A vicissicaudatan arthropod from the Silurian Herefordshire Lagerstätte, UK

Abstract

A new arthropod, Carimersa neptuni gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Konservat-Lagerstätte, UK. The head bears pedunculate eyes and five pairs of appendages. Triflagellate antennae are followed by two pairs of uniramous limbs each with an endopod bearing a pronounced gnathobasic basipod. The posterior two pairs of head limbs and all trunk limbs bear an endopod, exopod and filamentous exite. The trunk consists of 10 appendage-bearing segments followed by an apodous abdomen of four segments. The arthropod resolves as sister taxon to Kodymirus and Eozetetes + Aglaspidida. It is the first representative of Vicissicaudata reported from the Herefordshire Lagerstätte and the first Silurian example with well-preserved appendages. The preservation of a cluster of radiolarians apparently captured by the trunk appendages is the first direct association of predator and prey discovered in the Herefordshire fauna, and suggests that Carimersa was a nektobenthic form that used its gnathobasic basipods in microdurophagy.

Keywords: Arthropoda; Herefordshire Konservat-Lagerstätte; Nekton; Silurian; Vicissicaudata.

Figure 1.

Holotype of Carimersa neptuni, exoskeleton and soft parts (OUMNH PAL-C.376503): (a–e, i–u) ‘virtual' reconstructions (a–c,e,i–r,u are stereo-pairs); (f) specimen in rock. The exact boundary between structures such as body and limbs, as indicated by colour changes, involves some interpretation. (a) Head shield, dorsal view showing eyes. (b) Head shield, ventral oblique view showing eyes and hypostome/labrum complex, other soft parts omitted. (c) Dorsal view. (d) Left lateral view. (e) Ventral view. (f) Section through abdomen showing sediment filled body cavity and other features. (g,h) Cluster of radiolarians and a single specimen between trunk appendages 2–5. (i) Head in ventral view. (j) Anterior oblique view. (k) Left appendage 1, dorsal view of proximal part. (l,n–r) posterior oblique view of right head appendages 2 (l), 4 (n), 5 (o), trunk appendages 1 (p), 5 (q), 6 (r). (m) Left appendage 3, anterior oblique view. (s) Posteroventral view of the gut, head shield, eyes and hypostome/labrum complex; all other features omitted. (t) Right lateral oblique view of gut (showing posteriorly flexed oesophagus) and eyes; all other features omitted. (u) Left anterior oblique view. ab, abdomen; as, abdominal spine; ba, basipod; es, eye stalk; ey, eye; et, exite; ex, exopod; f1–3, flagella; ga, gastropod; gn, gnathobase; gs, genal spine; h1–h5, head appendages; gu, gut (or possibly body cavity); hs, head shield; lb, limb base; la, hypostome/labrum complex; os, oesophagus; pa, precoxal area; sp, spine(s); sr, strengthening rod; t1–t10, trunk appendages; tr, trunk; ts, tail spine. Numbers refer to trunk tergites (1–10) and abdominal segments (1–4). Arrows in (l–r) indicate podomere boundaries. Scale bars: a–f, i, j, o–u are 2 mm; g, h, k–n are 0.5 mm.

Figure 2.

The phylogenetic position of Carimersa neptuni among Vicissicaudata based on parsimony and Bayesian analyses (electronic supplementary material, figures S1, S2). Key synapomorphies are indicated.