Geology Reference
In-Depth Information
study of microstructures in thin sections requires mag-
nifications of at least x30 and the use of polarized light.
Staining techniques (Adams and MacKenzie 1998) and
cathodoluminescence microscopy are strongly recom-
mended. In-depth evaluations of microstructural fea-
tures are based on SEM data. A comparison of these
data with thin-section observations is sometimes diffi-
cult, partially because of different definitions of mi-
crostructures seen in SEM and in thin sections. The
terms listed in Box 10.7 are those commonly applied
to microstructures exhibited in thin sections.
(Middle Cambrian to Permian), Pentamerida (Late
Cambrian to Devonian), Rhynchonellida (Ordovician
to Holocene, abundant in the Mesozoic), Spiriferida
(Ordovician to Early Jurassic, extremely diverse dur-
ing the Devonian), and the Terebratulida (Carbonifer-
ous to Holocene), the most common brachiopods to-
day.
The body of brachiopods is enclosed by a valve ex-
cept for the pedicle which serves to attach the animal
to the substrate.
The larger valve on the ventral side of the body is
the pedicle valve, the smaller valve on the dorsal side
the brachial valve, taking its name from the brachia.
These are arm-like projections of the lophophore, a
feeding organ that also carries them.
10.2.4.1 Brachiopods
Brachiopods are bilaterally symmetrical sessile marine
organisms with an external shell consisting of two dis-
similar but equilateral valves. Brachiopods are major
contributors to the bioclastic content of shallow-ma-
rine limestones, particularly in the Paleozoic.
Shell structures:
All articulate brachiopods have
Low-Mg calcite shells. The shell grows continually
throughout the life of the animal; new material is laid
down around the larger rounded edge of the shell. The
original wall structure is commonly well preserved.
The calcitic shell is overlain by a thin chitinous layer
(periostracum), never preserved in fossil shells. The
calcitic shell consists of two layers - (1) a thin outer
primary cryptocrystalline layer of fairly constant thick-
ness, and (2) an inner, thicker layer that thickens with
growth and from which all internal structures are
formed. The primary layer is difficult to detect, but it
may be recognizable in shells of different brachiopod
groups. The boundary between primary and secondary
layers is commonly sharp.
The characteristic feature of brachiopod shells is a
foliated lamellar structure of the secondary layer ap-
pearing as thin parallel laths of calcite which are vari-
ably fused and amalgamated, commonly having a wavy
banded appearance (Fig. 10.39). The laminae are ori-
ented roughly parallel to the outer surface of the valve.
Morphology and classification:
Traditionally bra-
chiopods have been split up into two major groups, the
Inarticulata and the Articulata. The
Inarticulata
domi-
nated in the Cambrian and Ordovician; they received
their name from the fact that the two valves do not have
an articulating hinge and are united by muscles. Their
shells are composed of alternating layers of calcium
phosphate and chitin. All other brachiopods included
within the
Articulata
have articulating hinges and cal-
citic shells.
The main groups of the Articulata are the Stropho-
menida (Ordovician to Middle Jurassic; including the
Late Paleozoic productids and brachiopods that are at-
tached with one cone-shaped valve at the substrate,
while the other valve acts as a lid; Pl. 86/2), Orthida
Fig. 10.39.
Laminar microstructure of strophomenid brachiopod shells
characterized by wavy lamellae that are buckled
adjacent to the pseudopunctae. The latter feature distinguishes the shell from bivalve shells, which may exhibit laminar
microstructures as well. Middle Devonian (Givetian): Letmathe, Sauerland, Germany. Scale is 1 mm.
