Intracolony variation in zooid size in cheilostome bryozoans as a new technique for investigating palaeoseasonality

Variation in zooid size within colonies of fossil cheilostome bryozoans is presented as a potential source of information on palaeoseasonality. We base our approach on the inverse relationship between temperature and zooid size in bryozoans, and analyse the mean intracolonial coefficient of variation (CV) in zooid length, zooid width and zooid area (length×width) in a number of Recent bryozoan species collected from many seasonally different environments. A highly significant, positive correlation was obtained between the mean annual range of temperature (MART) experienced by the colonies and the mean intracolonial CVs in zooid lengths (R2=74.7%), zooid widths (R2=58.9%) and zooid areas (R2=80.0%). An algebraic equation derived from regression analysis of mean intracolonial CV of zooid area and MART is proposed as a new method of investigating the MART of ancient seas by assessing variation in zooid area within fossil cheilostome colonies. This technique is then applied to bryozoan colonies from two Neogene shallow-water deposits in Western Europe. Results from the Coralline Crag in south-eastern England reveal a moderate level of seasonality, in keeping with previous estimates of seasonality for British seas during the Pliocene. Results from the middle Miocene ‘faluns’ in north-west France suggest less seasonal variation in temperature than occurs in comparable seas today. We conclude that the technique represents a useful new approach that offers some benefits over other techniques of assessing seasonality in marine palaeoenvironments.