Late Ludfordian and early Přidoli monograptids from the Polish Lowland

Graptolites etched from the Mielnik-I wellcore (E Poland) reveal the main features of the development of monograptid faunas within the late Ludfordian-early Pi'idoli interval. Fifteen species and subspecies are described and Monograptus (Slovinograptus) subgen. n. as well as Neocolonograptus gen. n. are erected. Morphology of many species has been described adequately for the first time and their systematic position corrected. Four graptolite zones of the late Ludfordian are distinguished. The late Ludfordian fauna, which appears after the kozlowskii Event, is composed mainly of immigrants dominated by hooded monograptids. They reappear as a result of the Lazarus effect. Some of them initiated the lobate-spinose phyletic line terminating with Monograptus (Uncinatograptus) spineus, a highly characteristic index species. The lobate and the lobate-spinose types are accompanied by bilobate forms (Pseudomonoclimacis latilobus). The graptolite sequence indicates that the appeara nce of the early Pfidoli fauna was preceded by a biotic crisis, namely the spineus Event. Therefore this fauna is made up of a few holdovers and some new elements which developed from Pristiograptus dubius stem lineage (Neocolonograptus gen. n., Istrograptus Tsegelnjuk). This early assemblage, composed of bilobate forms, was later enriched by hooded monograptids, reappearing after the spineus Event. Both the late Ludfordian and early Pi'idoli assemblages abound in instances of homeomorphy and generic transitions. The latter occur due to independent acquisition by some forms of simple thecal characters which allows their assignement to a new genus. Another remarkable feature of Late Silurian monograptids is reducti on in the number of sicular rings, and establishing of a steady relation between one of the rings and the virgella. This character state was independently achieved as a structural grade, by all, otherwise unrelated, phyletic lines of Late Silurian monograptids.