Revising and refining the bryozoan zs-MART seasonality proxy

The zs-MART (zooid-size derived mean annual range of temperature) seasonality proxy is based on temperature-related variation in the sizes of zooids found in colonies of cheilostome bryozoans. It has been used in a number of studies for inferring palaeoseasonality. Here we present a revised version of the equation for the zs-MART proxy based on an appropriate regression analysis and an estimate of the error associated with predicted values. We use this improved version to recalculate data used in studies that have employed the zs-MART proxy and evaluate their conclusions in light of the revised estimates for seasonal regimes. In order to investigate trans-colony zooid-size profiles in zooid size, length and width, 1195 zooids from 40 transects through 19 fossil and Recent specimens of the malacostegine cheilostome bryozoan Acanthodesia were measured using a stereomicroscope with eyepiece graticule. The position of zooids within bifurcating rows of Acanthodesia colonies is found to influence zooid size, and this exemplifies potential problems for studies based on zooid-size transects and random sampling of zooids. Our demonstration that oscillatory signals in zooid size, possibly attributable to seasonal temperature regimes, can be detected whilst accounting for other sources of variation confirms that a more sophisticated zs-MART proxy may prove useful for constraining parameters in ocean models addressing questions about past climate. However, such a proxy will require more parameters than originally described along with larger raw datasets.