Dendrograptus avonleaensis, 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 : 312

Dendrograptus avonleaensis n.sp.

Figs. 5A, 6A,B

Material. HOLOTYPE AM F114646 a-b and PARATYPES AM F114650 and AM F114766 , all from F14, Bridge Creek .

Etymology. After the property “Avon Lea” on which F14 is located.

Diagnosis. Dendrograptus with robust funnel-shaped rhabdosome at least 15 mm long and 18 mm at its widest; branching in broad zones; stipes with a dorsoventral width of 0.50–0.60 mm and a lateral width of 0.20–0.40 mm; autothecal spacing 18–20 in 10 mm.

Description. The holotype ( Fig. 6B ) displays a distal array of at least 48 stipes arranged in a broadly conical fashion with two major branching zones. These distal stipes diverge some 3.5 mm above the visible base of the specimen, but they do not arise from a solid stem: tightly clustered stipes are visible in the densely packed area. The overall shape is, therefore, funnel-shaped with a proximal constriction. In one or two places the stipes are visible in profile when the autothecae, spaced at 18–20 in 10 mm, can be seen to be of simple denticulate type ( Fig. 4I ). The lateral stipe width is 0.40 mm proximally, falling to 0.20 mm most distally, suggesting some cortical thickening as the colony ages. Two other specimens (e.g., Fig. 5A) are fragments from beyond the zone of expansion. They show some of the above-described features but less well-displayed.

Remarks. As described by Chapman et al. (1996) some Dendrograptus species can be quite well ordered; this is one such species, having a broadly conical, funnel-shaped form and rough branching zones. Dendrograptus avonleaensis n.sp. has a superficial resemblance to Calyptograptus cyathiformis Spencer, 1878 from the Niagara Limestone, but that species has more frequent and irregular branching and a robust stem; the thecae of C. cyathiformis are not known. Dendrograptus avonleaensis differs from D. ashburniaensis n.sp. herein in that the latter has a thecal spacing of 36 in 10 mm, almost twice that of the former; and D. ashburniaensis develops a thickened stem proximally.

Figures

Fig. 6. A,B, Dendrograptus avonleaensis n.sp., respectively AM F114650 and holotype AM F114646a, both from BF14; C, Dictyonema cf. delicatulum, AM F114749, F14. Scale bars 1 mm.

Fig. 4. A–G, different types of bithecae: A, Bulman’s (1928, 1933) Type 1 with bitheca n+1 opening along the lateral wall of autotheca n. This is the most common of autothecal/bithecal relationships; B,C, Bulman’s (1928, 1933) Type 2 in which the bithecal aperture opens inside the autothecal tube or aperture; D, Bulman’s (1928, 1933) Type 3 where the n+1 bithecal aperture is located on the dorsal side of the aperture of autotheca n; E, Bulman’s (1928, 1933) Type 4 in which the bithecal tube n+1 passes between the dorsal wall of autotheca n and the ventral wall of autotheca n+ 1; F,G, Bulman’s (1928, 1933) Type 5 a modification of Type 4 in which the bithecal tube turns proximally or distally before the bithecal aperture is reached; H–M, autothecal variation in Dictyonema species; H, autothecal aperture is the termination of a simple tube; I, the ventral lip projects as a short, sharp denticle; J, the ventral lip projects as a spine which may or may not bifurcate; K, the ventral process is adorned with a large shield-like plate; L, the dorsal apertural process has plate-like process hanging more or less in front of the aperture; M, the apertural region is isolated from the ventral wall of the next autotheca; for the sake of simplicity in H–M the Type 1 bithecal arrangement is shown; fuselli omitted; a, autothecal aperture; b, bithecal aperture; these structures are described or commented upon in various places in the text.