Porifera-microbialites of the Lower Liassic (Northern Calcareous ...

Introduction 9
1. Introduction
In Upper Rhaetian time, reef evolution culminated in highly diverse ecosystems that notably
flourished along the northern shelf of the former Tethys ocean. Several of these buildups
constructed by branching corals and coralline sponges (Demospongiae with secondary basal
skeletons) are presently exposed in the Northern Calcareous Alps, where they are found both at
the northern and southern margins of the Upper Triassic Carbonate Platform (Flügel and
Kiessling 2002; Stanley 1988). Furthermore, the record of capping Lower Liassic sediments
show that all of the reefs abruptly disappeared at the Triassic-Jurassic boundary, when one of the
five most severe mass extinction occurred (Hallam 1990). The reasons for their sudden demise
are still under discussion and less is known yet about its effects on subsequent Liassic benthic
faunas. While it is postulated that faunal turnover took place also on land, the most severe
extinction rates are known from the marine realm, where they affected mainly invertebrate
groups like cephalopods, bivalves, brachiopods and corals (Hallam 1981; Benton 1991; Hallam
and Wignall 1997). In contrast to the concurrent demise of Rhaetian buildups that served as
“carbonate factories” less is known about the biodiversity in adjacent basins, where low
sedimentation rates prevailed. Since sea-level changes are discussed as playing the most
important role for the decease of many taxa, the fossil record of grey cherty deep water
limestones suggests widely spread populations of siliceous sponges persisting through the
Triassic-Jurassic boundary. As investigations by Mostler (1989a,b; 1990a,b) and Krainer and
Mostler (1997) have shown so far, these sponge faunas were dominated by taxa of the
Hexactinellida and non-lithistid Demospongiae. Further they have shown that the faunal
composition in these settings was hardly effected by the Triassic-Jurassic extinction event and
the total number of hexactinellid sponges continuously increased. Most of the skeletal remains
belong to collapsed non-rigid species, and Mostler had to determine nearly all taxa by solitary
spicules, especially by microscleres that were etched out of the limestones. Hence, the density of
sponge settlement could only be estimated, growth forms are mostly unknown, and collected
material could have been contaminated by dislocated spicules. In Liassic time, after the Rhaetian
reefs drowned, the rise of the sea level apparently triggered a diversification of sponges mainly
in the deep water realm south of the Upper Triassic Carbonate Platform. North of the platform,
sponge faunas occur in distal settings as well (e.g. Krainer and Mostler 1997), but they are also
known from Hettangian red limestones that onlapped the former reef slopes. There the
abundance of sponge spicules indicates that environmental changes and the lack of competition
by corals led to a radiation of sponges from deep water environments onto some of the drowned
buildups. Böhm (1992) already assumed that Liassic sponge diversity in the alpine region was
strongly influenced by the extant Triassic paleorelief.