Environmental change prior to the K–T boundary inferred from temporal variation in the morphology of cheilostome bryozoans

Fluctuations in the marine environment just prior to the K–T extinction event have been inferred from several geological sections around the world. Most previous studies have employed isotope or trace element proxies. This study uses morphological changes in erect and free-living cheilostome bryozoans as a proxy to investigate environmental change through the final stages of the Maastrichtian at the Nye Kløv section in Denmark. The metrics used are: (1) mean zooid size as a proxy for temperature; (2) intracolony variation in zooid size as a proxy for degree of seasonality; (3) density of defensive avicularia as a proxy for palaeoproductivity; and (4) colony size and asymmetry as proxies for unfavourable environmental conditions. Three semi-distinct phases in the benthic environment are evident: The lowest 3.5 m of the roughly 4.5 m section experienced apparently normal marine conditions. Next, low estimates of benthic seasonality, and highly symmetrical and large colonies with many avicularia indicate a time of increased environmental stability. Subsequent to this quiescence, the uppermost ~ 20 cm of the section witnessed environmental volatility and deterioration with mean zooid sizes in all species falling dramatically suggesting a rapid warming or dysoxic event, sharply increasing seasonality estimates implying unusual oceanographical states, and the growth of small, asymmetrical colonies with few avicularia all suggesting unfavourable conditions. These data therefore indicate that strong environmental perturbations occurred just prior to the K–T boundary in the Danish Basin. Such events may have contributed to biotic turnover at the K–T boundary because cause and effect in macroevolution can be delayed. However, potential mechanisms of turnover need to be robustly tested within a detailed palaeoenvironmental framework construct from a suite of independent proxies.