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

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new data from recent species to their fossil counterparts. Taking into account the classification of
living Hexactinellida that has recently been revised (Hooper and Van Soest 2002), the non-rigid
sponges from the Hettangian of Adnet seem to represent specimens of the Rossellidae and
Euplectellidae. A lophophytous substrate attachment is known from recent species of both
groups (Tabachnick 1991), whereas tubular growth, big hexactin and pentactin megascleres, and
varying types of dermal spicules allow attribution of some of the Adnet species to the family of
Euplectellidae and especially to the subfamily Euplectellinae. Other specimens show cup-like
shapes and a predominance of diactin parenchymalia, that are rather features of the Rossellidae.
Recent living Rossellidae often exhibit pentactin hypodermalia and anchor spicules, but in Adnet
only smooth diactins functioned as prostalia basalia. On the other hand, skeletal remains of any
sieve-plates, that may cover the osculum of euplectellid species, have not been detected in the
Adnet facies. Additionally, the hexactinellid specimens in the Schnöll facies display several
types of dictyonal skeletons. Small bulbous or globular specimens without a distinctly preserved
aquiferous system (dictyorhyse system?) probably belong to Euretidae. The tubular or cup-
shaped species with a schizorhyse system of canals more likely represent sponges of the family
Tretodictyidae.
While most Triassic reef building coralline sponges (Demospongiae) disappeared concurrent
with the demise of Upper Rhaetian coral reefs, sponge evolution and radiation persisted in
Liassic deep marine environments. But there is only a minor knowledge about extension and
taxonomic composition of these Lower Liassic sponge communities. Since the classification of
Adnet sponges is highly restricted it becomes difficult to compare this fauna with the few sponge
associations that were described by Mostler (1989a,b; 1990a,b) from basinal settings. In addition
to the eight morphotypes that were identified in Adnet, quantitative analyses have shown that
most of the observed sponge remains belong to the group of Lyssacinosida. The sponge faunas
from Liassic cherty limestones that Mostler has studied are also dominated by non-rigid species,
so that it appears quite obvious that colonization of slope settings, like the one at Adnet reef, was
most likely triggered by the immigration of species from nearby deep-water settings. The faunas
from such basinal limestones apparently show relatively high diversities (several lineages of
Hexactinosida, Lyssacinosida, Amphidiscosida), whereas in Adnet a lot of specimens exhibit
identically growth forms, that seem to reflect a high-quantity/low-diversity fauna. If that is true,
the Adnet fauna most likely represents a sponge association of mainly opportunistic taxa. In the
sense of Harries et al. (1996) such “taxa are persistent, common members in pre-extinction
environments, suppressed by competition from equilibrium species. […] opportunists are
capable of prolific population expansion and rapid biogeographical dispersal into stressed
environments. […] These are commonly pioneer species and represent an early phase of
recolonization of vacated ecospace.” Following up this assumption, mainly lyssacinosid species