Dictyonema williamsae, 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 : 314-315

Dictyonema williamsae n.sp.

Figs. 5D–F, 7B , 8A

Material. HOLOTYPE AM F114565 and three PARATYPES AM F114652 a-b , AM F114758 and AM F114772 a-b, all from F14, Bridge Creek .

Derivation of name.After Norma Williams of “Ashburnia”.

Diagnosis. Robust, conical Dictyonema rhabdosome, developed from a holdfast, each of four primary stipes with different branching patterns; stipes are connected by robust dissepiments, each of which is slightly arched distally; autothecae are aperturally isolated tubes without marked denticulation; autothecal spacing 13 in 10 mm; dissepimental spacing 6–7 in 10 mm; stipe spacing 8–12 in 10 mm; lateral stipe width maximum 0.30 mm; dorsoventral stipe width 0.60–0.70 mm.

Description. The rhabdosome is conical, some 14 mm high by 14 mm wide at the aperture of the cone. It derives from a small basal disc or holdfast. The most striking feature of the colony is the rectangular interstipe spaces defined by robust dissepiments and stipes of not much greater width; dissepiments are 0.15–0.20 mm wide and stipes in lateral view 0.15–0.30 mm. Of the several main stipes, which arise from the holdfast region, two produce at least 27 of the 34 peripheral stipes, so that one could say that at least three quarters of the cone is built by branchings from only two stipes. There are no obvious branching zones.

When seen in full profile the autothecae are aperturally isolated but without marked denticulation ( Fig. 5D,E). In places they are difficult to distinguish from partially preserved dissepiments which have a similar width; it is possible, though unproven, that the dissepiments are actually autothecal transfers from one stipe to the adjacent stipe.

Remarks. Dictyonema williamsae is an unusual form having a low thecal spacing which eliminates comparison with most other dictyonemids. Dictyonema inequabile Bulman from the late Llandovery of Scotland has a similar arrangement of stipes and rhabdosome, but differs in having a less regular meshwork and very variable lateral stipe widths and equally variable dissepimental widths. The autothecal type and spacing is unknown in D. inequabile . However, D. inequabile is undoubtedly the closest form to D. williamsae , most dictyonemids having a much higher thecal spacing. Dictyonema obpyriforme (Gurley in Bassler, 1909) has a similar stipe arrangement but all measurements differ markedly. Many of the Silurian dictyonemids described by Bassler (1909) and Ruedemann (1947) have stipes that are much too robust for comparison with D. williamsae even when the stipes have rectangular interstipe spaces. Of the species described by Bouěek (1957) Dictyonema elongatum is closest to D. williamsae in rhabdosomal and stipe dimensions, but its thecae appear to be simpler, its stipe spacing narrower (14–16 in 10 mm) and it dissepimental spacing wider (2–6 in 10 mm).

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. 8. A, Dictyonema williamsae n.sp., holotypeAM F114565, F14; B, Dictyonema falciferum, AM F114750, F14. Scale bars 1 mm.