Anthaspidelliden (sponzen) uit het Oostzeegebied

The anthaspidellids are an important Palaeozoic group of lithistids which are well-represented, both in number and in species, among the river- and ice-transported sponge erratics, found in secundary resting places in northern and western Europe (among others on Gotland, in the Netherlands and in northern Germany inclusive of the island of Sylt). These erratics originate from eroded, for a substantial part submarine, Palaeozoic sedimentary rocks in the eastern to northern part of the Baltic area. A brief review is given of the development of this group through the Ordovician period. The anthaspidellids were ubiquitous and not only inhabited the shallow open continental shelf, but were also common reef dwellers. By Middle Ordovician times many reefs and bioherms were built and dominated by anthaspidellids. One of the geologically oldest and first reported representative of this group is the genus Archaeoscyphia HINDE, 1889, the earliest Ordovician reef building metazoan. A well-preserved Baltic specimen of this genus is presented in this paper, together with some instructive examples of the younger Ordovician genus Calycocoelia BASSLER, 1927, which is a fairly commonly occurring sponge among the erratics of Baltic origin. All the specimens described here are from the excellent collection of Mr. U. von HACHT, Hamburg, and were collected from arenaceous fluvial deposits of young Neogene age on Sylt. Already by Late Ordovician, and certainly during Silurian times, the anthaspidellids had withdrawn from the reef environment and were replaced by algae, stromatoporoids and corals. Astylospongiids and hindiids flourished in the Silurian but apparently they never occupied the reefs as did their precursors, the anthaspidellids. Up to now bodily preserved Silurian sponges have not been reported from the carbonaceous stratified and biohermal bedrock of Gotland and Esthonia. In addition, loose lithistid spicules appear to be far from commonplace fossil remains within the same bedrock. Therefore, it seems likely that sponges formed only a minor benthic faunal element in the Baltic Silurian shelf sea. This may account for the apparent lack of unquestionable, Silurian Baltic material amongst the erratic sponge assemblages in Europe.