Archaeopetasus typicus ( Sipin, 1999 )

Kouchinsky, Artem, Alexander, Ruaridh, Bengtson, Stefan, Bowyer, Fred, Clausen, Sébastien, Holmer, Lars E., Kolesnikov, Kirill A., Korovnikov, Igor V., Pavlov, Vladimir, Skovsted, Christian B., Ushatinskaya, Galina, Wood, Rachel & Zhuravlev, Andrey Y., 2022, Early-middle Cambrian stratigraphy and faunas from northern Siberia, Acta Palaeontologica Polonica 67 (2), pp. 341-464 : 396-399

Archaeopetasus typicus ( Sipin, 1999)

Fig. 39.

1999 Sukharikhina typica Sipin 1999: 8–10 , fig. 1а–k.

Material.—Over one hundred calcium phosphatic coatings of sclerites, including figured SMNH X11130–11135, from sample 11/2B, basal Erkeket Formation, Khorbusuonka River, Siberia, Russia. Dokidocyathus regularis Zone, Tommotian stage (correlated with the upper part of Cambrian Stage 2).

Description.—Low-conical isometrical plate, 0.5–1.5 mm in diameter and 0.1–0.5 mm thick, with slightly undulating margin. The margin is usually gently arched at one or two sides but any bilateral symmetry is not evident. The convex (upper) side with a central domal elevation having a single blunt tip. The upper surface with scattered tubercles, ca. 10 μm in diameter. The opposite (lower) side concave smooth with a central circular cavity, 0.1–0.3 mm in diameter covered with tightly located low-relief tubercles, ca. 10 μm in diameter.

Remarks.—The interspace between the phosphatic coating of the opposite sides of the sclerite is void, probably due to dissolution of the filling material during chemical extraction. In insoluble residues, calcium phosphate coatings representing both sides often occur disarticulated. The upper side of the lower coating, when disarticulated or broken, exhibits a circular tubercle, ca. 0.2 mm in diameter. The latter is a counterpart of the cavity on the lower surface of the sclerite. The circular tubercle bears numerous pits tightly arranged over the entire surface. As was mentioned by Sipin (1999, 2001), the pits correspond to tiny flattened tubercles scattered on the opposite side. Sometimes two different sclerites are stacked or merge with their concave sides. It was interpreted as a “bivalved” fossil by Sipin (1999) in description of a new genus and species, Sukharikhina typica Sipin, 1999 , but reinterpreted herein as a taphonomic feature.

Stratigraphic and geographic range.— Nochoroicyathus sunnaginicus and Dokidocyathus regularis zones, Tommotian stage (correlated with the upper part of Cambrian Stage 2), Siberian Platform, Russia.