Dictyonema jenkinsi, 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

Dictyonema jenkinsi n.sp.

Figs. 7C–E , 11A,B

Material. HOLOTYPE AM F114716 and PARATYPES AM F11461 , AM F114747–8 and AM F114776–8 , all from F14, Bridge Creek .

Derivation of name. In honour of Dr Chris Jenkins who collected many of the graptolites in our collections.

Diagnosis. Large,?conical Dictyonema rhabdosome developed from basal holdfast; thecae with unique, platelike outgrowths of dorsal, apertural processes; autothecal apertures slightly isolated; bithecae bulbous aperturally.

Description. One rhabdosomal fragment ( Fig. 7E ) is of a holdfast with three or four main stipes developed from it; stipe division is close to the holdfast and, although this specimen is preserved in “plan” view, it suggests that a conical rhabdosome develops from the holdfast. Another large fragment of rhabdosome, of which Fig. 7C is a part, indicates an overall large colony perhaps 25 mm long. Stipes are spaced at 14 in 10 mm, have a lateral width of about 0.50 mm, and are connected by hair-like dissepiments spaced at 18–20 in 10 mm and by occasional anastomosis.

The stipe spacing is, therefore, slightly closer than the autothecal spacing, which is 16 in 10 mm. The autothecae have slightly isolated thecal apertures, a denticulate ventral apertural process ( Fig. 7D ) and a conspicuous dorsal apertural process, composed of fuselli, which grows to a plate-like form expanding away from the apertural region. Sometimes it hangs downward slightly, presumably overhanging the apertural region. The dorsoventral width is 0.80 mm, excluding the large process, but up to 1.30 mm including it. Bithecae can be detected on the holotype as bulbous growths positioned dorsally of the autothecal apertural region; they alternate along the stipe ( Fig. 7D ).

Remarks. The autothecal processes make D. jenkinsi a unique species. Similar plate-like growths are developed on D. peltatum Wiman, 1901 , but these are modified ventral apertural processes. The hydrodynamic function may be similar, of course, as suggested by preliminary work by one of us (R.B.R.). The bithecae may be of Bulman’s (1928) type 5; that is, not dissimilar to the bithecae in D. cavernosum Wiman, 1901 (see Bulman & Rickards, 1966).

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. 11. A,B, Dictyonema jenkinsi n.sp., AM F114748, respectively distal thecaeandproximalendclose to holdfast,F14.Scalebars 1 mm.