Direct evidence of competition is seldom available from the fossil record. Overgrowth relationships of encrusting marine organisms constitute an exception but have previously been reported in only temporally and geographically local occurrences. Results of overgrowths between members of two bryozoan clades, the Cyclostomata and the Cheilostomata, have been compiled for faunas distributed through the past 100 Myr. The cheilostomes have consistendy out-competed the cyclostomes, with approximately 66% overgrowdi success through the entire interval. This difference in success in direct interactions along with the Mid-Cretaceous rapid radiation of cheilostomes is interpreted as a factor in the Mid- to Late Cretaceous reversal from the previous diversification to stasis or gradual decline of cyclostome diversity.