Though the North Atlantic Ocean is a barrier to migration, interchange of warm-water shelf benthos between North America and Europe was a not unusual event in Tertiary time. Possible explanations for this are: (1) nearness of North America and Europe before they drifted apart later in the Cenozoic; (2) bridging of the North Atlantic by a continuous warm-water shelf; or (3) sweepstakes dispersal by currents, eastward across the northern Atlantic and westward nearer the equator.

Tertiary cheilostome bryozoans are particularly suitable for testing these hypotheses because, although they are among the least mobile marine invertebrates, they include types known to attach to seaweed and other floating objects. Data concerning resemblance, time and place of origination, and rate and direction of migration of middle Eocene to Oligocene faunas seem to harmonize best with the sweepstakes hypothesis of trans-Atlantic dispersal.