Callograptus bridgecreekensis, 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 bridgecreekensis n.sp.

Figs. 14A , 15A,B

Material. HOLOTYPE AM F114569 a-b from F14, Bridge Creek .

Derivation of name. After the stream at the type locality.

Diagnosis. Callograptus with a robust stem, consisting of numerous tubes, 10 mm long and 1.2 mm wide; no dissepiments; spacing of sharply denticulate autothecae 30–35 in 10 mm, bithecae small, inconspicuous tubes.

Description. A single, large rhabdosome 35 mm long by 27 mm wide is developed from a robust stem region, itself about 10 mm long and 1.2 mm at its widest. The stem may originate from a small holdfast about 2 mm wide. The stem comprises a considerable bundle of tubes, some ending as thecal apertures along its length ( Fig. 15B ). The rhabdosome has over 50 terminal stipes preserved. The stipes are parallel and divergence at branching points is at a low angle: stipe frequency is 19–21 in 10 mm. Branching is frequent and irregular but a relatively small number of “main stipes” seem to give rise to the many terminal stipes.

There are four, possibly five “main stipes” on the specimen: one (possibly two) arises quickly at the top of the stem region; the other three develop within 5 mm of the top of the stem. This arrangement of fan-like fronds, coupled with the presence of a robust stem, suggests an overall fan-shaped rather than conical colony. There are no dissepiments. Lateral stipe width proximally is 0.40–0.60 mm and most distally 0.20–0.30 mm. The dorsoventral stipe width is 0.40–0.50 mm. Autothecal spacing is 30–35 in 10 mm.

The autothecae are sharply denticulate, possibly slightly spatulate, but not spinose. Bithecae are small, inconspicuous tubes opening in the axil between autothecal aperture and the free ventral wall of the next autotheca.

Remarks. The arrangement of fan-like branches is not uncommon in Callograptus (e.g., the type species C. elegans ), and recalls to some extent Licnograptus Ruedemann, 1947 (wherein thecal details are unknown). Callograptus , like Dendrograptus , is uncommon in the Silurian. Callograptus bridgecreekensis differs from the two Silurian forms briefly discussed by Bulman (1928), and from the three species described by Bouěek (1957): C. conjunctus Bouěek exhibits stipe anastomosis and the stipes are more robust and widely spaced; C. flabellatus Bouěek (? = Dictyonema ) has numerous dissepiments; and C. scopatus Poěta is a markedly more robust species with very widely spaced stipes. Of the North American forms described by Bassler (1909), C. minutus Spencer, 1878 , C. multicaulis Spencer, 1878 and C. niagarensis Spencer, 1878 (see description below) have small, bushy rhabdosomes, and C. strictus Gurley, 1896 (? = Dictyonema ) has numerous dissepiments. Ruedemann (1947) considered an additional species, C. pulchellus Shrock, 1928 (see also Rickards & Wright, 1997), which has dissepiments but similar dimensions. The form described as Callograptus ? pulchellus Shrock subsp. 1 by Rickards & Wright (1997, fig. 4D) has more undulating, divergent stipes than C. bridgecreekensis , whereas C.? pulchellus Shrock subsp. 2 of Rickards & Wright (1997) is insufficiently known but is possibly referable to C. bridgecreekensis (although subsp. 2 is from the late Ludlow).

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.

Fig. 15. A,B, Callograptus bridgecreekensis n.sp.: A, AM F114569a, holotype, F14; B, paratype AM F114569b, F14. Scale bars 1 mm.