A new probable stem lineage crustacean with three-dimensionally preserved soft parts from the Herefordshire (Silurian) Lagerstätte, UK

Abstract

A new arthropod with three-dimensionally preserved soft parts, Tanazios dokeron, is described from the Wenlock Series (Silurian) of Herefordshire, England, UK. Serial grinding, digital photographic and computer rendering techniques yielded 'virtual fossils' in the round for study. The body tagmata of T. dokeron comprise a head shield and a long trunk. The head shield bears six pairs of horn-like spines and the head bears five pairs of appendages. The antennule, antenna and mandible are all uniramous, and the mandible includes a gnathobasic coxa. Appendages four and five are biramous and similar to those of the trunk: each comprises a limb base with an endite, an enditic membrane, and two epipodites, plus an endopod and exopod. The hypostome bears a large cone-like projection centrally, and there may be a short labrum. The trunk has some 64 segments and at least 60 appendage pairs. A very small telson has the anus sited ventrally in its posterior part and also bears a caudal furca. Comparative morphological and cladistic analyses of T. dokeron indicate a crustacean affinity, with a probable position in the eucrustacean stem group. As such the epipodites in T. dokeron are the first recorded in a eucrustacean stem taxon. The new species is interpreted as a benthic or nektobenthic scavenger.

Figure 1

Tanazios dokeron. (a–l) Holotype, almost complete specimen, OUMNH C.29601, ‘virtual’ reconstructions. (a–c,l) Dorsal and ventral stereo-pairs, lateral and anterolateral views, all ×4. (d,h,j) Head region and anterior part of trunk, anterior tip of head shield removed, anterior view; head region and anterior part of trunk, lateral part of head shield, outer part of pleural region, and trunk appendages one to six removed, anteroventral view; head shield and anterior part of trunk, anterolateral stereo-pair; all ×10. (e) Head region and anterior part of trunk, lateral part of head shield and outer pleural region removed, lateral view, ×9. (f) Anterior part of head, head shield removed, lateral view, ×14. (g) Head shield and anterior two trunk segments, dorsal stereo-pair, ×8. (i) Head and anterior part of trunk regions, head shield and appendages removed, posteroventral stereo-pair, ×16. (k) Head and anterior part of trunk regions, ventral stereo-pair, ×15. Abbreviations used in figures 1–3: A1, appendage 1: antennule; A2, appendage 2: antenna; A4, appendage 4; A5, appendage 5; AH, artificially filled-in head shield; AMS, anterior margin of segment; An, anus; AO?, atrium oris ?; AR, axial region; Ba, basipod; Co, coxa; CR, caudal ramus/rami; DE, dorsal epipodite; DF, dorsal flap; Ed, endite; EM1–EM3, enditic membrane elements 1 (proximal)–3 (distal); EM, enditic membrane (undifferentiated); En, endopod; Ex, exopod; FR, Fringe region; FG, food groove; HHS, contact of head with ventral face of head shield; HS, horn-like spines; Hy, hypostome; HyS, hypostomal spine; IPR, inner pleural region; La?, labrum ?; LB, limb base; LM, lateral margin of hypostome; Ma, mandible: appendage 3; MF, median furrow; MHS, margin of head shield; Mo, mouth; Oe, oesophagus; PF, pleural facets; PMS, posterior margin(s) of segment(s); SA, swollen area; SP, subconical projection; T1–T4, trunk segments 1–4; T7, trunk appendage 7; TA, trunk appendages (undifferentiated); Te, telson; Tr, trunk; VE, ventral epipodite; VB, ventrolateral bulge of head; VF, ventral flap; WP, wing process.

Figure 2

Tanazios dokeron. (a,b,r) Partially reconstructed specimen, OUMNH C.29602; (c–q,s) holotype, almost complete specimen, OUMNH C.29601. (a–r) ‘Virtual’ reconstructions; (s) holotype specimen in rock. (a,b,r) Anterior part of head region, anterior and ventral stereo-pairs, lateral view, all ×15. (c–e) Posterior part of trunk, the telson, and anterior part of caudal furca, dorsal, posteroventral and ventral stereo-pairs, ×18, ×13, ×18. (f) Posterior part of trunk, the telson and caudal furca, posteroventral view, ×10. (g) Antennae, anteroventral stereo-pair, ×13. Dotted lines here and on (h–j) indicate discrete sections of the appendage defined by presumed podomere boundaries; some sections probably equate to one podomere, others to more than one (see description). (h) Mandibles, posteroventral stereo-pair, ×13. (i,j) Third and fourth appendage pairs, posterodorsal stereo-pair, ×13. (k,q) Seventh trunk appendage pair, posterodorsal and posterolateral stereo-pairs, ×13, ×20. The endopod is incomplete distally. (l,o) Head region and most anterior part of trunk, with and without short gut section, ventral stereo-pairs, ×14. (m,n,p) Gut section, dorsal, ventral and lateral stereo-pairs, ×24. (s) Transverse section through the trunk in the region of the base of second trunk appendage, ×12.

Figure 3

Tanazios dokeron. Interpretations of morphology, based on computer reconstructions of OUM C.29601 (holotype) and OUM C.29602. (a,c) Head shield and anterior four trunk segments, dorsal and anterolateral views, ×8. (b) Seventh trunk appendage, posterodorsal view, ×15. The endopod is incomplete distally. (d) Telson and part of caudal furca, posteroventral view, ×32. (e,f) Hypostome and surrounding area, lateral and ventral views, ×18. Diagonal hatches on b and d–f indicate sliced surfaces.