DETERMINING THE ORIGIN OF INTRACAMERAL DEPOSITS IN THE ORTHOCERID GENUS ARIONOCERAS

Little work has been done to date on the origin of intracameral deposits in orthocerid cephalopods. This preliminary investigation focused on such deposits in Arionoceras orthocone fossils from the Silurian – Devonian beds of Tafilalt, Morocco (Kroger 2008,) in order to determine to what extent intracameral deposits were formed through biomineralization versus post-mortem diagenetic alteration.

Four specimens were cut longitudinally through the siphuncle, polished, etched with 5% hydrochloric acid, then examined under high magnification (1000x) on a binocular microscope, revealing 2 distinct regions within each chamber. A thin-section prepared from one specimen and examined under both plane polarized light and cross-polarized light on a petrographic microscope revealed distinctions among the septa, siphuncle and within the chambers. Examination under reflected light on a cathodoluminoscope revealed little, but under cathodoluminesence, two distinct Mn-Ca ratios were evident within the chambers. After all optical observations were made on one etched sample slice, it was coated with gold and imaged on a portable scanning electron microscope.

We found two different kinds of white, opaque deposits: finer, extremely delicate, thin triangular deposits along the chamber-facing side of the siphuncular wall in two chambers, and dozens of close, fine lattice layers radiating from the septal and siphuncular walls toward the middle of each chamber. We infer that these organized structures indicate biodeposition during life. The chamber centers were infilled with a clear mineral (possibly sparry calcite), as might be expected from post-burial diagenetic alteration. Distal (older) chambers in the specimens are almost completely filled with the white, opaque deposits, whereas the proximal (younger) chambers have increasingly higher ratios of clear to white deposits. SEM images show parallel cracks in the clear deposits, as would be expected from cleavage of sparry calcite formed in the chambers after burial.

Our results strongly point to internal biomineralization in the chambers of the orthocerids examined in this study. Analyses of more samples are needed to determine if this biomineralization is unique to these samples or more widespread through other orthocerids.