Middle to Upper Oxfordian sponges of the Polish Jura

Abstract

Oxfordian strata of the Polish Jura make part of the sponge megafacies in Europe and contain sponges within bedded rocks as well as in bioherms. The latter structures include "loose" bioherms, which owe their origin solely to the abundant occurrence of sponges, and "rigid" bioherms having an early-diagenetic framework. The Oxfordian sponge-bearing rocks have been deposited in open-shelf environments, generally below the storm wave base. For taxonomical purposes 68 species have been recognized in the sponge assemblage; 47 species of the class Hyalospongea, 16 of the Demospongea (primarily Lithistida), 4 of the Calcispongea, and 1 of the Sclerospongea. No more than 11 species are common in the assemblage; they account for 74% of the total number of specimens. A peculiar trend is discernible in this Oxfordian sponge assemblage: lithistids dominated at the beginning of the Oxfordian, but hyalosponges gradually increased in abundance through time and achieved pronounced dominance at the end of the Oxfordian. Recurrent changes in relative ·abundance of hexactinellid and lithistid sponges have in fact been recorded in the Middle and Upper Jurassic of Europe. The systematic part of the paper includes descriptions of all the collected species.