Byronia? bifida, Wrona, 2004

Wrona, Ryszard, 2004, Cambrian microfossils from glacial erratics of King George Island, Antarctica, Acta Palaeontologica Polonica 49 (1), pp. 13-56 : 50

Byronia? bifida sp. nov.

Fig. 25H, K–M.

Byronia ? sp. nov. A; Bengtson in Bengtson et al. 1990: 190, fig. 126. Holotype: Specimen shown in Fig. 25 L, ZPAL V. VI/35 S17 from erratic boulder Me66.

Type horizon: The inferred upper part of the Lower Cambrian, Botomian Stage, Syringocnema favus beds .

Type locality: Erratic boulders of Antarctic origin. Me 66 in glacio−marine Cape Melville Formation ( Lower Miocene ), King George Island , South Shetland Islands, West Antarctica .

Derivation of name: From Latin bifidus, split in two parts, referring to the deep split along the broad, lateral side of the tube.

Diagnosis.—Phosphatic tubular fossils, lenticular in transverse section. The conical tube has the proximal end constricted with a distinct margin, slightly deflected upwards. Distally, the tube expands in diameter and is deeply split along the broad lateral side. Apertural margins of both split walls are narrowly rounded and smooth.

Material.—Ten well preserved specimens from erratic boulders Me32, 33, 66. Figured specimens, ZPAL V. VI /33 S 19; 35 S 17; 39 S 8; 115 S 5.

Description.—Phosphatic, more or less conical tubes, gently curved and lenticular in cross−section. Proximal (basal) end of the tube is relatively small in diameter and more circular in cross−section than the widened distal end. Basal expanded margin may act as an attachment to the substrate: however, the basal attachment disc has not been recovered. Laterally, the broad sides are split distally to project aperturally as a bifurcating wall. Apertural margins of both split elements are narrowly rounded and smooth. The V−shaped area of slit could represent the proximal end that was covered by a weakly sclerotized, and thus very thin, external layer of the wall, which was incompletely preserved and could have been easily perforated or broken (Fig. 25 M). Outer surface is covered with densely spaced, fine, annular growth lines (Fig. 25 K). The growth lines are clearly visible in the translucent, thin wall covering the lateral slits. Inner surface of tubes is smooth. Broken tubes show relatively thick, phosphatic wall structure as a system of several thin layers.

Remarks.—These fossils are commonly interpreted as representing sessile tube−dwelling organisms with organic, partly mineralised (phosphatized) tubes ( Bischoff 1989; Bengtson in Bengtson et al. 1990). Splitting of the tube wall also occurs in other species of Byronia ( Bischoff 1989) , but the newly assigned species differs distinctly from the most similar Australian species, B. displosa Bischoff, 1989 in having a very prominent, long slit with smooth margins and narrowly rounded apertural edge, as well as a thin external layer covering the slit. The Australian specimens described and illustrated by Bengtson (see Bengtson et al. 1990) most probably represent tubes with unpreserved or damaged thin and weak wall in the slits. Occurrence.—Early Cambrian (Atdabanian/Botomian) Kulpara Formation and Parara Limestone, Horse Gully, Yorke Peninsula, Stansbury Basin, South Australia; allochthonous Early Cambrian (Botomian) boulders (Me32, 33, 66), King George Island, Antarctica.