A glyptocystitid cystoid affinity for the putative stem group chelicerate (Arthropoda: Aglaspidida or Xiphosura) Lemoneites from the Ordovician of Texas, USA

Abstract

The putative arthropod Lemoneites was originally described by Flower (1969) and assigned to the family Lemoneitidae, within the chelicerate stem group order Aglaspidida, because it possessed a semicircular carapace bearing eyes, a segmented body and a clubshaped telson. Flower (1969) suggested that Lemoneites combined characteristics of aglaspidids (eleven body segments) and synziphosurines (a postabdomen of three segments). Eldredge (1974) assigned Lemoneites to Xiphosura incertae sedis, but it was later excluded by Anderson and Selden (1997). Cladistic analyses have placed Lemoneites as “sister group to all other [eu]chelicerates . . . probably a common ancestor of Chelicerata” (Dunlop & Selden 1997, p. 230), as the possible ancestor of Xiphosura (Anderson & Selden 1997), or within a ‘chelicerate-allied’ clade of arachnomorph arthropods (Cotton & Braddy 2004). Other recent studies (e.g. Fortey & Rushton 2003) have used Lemoneites for comparative purposes with aglaspidids. Therefore, Lemoneites potentially occupies a crucial position in early chelicerate and/or xiphosuran evolution. However, these cladistic analyses coded Lemoneites based on Flower’s original description. Examination of the original specimens shows Flower’s description to be inaccurate in several important respects. The mode of preservation of Lemoneites has important implications for the interpretation of its anatomy. The fossils originate from the lower part of the Scenic Drive Formation (El Paso Group), of the Franklin Mountains, Texas (equivalent to trilobite zones H–J of Hintze (1953) and the early Arenig of the UK (Cooper & Fortey 1982; Fortey & Rushton 2003)). Lemoneites was found in association with a fully marine (mainly molluscan) assemblage. Gastropods are dominant (represented by the operculum of Ceratopea ankylosa, amongst other species) in addition to bivalves, cephalopods, trilobites, ostracods and rare brachiopods, monoplacophorans and polyplacophorans (Flower 1969). All of the fossils are preserved as three-dimensional silica replacements and were acid etched from a lens of massive dolomite containing fine sand laminations with minor cross-bedding (Flower 1969). Herein lies the first difficulty, as Flower (1969, p. 40) himself noted. If Lemoneites is an arthropod, it must represent an internal mould, however Flower noted that the associated molluscan fauna is preserved as casts in silica,