Telltale eyes: the lateral visual systems of Rhenish Lower Devonian eurypterids (Arthropoda, Chelicerata) and their palaeobiological implications

Abstract

The compound eyes of three taxa of Rhenish Lower Devonian eurypterids are examined and compared with those known from other eurypterids and the extant horseshoe crab Limulus polyphemus. The lateral eyes of the small species Rhenopterus diensti, a phylogenetically basal representative of the stylonurine clade, are characterized by a comparatively low number of lenses and high interommatidial angle Dφ (2.8°). The comparatively limited visual capacities of R. diensti are more similar to L. polyphemus than to its closer relatives of the eurypterine clade and perhaps this reflects a progression of lateral eye structure in the evolution of eurypterids as a whole. The number of eye facets in Adelophthalmus sievertsi is higher than that in the supposed ambush predator Acutiramus cummingsi, but lower than that in other 'swimming' eurypterids (Eurypterina).Due to poor preservation, no other eye parameters could be analysed in this species, but further morphological attributes and geographical distribution designate the mid-sized A. sievertsi as an able swimmer. A low interommatidial angle Dφ of less than 1°confirms that the visual capacities of Jaekelopterus rhenaniae are in line with an interpretation of this giant species as an active high-level predator. The inferred lifestyles of adult individuals of these three, co-occurring Rhenish eurypterids indicate niche differentiation avoiding to some degree the competition for food in their marginal marine to delta plain transitional habitats.

Computational biomechanical analyses demonstrate similar shell-crushing abilities in modern and ancient arthropods

“…Some of the oldest fossil arthropods display masticatory spines called gnathobases: tooth-like projections located on the proximal margins of cephalic and trunk appendages [2][3][4][5]. Gnathobases are common in various (typically predatory) arthropod clades throughout the Phanerozoic [2][3][4][5][6][7][8][9][10][11][12]. However, only one extant group has gnathobases developed on a series of appendages: the xiphosurids, true horseshoe crabs [1].…”

Abnormal xiphosurids, with possible application to Cambrian trilobites

“…Furthermore, paleontologists have been intrigued by the morphological similarities of L. polyphemus and fossil xiphosurids like Yunnanolimulus luopingensis Zhang et al, 2009 (Guanling Formation, China, Triassic;Hu et al, 2017), Mesolimulus walchi (Desmarest, 1822) (Solnhofen Limestone, Germany, Jurassic; Sekiguchi and Sugita, 1980;Smith and Berkson, 2005) and Limulus darwini Kin and Błaże-jowski, 2014 (Sławno Limestone, Kcynia Formation, Poland, Late Jurassic;Błażejowski, 2015). Finally, L. polyphemus is a useful modern analogue for exploring how extinct gnathobase-bearing arthropods consumed food, including large eurypterids (Selden, 1981;Poschmann et al, 2016), Sidneyia inexpectans Walcott, 1911(Zacaï et al, 2016Bicknell et al, 2018b;Bicknell and Paterson, 2018), and Alacaris mirabilis Yang et al 2018.…”

Vision in fossilised eyes

“…As shown here, the eye parameter had been applied in trilobites before (Fordyce & Cronin 1989McCormick & Fortey 1998). Its successful application, even to Cambrian arthropods, seems to have triggered a number of analyses of fossil compound eye systems in other arthropods of Cambrian time (Zhao et al 2013), trilobites Tanaka et al 2015), crustaceans (Schoenemann & Clarkson 2012a, b, c;Rust et al 2016;Vannier et al 2016) and eurypterids (Anderson et al 2014;Poschmann et al 2016). It may be concluded that this is indeed an effective way of investigating the structural parameters in the eyes of fossils.…”

“…Many of these tools can be applied, sometimes in a modified form, to fossils, and this allows an assignment to their ecological habitat and, in particular, the relative depth of the ocean they inhabited. This has been undertaken for the Chengjiang fauna , 2012a and Emu Bay eyes Paterson et al 2011), and also for the excellently preserved arthropods of Bundenbach (Lower Devonian, Hunsrück Shale), which could be assigned to waters up to 200 m depth (Rust et al 2016), as well as for eurypterids (Anderson et al 2014;Poschmann et al 2016) and for a variety of trilobites (Fordyce & Cronin 1989McCormick & Fortey 1998;Schoenemann et al 2008aTanaka et al 2015). Other visual systems in the fossil record, such as those of the Carboniferous shrimps Tealliocaris (Briggs & Clarkson 1985), remain to be investigated.…”