GRAINS: Skeletal Fragments: Arthropods
2003. "GRAINS: Skeletal Fragments: Arthropods", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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Class Trilobita — Early Cambrian (late Proterozoic?)-Late Permian
Most trilobites were mobile, benthic, detritus feeding, fully marine organisms with a limited salinity tolerance (they are not found in inferred salinity-stressed settings). A few groups of pelagic trilobites are known.
Although most common in shallow shelf settings, trilobites, especially eyeless forms, are also found in deeperwater environments. They are major rock-forming elements mainly in shallow shelf deposits of Cambro-Ordovician age.
Trilobite carapaces were composed of chitin with large amounts of calcium carbonate and variable amounts of calcium phosphate (up to 30% in some species). The carbonate consisted of calcite, probably with moderate to high Mg content.
Trilobites were characterized by exoskeletal carapaces with three lobes that extended the length of the organism. Carapaces were divided into a head shield (cephalon), an abdominal section (thorax) with 2 to 40 segments (sclerites), and a tail shield (pygidium).
The shields and segments were sharply recurved inwards along the margins of the organism. Carapaces were shed during growth stages (molting behavior) adding to the large numbers of trilobite grains in many sedimentary deposits.
Adult trilobites ranged in length from 0.1 to 75 cm; they average about 5 cm in length and 1-3 cm in width.
The segmented nature of the carapaces, coupled with trilobite molting behavior, means that these organisms are normally found as fragmentary remains. Individual segments typically are in the mm to cm length range and are less than a mm in thickness.
The recurved margins of trilobite shields and the multidirectionally curved forms of thoracic segments (sclerites) yielded fragments that commonly have characteristic “hook” or “shepherd’s crook” shapes.
Skeletal fragments have a homogeneous prismatic microstructure, with extremely fine (micrometer-scale) calcite prisms oriented perpendicular to the carapace surface. Typically, the wall appears smooth and uniform with no obvious crystals; trilobite fragments, however, show sweeping (undulose) extinction when rotated under cross-polarized light. Some trilobites may also have finely lamellar layers.
Many specimens show small perforations (canaliculi) that trend perpendicular to the skeletal walls.
Fine growth lines may be visible — they parallel the carapace surface but do not interrupt the continuity of calcite prisms
Trilobite fragments can be visibly multilayered, with thin inner or outer layers over the main carapace wall. Outer layer can be organic rich with a dark coloration in transmitted light.
Homogeneous prismatic wall structure (and consequent extinction behavior) of trilobites is similar to that shown by ostracodes and a few bivalves. Trilobite fragments, however, generally are larger than ostracodes and are more irregular in curvature than either ostracodes or bivalves.
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A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks