Description and interpretation of the internal structure of a Cambrian crustacean compound eye

 

Authors: Schoenemann B, Clarkson ENK, Castellani C, Waloszek D, Maas A, Meyer-Rochow VB

Published in: Bulletin of Geosciences, volume 89, issue 2; pages: 311 - 324; Received 7 March 2013; Accepted in revised form 21 January 2014; Online 19 March 2014

Keywords: Tetraconata, photoreceptor, vision, Arthropoda, Cambrian, apposition eye, photic environment,

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Abstract

The exceptionally good preservation of phosphatised, three dimensionally preserved “Orsten” arthropods permits insight into the internal morphology of ancient compound eyes. Analysis, presented here, of the stalked eyes of a Cambrian “Orsten” crustacean reveals structures such as a cornea, a crystalline cone, rhabdomers belonging to possibly five or six sensory cells and an absence of a gap (known as the clear-zone) between dioptric structures and the retina. All these features collectively suggest that this compound eye is of the apposition type. Thus, the principle of mosaic vision likely dates back to Cambrian times, more than half a billion years ago. Using well-established methods this eye can be characterised as dim-light adapted and is likely to have belonged to a benthic organism like many others known from fossils of the Cambrian Alum Shale of Sweden. This seems to be one of the oldest known apposition eyes, and the structural differences between it and eyes more closely conforming to the tetraconate system are likely to be related to the small size of the eye and the photic environment in which it had to operate. Differences from typical compound eye organisation, such as for instance a smaller number of retinula cells than the more typical eight found in most tetraconate arthropods, suggest that either the tetraconate system is not basal and universal, as is often assumed, or that modifications to the system enabling the eye to improve photon capture had already occurred during the Cambrian.

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