Trigonotarbus johnsoni, POCOCK, 1911

Jones, Fiona M., Dunlop, Jason A., Friedman, Matt & Garwood, Russell J., 2014, Trigonotarbus johnsoni Pocock, 1911, revealed by X-ray computed tomography, with a cladistic analysis of the extinct trigonotarbid arachnids, Zoological Journal of the Linnean Society 172 (1), pp. 49-70 : 57-58

TRIGONOTARBUS JOHNSONI POCOCK, 1911

1911 Trigonotarbus johnsoni Pocock , pp. 74–75, fig. 38, pl. 3, fig. 4.

1913 Trigonotarbus johnsoni Pocock ; Petrunkevitch, p. 98.

1949 Trigonotarbus johnsoni Pocock ; Petrunkevitch, pp. 250–255, figs 104–109, 111, 250–257.

1953 Trigonotarbus johnsoni Pocock ; Petrunkevitch 89–90, fig. 165.

1955a Trigonotarbus johnsoni Pocock ; Petrunkevitch p. 112, figs 77(2), 79(1–2).

1955b Trigonotarbus johnsoni Pocock ; Petrunkevitch p. 475.

1991 Trigonotarbus johnsoni Pocock ; Schultka, pp. 377, 381.

1996a Trigonotarbus johnsoni Pocock ; Dunlop, pp. 611–612, text-fig. 4.

Holotype

NHMUK In 31239. From Coseley near Dudley, Staffordshire, UK. Late Carboniferous (similis–pulchra zone, Westphalian B in the European stratigraphical scheme and Moscovian in an international stratigraphical framework; Wilson, 2005).

Additional material

NHMUK I. 7895, I. 13911, I. 13919, I. 13921, I. 15859, I. 15860, I. 15897, In 31240–41, In 31243, and In 31254– 55. All from the same locality as the holotype.

Emended diagnosis

Small, stubby-limbed trigonotarbids with triangular carapace, lacking lateral lobes, but with a raised median ridge bearing the eyes and a deep, inverted T-shaped depression behind the eyes. Rounded opisthosoma has laterally undivided tergite nine, apparently with fused sternites 9 and 10 bearing the pygidium at their anterior. (Emended from Petrunkevitch, 1955 b.)

Description

Trigonotarbus johnsoni was described in some detail by Pocock (1911) and Petrunkevitch (1949). Here, we redescribe it from restudy of the hand specimens ( Fig. 2 ) and tomographic reconstruction of NHMUK I. 15860 ( Fig. 3 , Files S2, S3). Except where noted, measurements derive from the latter. Small trigonotarbid; holotype 4.8 mm long (carapace 2.0 mm in length, opisthosoma 2.8 mm long by 2.5 mm wide); scanned specimen 6.3 mm in length (carapace 2.8 mm in length, opisthosoma 3.5 mm long by 3.6 mm wide). Carapace triangular with raised median area, increasing in width posteriorly ( Fig. 3A ). Raised median area drawn into anteriorly pointing, rounded clypeus, surrounded by zones of lower relief. Carapace with smooth lateral margins (see, e.g. NHMUK In 32139), i.e. no division into lobes, posteriorly demarcated by strong transverse groove; some specimens (e.g. NHMUK In 31241) reveal depression on median ridge behind the eyes and shaped like an inverted T that effectively merges into this groove. Closely spaced median eyes situated on median ridge, towards front of carapace (NHMUK In 31239; Fig. 2 ; NHMUK I. 15860, Fig. 3 ).

Ventral prosoma comprising triangular coxae, increasing in size posteriorly (e.g. NHMUK I. 13239, I. 15897, In 31241, Fig. 3 ; 1.1–1.3 mm), truncate against a central sternum. Sternum itself four-sided, almost diamond shaped. NHMUK In 31240 suggests a pointed anterior sternal margin. Tomographic reconstruction reveals a dorsoposteriorly angled, transverse groove between pedipalp coxae, and those of leg 1 ( Fig. 3D ). Medial boundary of leg 1 coxae developed into small, lip-like endites, with median anteriorly pointing projection between ( Fig. 3D , PR). Small, palaeognath (i.e. downward-hanging) chelicerae ventral to clypeus ( Fig. 3C ). Legs short and stubby, without strong ornament; 2–4 on right well preserved (L4 shown in Fig. 3B ): small, bulbous trochanter (0.8 mm on leg 4R, chosen as best preserved), femur (1.1 mm), patella (0.8 mm), and tibia (0.9 mm) of similar length, short metatarsus (0.3 mm), longer tarsus (0.8 mm).

Opisthosoma rounded; dorsal segmentation clear in model and NHMUK In 31243. Tergite 1 apparently expressed as anterior ridge on tergite 2; itself partially tucked under the carapace ( Fig. 3A ). No apparent fusion of tergites 2 and 3 into a diplotergite. Tergite nine narrow. Ventrally, small anterior and posterior operculae abut coxae of leg 4. Sternites strongly posteriorly recurved, especially towards the back, but sternite 4 expressed as straight transverse ridge, visible in both tomographic reconstruction and in NHMUK In 31241 and In 31240. Sternites 9 + 10 apparently fused (but see Discussion). Pygidium towards anterior rounded margin of sternites 9 + 10. No evidence of ventral sacs on underside of opisthosoma. Generally, no visible ornamentation (spines, pustules, tubercles, etc.) on either prosoma or opisthosoma.

Figures

Figure 1. Reconstructions of representatives of nine trigonotarbid families, shown to scale. A, Palaeocharinus rhyniensis (Palaeocharinidae). B, Archaeomartus levis (Archaeomartidae). C, Anthracomartus hindi (Anthracomartidae). D, Anthracosiro woodwardi (Anthracosironidae). E, Trigonotarbus johnsoni (Trigonotarbidae). F, Lissomartus schucherti (Lissomartidae). G, Aphantomartus pustulatus (Aphantomartidae). H, Eophrynus prestvicii (Eophrynidae). I, Kreischeria wiedei (Kreischeriidae). Scale bar = 10 mm.

Figure 2. Hand specimens of Trigonotarbus johnsoni from the NHMUK collections. Scale bars = 1 mm.

Figure 3. Tomographic reconstruction of Trigonotarbus johnsoni based on specimen NHMUK I. 15860. A, dorsal view. B, fourth right leg, podomeres labelled. C, ventral view. D, ventrolateral view of the coxo-sternal region. Abbreviations: 1–12, segment number; CE1−2, coxal endites 1–2; CH, chelicerae; CL, clypeus; FE, femur; L1−L4, legs 1–4; ME, median eye tubercle; MT, metatarsus; PA, patella; PP, pedipalp; PR, projection; TA, tarsus; TI, tibia; TR, trochanter. Scale bars: A, C = 2 mm, B = 1 mm, D = 0.5 mm.