Callograptus rigbyae, 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 : 319

Callograptus rigbyae n.sp.

Fig. 14B,C

Material. HOLOTYPE AM F114651 a-b, from BF28 from an unnamed tributary W of Bridge Creek ; and PARATYPE AM F114643 from W885, Bridge Creek .

Derivation of name. After Dr S. Rigby, well-known graptolite worker.

Diagnosis. Small, robust Callograptus with small holdfast or short stem; usually preserved in “plan” view, branching in well-marked zones, six of which give a 10 mm colony spread; dissepiments absent; autothecae 20 in 10 mm approximately.

Description. The rhabdosomes are spread in a “plan” view but are sufficiently well-preserved to conclude that dissepiments are absent. Stipes have a lateral width of 0.20– 0.30 mm and there are five branching zones in a distance of 5 mm from the origin resulting in rhabdosomes about 10 mm in diameter with 40 or so preserved peripheral stipes. Autothecae appear to be simple non-denticulate tubes spaced at approximately 20 in 10 mm: near the autothecal apertures the periderm is thin. At branching points the axil is sometimes infilled with the thin-walled autotheca terminating the preceding stipe. No bithecae have been seen. The origin is unclear but may consist of a small holdfast or a very short, twisted stem.

Remarks. Callograptus rigbyae resembles none of the Callograptus species discussed under the previous description. The obvious lack of a conspicuous stem region distinguishes it from C. bridgecreekensis n.sp. There is a presumed superficial resemblance with the Early Ordovician Staurograptus , which is a siculate, planktic genus. There is also some similarity between C. rigbyae and Dictyonema sp. A of Rickards & Wright (1997) from the late Ludlow near Mumbil, NSW, but the latter has well-developed dissepiments.

Figures

Fig. 14. A, Callograptus bridgecreekensis n.sp., AM F114569a, holotype, F14; B,C, Callograptus rigbyi n.sp., respectively AM F114651a, holotype, BF28, and paratype AM F114643, W885. Scale bars 1 mm. Note that in Fig. 14B the isolated autothecae of Cyclograptus? australis can be seen at bottom right and on Fig. 14C a specimen of Acanthograptus praedeckeri occurs at the top.