A Silurian short-great-appendage arthropod

Abstract

A new arthropod, Enalikter aphson gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Lagerstätte of the UK. It belongs to the Megacheira (=short-great-appendage group), which is recognized here, for the first time, in strata younger than mid-Cambrian age. Discovery of this new Silurian taxon allows us to identify a Devonian megacheiran representative, Bundenbachiellus giganteus from the Hunsrück Slate of Germany. The phylogenetic position of megacheirans is controversial: they have been interpreted as stem chelicerates, or stem euarthropods, but when Enalikter and Bundenbachiellus are added to the most comprehensive morphological database available, a stem euarthropod position is supported. Enalikter represents the only fully three-dimensionally preserved stem-group euarthropod, it falls in the sister clade to the crown-group euarthropods, and it provides new insights surrounding the origin and early evolution of the euarthropods. Recognition of Enalikter and Bundenbachiellus as megacheirans indicates that this major arthropod group survived for nearly 100 Myr beyond the mid-Cambrian.

Keywords: Arthropoda; Herefordshire Lagerstätte; Leanchoiliida; Megacheira; Silurian; exceptional preservation.

Figure 1.

(Opposite.) Enalikter aphson, virtual reconstructions. (a–c,k,o,x) Holotype, OUMNH C.29631, outstretched specimen, trunk somewhat dorsoventrally compressed; (a–c,k) complete specimen, (a) dorsal stereo pair, (b) left lateral view, (c) ventral view, (k) anterior-oblique stereo pair, (o) trunk appendage 11, posteroventral view, (x) trunk between appendages 8 and 9, cuticle translucent, dorsal view. (d–j,l,n,p–s,b1) OUMNH C.29633, almost complete outstretched specimen, trunk somewhat dorsoventrally compressed, estimated length 15.9 mm; (d,e,h,j,b1) head and anterior-most part of trunk, (d) ventral posterior-oblique view, (e) dorsal view, (h) ventral stereo pair, (i) lateral view, (b1) posterior view, (f) trunk between trunk appendages 6 and 11, ventral view, (g) complete specimen, (i) trunk between trunk appendages 11 and 12, ventral view, (l,n) telson, (l) dorsal stereo-pair, (n) lateral view, (p,s) head appendages 2, and 3, posteroventral views, (q) head, with head shield and soft tissue around the gut removed, dorsal view, (r) head, with head shield translucent, lateral view. (m,t–w,y,z,a1) OUMNH C.29632, complete, laterally flexed specimen, estimated length 11.0 mm; (m) trunk between trunk appendage 10 and anterior part of telson, ventrolateral stereo pair, (t–w,y) complete specimen, (t,v) with exopods, and with exopods removed, dorsal stereo pair, (u) ventral stereo-pair, (w) exopods removed, lateral view, (y) posterior-oblique view, (z) telson, posterior view, (a1) telson and posterior part of trunk, posterodorsal view. Scale bars are all 1 mm. an, anus; ap, anterior process; bs, button-like sternite; cn, central node; dpp, dorsal posterior process; dos, disc-like oral surface; dt, dome-like tergite; fs, faecal stream; gut, midgut/intestine; h1, h2, h3, head appendages 1, 2 and 3; h2en, head appendage 2 endopod; h2ex, head appendage 2 exopod; h3en, head appendage 3 endopod; h3ex, head appendage 3 exopod; hs, head shield; mnp, medial needle-like process; mo, mouth; oe, oesophagus; ptr, prominent trunk ridge; sar, subtriangular axial region; s, stomach; t, telson; t1, t2, t4, t8, t9, t11, t12, t14, trunk appendages 1, 2, 4, 8, 9, 11, 12 and 14; t1en, trunk appendage 1 endopod; t4en, trunk appendage 4 endopod; t8en, trunk appendage 8 endopod; t11en, trunk appendage 11 endopod; t11ex, trunk appendage 11 exopod; t11ef, trunk appendage 11 exopod filaments; t12en, trunk appendage 12 endopod; tef, trunk appendage exopod filaments; tr, trunk ridge(s); tst, transverse soft tissue; tst?, transverse soft tissue?; vb, ventral boss; vpp, ventral posterior process; wr, wedge-like region.

Figure 2.

A summary of the phylogenetic relationships and of topologies produced during phylogenetic analyses of Enaliktidae, which were consistent over all of them (see electronic supplementary material, text S1 and figure S1 for details).