradoc trilobite biofacies of western Hubei and Hunan, China

On the basis of representative collections made from seven measured section s in western Hubei and Hunan, four early Llanvirn and five late Llanvirn-ea rly Caradoc trilobite biofacies are distinguished along a bathymetric gradi ent from shallow to deep outer shelf The diversity and relative abundance o f mesopelagic cyclopygids gradually increased southwards or seawards. Accor ding to the facies association exhibited in the region, the upper limit of living depth for different forms is inferred as follows: 1) more than 70 m (M. (Microparia)); 2) less than 100 m (M. (Quadratapyge), Pricyclopyge, Asp idaeglina); 3) at approximately 100 m (Cyclopyge); and 4) more than 100 m. (Symphysops, Psilacella). Benthic forms were absent in the deep outer-shelf basin because of stagnant conditions. In shallower sites of the outer shel f, genera of nileids in the carbonate facies and raphiophorids in the clast ic facies were relatively abundant but some endemic forms, such as the asap hid Birmanites and remopleuridid Hexacopyge, became dominant during the lat e Llanvirn-early Caradoc. Rorringtoniids are mainly associated with deeper outer-shelf facies. Panderia is common in an early Llanvirn nileid fauna at Yichongqiao, Cili area, and may well indicate development of shelf carbona te buildups. A transgressive event is suggested by the sudden appearance of rich Cyclopyge in the early Caradoc at Jiuxi, Changde.