Geology Reference
In-Depth Information
Diving in tropical coral reefs resulted in the discov-
ery of puzzling sponges restricted to reef caves and deep
forereef areas with low light and strict oligotrophic con-
ditions in the West Atlantic and Indo-Pacific region (Fig.
10.35). These sponge, called
sclerosponges
by Hartman
and Goreau (1972), are characterized by a basal cal-
careous skeleton by which the animals are attached to
the substrate (Fig. 10.35). The living tissue is restricted
to a thin veneer upon or within the uppermost few mil-
limeters of the skeleton and contains, usually but not
always, tiny siliceous spicules. The taxa were first re-
garded as a separate class of the Demospongiae, but
subsequent to the recognition of the polyphyletic char-
acter of these sponges, they are now attributed to dif-
ferent families of coralline sponges.
The skeleton of these sponges consists of aragonite
or Mg-calcite. It is organized into tubes and calyces
(ceratoporellid type), tabulated tubes (chaetetid type),
a more or less regular network of the primary skeleton
with horizontal and vertical elements (stromatopor-
oid type), or a chambered skeleton (sphinctozoid or
thalamid type). Comparing the morphological criteria
of sclerosponges and other 'living fossils' with the cri-
teria of fossil groups of until then unassigned system-
atic position, such as the stromatoporoids and cha-
etetids, has now led to the assignment of these groups
as well as sphinctozoans and inozoans to the demo-
sponges. Some authors oppose this systematic treat-
ment and regard these groups primarily as different
grades of the morphological organization of skeletons
occurring independently in different sponge classes
(Reitner 1989; Wood 1991; Reitner and Wörheide
2002).
reefs and Late Triassic Dachstein reefs in the Northern
Calcareous Alps; Fig. 10.36, Pl. 116/2).
The first inozoans are known from the Carbonifer-
ous. Permian inozoan sponges were common both in
non-reef and reef environments (Rigby et al. 1989, Fan
et al. 1991, Rigby and Senowbari-Daryan 1996). The
group reached a maximum in the Late Triassic (Senow-
bari-Daryan et al. 1997) and again in the Middle and
Late Jurassic (Müller 1984). The last inozoans with a
rigid non-spicular skeleton were described from the
latest Cretaceous.
Stromatoporoids
Stromatoporoids are extinct fossils known from the
Early Ordovician to the Late Devonian and again in
the Mesozoic. The relations between Paleozoic and Me-
sozoic taxa are still open to discussion. Both Paleozoic
and Mesozoic stromatoporoids have been placed into
the Hydrozoa but are now recognized as Porifera. The
fossils are represented by calcified skeletons of domical,
bulbous, branching or columnar form, and ranging from
a few centimeters to meters in diameter. Internally the
skeleton exhibits elements more or less parallel and
perpendicular to the surface. Distinct aquiferous canal
systems (astrorhizae) occur at the surface and internally.
The canals converge towards a center and are radially
arranged. No traces of spicules have been found in Early
and Mid-Paleozoic stromatoporoids. In this feature Pa-
leozoic stromatoporoids differ from Mesozoic stromato-
poroids, some of which contain spicules. These taxa,
including many former 'hydrozoans' (see Pl. 81), are
placed into the Demospongiae. Many authors regard
the Paleozoic stromatoporoids as a separate class of
the Porifera with close relationships to other poriferans
secreting calcareous skeletons.
Overview of major groups of coralline sponges:
The
following text and the plates summarize the criteria,
distribution and significance of sphinctozoan and
inozoan sponges (Pl. 79), stromatoporoids (Pl. 80) and
chaetetids (Pl. 81) as well as archaeocyaths (Pl. 82).
Morphology and classification:
A glossary of the
morphological terms applied to the Paleozoic Stromato-
poroidea and a comprehensive overview of the current
state of classification, including a critical list of the gen-
era was provided by Stearn et al. (1999). Criteria used
in the taxonomic discrimination include the type of skel-
etal elements, their microstructure and features of the
exhalant water system represented by astrorhizal ca-
nals and central tubes. Some microstructural types are
identical with those of other coralline sponges and are
therefore regarded as primary skeletal structures.
Sphinctozoans and inozoans
Diagnostic morphological criteria of chambered
sponges (sphinctozoans) and non-chambered calcare-
ous sponges (inozoans) are shown in Plate 79. Sphincto-
zoans appear early in the Paleozoic, diversified and
dominated during the Permian and Triassic, and became
nearly extinct after the Cretaceous (Senowbari-Daryan
1990; review and extensive bibliography in Senowbari-
Daryan and Garcia-Bellido 2002). Sphinctozoans and
inozoans as well as other coralline sponges contributed
significantly to the formation of Late Paleozoic (e.g.
Permian Reef Complex in Texas, Pl. 145/1) and Middle
and Late Triassic reefs (e.g. Middle Triassic Wetterstein
Chaetetid coralline sponges and tabulozoans
Thin sections of Late Paleozoic and Mesozoic reef
limestones may exhibit sections of fossils composed
of calcareous tubes, sometimes with tabula-like hori-
zontal partitions. In the context of microfacies studies
