Geology Reference
In-Depth Information
including salinity, oxygenation, substrate and water
depths. The group is useful in the biostratigraphic
analysis of marine and nonmarine sediments, particu-
larly in freshwater environments where other fossils
may be lacking. The calcitic shells may be common in
Mesozoic and Cenozoic lacustrine limestones and
brackish-water carbonates as well as in marine shal-
low- and deep-marine limestones all through the Phan-
erozoic.
they sometimes show a characteristic overlap of one
valve by the other. The valves are articulated by teeth
and sockets or ridges and grooves along the dorsal
hinge line. The carapace is frequently pierced by a se-
ries of tiny canals (not obvious in thin sections). Inter-
nally many smooth valves show a recurved shelf (dup-
licature, calcified portion of the inner lamella) along
the free margins (best recognized in cross-sectional
views). The surface of the valves is smooth (Pl. 93/1)
or sculptured (Pl. 93/4) into ridges, ribs, spines or
nodes.
Fossil ostracods are defined by the shape of the cara-
pace, carapace surface sculpture, type of hinge, place-
ment of pores and muscle insertion scars. Only a few
of these criteria can be evaluated from thin sections.
Morphology: Ostracods are characterized by a bi-
valved shell (carapace) that is typically ovate or kid-
ney-shaped in outline. The shell is opened by a dor-
sally located ligament and closed by a set of adductor
muscles. The carapace encloses the soft parts and con-
sists of two valves which overlap each other along the
ventral margin or at all margins. The valves are com-
monly equally shaped but of a slightly different size;
Shell structure: The calcified carapace consists of
an outer and an inner calcitic lamella. The microstruc-
Plate 93 Crustacean Arthropods
Ostracods (-> 1-4) can be recognized in thin sections of limestones by small calcitic shells consisting of more
or less equally shaped but slightly differently sized valves (-> 1), sometimes showing finely prismatic micro-
structures (-> 2) or shell ornamentation (-> 3, 4).
Balanids (barnacles) are sessile cirripedians protected by a calcitic multi-plate skeleton consisting of a massive
or hollow basal plate (-> 6) and articulated wall plates (-> 6, 7). Balanid shells are often associated with
bryozoans and coralline algae (-> 7) as well as benthic foraminifera.
Favreinids (-> 5) are fecal pellets of decapod crabs. These microcoprolites can be easily recognized by their
sieve-like structure caused by calcite-filled canals that form specific patterns.
1
Ostracod-bivalve wackestone. Ostracods are represented by disarticulated valves (black arrows) and complete tests
(white arrows). The percentage of disarticulated carapaces provides a measure of post-mortem modification by tapho-
nomic processes. The bivalve shells (BS) are replaced by coarse calcite. Late Jurassic: Margas de Terenas, Ribadresella,
Spain.
2
Ostracod microstructure . The thick shell consists of two valves showing distinct overlap (O). The carapace exhibits a
finely prismatic microstructure. Note the contrast between the well-preserved ostracod shell and the strongly recrystal-
lized sedimentary matrix. Silurian: Gotland Island, Sweden.
3
Paleozoic pelagic ostracods. The thin section shows complete and disarticulated valves of highly spinose ostracods
belonging to the Entomozoacea (Myodocopida). Entomozoans are common in Devonian basinal sediments and are
interpreted as having been planktonic. The sample comes from limestones deposited in an open-marine deep-shelf
environment. Late Devonian (Famennian): Menorca Island, Spain.
4
Pelagic entomozoan ostracods. Note the spinose ornamentation (arrows) of the shells. Same locality as -> 3.
5
Favreinids. Fecal pellets of decapod crabs. Poorly sorted grainstone consisting of Favreina (cross sections, CS, and
longitudinal sections, LS), tiny peloids and large rounded intraclasts. This microfacies is common in the lagoonal
environments of inner ramps affected periodically by storms. Lowermost Cretaceous (Purbeck facies, Berriasian): Sub-
surface, Kinsau well, Bavaria, Germany.
6
Balanid shell. Cross section of the basal plate. The hollow plate is subdivided by partitions (black arrows). Some wall
plates are displayed in the upper part of the cross section. The barnacle and bryozoans (B) are encrusted by coralline red
algae (white arrows) forming a macroid. Multitaxon macroids indicate unstable substrates. The sample comes from
warm-temperate carbonates (Nebelsick 1989). Late Tertiary (Early Miocene): Eggenburg, Austria.
7
Balanid-bryozoan-coralline algal association. The poorly sorted bioclastic packstone consists of balanids (white arrow
pointing to an oblique section showing some wall plates), bryozoans (B) and coralline red algae (black arrow). This
association is a common attribute of Cenozoic temperate and cold-water carbonates. The irregularly shaped open vugs
(white) are caused by freshwater leaching. Late Tertiary (Early Miocene): Zogelsdorf near Eggenburg, Austria.
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