Girvanella sp.

Wrona, Ryszard, 2004, Cambrian microfossils from glacial erratics of King George Island, Antarctica, Acta Palaeontologica Polonica 49 (1), pp. 13-56 : 20-22

Girvanella sp.

Fig. 4.

Material.— A number of phosphate replacements of filaments or tubes (coats of sheaths) in residuum of erratics Me32, 33, 40, 52, 66. Figured specimens, ZPAL V. VI/25 S12, 50; 28 S26, 35 S4 ; 37 S14 ; 38 S2 ; 39 S29 ; 103 S12 ; 107 S8 ; 117 S1, 2; also see Wrona 1989, pl. 7: 6, and calcified filaments in thin sections, ZPAL Ac. I / M11 ; see Wrona and Zhuravlev 1996, pl. 8: 7; fig. 3b, c.

Description.—Irregularly coiled, unbranched cylindrical tubes or sheath replacements, now phosphatised, but probably originally calcareous, 9.8–12.8 µm in diameter. The tubes are fairly equal in diameter along the entire length of the sheath, and not branched. Tapering has not been observed. The most common types are irregular encrustations on shell or clast surfaces (Fig. 4 A, C). They are usually tightly packed, spirally coiled (Fig. 4 A) or labyrinthically tangled (Fig. 4 C). Somewhat more rare are bundles of irregularly, more or less tightly tangled cylindrical tubes (oncoids?) (Fig. 4 B), which may have smooth external surfaces or may possess a sculpture such as annular ribbing (Fig. 4D). This is interpreted as a reflection of the fine, cellular structure of the living cyanobacterial filaments ( Danielli 1981; Peel 1988).

Remarks.—The described forms are closely similar to the non−reef habits of Girvanella distinguished by Pratt (2001: fig. 1). The phosphatized filaments show a most striking similarity to silicified filaments encrusting various hard substrates, such as metazoan skeletons and bioclasts ( Pratt 2001: fig. 2D– F). At the same time, however, specimens observed in thin sections of reef limestone erratics show calcified micritic−walled tubes ( Wrona and Zhuravlev 1996). Antarctic forms have the same characteristic features as the type material of G. problematica ( Edhorn 1979) from the Mid−Ordovician of Scotland ( Wood 1957), but are about half the diameter. However, they closely match the specimens of Girvanella sp. described from the Parara Limestone, Curramulka ( Bengtson et al. 1990), and other narrow forms referred to G. problematica by Danielli (1981). They also compare well in the external diameter filaments (9–22.5 µm; mean 15.1 µm) specimens re−

Fig. 4. Mineralized sheaths of filamentous cyanophyte Girvanella sp. A . Irregular encrustation on subspherical element, ZPAL V. VI /117 S 1, erratic Me66; Ą A 1, outer oblique view; A 2, enlargement of densely tangled phosphate filling (replica) of filament; A 3, detail of the same specimen; A 4, the same encrustation on smooth surface of calcareous shell (now dissolved) in other (inner), concave side; A 5, enlargement showing concentrically coiled phosphate filling of filament; A 6, detail showing tightly packed unbranched phosphatized thread. B. Specimen ZPAL V. VI /117 S 2, erratic Me66; B 1, bundle of irregularly tangled tubular phosphate replacements of calcareous sheaths; B 2, the same from the other side. C. Specimen ZPAL V. VI /37 S 14, erratic Me33;

C 1, loosely tangled phosphatized tubular sheath, with annulate feature interpreted as a representation of cell structure; C 2, detail of annulation. D. Specimen ZPAL V. VI /35 S 4, erratic Me33; D 1, phosphate filling of calcareous sheaths, now dissolved, as coiled encrustation on bioclast (hyolith conch); D 2, enlargement showing unbranched phosphatized thread labyrinthically tangled; D 3, detail of phosphate filling; showing fine−grained apatite.

ferred to G. problematica from the Ordovician of China ( Riding and Fan 2001). This species has been reported from the Cambrian by several authors (e.g., Edhorn 1979; Danielli 1981; Cherchi and Schroeder 1984).

Occurrence.—Allochthonous Early Cambrian (Botomian) boulders (Me33, 66), King George Island, Antarctica.