A PALEOBIOGEOGRAPHIC LEAK ACROSS NORTH AMERICA DURING THE EARLY DEVONIAN (LOCHKOVIAN) DOCUMENTED BY STROMATOPOROIDS

The stromatoporoid genus Habrostroma first appeared in the Late Silurian (Pridoli) of Estonia and the Appalachian Basin, and became extinct during the Middle Devonian. It is a particularly prominent genus during the early Early Devonian (Lochkovian Age) in various parts of North America. Previous paleobiogeographic reconstructions of North America, (also known as the Laurussia Plate) for the Lochkovian show the continent divided between two faunal realms: 1) Eastern Americas Realm—area today east and south of the Transcontinental Arch and Canadian Shield; and 2) Old World Realm—area today west and north of the Transcontinental Arch and Canadian Shield.

Three closely related species of Habrostroma are found in the Lochkovian of North America: H. centrotum (Girty), H. microporum (Girty), and H. consimile (Girty). All three species occur in the Manlius and Coeymans Formations of New York. Habrostroma centrotum has also been found in the LaVale Member of the Keyser Formation of Virginia, the Beck Pond Limestone of Maine, and in Lochkovian-age olistostromes within the middle Lower Devonian (Pragian) Stuart Bay Formation of Bathurst Island, arctic Canada. In each area Habrostroma is the most common genus present, and in many cases includes the majority of specimens. The Bathurst Island occurrence is anomalous, in that it is the only one in the Old World Realm.

How did H. centrotum occur in both realms at the same time? Were there connections across the Transcontinental Arch or Canadian Shield, or were they around the edges of Laurussia? The former hypothesis was leant some credence through the discovery of Devonian fossils in kimberlite xenoliths in the Canadian shield of Northwest Territories, Ontario and Quebec, over 300 km from the closest present-day Devonian outcrops, indicating the presence of a seaway there during at least part of the Devonian. This seaway allowed at least one stromatoporoid species to breach the barrier, while preventing other taxa from doing so.