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

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. •

Vision in fossilised eyes

“…Hexagonal facets in densely packed compound eyes (as known from modern crustaceans and insects such as dragonflies or bees) are represented, for example, in Cindarella eucalla (Ramskö ld et al 1997;Schoenemann & Clarkson 2012a, b, c;Zhao et al 2013). We find the same in an assumed anomalocaridid from Emu Bay , an unidentified arthropod , and some excellently preserved crustaceans from the Upper Cambrian Orsten biota (Schoenemann et al , 2014Parker et al 2013;Schoenemann 2013). …”

“…1997; Schoenemann & Clarkson 2010, 2012a, b, c, 2015; Paterson et al . 2011; Schoenemann et al 2011, 2014; Lee et al 2012). Moreover, even internal sensory structures have been described recently from the compound eyes of Devonian trilobites (Schoenemann & Clarkson 2013).…”

“…There was a great Palaeozoic diversification of different kinds of eye, especially in the Ordovician, and particularly those of trilobites. These come mainly from the early to late Cambrian (Ramsköld et al 1997;, 2012a, b, c, 2015Paterson et al 2011;Schoenemann et al , 2014Lee et al 2012). Moreover, even internal sensory structures have been described recently from the compound eyes of Devonian trilobites (Schoenemann & Clarkson 2013).…”

“…They contain phosphatised, three-dimensionally preserved crustaceans and other fossilised organisms that present finest details, such as bristles and compound eyes. Analysis of these eyes has shown that these early crustaceans possessed focal apposition eyes (Schoenemann 2013;Schoenemann et al 2014), with a structure comparable to that of modern diurnal arthropods such as bees. Parker et al (2013) were able to show that amongst these crustaceans, a sophisticated yellow cornea had been developed, filtering blue scattering light and enhancing the contrast underwater.…”

Compound eyes of insects and crustaceans: Some examples that show there is still a lot of work left to be done

“…As instruments to detect light, the compound eye has thus had a very long history that may even stretch back as far as the neo-proterozoic, that is, more than 540 millions years ago. However, when exactly the first compound eye appeared, which structural component or components it evolved out of and to what extent the compound eyes nowadays present in the various arthropod taxa are related, these are still largely unresolved and hotly debated questions (Nilsson, 1989;Melzer et al, 1997;Gehring & Ikeo, 1999;Oakley & Cunningham, 2002;Oakley, 2003;Harzsch & Hafner, 2006;Harzsch et al, 2007;Lee et al, 2011;Schoenemann et al, 2014).…”