PEERING INTO THE EYES OF TRILOB1TES USING EBSD

摘要

The compound eyes of trilobites, an extinct group of marine arthropods, contain many sub-millimeter sized calcium carbonate lenses whose microstructures are the key to understanding how this ancient visual system functioned. The lenses had to be constructed by the animal very precisely in order to avoid double refraction but the mechanisms by which they focused light remain controversial. Optical models of the schizochroal eyes that typify phacopine trilobites, accepted since the 1970s, have assumed that the calcite of each lens had a uniform crystallographic orientation and light was focused onto underlying photoreceptor cells via intralensar structures. These models, and others that have been developed subsequently, were tested by mapping the lenses of different trilobite species by Electron Backscatter Diffraction (EBSD) and using both carbon-coated and uncoated cross-sections of the lenses; the latter within the SEM operated in low vacuum mode. Some species did indeed form their lenses from calcite of a uniform orientation and focusing was assisted by using intralensar structures of different chemical compositions. Other species had lenses with a very different and highly intriguing microstructure whereby light may have been 'guided' to a central single crystal using an outer lens unit composed of calcite with a radially arranged c axis. This study highlights a novel application of EBSD to the flourishing field of biomineralization and illustrates how crystallography can be used to help understand functional morphology.