Bunodes

Bunodes

sp.

detail are not preserved in these fossils. Examples of LCEs in non-biomineralizing taxa are comparatively less common but still highly significant and include the exceptionally preserved visual surfaces of radiodontans ( Lee et al., 2011; Paterson et al., 2011), numerous stem-group euarthropods ( Ortega-Hernández, 2015; Strausfeld et al., 2016), phosphatized larvae ( Castellani et al., 2012) and euchelicerates ( Poschmann, 2006; Schoenemann, 2006; Schoenemann & Clarkson, 2008, 2017; McCoy et al., 2015; Miether & Dunlop, 2016; Poschmann et al., 2016). In this context, the synziphosurines represent the earliest crown-group chelicerates that possess LCEs (Lamsdell, 2013), which, when combined with their early divergent phylogenetic position, makes them significant for understanding the early evolution of vision in a major group of extant euarthropods. However, the fossil record of LCEs in synziphosurines requires revision.

The evolutionary relationships of synziphosurines in Euchelicerata remain somewhat controversial, partly because most representatives are known only from the dorsal exoskeleton, and there are remarkably few instances of exceptional limb preservation ( Moore et al., 2005a, b, 2007; Briggs et al., 2012), especially when compared to true horseshoe crabs ( Bicknell et al., 2019). For example, the suborder Synziphosurina (sensu Eldredge, 1974), typified by forms that lack a fused opisthosoma, is now considered paraphyletic based on the results of recent phylogenetic analyses and most probably includes stem-group representatives of Euchelicerata, Xiphosura and Arachnida (Lamsdell, 2013, 2016; Lamsdell et al., 2015). Despite these limitations, synziphosurines have been studied for 160 years and are therefore anatomically well documented. One aspect of their exoskeletal morphology that has drawn particular interest is the apparent lack of LCEs in several synziphosurine taxa, making their representatives ‘mostly blind forms’ according to the traditional literature ( Størmer, 1952: p. 632), a condition that is generally regarded as secondarily derived (e.g. Dunlop & Lamsdell, 2017). Reports of LCEs have become more frequent in recent times ( Stürmer & Bergström, 1981), with taxa such as Bunodes sp. in Bergström (1975), Cyamocephalus loganensis Currie, 1927 , Legrandella lombardii Eldredge, 1974 , Limuloides limuloides ( Woodward, 1865) , Pseudoniscus clarkei Ruedemann, 1916 and Pseudoniscus falcatus ( Woodward, 1868) having been reported to possess lateral ocular features ( Fig. 1; Størmer, 1934; Eldredge, 1974; Bergström, 1975; Dunlop & Selden, 1998; Rudkin & Young, 2009; Selden et al., 2015). Other forms may have possessed putative LCEs, including Drabovaspis complexa (Barrande, 1872) , Pasternakevia podolica Selden & Drygant, 1987 , Weinbergina opitzi Richter & Richter, 1929 and Willwerathia laticeps ( Størmer, 1936) ( Lehmann, 1956; Chlupáč, 1963, 1965; Stürmer & Bergström, 1981; Anderson et al., 1998; Krzemiński et al., 2010; Ortega-Hernández et al., 2010). Problematically, several of these taxa have not been revised recently, and thus evidence for LCEs has not been corroborated since their original description.

Here, we re-examine the morphology of the seven synziphosurine taxa previously reported to possess LCEs and present the first direct evidence for LCEs in the synziphosurine Pseudoniscus roosevelti Clarke, 1902 . We also compare these features with the LCEs of the fossil crown-group euchelicerates Houia yueya ( Lamsdell et al., 2013) , Kasibelinurus amicorum Pickett, 1993 , Lunataspis aurora Rudkin et al., 2008 and the xiphosurid Limulus polyphemus (Linnaeus, 1758) . We use these comparisons to explore the phylogenetic distribution of different LCE morphologies in synziphosurines and xiphosurans, and thus investigate the ancestral condition of ocular structures in Euchelicerata.

INSTITUTIONAL ABBREVIATIONS

AM F, Australian Museum , Sydney, NSW, Australia ; AMNH, American Museum of Natural History , New York City, NY, USA ; GSM, Geological Survey of Britain , Keyworth, Nottinghamshire, UK ; MM, Manitoba Museum , Winnipeg, Manitoba, Canada ; NHMUK IA, The Natural History Museum , London, UK ; NIGP, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China ; NYSM, New York State Museum , Albany, NY, USA ; UNE.NHM.Z, University of New England Natural History Museum (Zoology Collection), Armidale, NSW, Australia .