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, 9 (1), 17797

Insights Into the 400 Million-Year-Old Eyes of Giant Sea Scorpions (Eurypterida) Suggest the Structure of Palaeozoic Compound Eyes

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Insights Into the 400 Million-Year-Old Eyes of Giant Sea Scorpions (Eurypterida) Suggest the Structure of Palaeozoic Compound Eyes

Brigitte Schoenemann et al. Sci Rep.

Abstract

Sea scorpions (Eurypterida, Chelicerata) of the Lower Devonian (~400 Mya) lived as large, aquatic predators. The structure of modern chelicerate eyes is very different from that of mandibulate compound eyes [Mandibulata: Crustacea and Tracheata (Hexapoda, such as insects, and Myriapoda)]. Here we show that the visual system of Lower Devonian (~400 Mya) eurypterids closely matches that of xiphosurans (Xiphosura, Chelicerata). Modern representatives of this group, the horseshoe crabs (Limulidae), have cuticular lens cylinders and usually also an eccentric cell in their sensory apparatus. This strongly suggests that the xiphosuran/eurypterid compound eye is a plesiomorphic structure with respect to the Chelicerata, and probably ancestral to that of Euchelicerata, including Eurypterida, Arachnida and Xiphosura. This is supported by the fact that some Palaeozoic scorpions also possessed compound eyes similar to those of eurypterids. Accordingly, edge enhancement (lateral inhibition), organised by the eccentric cell, most useful in scattered light-conditions, may be a very old mechanism, while the single-lens system of arachnids is possibly an adaptation to a terrestrial life-style.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Examples of eurypterid and limulid morphology and comparison of Jaekelopterus lateral eyes to other arthropod compound eyes. (a) Reconstruction of Jaekelopterus rhenaniae (Jaekel, 1914) (modified from Braddy et al. 2008; drawing by S. Powell, with permission. (b) Tachypleus gigas (Müller, 1785), Limulida [© Subham Chatterjee/CC BY-SA 3.0 (via Wikimedia Commons)]. (c–e) Examples of specimens (c: GIK 186, d: GIK 188, e: GIK 190). (fh) Compound eye of a hornet Vespa crabro germana Christ, 1791. (i–k) Compound eye of Carcinoscorpius rotundicauda (Latreille, 1802), Limulida, exuviae. (l) Compound eye of the trilobite Pricyclopyge binodosa (Salter, 1859), Ordovician, Šárka formation, Czech Republic. (m) Ommatidium of the trilobite Schmidtiellus reetae Bergström 1973, base of the Lower Cambrian, Lükati Fm., Estonia. (np) Cones (lens cylinders) of the internal side of the compound eye of C. rotundicauda (exuvia). (q) Impressions of the exocones in Jaekelopterus rhenaniae (Jaekel, 1914), (GIK 186a). (rt) Exocones of J. rhenaniae (GIK 186), note the regular arrangement compared to (i). (u) Schematic drawing of the ommatidium of an aquatic mandibulate (crustacean), and of the ommatidium of a limulid. (w) schematic drawing of several ommatidia of a limulide. (x) Schematic drawing of crossections of (v), the limulide after. (y) Visual unit of a limulide (redrawn and changed after). cc, crystalline cone; ec, eccentric cell; exc exocone; l, lens; p, pigment; pc, pigment cells; rc, receptor cells; rh, rhabdom; su sensory unit.
Figure 2
Figure 2
The ommatidium of Jaekelopterus rhenaniae (Jaekel, 1914). (ad) The specimen, where the first ommatidium was discovered (red arrow), d: brown: rhabdom with eccentric cell dendrite in the centre, yellow: receptor cells, black spots: possibly screening pigments. (b) shows the rosette of the ommatidium in crossection under different contrasts. Note the bright spot of the presumed eccentric cell in the centre of the rhabdom. (c,d,f) Rosette of (a) magnified, and its schematic drawings. (e) Ommatidium of the trilobite Schmidtiellus reetae Bergström 1973 and its homogenous rhabdom. (gj) Cross sections of ommatidia of Limulus (quoted from Figs. 2 and 3)), [white circle indicates the rosette, yellow circle the pigmental periphery, blue arrow the situation of the rhabdom, (j). (k–o) SEM of the compound eye of Jaekelopterus rhenaniae (Jaekel, 1914), total aspect (k) to one rosette (mo) and individual rhabdom (o), [white circle indicates the rosette of receptor cells, yellow circle the pigmental periphery, yellow arrow the situation of the rhabdom, (n)]. (p–u) Different sensory rosettes of J. rhenaniae (Jaekel, 1914) and their interpretative drawings. Note the bright patch in the centre of the rhabdom, comparable to (ad), and (gj). (q–r) In black and white to show contrasts different from (s–u) in colour, [white circle indicates the rosette of receptor cells, yellow circle the pigmental periphery, blue arrow the situation of the rhabdom]. p, q GIK 188; (rt), w, u GIK 190; ec eccentric cell; exc exocone, erc, element of receptor cell forming the outer part of the rhabdom; p, pigment cells; rc, receptor cells; rh, rhabdom; su sensory unit. Blue arrows indicate the dark rhabdomeric ring with the relics of the presumed dendrite of the eccentric cell inside.

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