GEOCHEMICAL ANALYSIS OF COLOR BANDING IN UPPER TRIASSIC GASTROPODS FROM PERU

Upper Triassic silicified gastropods: Omphaloptcha jaworski, O. speciosa, Promathilda teretrina, Neritaria. ninacacana, N. dicosmoides, from the of the Pucara` Group carbonate rocks in the Peruvian Andes are characterized by distinctive shell markings thought to represent original color banding. Like modern taxa, the banding may have been brightly colored as in some modern tropical counterparts. Freed from the matrix by treatment in 5% acetic acid, shells of these mollusks show patterns reminiscent of red-brown color patterns of some rhychonelliform brachiopods previously reported from the Devonian of Alabama. Initial SEM and EDS analysis of the shells from Peru yielded trace elements of iron and sulfur, similar to those found in the Devonian brachiopods. Later SEM and EDS analysis showed the trace elements to be replacement material in the form of pyrite preservation of the color patterns. Research then concentrated to answering the question of why the banding is associated with pyrite preservation, while the rest of the originally aragonitic shell was replaced by silica. Possible answers come from the organic structures of hydrocarbon (commonly used by living organisms in production of pigments) attracted iron after death of these mollusks. The most likely of these hydrocarbons would be porphyren, commonly found in the shells of mollusks, except monoplacophorans and cephalopods. Most important for this question, porphyrin is used to produce colors of browns and reds. The colors depend upon the attraction of metals such as iron, vanadium, copper, and magnesium, which render the color and make the porphyrin structure stable. Porphyren may transform during re-crystallization of the pigment into pyrite, as the rest of the shell becomes preserved in silica. Observation of both Triassic gastropods and bivalves from Peru also shows some of the prismatic layers to be poorly preserved to non-existent, and results in a hollowness between the surface of the shell and the preserved necreous layer, so the only layer color could be preserved is the persiostracum. There is little knowledge on this field. Future investigation may reveal color pattern preservation are related to the preservation of the persiostracum in mollusks inhabiting shallow tropical lagoon environments where these mollusks are interpreted to have lived.