Comparison of Late Ordovician and Late Wenlock Graptolite Extinctions and Recoveries

Most graptolites lived in or close to hypoxic marine environments which existed along certain coastal margins and in shelf basins in which abundant nutrients were supplied from land and resultant organic productivity was high. Graptolites nearly became extinct during the Late Ordovician and Late Wenlock following relatively brief but significant periods of sea level fall and significant reduction of hypoxic environments. Only a few lineages survived the loss of the hypoxic marine environments in both events. Many of the graptolites that survived habitat loss in both extinction events appear to have been those living near the ocean surface and/or those without close connection to hypoxic environments. As sea level rose after an interval of low sea level at both time intervals, hypoxic habitats expanded gradually and biodiversity increased slowly initially. Biodiversity increased significantly as hypoxic water environments expanded and many new lineages appeared in both events. Hypoxic environments persisted in a South China shelf basin throughout most of the Late Ordovician sea level changes. Species originations and extinctions continued in this basin in a pattern somewhat similar to the pre-extinction event pattern. Certain new lineages that developed as sea level rose came from stocks in this basin. Post- Late Wenlock graptolite diversification seems to have taken place among lineages that persisted in and then expanded in shelf margin hypoxic environments. Similarity in marine hypoxic water environmental changes in both events is reflected in the pattern of biodiversity recovery after both extinction events.