Ordovician erratic sponges from Gotland, Sweden

Erratic Ordovician sponges (Porifera) from Gotland in the collections of the Swedish Museum of Natural History in Stockholm and the Natural Museum in Visby were subject of an inventory, the results of which are presented in this paper. The sponges are part of an extensive and diverse Upper Ordovician sponge fauna that lived partly in tropical shelf areas and partly in a continental basin of the palaeocontinent Baltica. As a result of their transportation most of these sponges are isolated silicified bodies without adhering sediment. Since no major Ordovician sponge fauna has been found in solid rocks of Baltoscandia, the source area of these erratics is unknown. The sponges from Gotland were deposited during the Weichselian glaciation. They have been compared with erratic Ordovician sponges from Germany, The Netherlands, and Poland by a quantitative method of research. Generic composition of the sponge association from Gotland is identical to that from Sadewitz/Sawidowice (Poland), which was deposited during the Saalian glaciation. There is also a similarity to the majority of sponges in the Dutch/German border region that were deposited in glaciofluvial sediments during the Menapian (Early Pleistocene). The characteristics of the sponges from the Lausitz area (Germany) and from the Isle of Sylt (Germany), which were deposited in Middle Miocene and Late Pliocene time, respectively, are different. A supposed bipartition of these associations is discussed and related to the palaeogeography and palaeoecology in the Late Ordovician. Extinction of the Baltic sponge fauna is possibly related to the glaciations during the Hirnantian (Late Ordovician). The importance of this fossil sponge fauna is emphasized.