Geology Reference
In-Depth Information
Box 4.6.
Major microstructure categories
occurring in invertebrate and algal skeletons (after Wendt 1990 and Majewske
1974). The organic matrix within the hardparts is an important control on the preservation of microstructures and the
post-mortem destruction (Glover and Kidwell 1993; Cuif et al. 1999). Organic-rich microstructures (OR) of mollusks
are in fact commonly preserved only as molds; organic-poor microstructures (OP) are often fairly well-preserved.
0dimensional
Microgranular:
Irregularly interlocked anhedral grains without preferred orientations. Aragonite or calcite. Com-
mon in cyanobacteria (Pl. 7/6) and calcareous algae, foraminifera (Pl. 68), calpionellids, serpulids; uncommon in
mollusks; rare in corals and coralline sponges (Pl. 79/1, 6).
1dimensional
: Thin, elongate crystals with length/width ratios between 1:5 and 1:25.
Irregular fibrous:
Randomly oriented individual fibers. Aragonite or calcite. Foraminifera, coralline sponges, ostra-
cods, trilobites (Pl. 94/3), mollusks.
Orthogonal
(fibro-normal): Crystals oriented parallel to each other and perpendicular to the surface of the skeletal
elements. Aragonite or calcite. Calcareous algae, foraminifera, coralline sponges, rugose and tabulate corals, ostra-
cods, trilobites, mollusks.
Clinogonal
(trabecular): Crystals diverging at low angles from a central axis or several aligned centers. Usually
aragonite, rarely calcite. Coralline sponges, hydrozoans, and rugose, tabulate and scleractinian corals.
Spherulitic
: Spherical or hemispherical aggregates of crystals, centrically or excentrically arranged. Usually arago-
nite, very rarely calcite. Coralline sponges, hydrozoans, octocorallia; very rare in bivalves.
2dimensional
: Planar or curved blades which may be composed of subcrystals with the same optical orientation.
Lamellar
(foliated): Very thin lamellae regular or more or less parallel to surface of skeletal elements, or arranged in
stacks of slightly different optical orientation. Mostly calcite, rarely aragonite. Hydrozoans, rugose and tabulate
corals, serpulids (Pl. 92/6, 8), brachiopod shells and spines (Pl. 86/1, 6), tentaculitids (Pl. 91/7), bryozoans (Pl. 85/4),
gastropods, bivalves (OP).
Nacreous
: Thin, organic-rich tablets composed of subcrystals of the same optical orientation, oriented parallel to the
surface of skeletal elements or growth lamellae. Aragonite. Found only in mollusks (inner layer of certain bivalve
and gastropod shells; major parts of cephalopod shells; OR).
Crossed lamellar:
Thin interfingering or branching first order lamellae composed of parallel-arranged second order
lamellae. The optical orientation of the first-order lamellae alternates. Commonly aragonite, rarely calcite. Found
only in mollusks (OP; bivalves, gastropods: Pl. 149/5; hyolithids).
3dimensional
: Large crystals (about 0.1 mm to several cm in size).
Normal prismatic:
Polygonal prisms arranged parallel to each other and perpendicular or oblique to the surface of
skeletal elements. Each prism is an individual crystal with unit extinction. Usually calcite, rarely aragonite. Inner
carbonate layer of most punctate and impunctate brachiopods, outer carbonate layer of bivalves (Pl. 87/2, 5, 8; OR)
and gastropods, rostra of belemnites (Pl. 90/7), ostracods (Pl. 93/2).
Optical uniform:
Morphologically well-defined skeletal elements or whole shells appear to be single calcite crystals
in the light microscope. Calcite cleavage visible under ordinary light. Typical of echinoderms (Pl. 95/3) and spicules
of some calcareous sponges. Extremely rare in foraminifera and bivalves.
(gastropods, cephalopods) and thin bivalved shells (bra-
chiopods, bivalves, ostracods) are highly subject to in-
tact transport of the skeleton, whereas multi-element,
tightly sutured (echinoids, some crinoids) and loosely
articulated skeletons (most crinoids, trilobites) are sub-
ject to disarticulation and fragmentation (Brett and
Baird 1986). Out-of-habitat transport of fossils is com-
monly associated with settings affected by high-energy
currents or steep depositional slopes.
Time-averaging
is the mixing of skeletal elements
of non-contemporaneous organisms (Walker and Bam-
bach 1971). Mollusks and benthic foraminifera from
near-shore marine settings are time-averaged over de-
cades to hundreds of years, and organisms living on
carbonate platforms and in reefs over a time scale of
decades to ten thousands of years (Kidwell and Behrens-
meyer 1993).
A model showing the distribution of the relative
abundance of the types of time-averaging in different
marine environments was developed by Fürsich and
Aberhan (1990). However, despite all these constraints,
death assemblages of resistant skeletal elements are ex-
cellent records of environmental distribution and com-
munity composition for many major groups (Kidwell
and Flessa 1996).
Many skeletal grain types have specific preserva-
tion potentials within a given depositional environment,
depending on the susceptibility of grains to mechani-
cal destruction, duration of deposition/exhumation/re-
