Early Devonian Decrease in Paleogeographic Distribution and Diversity of Stromatoporoids

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 Early Devonian the overall geographic extent of stromatoporoids, and their generic diversity, declined. In parallel, the abundance and distribution of reefs decreased drastically. 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 near total absence of stromatoporoids in the middle Early Devonian (Pragian Age) of North America? To answer that question, we examined six lines of inquiry: (1) eustasy, (2) tectonics, (3) xenoliths in kimberlites in the Canadian Shield, (4) similar events in the Ordovician of North America, and the Ordovician through Mississippian of North China, (5) the validity of the existing data, (6) 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 the absence of stromatoporoids from North America The Antler Effect proposes that during most of the Early Devonian in North America there were no orogenies. Instead there was epeirogeny, in which the central part of the continent was bowed up, raising it above sea level, restricting seas to the continental margins, however, nearly no stromatoporoids are represented in the Pragian margin of North America. Although the Hudson Bay and Moose River basins contain Paleozoic strata, there are no Pragian-age strata in the Hudson Bay Basin, and Pragian-age strata in the Moose River Basin are siliciclastic, representing environments uninhabitable by stromatoporoids. It does contain numerous kimberlites in which there are xenoliths composed of normal-marine sedimentary rocks, some of which are Devonian in age, however, no Pragian-age stromatoporoids have been found. Of the three ages of the Early Devonian (Lochkovian, Pragian, Emsian), collections made world-wide show the lowest number of genera for the Pragian. If these data were adjusted to include genera not collected nor described from a particular age, but present earlier and later in the geologic record (i.e., Lazarus taxa), the Pragian is not so markedly different from the two other ages. If the data are considered in terms of the duration of the three ages, the Pragian yields the highest number of genera per million years. The presence of bona fide stromatoporoids in the Pragian of central Nevada and central New York merits further systematic study. The complex, and sometimes conflicting, lines of evidence available in the literature do not fully explain why there are nearly no Pragian-age stromatoporoids in North America.