Geology Reference
In-Depth Information
Preservation of non-rigid soft sponges has been re-
corded from Silurian, Devonian and Jurassic mud
mounds (Bourque and Gignac 1983; Flajs and Hüssner
1993; Reitner and Keupp 1991).
meters) are the large principal frame-building elements.
Microscleres (typically 10 to 100
m long) commonly
occur freely within the soft parts of the sponge. Major
architectural spicule types differentiated according to
the number of axes are monaxons (with a single axis),
triaxones (basically consisting of six rays and three axes
extending at right angles from a central point; restricted
to hexactinellids), tetraxones (four rays radiating from
a central point), and polyaxons (several rays approxi-
mately of equal length extending from a central point).
Desmas are irregularly shaped spicules forming the
skeletal network of lithistid sponges. Further morpho-
logical differentiations use the number of points or rays
on the spicule; resulting terms are often characterized
by the root 'actine'. Sponge spicules may show a cen-
tral canal (Pl. 112/1). Cross sections of calcitized spi-
cules can be confused with radiolarians. Drawings of
common spicule types which help to
typify spicules in
thin sections
and a list of relevant papers can be found
in Rigby (1985).
Sponge spicules
The fossil record of many sponge groups, particu-
larly those of siliceous sponges, consists mainly of spi-
cules which are studied in standard micropaleontologi-
cal samples, but can sometimes also be recognized in
thin sections of limestones (Keupp et al. 1989; see Pl.
78/2, 3).
The composition and general architecture of spicules
are used to classify sponges into classes. The form of
the spicules and the manner in which they are com-
bined to construct the skeletal structure are the basis
for further systematic subdivisions.
A complex terminology has been developed to de-
scribe the morphological variations of mega- and micro-
scleres. Megascleres (length > 100
m to several milli-
Plate 78 Siliceous Sponges
'Siliceous sponges' is a non-taxonomic term referring to sponges characterized by a skeleton consisting of sili-
ceous (opaline) spicules. These sponges comprise taxa belonging to two polyphyletic systematic units with
spicular skeletons, the Hexactinellida and the Demospongiae. Both groups include important reef builders (-> 1,
4, 6), include organisms living on soft and firm bottoms, and contribute to the formation of deep-shelf and
basinal sediments (-> 2). Spicules can be differentiated according to the number of axes and rays and the shape
of the scleres (-> 3). Most figures in the plate display hexactinellid sponges whose siliceous skeletons have been
partly replaced by calcite.
1
Sponge reef limestone
exhibiting hexactinellid sponges with a rigid skeleton. Hexactinellida (or 'glass sponges') are
characterized by triaxon spicules consisting of six rays intersecting at right angles. Arrows point to the canal system. The
sponge is strongly bored (B). Late Jurassic (Treuchtlingen limestone, Kimmeridgian): Bonhof, southern Bavaria, Ger-
many.
2
Spiculite
composed of predominantly monaxone megascleres. Causes for concentrations of spicules are in-place deposi-
tion of spicules derived from the disintegration of soft-bodied demosponges, or an accumulation of spicules of decaying
soft sponges within organic mats. Some current transport is indicated by parallel alignment of the spicules. Early Jurassic:
Roman mosaic stone found in Kraiburg, southern Bavaria, Germany.
3
Different types of sponge spicules
. Megasclere spicules are classified according to the number of axis and to the number
of points or rays on the spicule. C - Monaxon spicule with stalked rings (cricorhabd), M - Monaxon spicule pointed at one
end (style), TE - Tetraxon spicule exhibiting one long and three short rays (triaenes), TR -triaxone spicule with three rays
in one plane (triactines). The siliceous spicules have been dissolved; the molds are filled with granular calcite cement. The
good preservation of the spicules points to slow dissolution and rapid cementation. Late Triassic: North of Isfahan, Iran.
4
Growth form.
Tabular platy hexactinellid siliceous sponges covering fine-grained micritic matrix (coverstone). The arrow
points to the canal system within the wall. The breccioid texture of the matrix is due to burrowing followed by dissolution.
Same locality as -> 1.
5
Constructional grade.
Some hexactinellid sponges exhibit internal segmentation similar to sphinctozoid demosponges
(see Pl. 79). The sample is selectively silicified (white areas). Late Triassic: North of Isfahan, Iran.
6
Preservation of hexactinellid skeleton.
A characteristic feature of sponge structures in thin sections is the fine network
apparently consisting of discontinuous horizontal and vertical elements. Note the voids within the sponge skeleton caused
by dissolution of the skeleton prior to the lithification of the lime mud between the spicules. Same locality as -> 1.
-> 3, 5: Courtesy of B. Senowbari-Daryan (Erlangen)
