Allonnia sp.

Wrona, Ryszard, 2004, Cambrian microfossils from glacial erratics of King George Island, Antarctica, Acta Palaeontologica Polonica 49 (1), pp. 13-56 : 26-28

Allonnia sp.

Fig. 6M–R.

Material.—A dozen phosphatic steinkerns of isolated sclerite rays, often with broken distal ends from erratic boulders Me33, 66. Figured specimens, ZPAL V. VI /26S18; 37S2; 37S9; 105S7; 107S20; 115S8.

Description.—Sclerites distinctly bilaterally symmetric, having two equal rays (formula: 2+0), slightly bent in the basal plane. Distal end tapers to a point. Foramina round and variable in size but relatively small (Fig. 6O, Q). Outer surface of ray steinkerns, except for a small area around the basal pore, is cov−

Fig. 6. A, B, G–K. Internal moulds of Chancelloria sp. A . disarticulated sclerite. A. Marginal ray, ZPAL V. VI /105S19, erratic Me66. B. Marginal ray, Ą ZPAL V. VI /105S16, erratic Me66. C. Broken internal mould of Chancelloria sp. B marginal ray, ZPAL V. VI /105S17, erratic Me66. D. Internal mould of Allonnia ex gr. A. tripodophora Doré and Reid marginal ray, ZPAL V. VI /103S15, erratic Me33. E. Internal mould of Chancelloria racemifundis Bengtson marginal ray, ZPAL V. VI /36S10, erratic Me33. F. Internal mould of Allonnia ex gr. A. tripodophora Doré and Reid marginal ray, ZPAL V. VI /103S11, erratic Me66. G. Central ray, ZPAL V. VI /35S18, erratic Me33. H. Central ray, ZPAL V. VI /108S12, erratic Me66. I. Central ray, ZPAL V. VI /103S5, erratic Me33. J. Articulated sclerite, ZPAL V. VI /37S15, erratic Me66. K. Internal mould of the central ray, ZPAL V. VI /105S3, erratic Me66. L. Chancelloria racemifundis Bengtson. Internal mould of the central ray, ZPAL V. VI /36S8, erratic Me66. M– R. Allonnia sp. M. Internal mould of the twins articulated sclerite, ZPAL V. VI /105S7, erratic Me66. N. Internal mould of the articulated sclerite, ZPAL V. VI /26S18, erratic Me33. O. Single sclerite, ZPAL V. VI /37S2, erratic Me66, dissociated on the SEM stub in two rays: left (O 1) and right (O 2). P. One ray of the twins sclerite, ZPAL V. VI /107S20, erratic Me66. Q. Partly broken articulated specimen, ZPAL V. VI /115S8, erratic Me66. R. Internal mould of the articulated sclerite, ZPAL V. VI /37S9, erratic Me33. S. Chancelloria sp. A , partly broken articulated sclerite, ZPAL V. VI /28S24, erratic Me33. T. Chancelloria racemifundis Bengtson, ZPAL V.VI /37S6, erratic Me33; T 1, partly broken articulated sclerite in basal view; T 2, detail of the basal foramina.

ered with characteristic ornamentation of minute, robust spinules (Fig. 6M), which may reflect porosity of the sclerite wall. Such spiny ornamentation is similar to irregularly spaced pillars on the Eremactis mawsoni steinkern surface, interpreted as being originally diagenetic ( Bengtson et al. 1990: figs. 35A, B and 36G). These double rays can be fused, with a small attachment surface (Fig. 6O, Q), as is common in this genus, or with a large attachment area (Fig. 6M, P), in nearly parallel position (Fig. 6N, R). The twin rays, fused along their length, resemble certain sclerites of Eremactis conara Bengtson and Conway Morris ( Bengtson et al. 1990: fig. 32E, F), but their basal foramina remain simple and round, without any pronounced structures like circular ridge, or spherulitic surface. The Antarctic Allonnia sp. rays have triangular cross−section in their proximal (fused) part and a more rounded to circular cross−section at the distal end.

Remarks.—Similar chancelloriid sclerites with formula 2+0 were first assigned to the genus Dimidia by Jiang (in Luo et al. 1982) and synonymized by Qian and Bengtson (1989) with a broad concept of the genus Allonnia . A number of internal moulds with large lateral facet on each ray, which reflects a composition of twin rays sclerite, may also represent elements of the Antarctic Allonnia sp. Such sclerites with large fusion facet and triangular cross−section are an exclusively abundant and characteristic constituent of the Antarctic Lower Cambrian chancelloriid assemblage.

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