Dictyonema venustum Lapworth, 1881

Rickards, R. B., Chapman, A. J., Wright, A. J. & Packham, G. H, 2003, Dendroid and Tuboid Graptolites from the Llandovery (Silurian) of the Four Mile Creek Area, New South Wales, Records of the Australian Museum 55 (3), pp. 305-330 : 315-316

Dictyonema venustum Lapworth, 1881

Figs. 7A , 9D , 10B–D

1881 Dictyonema venustum Lapworth ; Lapworth, p. 171–172, pl. 7, fig. 1a–c.

1928 Dictyonema venustum, Lapworth , emend.; Bulman, p. 61– 63, pl. 5, figs. 6–8, text-fig. 34.

Material. AM F114648a-b, AM F114649, AM F114717ab and AM F114873, all from F14; AM F114751 from BF 28, all Bridge Creek.

Diagnosis. Large Dictyonema characterized by a striking rectangular meshwork of stipes and fairly robust dissepiments; at least 30 mm long, possibly conical, with irregular stipe branching; stipes 10–18 in 10 mm; denticulate autothecae spaced at 20–30 in 10 mm; dissepimental spacing 5–8 in 10 mm.

Description. Our largest specimen is a fragment of a large rhabdosome but it is not possible to say whether or not it is conical.A striking rectangular meshwork of stipes and fairly robust dissepiments typifies the colony. Stipes have a lateral width of 0.20–0.35 mm and a dorsoventral width of about 0.40 mm, whereas the dissepimental width is 0.10–0.25 mm. Stipe spacing ranges from 10–18 in 10 mm, the mean being 14–15 and the lower values nearer the proximal end. Stipe branching seems quite irregular. There is a limited amount of anastomosis in the stipe pattern. No bithecae have been detected.

Remarks. In most respects our material is close to Bulman’s (1928) redescription of the type and other material, differing only in having a slightly closer thecal spacing (20 in 10 mm compared with 16–17 in 10 mm) and slightly more slender dissepiments. The irregular branching mode is very similar to that on the Welsh and Scottish material described by Bulman (1928: p. 643), as is the stipe spacing, stipe widths and dissepimental spacing.

The largest specimen exhibits a damaged and repaired area ( Fig. 10B , arrowed). In this region are two short, incomplete stipes whereas one very robust stipe tracks left to right, crosses one stipe and then anastomoses with the next one, which then terminates. The stipe crossed over by the robust, laterally-directed stipe continues to grow and fills the gap caused by the damage, so that by the rhabdosomal periphery a normal stipe arrangement is restored. The most likely explanation of the damage is that a hole was punched into the stipe array, separating the stipes and breaking off some of them.

Figures

Fig. 7. A, Dictyonema venustum, AM F114751, BF28; B, Dictyonema williamsae n.sp., holotype AM F114565, W885; C– E, Dictyonema jenkinsi n.sp., respectively AM F114747, F14, holotype AM F114716, BF24; and AM F114761, F14, holdfast region attached to roughly circular object, possibly a pebble; F,G, Dictyonema paululum australis n.subsp., AM F114757, parts of holotype,BF14.Scale bars 1 mm;on B hatching indicatesspecimen obscured by sediment, stipple indicates badly preserved area, and horizontal shading indicates fractures in rock.

Fig. 9. A–E, illustrating contrasting stipe and dissepimental patterns in distal parts of colonies: A, Dictyonema cf. delicatulum, AM F114749; B, Dictyonema williamsae n.sp.,AM F114565,holotype; C, Dictyonemafalciferum,AM F114750; D, Dictyonemavenustum, AM F114648a, F14; E, Dictyonema paululum australis n.subsp., holotype AM F114757. Scale bars 1 mm.

Fig. 10. A, Dictyonema falciferum, AM F114750, F14; B–D, Dictyonema venustum, respectively AM F114648 and AM F114873, both BF14, and AM F114751, BF28. Scale bars 1 mm; arrow on B indicates damaged area discussed in text.