Geology Reference
In-Depth Information
Shells consisting of calcite prisms are a common fea-
ture of rhynchonellid brachiopods. The basic types of
brachiopod shell microstructures were summarized by
Williams (1990).
In punctate brachiopods the shells are perforated by
tiny open tubes extending almost to the outer surface
(e.g. terebratulids). In pseudopunctate brachiopods
(most strophomenids, including productids and chon-
etids) the shell has solid rods of calcite reminiscent of
true punctation that end within the fibrous layer; the
fibers contributing to the formation of pseudopunctae
are buckled into a series of superimposed cones. In
impunctate brachiopods the shells are imperforate and
dense all the way through (pentamerids, most rhyn-
chonellids and spiriferids). The first punctated (endo-
punctated) shells occur in the Ordovician. Pseudo-
punctate and impunctate shells are already known from
the Cambrian.
chiopods are also found in temperate and cool waters
of normal salinity. Many articulate species are deep-
water specialists and occur in depths down to several
hundreds of meters and more. Inarticulate forms, e.g.
Lingula , live in relatively shallow coastal waters.
Brachiopod shells as environmental recorders: Bra-
chiopod shells are widely used as paleoenvironmental
indicators in the reconstruction of ancient seawater tem-
peratures (Veizer et al. 1986; Curry and Fallick 2003).
Because Low-Mg calcite is less susceptible to diage-
netic alteration, it is more likely that brachiopod shells
have retained the isotopic composition in equilibrium
with the environment in which the organisms lived.
Possible diagenetic alterations are commonly deter-
mined by cathodoluminescence (Barbin and Gaspard
1995).
Distribution: Brachiopods appeared in the Cambrian,
were abundant in the Paleozoic and most diversified in
the Devonian. There was a distinct drop in the number
of taxa following the Permian-Triassic extinction event,
but brachiopods continued to be part of the shelf fau-
Ecology: Brachiopods are active filter feeders com-
monly attached to hard and firm substrates. Modern
brachiopods live in all parts of the sea as sessile organ-
isms attached to the substrate by a pedicle. Today bra-
Plate 86 Brachiopods
The plate displays thin sections of articulate brachiopods characterized by two valves that are bilaterally sym-
metrical but dissimilar in size and shape (-> 1, 3).
Internal structures are represented by a calcareous framework (brachidium) that supports the food-gathering
system of the soft body. This framework takes several forms, e.g. calcareous ribbons coiled in helical spirals
(-> 1), or elements supporting the brachidium (-> 3).
Characteristic microstructures distinguish brachiopod shells from bivalve shells (-> 1, 5).
1
Microstructure and internal elements: Cross sections of articulate spiriferid brachiopods. Note the well-preserved inter-
nal brachialia (B), the median septum (MS) and the low-angle finely layered microstructure of the calcitic impunctate
shell (arrow) characteristic of brachiopods. SMF 8. Middle Triassic (Anisian): Olang Dolomites, Southern Alps, Italy.
2
Richthofenia . Richthofeniacean brachiopods (order Strophomenida) differ from other brachiopods by a cone-shaped pedicle
valve fixed by the apex to the seafloor and anchored by spines. The brachial valve acts as a lid-like structure. These
brachiopods, together with sponges and microbes, contributed to the formation of Permian reefs in tropical and temperate
paleolatitudes. The group is restricted to the Permian. Oblique section exhibiting the open framework of the pedicle valve.
Late Permian: Chios Island, Greece.
3
Internal elements. Cross section of a smooth brachiopod shell cut parallel to the hinge line exhibiting the larger pedicle
valve (right) and the smaller brachial valve. Note the broken interior double plates (arrows) corresponding to supporting
elements of the brachialia. Bioclastic wackestone. Early Devonian (Emsian): Erfoud, southern Anti-Atlas, Morocco.
4
Brachiopod coquina. Most shells are cut longitudinally. Note microbial encrustations (arrow). Rock-building accumula-
tions of brachiopod shells are common on shelves and in slope settings. Flanking beds of a microbial-stromatoporoid
buildup. Late Devonian (Frasnian): Kielce, Holy Cross Mountains, Poland.
5
Microstructure. Tangential section of a punctate articulate brachiopod exhibiting very small tubules (punctae, P) within
the shell. Early Jurassic (Hierlatz limestone): Maurach near Innsbruck, Tyrol, Austria.
6
Brachiopod spines: Some pseudopunctate brachiopods yield spines that are found as displaced elements in the sediment.
Diagnostic criteria are the hollow central canal surrounded by concentric inner and thick radial-fibrous outer layers.
Transverse section. These spines are common in Carboniferous and Permian shelf carbonates. Early Carboniferous: Holy
Cross Mountains, Poland.
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