Taxonomic revision of Late Triassic 'Lithocodium aggregatum Elliott' (Northern Calcareous Alps, Austria)

摘要

Late Triassic forms of Lithocodium aggregatum Elliott are revised taxonomically based on thin-section material from the Late Rhaetian of Mount Steinplatte, Tyrol, and Adnet, Salzburg. They are interpreted as the boring galleries (chambers and canals) of sponges (ichnogenus Entobia), excavating into microbial-oncoidal crusts and framebuilders. From the roof of these chambers, branching canals exhibiting successively decreasing diameters transect the crusts, while pioneering filaments bored downwards from the chamber bases into the bioclastic substrate. The traces of this activity are preserved as a micritic network. Irregular globular bodies displaying a thin microcrystalline wall occur inside the cement-filled chambers. These are interpreted as remnants of the former sponges (ectosome). The occurrence of abundant boring sponges in Late Triassic times could be linked to the biological crisis postulated around the Triassic-Jurassic boundary.