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 bas e our approach on the inverse relationship between temperature and zooid si ze in bryozoans, and analyse the mean intracolonial coefficient of Variatio n (CV) in zooid length, zooid width and zooid area (length x width) in a nu mber of Recent bryozoan species collected from many seasonally different en vironments. A highly significant, positive correlation was obtained between the mean annual range of temperature (MART) experienced by the colonies an d the mean intracolonial CVs in zooid lengths (R-2 = 74.7%), zooid widths ( R-2 = 58.9%) and zooid areas (R-2 = 89.0%). An algebraic equation derived f rom 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 asses sing variation in zooid area within fossil cheilostome colonies. This techn ique is then applied to bryozoan colonies from two Neogene shallow-water de posits in Western Europe. Results from the Coralline Crag in southeastern E ngland reveal a moderate lever of seasonality, in keeping with previous est imates of seasonality for British seas during the Pliocene. Results from th e middle Miocene 'faluns' in north-west France suggest less seasonal variat ion in temperature than occurs in comparable seas today. We conclude that t he technique represents a useful new approach that offers some benefits ove r other techniques of assessing seasonality in marine palaeoenvironments. ( C) 2000 Elsevier Science B.V. All rights reserved.