Dictyonema paululum australis, Rickards, R. B., Chapman, A. J., Wright, A. J. & Packham, G. H, 2003

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 : 316

Dictyonema paululum australis n.subsp.

Figs. 7F,G , 9E , 12A

Material. HOLOTYPE AM F114757 a-b and PARATYPES AM F114758 and AM F114773–4 , all from BF14, Bridge Creek .

Derivation of name. Suggesting that it is a geographical subspecies.

Diagnosis. Conical Dictyonema paululum with 16 stipes in 10 mm; autothecae denticulate and spaced at 20 in 10

mm; dissepiments spaced at 12–20 in 10 mm; stipe lateral width 0.20 mm and dorsoventral width 0.50 mm.

Description. The largest specimen is about 30 mm long and all the rhabdosomal fragments are characterized by slender stipes with a marked parallel arrangement connected by very slender dissepiments of 0.05 mm. Branching is very approximately zoned, every 1.5–2.5 mm, being more widely spaced distally. Dissepimental spacing is closest distally, at 20 in 10 mm, about one per autotheca, but proximally may rise to 12–13 in 10 mm. The autothecae are certainly denticulate, even spinose in profile, and the termination may possibly be spatulate rather than pointed. It is difficult to distinguish autothecal processes from dissepiments in this material. The overall appearance of the stipes is gently undulating with a serrated margin in places ( Fig. 7G ).

Remarks. Dictyonema paululum australis occurs at the same late Llandovery level as the nominate subspecies and, although very elegant and slender, is fractionally more robust than that form, with a more varied dissepimental spacing. In the nominate subspecies, the stipe spacing is slightly wider (16 in 10 mm compared with 20 in 10 mm) and the rectangular interspaces are narrower and often longer. The differences seem consistent in all material, and we regard D. p. australis as a geographical subspecies of D. paululum Bulman, 1928 .

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. 12. A, Dictyonema paululum australis n.subsp.,AM F114757, holotype, BF14; B, Dictyonema warrisi n.sp., holotype AM F114647a, F14. Scale bars 1 mm.