Dendrograptus ashburniaensis, 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 : 313-314

Dendrograptus ashburniaensis n.sp.

Fig. 5B,C

Material. HOLOTYPE AM F114567 and PARATYPE AM F114566 , both from F14, Bridge Creek .

Etymology. After the nearby property of “Ashburnia”, at the former Four Mile Creek post office.

Diagnosis. Dendrograptus with an unusually high thecal spacing of 36 in 10 mm, and irregular and frequent branching from a robust stem region.

Description. The overall colony shape may be irregularly dendroid, developed from a robust stem about 1 mm thick. Branching is frequent and irregular, at least once each 1–2 mm of stipe. The lateral stipe width decreases immediately away from the stem or main branches to approximately 0.30 mm; the dorsoventral width is about the same. Autothecae are visible where branches have been turned into the profile position, and seem to be of simple denticulate type, numbering 36 in 10 mm on both the specimens available. No trace of bithecae has been found. The specimen illustrated in Fig. 5B may have a relatively narrow holdfast at its base. The holotype has at least 21 terminal stipes after only 13 mm of growth, but the number at this stage could be much greater as several branches are broken.

Remarks. Most Dendrograptus species have thecae relatively widely spaced in the range of 12–20 in 10 mm, so D. ashburniaensis is unusual in this respect with its thecal spacing of 36 per 10 mm. Dendrograptus species are uncommon in the Silurian and we can equate our Bridge Creek specimens with none of them. Dendrograptus parallelus Shrock, 1928 from the Silurian of Indiana has a similar rhabdosome but the branching is much more widely spaced. Dendrograptus phainotheca Gurley, 1896 also has very widely spaced branching and an autothecal spacing of only 18 in 10 mm. Described Ordovician species have wider thecal spacings than D. ashburniaensis and mostly narrow stipes with more widely spaced branches. Dendrograptus avonleaensis n.sp. is the nearest form in general appearance but differs in having a much wider thecal spacing (see previous description).

The species described as Dendrograptus sp. B . by Rickards et al. (1995) has more widely spaced branching points than D. ashburniaensis , and D. sp. B. may be roughly compared with D. parallelus . Of the other Australian Dendrograptus species , D. sp. of Rickards & Wright (1997) from the inexpectatus or kozlowskii Biozone (Ludlow, Silurian) has a similar branching frequency but a much wider thecal spacing (15–20 in 10 mm). Dendrograptus ashburniaensis and D. avonleaensis are the best-preserved Australian Dendrograptus species so far described.