The stromatoporoid Habrostroma centrotum (Girty) is found in Lochkovian-age rocks of Virginia, New York, Maine, and Bathurst Island, Canada, and it is the most common stromatoporoid species present in those strata. Bathurst Island is believed to have been part of the Old World Realm, which included arctic and western Canada, and the western US. The three eastern US localities were part of the Eastern Americas Realm, which also extended into maritime Canada and the south-central US. Historically, these realms have been defined on the basis of rugose corals, brachiopods, ammonoids, and conodonts, separated by a land barrier formed by the Canadian Shield and Transcontinental Arch. Two realms should contain faunas that do not share many genera in common, and certainly do not have shared species. How then could H. centrotum co-occur in both realms?

Minimal breaching of a barrier can occur by means of two dispersal pathways: 1) filter—a region of limited habitats that allows the passage of only those organisms able to tolerate those habitats; or the more restrictive 2) sweepstakes route—a normally intolerable barrier (e.g., dry land) that is crossed due to the chance combination of favorable circumstances that involve only a portion of the life history of an organism. Whether H. centrotum was able to establish a population in both realms simultaneously by larval drift around the continent of Laurussia, or by transport through an extremely shallow seaway across Laurussia, neither previously defined breaching mechanism fits perfectly. A habitat that would allow the passage of only one species is too restrictive to be considered a filter, and it would appear unlikely that one generation of larvae would drift all the way between the two regions of habitation, as would be required for a sweepstakes route. A highly selective paleobiogeographic "leak" in the Laurussian barrier is suggested.