ORIGINATIONS AND EXTINCTIONS OF STROMATOPOROID GENERA AND THEIR ROLE IN THE FRASNIAN-FAMENNIAN EXTINCTION

Stromatoporoids reached a peak of diversity, abundance, and geographic extent during the Givetian and Frasnian Ages of the Devonian, but were decimated by the Frasnian-Famennian mass extinction. They survived into the Famennian, only to become totally extinct by the end of the Devonian--post-Devonian “stromatoporoids” are not related. Examination of the fossil record of Devonian stromatoporoid genera reveals that both a decrease in originations and an increase in extinctions took place prior to the Frasnian-Famennian extinction. Through the Early and early Middle Devonian (Lochkovian-Eifelian) genus originations outnumbered extinctions; however, during the second half of the Middle Devonian (Givetian) extinctions exceeded originations by more than a 2:1 ratio. In the Frasnian extinctions outnumbered originations by more than a 4:1 ratio, and this situation continued into the Famennian where the ratio is 3:1. In addition, there were fewer genus originations in the Frasnian (4) than in any other Devonian age except for the Pragian (2)

The disproportionately large role of extinctions in the Frasnian and Famennian are not surprising, as both preceded major extinction events, probably due primarily to global cooling, indicated by glaciations in Gondwana (e.g., South America). The Givetian, with 6 originations and 13 extinctions, is not as easily explained. The decrease in originations is part of a downward trend--there were 11 originations in the Emsian, 8 in the Eifelian, 6 in the Givetian, and 4 in the Frasnian, but the 13 extinctions of the Givetian represent a significant increase over the 4 each of the Emsian and Eifelian. The Givetian extinction event is probably due to the loss of endemism near the end of the Middle Devonian during the Taghanic Onlap, when the Old World and Eastern Americas Realms merged, resulting in fewer genera in the Frasnian. Reduced generic diversity put the stromatoporoids more at risk in the Frasnian extinction event.