Broad patterns of originations and extinctions of genera, as well as families and higher groups, have always interested those who study the fossil record (e.g., Sepkoski, 1984). They record an important part of the major changeovers, and thus the dynamics, of marine ecosystems over time (Droser et al., 1996; Droser and Sheehan, 1997). This seems especially true for the Paleozoic, when brachiopods were the dominant shelly animals on the seafloor in tropical, temperate, and even cold water settings. Attempts have also been made to determine turnover patterns at the species level (Patzkowsky and Holland, 1997), though this is a much more difficult task, as the validity of species depends a great deal on the skills of the taxonomist. A similar problem is the comparative analysis of diversification data based on a single continent, e.g., North America, as related to others (Miller, 1997a, b); though Laurentia is probably better studied than most areas except western Europe. The exercise of studying broad-scale generic gains and losses for the brachiopods is at the present time preliminary (only three volumes of the revised Treatise are published). The 1965 Treatise contains fewer than 25% of the genera known in detail and described today, with an almost exponential increase in taxonomic description since the 1960s (Williams, 1996). Since then, there have been dramatic revisions and re-interpretations of the evolutionary history of the major brachiopod families, as a new generation of brachiopod workers arrived and matured. We also have a considerably improved knowledge of molecular relationships within the Brachiopoda (Cohen and Gawthrop, 1996). Sound taxonomy is the fundamental basis for sound theoretical discussion of the nature and origins of major changeovers in phyla such as the Brachiopoda. Unfortunately, there are presently relatively few, active brachiopod specialists, as taxonomy has given way to other, more general interests.
Abundance, movements and size of gadoids (Teleostei) in the Severn Estuary