Geology Reference
In-Depth Information
Trilobites in thin sections:
The most common trilo-
bite fragments seen in thin sections are dissociated tho-
rax fragments (Pl. 94/2, 4). Because the shields and
segments are turned under on the ventral side, in thin
sections the inflected margins of the segments look like
tiny hooks causing the appearance of 'shepherd's
crooks' (Pl. 94/1, 3).
Microstructure and recognition of echinoderms in thin
sections:
The echinoderm skeleton is made of plates,
spines or sclerites. Each skeleton element consists of a
three-dimensional porous network composed of crys-
tallographically uniformly oriented carbonate crystals
(stereom: a latticework constructed from lathes of High-
Mg calcite) and organic tissue (stroma) within the
meshes of the network (Fig. 10.52). Skeleton elements
exhibit unit extinction between crossed nicols and cor-
respond to a single calcite crystal. The Mg-calcite ele-
ments transform to calcite with some changes in the
preservation of the original microstructure (Dickson
2001).
Echinoderm grains are usually preserved as large
single crystals of calcite. The sparry preservation is
found irrespective of whether the primary porous net-
work is filled with syntaxial calcite or with pyrite, he-
matite, glauconite or other minerals. Micritic preser-
vation of echinoderm fragments is related to crystal
diminution by diagenetic dissolution (Evamy and
Shearman 1965; Neugebauer 1978; Neugebauer and
Ruhrmann 1978; Richter 1974).
Basics: Trilobites
Kaesler, R.L. (1987): Trilobitomorpha. - In: Boardman, R.S.,
Cheetham, A.H., Rowell, A.J. (eds.): Fossil invertebrates.
- 221-240, Palo Alto (Blackwell)
Moore, R.C. (1959): Arthropoda 1. Trilobita. - Treatise on
Invertebrate Paleontology. Part I, 560 pp., Lawrence (Geo-
logical Society of America)
Further reading:
K126
10.2.4.9 Echinoderms
Echinoderms are marine invertebrates with a multi-plate
calcareous internal skeleton embedded in the skin, a
marked five-rayed symmetry and a water vascular sys-
tem through which the water is circulated in the body.
With few exceptions, modern echinoderms are benthic
bottom dwellers. A few can swim or float.
The phylum Echinodermata, known since the Cam-
brian, embraces many major subgroups, some of which
are important rock builders during the Phanerozoic (e.g.
crinoids, echinoids). Echinoderm fragments are present
in limestones formed in shallow-marine as well as in
deep-marine environments. Most Paleozoic echino-
derms were sessile.
Usually echinoderm skeletons disarticulate and the
skeletal plates are scattered in the sediment. Individual
plates are commonly millimeter-sized to a few centi-
meters in maximum dimension. In thin sections, echi-
noderm fragments are easily recognized by the uniform
extinction, the dusty appearance and often gray or yel-
lowish colors in transmitted light, and the calcite lat-
ticework. Many echinoderm plates exhibit a finer more
dense meshwork towards the plate surfaces, appearing
Plate 94 Trilobites
The plate displays trilobite limestones formed in low- and high-energy settings of open-marine platforms. Dif-
ferent skeletal parts occur in texturally different limestones, pointing to transport sorting. Grainstones yield
head-shield and thoracal fragments (-> 1, 3). Wackestones commonly contain variously sized thoracal frag-
ments (-> 2). Packstones are formed by accumulations of pygidia (-> 7).
1
Trilobite rudstone.
Various trilobite sections: Semicircular section (1) of a trilobite head shield (cephalon). Note well-
differentiated axial region (glabella, G) and long spines (S). Recurved thoracal segments (2). Open-marine platform.
Early Devonian: Hamar Laghdad, Erfoud, southern Anti-Atlas, Morocco.
2
Trilobite wackestone.
Poorly sorted trilobite debris in micritic matrix. Same locality as -> 1.
3
Microstructure.
Characteristic 'shepherd's crook' structure, caused by recurved lateral borders of thoracic segments. The
homogeneous prismatic microstructure encloses fine pores (arrows) running perpendicular to the surface. Intergranular
spaces between the trilobite fragments are filled with radiaxial calcite cement (RC). Same locality as -> 1.
4
Typical criteria.
Recurved trilobite fragment (T) and echinoderm (E) bioclast within a micritic matrix. Same locality as -> 1.
5
Taphonomy
. Microbially encrusted trilobite fragment. Late Silurian: Valentin-Törl, central Carnic Alps, Austria.
6
Typical criteria.
Trilobite spines within a fine-grained quartz sandstone. Note the fine prismatic microstructure of the thin
elements. Middle Cambrian (
Paradoxides
Sandstone): Öland Island, Sweden.
7
Trilobite packstone.
Densely packed and parallel arrangement of trilobite fragments forming a coquina. Most fragments
are parts of tail shields of trilobites living near mud mounds. SMF 12-T
RILO
. Same locality as -> 1.
