main marine extinction pulse begins abruptly with the subsequent deposition of the Hangenberg Black Shale, a layer of organic material deposited in anoxic deep-water environments. This is correlated with the beginning of the LN spore zone, indicated by the first occurrence of Verrucosisporites nitidus. However, in some areas the boundary between the LE and LN zones is unclear and possibly based on geography more than chronology. The black shale was deposited during a large marine transgression (sea level rise), as indicated by flooding reducing the input of terrestrial spores and increasing eutrophication. The Hangenberg Black Shale corresponds to the Postclymenia zone (UD VI-E), an ammonoid genozone based on massive extinctions within the group, rather than new occurrences. This is also the case for the costatus–kockeli Interregnum (ckI) conodont zone. Foraminifera disappear from the fossil record during the black shale interval. Uranium-Lead dating of ash beds in Poland provide dates of 358.97 ± 0.11 Ma and 358.89 ± 0.20 Ma below and above the black shale. This constrains the main marine extinction pulse to a duration of 50,000 to 190,000 years. Glaciation - the middle crisis interval In the middle crisis interval, the black shale grades into a thicker deposit of more oxygenated shallow-water sediment. It may be represented by shale (Hangenberg Shale) or sandstone (Hangenberg Sandstone), and fossils are still rare. These layers are still within the ckI conodont zone and LN spore zone, and foraminifera are still absent. However, ammonoid fossils switch over to the lower Acutimitoceras (Stockumites) genozone (UD VI-F), indicating that post-Devonian ammonoids were beginning to diversify after the main extinction pulse. A major marine regression occurred during the middle crisis interval, as indicated by the increased amount of erosion and river-supplied siliciclastic material. Some areas even show deep incised valley fill deposits, where rivers have cut into their former floodplains. Strata in Morocco suggest that the sea level fell by more than 100 meters (328 feet) during the middle crisis interval. This regression was caused by a cooling episode, and time-constrained glacial deposits have been found in Bolivia and Brazil (which would have been high-latitude areas), as well as the Appalachian Basin (which would have been a tropical alpine environment). These are known to have been deposited within the LE and/or LN spore zones, which are difficult to distinguish outside of Europe. Less well-constrained glacial deposits have also been found in Peru, Libya, South Africa, and