Dictyonema muirae, 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 : 317-318

Dictyonema muirae n.sp.

Fig. 13A–D

Material. HOLOTYPE AM F114762 and PARATYPES AM F114763–4 and AM F114775 , all from F14, Bridge Creek . PARATYPE AM F14779 from BF24, Wallace Creek .

Derivation of name. After Lucy Muir, in recognition of her skilled graptolite collecting.

Diagnosis. Dictyonema with unusually close thecal spacing of 40 in 10 mm, of denticulate autothecae, and with very unusual isolated, curved bithecae; sparse dissepiments.

Description. The rhabdosome grows from a holdfast with rapid and irregular branching taking plate ( Fig. 13D ). Irregular branching is maintained in the specimens we have, and perhaps because of the frequency of branching, dissepiments are difficult to identify and possibly few in number. Stipe spacing is 14–20 in 10 mm depending upon how divergent the growth is at any point. The dorsoventral stipe width is about 0.60 mm, and the lateral stipe width 0.30–0.40 mm. The autothecae are markedly denticulate ( Fig. 13B ) and are spaced at 40 in 10 mm. Associated with the autothecae ( Fig. 13C ) are curved, narrower tubes, the apertures of which face distally. These are probably bithecae and they may be a modified form of Bulman’s (1928) Type 5 where the bithecal apertural region extends in an isolated manner. The overall colony form is not known.

Remarks. The combination of very close autothecal spacing with unusual bithecae makes D. muirae a unique, unusual species of Dictyonema . The irregular, frequent, and divergent branching, coupled with the presence of dissepiments, supports attribution to Dictyonema rather than Callograptus . The individual stipes look not unlike robust Dendrograptus species.

Figures

Fig. 13. A–D, Dictyonema muirae n.sp., respectivelyAM F114763, BF14, AM F114775, F14, holotype AM F114762, F14, and AM F114764, BF14 proximal part of colony attached to pebble of phosphate; E–G, Dictyonema warrisi n.sp., respectively AM F114647, F14, holotype AM F114717, W885 and AM F114647, F14, with stipes more proximal than E; H, Dictyonema sp. 1,AM F11475, F14. Scale bars 1 mm; shading on C indicates badly preserved area.