Allatheca sp.

Skovsted, Christian B. & Peel, John S., 2007, Small shelly fossils from the argillaceous facies of the Lower Cambrian Forteau Formation of western Newfoundland, Acta Palaeontologica Polonica 52 (4), pp. 729-748 : 740-741

Allatheca sp.

Fig. 5J, K.

Material.—NFM−703, NFM−704, and 29 additional specimens from sample JSP1982−01.

Description.—Circular to sub−rectangular operculum, 0.6 to 1.0 mm in diameter. External surface convex with concentric growth lines around a sub−central summit. Dorsal and ventral regions are not clearly differentiated. The internal surface is divided by a sharply defined furrow into a relatively narrow marginal region and a circular to sub−triangular central region bounded by a concentric ridge raised above the marginal zone. The floor of the marginal region is smooth and slope slightly outwards. The concentric ridge is differentiated into a curved dorsal edge with two weakly defined swellings, a straight ventral edge, and more strongly developed lateral edges that sometimes slightly overhang the marginal zone of the operculum.

Discussion.—The circular to sub−rectangular opercula described above resemble opercula of Allatheca sp. from the Tommotian Stage of Siberia ( Rozanov et al. 1969: pl. 11: 4, 8; Meshkova 1974: pl. 11: 12; Dzik 1994: fig. 15) in having a thickened central region with ridges raised above the interior surface and in lacking cardinal processes. Allatheca is also known to occur in the Lower Cambrian of Sweden and England ( Rozanov et al. 1969: 139–140) as well as eastern (Avalonian) Newfoundland ( Landing 1988). The conch of Allatheca is a slowly expanding cone with densely set growth lines, straight or slightly curved dorso−ventrally and has a circular to sub−circular cross−section ( Landing 1988; Missarzhevsky 1989).

The current specimens can only be compared in detail to specimens from Siberia due to lack of data on the internal morphology of opercula from other areas. The material from the Forteau Formation differs from the Siberian specimens in terms of its sub−rectangular shape and by the lack of radial ribs on lateral parts of the concentric ridge. The difference in age between the Siberian (Tommotian) and Newfoundland (Dyeran, Botomian−equivalent) material is also considerable. Hyolith opercula of apparently identical morphology from strata of Adtabanian equivalent age in northwest China were referred to Neogloborilus Qian and Zhang, 1983 ( Qian et al. 2001). In terms of morphology and age, the Chinese specimens are more compatible with the Newfoundland opercula. However, Neogloborilus is presently poorly understood and its relationship to Allatheca remain to be resolved. The new material from Newfoundland is here referred to the better known genus Allatheca .

Allatheca sp. differs from the widespread Hyptiotheca Bengtson, 1990 (see review in Malinky and Skovsted 2004) by the presence of cardinal processes and well defined furrows and rooflets in Hyptiotheca , and by the more strongly developed, overhanging edges of the central region in Allatheca sp. A concentric ridge raised above the internal surface is also present in the opercula of Conotheca australiensis Bengtson, 1990 , but this species also has a pair of well defined cardinal processes and the ridge is more strongly differentiated into clavicles. Opercula of Petrasotheca minuta Landing and Bartowski, 1996 from the Taconic Allochthon of New York differ from Allatheca sp. by the dorso−lateral rather than ventro−lateral position of the concentric ridge. The small swellings on the dorsal edge of the concentric ridge in Allatheca sp. are situated in approximately the same position as the cardinal processes of Hyptiotheca and Conotheca , and this form could be viewed as intermediate between the orthothecid−like Conotheca and the hyolithid−like Hyptiotheca . Dzik (1994) derived all younger orthothecid hyoliths from Allatheca . No conch from western Newfoundland can presently be associated with Allatheca sp.