The ecological, significance of variation in the generic composition of Rawtheyan (late Ordovician) trilobite faunas from North Wales, U.K.

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Abstract
Contemporaneous trilobite faunas from Rawtheyan rocks forming the local base to the Ashgill Series change in their generic composition along the outcrop between Bala and Corris. The change, although overall a very marked one, is progressive and is held to reflect original ecological control along a depth gradient between a locally elevated area around Bala and the deeper central part of the Welsh Basin. Along this intrabasinal slope three distinctive trilobite generic associations–the Ceraurinella‐Encrinuroides Association, the Nankinolithus‐Opsimasaphus Association and the Novaspis‐cyclopygid Association–succeeded each other in gradually deeper water. Faunas of all three associations differ markedly from contemporaneous faunas of shallower shelf environments.