By Paul Wignall
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Additional resources for Benthic palaeoecology of the Late Jurassic Kimmeridge Clay of England
The range of the association at Westbury (textfig. 4), includes the Pentacrinus bed (Gallois and Cox 1976; Birkelund et al. 1983) named after the crinoid, Balanocrinus subteres, which is also common at this horizon. The high diversity of this association and the rarity of opportunists indicates a stable environment. However, infrequent perturbations presumably affected the fauna as low diversities, caused by prolonged competition, do not occur (Huston 1979; text-fig. 2). Alternatively, the diversity could be due to time-averaging and condensation although this might be expected to have produced shellier sediments than those in which this association occurs.
The relative abundance of Chondrites cannot be quantified, although it is consistently present in association E8. TEXT-FIG. 40 WIGNALL: KIMMERIDGE CLAY BENTHIC PALAEOECOLOGY pectinatus Zones (text-figs 5 and 10), most commonly on discrete bedding planes in organic-rich shales. Beneath the Washing Ledge Stone Band of the autissiodorensis Zone (text-fig. 5), association E4 occurs in three organic-rich shales containing bedding planes covered in the ammonite Nannocardioceras. Nannocardioceras has not been included in the benthic faunal analysis, but its distribution within the sediment suggests that it was probably a nektobenthic form living close to the substrate (see discussion above, p.
A10 Protocardia morinica: Lingula ovalis Association (15 samples, 3218 specimens, 53 species, 2 3 % average fragmentation, H = 217) P. morinica dominates this association forming nearly a quarter of the total fauna (text-fig. 21). The remainder of the fauna occurs in slowly decreasing proportions and thus exhibits a high equitability. Filter feeding bivalves are dominant, although up to 60 % herbivores and deposit feeders may occur in some samples (text-fig. 13) mainly due to local abundances of Procerithium.