Geology Reference
In-Depth Information
replacement of the former sponge tissue by
in situ
pre-
cipitated carbonate cement (automicrite, Sect. 4.1.2) and
(b) early or late dissolution and replacement of the sili-
ceous skeleton by calcite. Dissolution depends on pres-
sure, temperature and the type of modification. Skel-
eton opal is relatively stable under normal marine con-
ditions (pH about 8). Dissolution, therefore, requires
changes in pH values and an increase in carbonate al-
kalinities. An increase in pH values may be caused by
(1) bacterial degradation of organic matter including
ammonification, or desulfurication triggered by sulfur
bacteria, or (2) removal of CO
2
due to changes in physi-
cal parameters. The importance of these processes in
pH changes is still a matter of discussion (Fritz 1958;
Reitner et al. 1995; Rehfeld 1996).
from soft bottom deeper-water environments and are
abundant in Late Jurassic ramp and basinal settings
where they formed extensive and widely distributed
reefs.
During the Late Cretaceous a major turnover in
demosponges took place, as demonstrated by the highly
diverse sponge faunas of the Chalk facies in northwest-
ern Europe. The adaption of siliceous sponges to deep-
water environment was continued during the Tertiary.
Coralline calcareous sponges
Morphology and classification:
Coralline sponges
are defined as sponges with a secondary rigid calcare-
ous skeleton in addition to a spicular skeleton. Rigid
calcareous skeletons occur in different sponge groups
including sphinctozoan and inozoan sponges, stromato-
poroids and chaetetids as well as archaeocyaths. The
great debate on the assignment of these groups to the
major sponge classes was strongly triggered by study-
ing modern sclerosponges that exhibit morphological
criteria occurring in different fossil groups.
Distribution of siliceous sponges:
During the Paleo-
zoic siliceous sponges flourished in quiet-water envi-
ronments, contributing to the formation of mud mounds.
In the Mesozoic siliceous sponges are abundant in the
Jurassic and Cretaceous. Liassic sponges are known
Plate 82 Archaeocyatha
Archaeocyaths represent an extinct group of benthic, sessile organisms that lived in Cambrian equatorial open-
shelf and reef environments. The cup-shaped, vase-like or cylindrical calcareous skeletons are characterized by
a large central cavity surrounded by a concentric porous double wall (->1, 2) or by a porous single wall (-> 3).
The space between the inner and the outer wall (intervallum) is subdivided by radial-longitudinal and/or radial-
transverse partitions (septa, flat tabulae, curved cysts). The arrangement of these partitions, their pore systems
and the constructional pattern are of taxonomic importance (-> 1, 2).
Archaeocyaths are often associated with calcimicrobes (-> 4, 5) and form the substrate for microbes and
submarine carbonate cements, thus contributing to the formation of Early Cambrian reef mounds and reefs.
1
Double-walled archaeocyaths:
Various sections of double-walled archaeocyaths. Cross- and oblique sections (CS, OS)
show the central cavity bounded by porous inner and outer walls. The intervallum is subdivided by septa. The septa are
finely porous, as shown in a tangential section (TS). These structures exhibit the characteristic archaeocyathid construc-
tional type of the skeleton. Other constructional types used in taxonomic differentiations are indicated by black and white
arrows (vesicular and dictyonid network respectively). The lower part of the figure below the wavy line shows a parallel
section to the upper part taken within a distance of about 2 mm. Note the apparently different filling of the central cavity.
Early Cambrian: Western Antarctica.
2
Archaeocyath morphology:
Cross section (CS) of double-walled archaeocyaths and oblique section (OS) of archaeocyaths
corresponding to another constructional type characterized by septa arranged in a network form. The white arrow points
to tubular cyanobacteria, the black arrows to juvenile archaeocyathids. SMF 6. Early Cambrian: Sardinia, Italy.
3
Single-walled archaeocyath:
Oblique cross section. The tubelike structures have been described as
Proaulopora
(P) and
were referred to green and red algae and cyanobacteria as well. This genus indicates an Early Cambrian age. Early
Cambrian: Same locality as -> 2.
4
Associated calcimicrobes:
Many archaeocyaths occur in association with calcimicrobes that use the archaeocyaths as a
substrate. The figure shows
Epiphyton
. The disintegration of the erect-growing micritic branches of
Epiphyton
and other
calcimicrobes provides coarse silt to very fine sand-sized fragments that lack characteristic microstructures and therefore
correspond to peloids. Cyanobacterial peloids contributed greatly to the production of carbonates deposited adjacent to
Early Paleozoic reef mounds (Coniglio and James 1985). Same locality as -> 2.
5
Associated calcimicrobes:
Widely distributed Cambrian calcimicrobes are
Renalcis
(R) and
Epiphyton
(E).
Renalcis
is
characterized by clusters of thick-walled botryoidal chambers, with the larger ones enveloping smaller ones.
Epiphyton
exhibits micritic, narrow dendritic filaments. Both taxa are known from Early Cambrian to Late Devonian. Same locality
as -> 2.
-> 2-5: Courtesy of M. Boni (Napoli) and T. Bechstädt (Heidelberg)
