Early Devonian (Pragian) decrease in global generic diversity of stromatoporoids, and their extreme decrease in paleogeographic distribution in North America

Stromatoporoid sponges flourished throughout the Silurian Period, during the Middle Devonian Epoch, and the first half of the Late Devonian Epoch (Frasnian Age). Their prime habitats were in shallow carbonate seas of the tropics and sub-tropics. During the middle Early Devonian (Pragian Age) there was a global decrease in stromatoporoid generic diversity and a decrease in abundance, especially in North America. Throughout the Early Devonian there is a continuous presence of stromatoporoids in the fossil record of Europe, Asia, and Australia; however, this is not true for North America. Why then is there a dearth of stromatoporoids in the middle Early Devonian (Pragian Age) of North America? To answer that question, we examined five lines of inquiry: (1) eustasy; (2) tectonics; (3) xenoliths in kimberlites in the Canadian Shield; (4) the validity of the existing data; (5) reported Pragian stromatoporoids in Nevada and New York. Eustatic sea level in the Early Devonian was the lowest of the Devonian epochs, at times seemingly withdrawing seas from continents; however, there is no evidence for continental glaciation during the Early Devonian that might explain a drop in sea level, resulting the near-absence of stromatoporoids from North America. Instead there was epeirogeny, in which the margins of the Laurussia plate moved downward, while the central part of the plate was bowed upward, raising it above sea level, restricting seas to the continental margins. This tectonic change caused a regression of the sea from most of the continent, resulting in a widespread unconformity, where upper Silurian through upper Lower Devonian (lower Emsian Age) strata are absent. The Canadian shield, known for its exposures of Precambrian rocks, contains numerous Mesozoic-age kimberlites in which there are xenoliths composed of normal-marine sedimentary rocks, some of Devonian in age; however, no Pragian-age stromatoporoids have yet been found. The documented presence of undescribed stromatoporoids in the Pragian of central Nevada and central New York merits further systematic study. The complex, and sometimes conflicting, lines of evidence do not fully explain why there are nearly no Pragian-age stromatoporoids in North America.