George R. Clark

Mollusk Shell: Daily Growth Lines

Ridges forming the concentric sculpture on the shells of laboratory-grown specimens of Pecten diegensis Dall show daily periodicity. Missing growth lines account for all scatter in the data, so that the maximum, not the average, line count is most representative. The variation in spacing between growth lines can be correlated among specimens.

Pyritization in the shells of living bivalves

Pyrite with characteristic brass-yellow luster was found coating parts of the outer surface of living Mercenaria mercenaria and Geukensia demissa in a Georgia tidal marsh. Sections through one shell revealed additional pyrite replacing aragonite in parts of the outer shell layer, especially around fractures, borings, and concentrations of organic matrix. This is the first report of the precipitation of massive (metallic luster) pyrite in modern sediments and the first indication that pyritization can commence in a mollusk shell even before the death of the mollusk.

Organic matrix taphonomy in some molluscan shell microstructures

Abstract The organic matrix of selected specimens of cephalopods and bivalves was observed by scanning electron microscopy upon shell sections etched and preserved by critical point drying. For specimens ranging in age from Pennsylvanian to Recent, it was noted that physical preservation of the organic matrix was not strongly dependent on age. Matrices were also found to have degraded at different rates in different shell microstructures, even in the same shell. The type of degradation had also varied, even for the same kind of microstructures. The occurrence of preserved organic matrix may be more widespread among molluscan fossils than commonly thought.

Observations on the tunnel morphology of Heterocerus brunneus Melsheimer (Coleoptera; Heteroceridae) and its paleoecological significance

The burrow structures of Heterocerus brunneus Melsheimer and its larvae are described from both field (northeastern Kansas) and laboratory habitats. These are further interpreted in light of known North American heterocerid biology. Two important observations are that significant features of any one burrow varied with the consistency (especially the water content) of the substrate, and that different insects were making superficially similar burrows at the same field site. It would require exceptional preservation, or associated body parts, to positively identify fossil traces of these structures as heterocerid burrows, but their general pattern should be readily recognized as foraging traces in even the most degraded preservations.

Microstructure and composition of the periderm of conulariids

Abstract. Transmitted light and scanning electron imaging of sectioned specimens of Conularia and Paraconularia, prepared using HCl etching and critical point drying, revealed that their periderm is composed of extremely thin (approximately 0.5–3 μm), variably distinct microlamellae that are alternately organic poor and organic rich. Organic-rich microlamellae are cross-connected by slender strands of organic matter originally embedded in calcium phosphate, which in etched specimens has been dissolved. Microlamellae may be organized in thicker (approximately 5–75 μm) layers, or macrolamellae, that vary in color and organic matter content, possibly owing to changes in the ambient paleoenvironment. Thickening of the periderm to form transverse ribs and internal carinae was achieved through gradual thickening of individual microlamellae. In the core of the transverse ribs and internal carinae the distinction between organic-rich and organic-poor microlamellae may be reduced, owing to organic material becoming dominant over (former) mineral matter or vice versa. Combined with observations of plicated aperture closure in thin-walled conulariids, including Archaeoconularia slateri (Reed, 1933) (Upper Ordovician, Scotland) showing smooth folding of midline carinae through angles greater than 90°, these results suggest a structure and original flexibility in the organic-rich biocomposite forming the conulariid periderm that supports its homology to the chitinous lamellar periderm of coronate scyphozoans.

Pyritization in Shells of Living Bivalves: ABSTRACT

Several specimens of Mercenaria mercenaria and Guekensia demissa collected alive in early August from tidal marshes on St. Catherines Island, Georgia, were found to have a prominent brass-yellow material on part of the outer surface of their shells. On some individuals this may occur as a surficial coating, for a portion flaked off one specimen revealed an apparently unaffected outer shell surface. In another specimen, however, thin sections and sections examined by scanning electron microscopy (SEM) showed the material extended into the shell, following planes of weakness, such as growth lines and microborings, and replaced aragonite. In thin section, the material is opaque to transmitted light and appears much like framboidal pyrite under oblique reflected light. Studies by SEM reveal a massive outer region grading into loosely packed masses of spheroids about 0.1µm in diameter. Energy dispersive X-ray analysis identified iron and sulfur as the principal components of this material. Preliminary X-ray diffraction studies on mechanical preparations of the outermost region of the shell were hampered by the low concentration of the material relative to aragonite; despite this, the strongest pyrite peak (311) was detected in both diffractometer and powder camera results. No other iron sulfides could be identified. These observations strongly suggest that we are observing the process of calcium carbonate replacement by pyrite in a modern sedimentary environment. End_of_Article - Last_Page 432------------