Fossil record

Cheilostomes first appear in the fossil record around the Upper Jurassic (~155 mya) (Taylor 1994; Pohowsky 1973) and continued to dominate the fossil bryozoa through the Tertiary and, of course, in present day. They appear all over the world in deposits from the Upper Cretaceous (Ostrovsky et al. 2006).  For around 65 million years following the mid-Cretaceous, the Cheilostomata underwent rapid diversification likely due to the emergence of shot-lived larvae interrupting gene flow between populations (see PHYLOGENY).  

O’Dea and Okamura (2000) suggested that size of zooids in the fossilized cheilostomes could be used as a proxy for measuring paeleoseasonality.  Colonies growing in warm water tend to produce smaller zooids, while those in cold water grow larger zooids.  This can be seen both in differences within one colony (genealogically) (O’Dea & Okamura 1999), between colonies of the same species growing in different climates (Ryland1963), and within one species over time (Okamura & Bishop 1988).  Thus fossil cheilostomes may be of great use in estimating ancient sea-temperature fluctuations, with a higher variation in zooid size within a colony indicating a larger mean annual temperature range (O’Dea & Okamura 2000).