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.
Highlights
Eurypterids, popularly known as sea scorpions, possess conspicuously large compound eyes
In the xiphosuran compound eye, less ordered than a typical tetraconate compound eye, the dioptric apparatus consists of cuticular, cone-shaped lens-cylinders, which form a pattern of separated domes, visible if the cuticle is removed from the receptors (Fig. 1n–p)
Our results clearly show that the typical shape of exocones in the eurypterid eye is similar to those of Limulus (Fig. 1n–u) and these exocones may have functioned in the same way as lens cylinders
Summary
Eurypterids, popularly known as sea scorpions, possess conspicuously large compound eyes. A fully grown Jaekelopterus (Fig. 1a) was a giant even when compared to other large arthropods such as the well-known Cambrian Anomalocaris (~1 m long2), or the meganeurid griffenflies (~75 cm wing span) of the Permo-Carboniferous[1,3,4], or Hibbertopterus, another eurypterid, which probably was about 1.60 m long[5]. Tollerton[23] defined nine different shapes of eurypterid eyes, ranging from small lunar shaped examples to domed ovoid types, of which some even had a frontally overlapping visual field (e.g., J. rhenaniae). The latter is typical for predators, allowing stereoscopic vision, which is imperative for the estimation of distances, volumes etc. Nothing has been documented about the internal structure of the eurypterid visual system, which might tell us about function and phylogenetic context
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