Article: Caradoc marine benthic communities of the south Berwyn Hills, North Wales

The Upper Ordovician (mid Caradoc: Soudleyan-Longvillian) clastic rocks of the south Berwyn Hills, North Wales, contain an abundant and diverse macrobenthic fauna dominated by epifaunal brachiopods. Based on studies in the south Berwyns, Shropshire, and Snowdonia, four communities are recognized and examined in terms of composition and related palaeoenvironmental parameters. It is concluded that the communities are intergrading and exhibit a close correlation with substrate and a broad correlation with depth, distance from shore, turbulence, and rates of sedimentation. The low diversity Howellites community was best developed on muddy silt and silty mud substrates in low energy turbid conditions and in water depths of less than 25m. The Dinorthis community exhibits a low to moderate diversity and based on the balance of constituent genera and relationship to substrate is divisible into two sub-communities: the Dinorthis sub-community was best developed on shifting coarse sand substrates in high energy, non-turbid, well-oxygenated environments of water depths of less than approximately 10m. The Macrocoelia sub-community was developed on finer sand substrates deposited in lower energy conditions and slightly more offshore in deeper water (25 m). The Dalmanella community developed on non-turbid, well-oxygenated, mobile fine sand substrates in water depths of 25 m or less and in areas of reduced sedimentation. The Nicolella community inhabited a variety of substrates but developed best on calcareous silt and fine sands. Energy conditions were variable at any one time but in general low-energy conditions prevailed, sedimentation rates were low and water depth was in the order of approximately 30 m. The communities are examined in terms of their stratigraphical distribution within the Berwyn succession and are discussed in relation to previously described Lower Palaeozoic communities. It is suggested that benthic faunas progressively migrated into deeper waters throughout the Lower Palaeozoic.