THE SAGA OF DEVONIAN STROMATOPOROIDS: ORIGINATIONS, EXTINCTIONS, DIVERSITY, AND PALEOBIOGEOGRAPHY

The Devonian began with the Lochkovian Age, in which 24 stromatoporoid genera occurred. In the Pragian Age diversity dropped to 17 genera, but rose in the Emsian to 36 genera. Stromatoporoids reached a peak of diversity in the Eifelian Age, with 38 genera, dropping back to 36 genera in the Givetian Age. There were 30 genera in the Frasnian Age, but abundance dropped to 18 genera, following the Frasnian-Famennian (F-F) mass extinction. There are 27 stromatoporoid genera represented in the Devonian that originated in the Ordovician or the Silurian. Genus originations exceeded extinctions from the Lochkovian through the Eifelian, but the trend reversed, with extinctions outpacing originations from the Givetian through the Famennian, with total extinction of the stromatoporoids at the close of the Devonian--post-Devonian “stromatoporoids” are not related.

Sea-level curves for the Devonian indicate lowstands in the late Pragian and late Emsian, followed by a general episodic rise of sea level through the Frasnian. Overall, sea level fell during the Famennian, but this trend was punctuated by brief rises. The Eifelian through Frasnian increase in sea level appears to accompany a global warming trend, followed by cooling associated with the Famennian sea-level fall, related to glaciations in Gondwana (e.g., South America).

The disproportionately large role of extinctions in the Frasnian and Famennian is not surprising, as both preceded major extinction events. The Givetian, with 6 originations and 11 extinctions, is not as easily explained. The decrease in originations is part of a downward trend—there were 11 originations in the Emsian, 9 in the Eifelian, 6 in the Givetian, and 5 in the Frasnian, but the 11 extinctions of the Givetian represent an increase over 6 for the Emsian and 8 for the 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 F-F extinction event.