Dailyatia ajax Bischoff, 1976

Wrona, Ryszard, 2004, Cambrian microfossils from glacial erratics of King George Island, Antarctica, Acta Palaeontologica Polonica 49 (1), pp. 13-56 : 30

Dailyatia ajax Bischoff, 1976

Figs. 8A–E, 9, 11, 12, 15F, K.

Dailyatia ajax Bischoff ; Demidenko in Gravestock et al. 2001: 114, pl. 8: 4–7 [full synonymy].

Material.—A number of isolated, variously shaped sclerites from erratic boulders Me30, 32, 33, and 66. Figured specimens, ZPAL V.VI/25S2; 26S13; 32S5; 35S22; 36S3; 37S2; 39S7; 39S27; 55S55; 105S7; 106S2; 108S4.

Description.—Minute sclerites of variable shape, subpyramidal to conical with prominent apex and radial folds and plicae. Sculpture consists of evenly and densely spaced carinate ribs with a distinct cancellate ornament. Sclerites of type A, with rectangular transverse cross−section, are absent in the Antarctic material. Sclerites of type B have subovate transverse cross−section (Figs. 11D, 12D) and prominent radial folds and plicae, and are strongly curved and twisted. Sclerite of type C are triangular (Figs. 8A–E, 11A–C, 12A) or crescentic (Fig. 8C) in trnsverse cross−section, with moderate curvature and torsion, and without radial folds. Well developed reticulate ornamentation on the juvenile part of the sclerite (Figs. 8A, C, 11D, 12A 4, B) has been interpreted as representing epithelial cell imprints (e.g., Bengtson et al. 1990; Conway Morris and Chen 1990). The multilamellar wall structure is well visible in the broken and polished cross−sections (Figs. 12C, 15K), and the extension of single laminae can also be easily observed as longitudinal growth lines on the external (Fig. 11A 5, A 6) and internal (Fig. 12C 4) surfaces of the sclerite wall. Transverse sections of wall folds also shows that they were internally overgrown with phosphate laminae (Figs. 12C 2, 15K). However, those laminae neither enclose the entire internal sclerite circumference (see longitudinal growth lines in Fig. 12C 4), nor extend throughout the entire interior hight of the sclerite.

Remarks.—The sclerite morphology fits well the description by Bischoff (1976), but only two morphotypes are recognised. They resemble fairly closely D. ajax recovered from the Lower Cambrian Ajax and Wilkawillina Limestones, Flinders Ranges ( Bischoff 1976) and Kulpara Formation, Parara Limestone, Koolywurtie Limestone Member and Ramsay Limestone, Yorke Peninsula (Gravestock et al. 2001), South Australia, and Todd River Dolomite, Amadeus Basin, Northern Territory, central Australia ( Laurie and Shergold 1985; Laurie 1986). The excellently preserved Antarctic sclerites of D. ajax , with epithelial imprint and multilamellar wall structure, reveal their mode of growth and make possible a modified reconstruction ( Fig. 9 ) of the relationship between sclerite element and secretory tissue earlier proposed by Conway Morris and Chen (1990). The epithelial tissue envelops the whole sclerite in the initial stage of growth ( Fig. 9A ), as its presence is reflected by cellular imprints; the epithelial tissue enfolds the basal part of the sclerite exterior (reflected by cellular polygonal imprints), and their secretory zone probably lines the interior of the sclerite secreting basal−internal accretion of phosphate laminae ( Fig. 9C, E ). The sclerite wall has more or less the same thickness over the entire sclerite, since the secretory zone of the soft tissue was restricted to the wide basal part of the interior sclerite.

Occurrence.—This species is restricted to the Early Cambrian (Atdabanian–Botomian) of the Kulpara Formation, Parara Limestone, and Koolywurite Limestone Member, Yorke Peninsula, Stansbury Basin and Ajax Limestone, Flinders Ranges, Arrowie Basin , South Australia; allochthonous Early Cambrian (Botomian) boulders (Me30, 32, 33, and 66), King George Island, Antarctica.

Figures

Fig. 9. A. Energy dispersive spectrum (EDS) for Dailyatia ajax Bischoff sclerite. B–E. Diagrammatic reconstruction of the relationship between Dailyatia sclerite element and secretory epithelium during growth, based on the multilamellar wall structure and the polygonal pattern covering the entire sclerite, except for a nipple−like termination of the apex.