Lethaia Seminar

Lethaia, Vol. 38, pp. 293–296. Oslo, 2005 09 12

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

RACHEL A. MOORE AND SIMON J. BRADDY

The putative arthropod Lemoneites was originally described byFlower (1969) and assigned to the family Lemoneitidae, within thechelicerate stem group order Aglaspidida, because it possessed asemicircular carapace bearing eyes, a segmented body and a club-shaped telson. Flower (1969) suggested that Lemoneites combinedcharacteristics of aglaspidids (eleven body segments) and synzi-phosurines (a postabdomen of three segments). Eldredge (1974)assigned Lemoneites to Xiphosura incertae sedis, but it was laterexcluded by Anderson and Selden (1997). Cladistic analyses haveplaced Lemoneites as “sister group to all other [eu]chelicer-ates . . . 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 ofarachnomorph arthropods (Cotton & Braddy 2004). Other recentstudies (e.g. Fortey & Rushton 2003) have used Lemoneites forcomparative purposes with aglaspidids. Therefore, Lemoneitespotentially occupies a crucial position in early chelicerate and/orxiphosuran evolution. However, these cladistic analyses codedLemoneites based on Flower’s original description. Examination of

the original specimens shows Flower’s description to be inaccuratein several important respects.

The mode of preservation of Lemoneites has important impli-cations for the interpretation of its anatomy. The fossils originatefrom the lower part of the Scenic Drive Formation (El Paso Group),of the Franklin Mountains, Texas (equivalent to trilobite zones H–Jof Hintze (1953) and the early Arenig of the UK (Cooper & Fortey1982; Fortey & Rushton 2003)). Lemoneites was found in associationwith a fully marine (mainly molluscan) assemblage. Gastropods aredominant (represented by the operculum of Ceratopea ankylosa,amongst other species) in addition to bivalves, cephalopods,trilobites, ostracods and rare brachiopods, monoplacophorans andpolyplacophorans (Flower 1969). All of the fossils are preserved asthree-dimensional silica replacements and were acid etched from alens of massive dolomite containing fine sand laminations withminor 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 thatthe associated molluscan fauna is preserved as casts in silica,

Fig. 1. Camera lucida drawings of specimens in dorsal view. &A. Lemoneites mirabilis Flower 1969, holotype NMMNH 1207. &B. Lemoneitessimplex Flower 1969, holotype NMMNH 1209. &C. Lemoneites ambiguus Flower 1969, holotype NMMNH 1213. &D. Lemoneitesgomphocaudatus Flower 1969, paratype NMMNH 1211 (holotype missing). pro = putative ‘prosoma’, e = putative ‘eye’, t = putative ‘telson’,c = constriction of ‘telson’, tu = tubercle. Scale bar = 1 mm.

DOI: 10.1080/00241160510013277 # 2005 Taylor & Francis Group Ltd

implying a similar preservation pathway for Lemoneites. Anunmineralized arthropod would be expected to degrade, collapseand be flattened in a carbonate depositional environment. Thenon-mineralized cuticle of chelicerates does not lend itself to sili-cification in the same way as the biomineralized exoskeleton oftrilobites and crustaceans. Silicified chelicerates are unknown; eventhe Early Devonian Rhynie Chert arachnids are preserved as organiccuticle films within chert (Trewin 1994; Dunlop 1997), althoughthis hot-spring setting does not equate to the secondary diageneticconditions that formed the El Paso dolomite. Synziphosurines andaglaspidids are commonly preserved as flattened fossils. The onlysynziphosurines preserved in any significant relief are Bunodeslunula Eichwald, 1854, Pseudoniscus aculeatus Nieszkowski, 1859and Legrandella lombardii Eldredge, 1974, as are the Cambrianstrabopids Paleomerus Størmer, 1956 (see also Tetlie & Moore2004) and Strabops Beecher, 1901.

Flower (1969) described Lemoneites as having two small eyes oflow relief but did not label these features. The prosoma wasdescribed as ‘smoothly curved’ anteriorly with a ‘rostrum-like

structure’ on the ventral surface. He noted that Lemoneites haseleven body segments – eight in the preabdomen and three in thepostabdomen. The body segments consist of an anterior ring, adeeply recessed medial band and a raised posterior ring, whichcurls above and overlaps the anterior ring of the next posteriorsegment. Based only on NMMNH 1209, the three postabdominalsegments of Lemoneites were described by Flower (1969, pl. 8,fig. 13) as annular (i.e. fused, ring-like segments separated bynarrower inter-ring regions) (Figs 1 & 2).

Flower (1969) erected four species of Lemoneites: L. mirabilis,L. simplex, L. gomphocaudatus, and L. ambiguus (Figs 1 & 2), basedon eight specimens, largely defined on the form of their telsons.L. mirabilis has a rod-like telson with lateral wrinkles and a raised,blunt and incomplete transverse band near the tip (Figs 1A, 2A–B).L. simplex has a club-shaped telson with no ornament (Figs 1B,2C–D). L. gomphocaudatus has a club-shaped telson ornamentedwith bilaterally symmetrical pustules (Figs 1D, 2G–H), interpretedby Flower (1969) as a genuine feature rather than taphonomicovergrowth. The telson of L. ambiguus is unknown.

A B C D

E F G H IFig. 2. Photographs of original specimens. &A. Lemoneites mirabilis Flower 1969, holotype NMMNH 1207, dorsal view. &B. Lateral view ofprevious specimen. &C. Lemoneites simplex Flower 1969, holotype NMMNH 1209, dorsal view. &D. Ventral view of previous specimen.&E. Lemoneites ambiguus Flower 1969, holotype NMMNH 1213, dorsal view. &F. Lemoneites ambiguus Flower 1969, paratype NMMNH1214, dorsal view. &G. Lemoneites gomphocaudatus Flower 1969, paratype NMMNH 1211, dorsal view. &H. Lemoneites gomphocaudatusFlower 1969, paratype NMMNH 1212, lateral view. &I. Dorsal view of previous specimen. Scale bar = 1 mm.

294 Lethaia Seminar LETHAIA 38 (2005)

The holotype of L. gomphocaudatus (NMMNH 1210) couldnot be located, but an additional poorly preserved (originallyunfigured) specimen was found in the collection of the NewMexico Museum of Natural History and Science (NMMNH),Alburquerque, New Mexico (see Fig. 2 legend for specimennumbers), where all the material is now held (Rowland & Neville1999).

Restudy of the specimens has found no evidence for eyes in anyof the material; the structures in NMMNH 1207 and 1213, whichwere probably interpreted as eyes by Flower, are merely irregularblobs of silica on the supposed carapace (Fig. 1A, C, ‘e’), whichitself shows an irregular anterior margin in all the material, and isnot ‘smoothly curved’ as described by Flower. The so-called‘rostrum-like structure’ noted by Flower in NMMNH 1207, 1209,1211 and 1214, shows no consistent morphology and is alsointerpreted here as irregular regions of silica. In NMMNH 1207the anterior ring of the postabdominal segments have becomedecoupled (Fig. 1A). This specimen also shows the ventral surfaceof the postabdomen as a flat plane (Flower 1969, pl. 8, fig. 4),similar to the preabdomen in all the other material. Thus, theevidence that only the postabdominal segments were annular isequivocal.

Reinterpretation of Lemoneites

Restudy of Lemoneites suggests that Flower’s (1969) originalinterpretation of its morphology as consistent with that of either anaglaspidid or synziphosurine is incorrect. Firstly, the body has anunusual segmentation pattern, divided into raised rings and narrowinter-ring regions, unlike any merostome (i.e. Xiphosura orEurypterida) or aglaspidid, even accounting for the unlikelypreservation as an internal mould (see above), which consist of flatsegments separated by flexible membranes enabling articulation(i.e. tergites may overlap to some degree but they are not raised).Secondly, the putative eyes and rostrum-like structure are based onpreservational artefacts (i.e. irregular regions of silica). Finally, the

form of the telson is unlike that of any known merostome oraglaspidid. The constriction near the tip of the telson in L. mirabilis(Fig. 1A, c) most likely represents a boundary between two separateunits. The pustular ornament of the telson in L. gomphocaudatus(Fig. 1D), and the cylindrical form of the telson of all Lemoneitesspp. are unlike any merostome or aglaspidid, where the telsonconsists of one unit, is unornamented, typically triangular incross-section, and generally long and styliform or short andtapering (modified into a broad, paddle-shaped telson in someEurypterida).

Consequently, Lemoneites should be excluded from futureanalyses of chelicerate and/or xiphosuran phylogeny. This leavesthe oldest unequivocal synziphosurines as Venustulus wauke-shaensis from the Late Llandovery Waukesha Lagerstatte ofWisconsin (Moore et al. 2005) and Bembicosoma pomphicus fromthe Late Llandovery of the Pentland Hills, Scotland (Anderson &Moore 2004).

Many of the features in Lemoneites imply a non-arthropodaffinity. As the evidence that only the postabdominal segmentswere annular is equivocal (see above), we interpret that partialobliteration (e.g. due to selective decay) of the ventral preabdomenin all specimens and the ventral postabdomen in some specimensoccurred in an originally radially-symmetrical animal.

This inferred original radial symmetry and the nature of thesegmentation indicates a pelmatozoan echinoderm affinity, speci-fically a dichoporite rhombiferan cystoid, which possess a flexibleproximal stem differentiated into inner and outer rings with well-developed flanges (Fig. 3), and a relatively rigid distal stem(B. Lefebvre personal communication 2004). The irregular natureof the anterior prosomal margin indicates that it formed part ofa larger structure, i.e. the basal portion of the theca (possibly withhypertrophied basal thecal plates), the fragile arms and otherdiagnostic features lacking. The missing portion of the theca maybe explained if it retained some calcite, which was destroyed by theacid preparation of the fossils (C. Paul personal communication2004). The abdomen represents the proximal region of the stem,and the telson represents the proximal-most portion of thedistal region of the stem. Indeed, Lemoneites is very similar tothe proximal stem of a glyptocystitid cystoid (see Paul 1968,fig. 15); the proximal stem, adjacent to the theca, tapers rapidlyand consists of annular columnals, smaller ones fitting insidelarger ones, with a large lumen, the distal stem consists ofnarrow columnals, the larger ones often with a median keel, and anarrow lumen (C. Paul personal communication 2004). Brett(1981) discussed the form of the attachments of pelmatozoanechinoderms, noting that some stereomatic outgrowths may occuras knobs, as in the telson of L. gomphocaudatus. Indeed, suchorganisms are typical in the Early Ordovician and are of compar-able size to Lemoneites. One dichoporite rhombiferan cystoid(Cuniculocystis) has even been described from the El Paso Group ofTexas (Sprinkle & Wahlman 1994), although it differs in overallmorphology to Lemoneites. Glyptocystitids have been reportedfrom older or equivalent strata in the area; Cheirocystella antiquaoccurs in trilobite zone D, near the base of the Fillmore Limestonein Utah (Paul 1972).

The fragmentary nature and poor preservation, due in part to theacid preparation of Lemoneites spp., indicates that there areinsufficient defining characters to justify four separate species. Forthis reason we suggest the synonomy of the three junior specieswith L. mirabilis, which itself should be referred to the Glypto-cystitida Bather, 1899, although a redescription of this taxon mayneed to await further discoveries, prepared to reveal its completemorphology.

Acknowledgements. – We thank Paul Selden for initially suggestingthis line of enquiry. We thank Derek Briggs and Erik Tetlie for theircomments on the manuscript. We also thank Joseph Botting,Bertrand Lefebvre and Chris Paul for useful discussions. LyallAnderson and one anonymous referee reviewed the manuscript.Lyall Anderson (Museum of Scotland) kindly provided the imagefor Fig. 3. RAM was funded by a NERC research studentship (NER/S/A/2001/05999).

Fig. 3. Photograph of the Ordovician cystoid Dendrocystis scotica(Bather) from the ‘Starfish Bed’ of South Threave, Ladyburn,Girvan, Ayrshire, Scotland, NMS G.1957.1.25. Specimen whitenedwith ammonium chloride. Scale bar = 1 mm.

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Rachel A. Moore, Department of Earth Sciences, University ofBristol, Wills Memorial Building, Queen’s Road, Bristol BS81RJ, UK, Current address [ramoore@ku.edu], Department of Geol-ogy, University of Kansas, Lindley Hall, 1475 Jayhawk Blvd,Room 120, Lawrence, KS 66045-7613, USA; Simon J. Braddy[s.j.braddy@bristol.ac.uk], Department of Earth Sciences, Universityof Bristol, Wills Memorial Building, Queen’s Road, Bristol BS81RJ, UK.

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