The process that would BEST account for the pattern of evolution in bryozoans, as described, is punctuated equilibrium.
Punctuated equilibrium is a theory in evolutionary biology that suggests that species experience long periods of stability (stasis) interrupted by short periods of rapid change, often in response to significant environmental shifts or other factors. In the case of bryozoans, they remained unchanged for 40 million years and then suddenly experienced rapid genetic change, which aligns well with this theory.
In contrast, gradualism would imply a slow and gradual change over time, which does not fit with the pattern of sudden change observed in bryozoans.