Geology Reference
In-Depth Information
Box 10.5.
Describing sponges in microfacies thin sections
involves a bewildering number of valid and outdated terms
for sponge groups, some of which are systematic units, whereas others are combined by rather loose definitions. The
definitions listed in the table are based mainly on the articles included in the comprehensive treatise edited by Hooper
and Van Soest (2002).
Archaeocyatha:
Extinct sponges characterized by calcar-
eous cups and a non-spiculate skeleton consisting of
Mg-calcite. Now regarded as a group (class?) of Demo-
spongiae with uncertain affinities.
Calcarea:
Porifera have a skeleton composed entirely of cal-
cium carbonate. Characterized by free, rarely linked di-,
tri-, tetra- and/or polyactine spicules to which a basal
skeleton can be added. Comprise less than 5% of mod-
ern sponges.
Calcareous sponges:
The non-systematic term is used in a
rather loose manner for 'calcisponges' exhibiting cal-
cite spicules or a calcareous skeleton.
Chaetetida:
Formerly regarded as a group of tabulate cor-
als. Now considered as a name for a constructional grade
occurring in different sponge groups. In microfacies
studies the name is commonly used in an informal way
referring to fossils composed of densely packed, thin
calcareous tubes with horizontal partitions.
Coralline sponges:
A polyphyletic group with affinities
both to Demospongiae and Calcarea.
Demospongiae:
Porifera with siliceous spicules and/or fi-
brous spongin skeleton, or occasionally without a skel-
eton. Spicules are monaxon (either monactine or
diactine) or tetraxon (tetractine), never triaxon. Com-
prise about 85% of all living sponges. Minor groups
have developed a hypercalcified basal skeleton consist-
ing of aragonite or Mg-calcite in addition to other ele-
ments, or exhibit a solid aragonite structure lacking free
spicules.
Hexactinellida:
Sponges defined by siliceous triaxon
hexactine spicules or derivations from that basic form.
Comprise about 7% of the described recent sponges.
Important members of deep-marine communities.
Inozoa:
Originally a name for coralline sponges without
segmentation. Now applied to non-segmented calcare-
ous sponges with spicules. Note that the term 'inozoid'
is used in a non-systematic context referring to non-
segmented calcareous sponges, but also as the name
for a systematic group.
Ischyrospongia:
A name introduced for a group which
should include stromatoporoids, sclerosponges and
pharetronids. The term should be abandoned but is still
in use in papers dealing with microfacies of Paleozoic
limestones.
Lithistids:
A polyphyletic group of demosponges charac-
terized by articulated spicules (desmas) forming a rigid
skeleton.
Pharetronids:
A term including Sphinctozoa and Inozoa.
Also used for calcareous sponges exhibiting spicules.
Sometimes considered as a subgroup of the Calcarea.
The term is now only rarely used.
Sclerosponges
: A polyphyletic group of demosponges char-
acterized by a calcareous basal skeleton with embed-
ded siliceous spicules and different constructional grades
resembling those of stromatoporoids and chaetetids.
Siliceous sponges:
Commonly used as an informal term
for sponges with a skeleton composed of siliceous spi-
cules. Include demosponges as well as hexactinellids.
Sphinctozoa:
Originally proposed as a term for chambered
sponges with a rigid calcareous skeleton. Now defined
by an external rigid skeleton and chambered organiza-
tion. The name should not be used in a systematic con-
text; it includes sponges that are polyphyletic in origin.
Chambered construction has developed independently
several times in different sponge groups and occurs in
demosponges, Calcarea and hexactinellids. Most
'sphinctozoans' are attributed to the Demospongiae
.
Stromatoporoidea:
Until the 70s of the twentieth century
commonly regarded as hydrozoans. Giving credit to the
affinities between the stromatoporoid skeletal structure
and that of some recent sclerosponges, the group is now
placed into the Porifera, and regarded as being related
to Demospongiae, or as a separate class of sponges.
Thalamida:
Synonymous with Sphinctozoa.
The skeleton consists of discrete, articulated or fused
spicules (Fig. 10.33) and/or a hypercalcified mineral-
ized basal skeleton.
terpreted as grades of construction and not phylogenetic
clades, although the assignment of these groups and their
lower taxa to the established classes is largely unre-
solved.
Major groups:
The subdivision of sponges into sili-
ceous and calcareous sponges is anything but correct,
but in the context of facies analyses still in use. In fact,
there are three distinct classes (Hexactinellida, Demo-
spongiae, and Calcarea, all appearing in the Late Pro-
terozoic), and the extinct Cambrian class Archaeo-
cyatha having suspected affinities wih Demospongiae.
Several predominantly fossil groups including the
Sphinctozoa, Stromatoporoidea and Chaetetida are in-
Ecological constraints on modern sponges:
Sponges
are benthic, almost exclusively marine organisms. They
occur at all latitudes, and from the intertidal to the deep
sea, down to more than 5000 meters. A small group of
freshwater sponges originated in the Cretaceous. The
distribution of modern sponges in shallow and deep-
marine environments is controlled by nutrients, sub-
strate, sediment input, and water energy:
