CENTERS OF ORIGINATION OF EARLY SILURIAN (LLANDOVERY) STROMATOPOROIDS

Stromatoporoids suffered a severe, although not total, extinction during the latest Ordovician (Ashgill Epoch). Many genera became extinct by the end of the Rawtheyan Age, as did a smaller group by the end of the ensuing Hirnantian Age (the end of the Ordovician). The Ordovician stromatoporoid fauna was dominated by genera in order Labechiida, with a few genera representing order Clathrodictyida also present.

The return to pre-extinction levels of abundance and diversity during the Early Silurian (Llandovery Epoch) was at first a gradual process during the Rhuddanian Age and the first half of the subsequent Aeronian Age. In the Rhuddanian two Ordovician hold-over clathrodictyid genera, Clathrodictyon and Ecclimadictyon, prevailed with very rare occurrences of labechiids Pachystylostroma, Labechia, and Forolinia. In the late Rhuddanian the first actinostromatid (Plectostroma) appeared.

The late Aeronian saw a dramatic increase in the number of stromatoporoid genera. This diversification process continued at a slower pace during the Telychian Age. It was during the late Aeronian that the clathrodictyids Stelodictyon, Oslodictyon, Plexodictyon?, and Petridiostroma originated, as did actinostromatids Pachystroma and two new genera, and the first stromatoporids, Lineastroma and Syringostromella. Additional genera that appeared first in the Telychian include three clathrodictyids (Actinodictyon, Gerronostroma, and a new genus), two actinostromatids (Desmostroma, Densastroma), and a stromatoporid (Eostromatopora).

The center of origination during the Rhuddanian and early Aeronian was Estonia, but it shifted to the Michigan Basin for the late Aeronian. Data are incomplete for the Telychian, but for at least the late Telychian either the Michigan Basin or eastern Iowa served as a source for the most new genera.