Article: Magnesium-rich intralensar structures in schizochroal trilobite eyes

The interpretation of the lenses of schizochroal trilobite eyes as aplanatic doublets by Clarkson and Levi-Setti over 30 years ago has been widely accepted. However, the means of achieving a difference in refractive index across the interface between the two parts of each lens to overcome spherical aberration has remained a matter of speculation and lately it has been argued that the doublet structure itself is no more than a diagenetic artefact. Recent advances in technologies for imaging, chemical analysis and crystallographic characterization of minerals at high spatial resolutions have enabled a re-examination of the structure of calcite lenses at an unprecedented level of detail. The lenses in the eyes of the specimen of Dalmanites sp. used in the original formulation of the aplanatic doublet hypothesis are shown to have undergone diagenetic alteration, but its products reflect original differences in mineral chemistry between the upper lens unit and lower intralensar bowl. The turbidity of the bowl and of the core within the upper part of the lens are the result of their greater microporosity and abundance of microdolomite inclusions, both of which were products of diagenetic replacement of original magnesian calcite in these areas. Such a difference in magnesium concentration in the original calcite has long been postulated as one of the ways by which the interface between these lens units could have produced an aberration-free image and the present study provides the first direct evidence of such a chemical contrast, thus confirming the doublet hypothesis.