Abstract
The Ordovician Period saw the most sustained, steep rise in marine biodiversity in the history of life on Earth and set the ecological pattern for the rest of the Palaeozoic Era. The long history of research, wide variety of depositional settings and juxtaposition of terranes with very different palaeobiogeographical histories makes the Ordovician of the British Isles an excellent laboratory in which to study this change. A database approach to analysis of trilobite biodiversity change is described using a simple compilation of stratigraphical ranges of 663 species of Avalonian trilobites, and a much more sophisticated information‐rich relational database based on the fundamental information of the fossil record–species at localities. The latter includes 2001 occurrence records of 617 trilobite species at 508 localities in the Welsh Basin. The first ever species‐level diversity curves for the whole Ordovician of an entire region are presented. Both databases reveal an overall increase in species‐ and genus‐level diversity through the Ordovician. Random resampling tests and environmental information are used to remove sampling effects from the Welsh Basin diversity curves. The main features of trilobite biodiversity change in Avalonia are the following: a late Arenig–early Llanvirn increase, during which time the highest species‐to‐genus ratios occur, is contemporaneous with a rise in global diversity and rifting of Avalonia from Gondwana; a late Abereiddian dip is followed by recovery during the Llandeilian–early Caradoc; decline during the late Caradoc–early Ashgill is at least partly attributable to lack of preserved rock and restriction of most of the preserved faunas to deep‐water environments. The greatest diversity occurs in the palaeoenvironmentally differentiated Rawtheyan and is followed by the diversity crash seen in the Hirnantian throughout the world. Copyright © 2001 John Wiley & Sons, Ltd.