Geology Reference
In-Depth Information
Plate 81 Calcareous Chaetetid Sponges and Other Still Disputed Taxa
The plate shows a selection of Triassic and Jurassic fossils characterized by uniform or differentiated calcareous
tubes and sometimes summarized under the name 'tabulozoans'. This term should be forgotten. With the discov-
ery of living, very similar animals (sclerosponges and related forms), it was learned that most of these fossils are
sponges although the former systematic assignment of some taxa to hydrozoans (-> 3, 5) has not yet been
completely rejected.
Figs. (->1-4 and 6-8) display some common chaetetid coralline sponges. The skeleton of chaetetids is formed
in two steps. Skeleton formation starts with the precipitation of a spicular primary skeleton, followed by the
formation of a secondary skeleton consisting of calcareous walls and horizontal partitions (see schematic draw-
ings on this page). Important diagnostic criteria used in taxonomic differentiations are the morphology and mode
of the formation of the tubes, the primary mineralogy and microstructure of the walls, the type of the horizontal
partitions (tabulae; -> 8) and the mode of the formation of new tubes (-> 7, 8, and schematic drawings on this
page).
Chaetetids range from Ordovician to Miocene and peaked in the early Late Carboniferous, where they formed
shallow-marine biostromal reefs. These reefs were constructed more or less only by chaetetids, in contrast to
Late Jurassic reefs where chaetetids played only a small part in the association of stromatoporoid sponges and
corals. Chaetetids in Early Cretaceous rudist reefs were usually dwellers.
1
Different types of coralline calcareous sponges.
The reef limestone contains an assemblage of spongiomorphids (S),
sphinctozoan sponges (SS), inozoan sponges (IS) and skeletons composed of calcareous tubes (formerly summarized as
tabulozoans, T). The latter exhibit the chaetetid constructional type known from sclerosponges. Note the different preser-
vation of spongiomorphids and inozoans as compared with sphinctozoans and chaetetids, pointing to differences in the
original skeleton mineralogy. Many Mesozoic chaetetids had an aragonitic skeleton (Wendt 1990; Mastrandrea and Russo
1995). Spongiomorphids have been regarded as corals or hydrozoans but an assignment to sponges is now under discus-
sion. Late Triassic (Dachstein reef limestone, Norian): Hoher Göll, Bavarian Alps, Germany.
2
Preservation of chaetetid skeletons:
The infill of micrite into the peripheral part of the skeleton simulates the existence of
a morphological differentiation.
Blastochaetetes
sp. Same locality as -> 1.
3
Astrostylopsis
(A), a sponge with Mg-calcite skeleton formerly regarded as hydrozoan, encrusted by a chaetetid (C;
Bauneia
sp.) with preserved aragonitic skeleton. Note the difference in color in transmitted light between Mg-calcite and
aragonite. Late Jurassic: Jebel Zhaguan, Tunisia.
4 Oblique longitudinal section of a chaetetid. Same locality as -> 1.
5
Ellipsactinia
(E), sometimes regarded as a hydrozoan, and
Astrostylopsis
(A) assigned to coralline sponges. The construc-
tional pattern of
Ellipsactinia
corresponds to the stromatoporoid type. Ellipsactiniids contributed significantly to the
formation of Late Jurassic and Early Cretaceous platform-margin reefs. Late Jurassic: Central Apennines, Italy.
6
Ptychochaetetes globosus
Koechlin exhibiting distinct growth zonation. Late Jurassic (Tithonian): Krahstein near
Mitterndorf, Styria, Austria.
7
Ptychochaetetes
. Longitudinal section. The mode of the formation of new tubes (here basal budding) is used as a generic
criterion. Late Jurassic (Kimmeridgian): Vöksen/Deister, Lower Saxony, Germany.
8
Neuropora,
a chaetetid sclerosponge formerly attributed to bryozoans. Longitudinal section exhibiting densely spaced
cross partitions. New tubes are formed by intramural budding. Same locality as -> 6.
9
Spongiomorpha ramosa
Frech showing a fine reticulate pattern consisting of crenulated vertical elements. The assign-
ment of spongiomorphids to hydrozoans is now questioned and sponge affinities preferred. Spongiomorphids were com-
mon in deeper subtidal parts of Triassic to Early Cretaceous reefs and platforms. Note that the term 'spongiomorphid' is
also loosely used for sponge-like fossils! Late Triassic (Rhaetian): Steinplatte, Tyrol, Austria.
-> Figure on this page: Cremer (1993)
