Geology Reference
In-Depth Information
these fossils have often been called 'tabulozoans', us-
ing a non-systematic term proposed by Kühn (1942).
Plate 81 displays some of these taxa that were formerly
assigned to various fossil groups, including hydrozo-
ans, tabulate corals and sponges. Most taxa, particu-
larly the chaetetids, are now recognized as sponges re-
lated to calcified Demospongiae.
The
chaetetids
compose a small group of organisms
that was presumed to be tabulate corals, but are now
attributed to sponges as a result of the discovery of a
recent representative. The rigid skeleton corresponds
to a cluster of closely packed nonporous calcareous
tubes with horizontal floors (tabulae).
and about 600 mm. The average diameter of adult forms
is 5 to 25 mm. The height can be more than 300 mm.
The classification is based on ontogenic develop-
ment stages, intervallum structures and pore systems.
Generic determination needs oriented transverse, lon-
gitudinal and tangential thin sections. Transverse
sections can be confused with corals and dasyclad
green algae, but most corals lack a distinct double
wall. Transverse sections of dasyclads are smaller,
and the simple calcareous walls of the algae are pene-
trated by small pores.
Ecology and facies:
Archaeocyaths were exclusively
marine filter-feeders living on fairly energetic ramps
and platforms. Solitary and colonial archaeocyaths were
important constituents of the first metazoan reef com-
munities but also occur in subtidal bottom-level asso-
ciations. Biohermal and biostromal reefs are believed
to have been formed in shallow water depths (about
20-50 m). Depth estimates are based on the abundant
association of archaeocyaths and various calcimicrobes
(
Renalcis
,
Epiphyton
; sometimes quantitatively more
important than archaeocyaths), current-produced sedi-
mentary structures, and the abundant fragmentation and
resedimentation of archaeocyathan skeletons. Associ-
ated biota include trilobites, hyalithids, various shells
and fossils with supposed cnidarian affinities. Many
limestones with archaeocyaths are boundstones or bio-
clastic and peloidal floatstones and wackestones.
Spicules have been described from Mesozoic chae-
tetids. Fossil chaetetids appear in the Ordovician. They
contributed to the development of Paleozoic (particu-
larly Late Carboniferous) and Mesozoic reefs, both
overgrowing and serving as substrate for other reef-
building organisms.
Chaetetid coralline sponges were important reef-
builders both in the Paleozoic and Mesozoic. In Middle
Carboniferous platform and platform-margin settings
they formed small mounds and broad low banks where
they were often associated with beresellid and ungdar-
ellid algae (see Pl. 56/6, 7 and Pl. 60/2, 5). In the Trias-
sic, Late Jurassic and Early Cretaceous chaetetids lived
in open-marine platform environments and in reefs.
Archaeocyaths
This is an extinct group of Cambrian fossils now re-
garded as Demospongiae with uncertain affinities (Pl.
82/1-3). The group appears near the base of the Cam-
brian (Early Tommotian) and had a worldwide distri-
bution in the Early Cambrian (Atdabanian and Botom-
ian). Some relicts persisted in Middle and Late Cam-
brian. Compositional differences of faunas from Sibe-
ria, Europe, Northern America, North Africa and Aus-
tralia indicate distinct provincialism.
Basics: Sponges
(publications which assist in taxonomic dif-
ferentiation are marked by an asterisk)
Overviews
Broadhead, T.W. (ed., 1983): Sponges and spongiomorphs.
Notes for a short course. - 220 pp., Knoxville (Univer-
sity of Tennessee)
Hartman, W.D., Wendt, J., Wiedenmayer, F. (1980): Living
and fossil sponges. - Sedimenta,
8
, 274 pp.
Hooper, J.N.A., Van Soest, R.W.M. (eds., 2002): Systema
Porifera. A guide to the classifications of sponges. 2 vol-
umes. - 1708 pp., New York (Kluwer Academic/Plenum)
Reitner, J., Keupp, H. (eds., 1991): Fossil and recent sponges.
- 595 pp., Berlin (Springer)
Morphology and classification:
The skeleton con-
sisting of microgranular Mg-calcite corresponds to in-
verted cones or cups exhibiting a distinct central cav-
ity surrounded by porous walls formed by one or two
elements . The 'intervallum' between the outer wall and
the highly variable inner wall contains a variety of ra-
dially or irregularly arranged partitions (blade-like
septa; irregular curved plates or subradial pseudosepta).
These elements can be connected by bars. The lower
end (tip) of the cone was buried in soft sediment or,
more commonly, rooted or encrusted onto hard sub-
strates. The diameter of the cups changes with the on-
togenetic stages and varies between a few millimeter
Siliceous sponges
Brachert, T. (1991): Environmental control on fossilization
and siliceous sponge assemblages: a proposal. - In:
Reitner, J., Keupp, H. (eds.): Fossil and recent sponges.
- 543-553, Berlin (Springer)
*Krautter, M. (1997): Aspekte zur Paläökologie postpaläo-
zoischer Kieselschwämme. - Profil,
11
, 199-324
Krautter, M. (2002): Fossil Hexactinellida: an overview. -
In: Hooper, J.N.A., Van Soest, R.W.M. (eds.): Systema
Porifera. A guide to the classifications of sponges, 2. -
1211-1222, New York (Kluwer Academic/Plenum)
