Arthropod eyes: The early Cambrian fossil record and divergent evolution of visual systems.

摘要: Four types of eyes serve the visual neuropils extant arthropods: compound retinas composed adjacent facets; a surface populated by spaced eyelets; smooth transparent cuticle providing inwardly directed lens cylinders; and single-lens eyes. The first type is characteristic pancrustaceans, which comprise lenses arranged as hexagonal or rectilinear arrays, each crowning 8-9 photoreceptor neurons. Except for Scutigeromorpha, second typifies Myriapoda whose relatively large eyelets surmount numerous photoreceptive rhabdoms stacked together tiers. Scutigeromorph are facetted, some dozen neurons modified apposition-type eye. Extant chelicerate single-lensed except in xiphosurans, lateral with outer an inner one regular arrays cylinders. This account discusses whether these disparate eye speak against divergence from ancestral type. Previous considerations evolution, focusing on trilobites facet proliferation xiphosurans myriapods, have proposed that mode development those taxa distinct pancrustaceans plesiomorphic condition facetted evolved. But recent discovery enormous regularly belonging to early Cambrian radiodontans suggests high-resolution superior optics may be ground pattern organization arthropods, predating evolution arthrodization jointed post-protocerebral appendages. Here we provide evidence stem-group euarthropods can understood terms morphologies diverging this radiodontan-type pattern. We show certain groups apposition relate fixed mobile eyestalks, whereas other reveal concomitant sessile equipped typical xiphosurans. Observations fossil material, including eurypterids, support proposition was arthropods include possible precursors mandibulate latter eyes, now sessile, their underlying optic lobes exemplified scutigeromorph chilopods, stalked more elaborate typifying Pancrustacea. Radical demonstrated doublet sessile-eyed special arachnids. Different discussed respect modes life extinct species possessed them, comparing counterparts centers might served.