Archaeopetasus pachybasalis Kouchinsky, 2022

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 : 394-396

Archaeopetasus pachybasalis Kouchinsky sp. nov.

Figs. 37 , 38 .

Zoobank LSID: urn:lsid:zoobank.org:act:FCA618F0-6F22-4EEA-8E16-9BBEE18FDE6B

Etymology: From the Greek pachys, thick and Latin basalis, having a pedestal; for the thick basal part of sclerite.

Type material: Phosphatised disarticulated sclerites: holotype SMNH X11113 ( Fig. 37 ) and paratypes SMNH X11114‒11125 ( Fig. 38 ), from the type locality and horizon .

Type locality: locality 21/21 (71°35’N, 127°21’E) on the right bank of the Lena River , ca. 500 m upstream from the mouth of the Ulakhan-Ald’yarkhay Brook, northern Siberia, Russia .

Type horizon: Basal Middle Member of the Tyuser Formation, 21–22 m from the base of the the Tyuser Formation, Dokidocyathus regularis Zone, Tommotian stage , correlated herein with the upper part of Cambrian Stage 2.

Material.— Several hundred phosphatised sclerites and phosphatic coatings, including type material and figured SMNH X11126–11129 , from samples 21/21 and 21/22, Tyuser Formation , Lena River, Siberia, Russia. Dokidocyathus regularis Zone, Tommotian stage .

Diagnosis. —Broadly flaring smooth or tuberculated conical sclerite having undulating brim and thick protruding basal part, which is folded and funnelling towards the middle.

Description.—Sclerites, 0.5–1.5 mm in diameter, having funnel- or hat-shaped smooth upper surface and doughnut- or flower-shaped plicated basal part. The upper surface narrows abruptly into a single centrally situated tubular extension with a pointed tip. The upper surface is covered with flattened tubercles which tend to be elongated and aligned along the undulating margin of sclerite. Fragmentarily preserved spines are 0.3–0.6 mm in diameter and 0.5–1.5 mm long. They are also covered concentrically with elongated tubercles. Inner surface of spines is covered in places with tightly located flattened tubercles, ca. 10 μm in diameter. Basal part subdivided by several irregular but generally radially directed folds into several partially separated sectors funnelling towards the center of the lower side of sclerite. There is a circular deep pit in the middle of the lower side leading to a channel, ca. 0.3 mm in diameter inside the tubular extension of the upper surface of sclerite, thus forming inner layer of the tube. Interspace between phosphatic coatings of the opposite sides of sclerite often preserve aciculated elements oriented subperpendicular to the inner surface of phosphatic coat/wall.

Remarks.— Archaeopetasus excavatus Conway Morris and Bengtson in Bengtson et al., 1990, different in having isometrical granules on the external surface of sclerite, cone-shaped central spine, and relatively thin basal part. Archaeopetasus typicus ( Sipin, 1999) has a central boss-like elevation which never extends as a spine and a well-defined central pit with granulated surface on the lower smooth surface of sclerite.

Similar fossils from the Tommotian stage of Aldan and Lena rivers were figured by Missarzhevsky in Rozanov et al. (1969: pl. 7: 3, 15, 16) and Missarzhevsky (1989: pl. 24: 12) and tentatively interpreted as opercula of Coleolella billingsi ( Sysoev, 1962) , a problematic calcareous tubular fossil with circular cross-section and transversal flanges Fig. 67G ). The fossil from the same locality is considered by Repina et al. (1974: pl. 19: 11) an attachment disk of Hyolithellus vladimirovae Missarzhevsky in Rozanov and Missarzhevsky, 1966.

Reinterpreted herein as a sclerite, the fossil is similar to sclerites of Nidelric pugio Hou, Williams, Siveter, Siveter, Gabbott, Holwell, and Harvey, 2014 , from Chengjiang Lagerstätte and Nidelric gaoloufangensis Zhao, Li, and Selden, 2018 , from Guanshan biota, Cambrian Series 2, Stage 4, Palaeolenus Zone ( Zhao et al. 2018) . The latter forms show single-element spines with rounded bases, ca. 0.5 mm in diameter and belong to a group probably allied to chancelloriids.

Stratigraphic and geographic range.—Known from the type horizon and locality only.

Figures

Fig. 37. Sclerites of chancelloriid Archaeopetasus pachybasalis Kouchinsky sp. nov., from the lower Cambrian Tyuser Formation, lower reaches of the Lena River, Siberia, Russia; sample 21/21. Holotype specimen, SMNH X11113.A1, upper view; A2, A3, lateral view; A4, lower surfaces of sclerite. Scale bar 500 µm.

Fig. 38. Sclerites of chancelloriid Archaeopetasus pachybasalis Kouchinsky sp. nov., from the lower Cambrian Tyuser Formation, lower reaches of the Lena River, Siberia, Russia; sample 21/21. A–H, J–L, P. Basal part of phosphatized sclerites, SMNH X11114–11125, respectively. A1, upper surface; A2, J2, oblique view on the upper surface; J1, L1, lateral view; C1, F1, lower surface; C2, F2, L2, oblique view on the lower surface of sclerites.I, M–O.Apical spines detached from the basal part of sclerites, SMNH X11126–11129, respectively. Scale bar 500 µm.

Fig. 67. Anabaritids, internal moulds (A–F) and other problematic tubes G–I) from the lower Cambrian Kessyusa Group (B–D, H, I) and Erkeket Formation (A, E–G), Khorbusuonka River, Siberia, Russia; samples 11/16.05 (A), 11/2B (E, F), 15/23 (B–D, H, I), and 20/1B (G). A. Anabarites tristichus Missarzhevsky in Rozanov et al., 1969, SMNH X11315. A1, transversal view of the tube; A2, lateral view. B. Anabarites hexasulcatus (Missarzhevsky, 1974), SMNH X11316. C. Selindeochrea ternaria Missarzhevsky inRozanov etal.,1969),SMNHX11317.D.Cambrotubulus sp., SMNH X11318.E, F. Anabarites korobovi (Missarzhevsky in Rozanov and Missarzhevsky, 1966), SMNH X11319 and 11320, respectively. G. Coleolella billinsi (Sysoev, 1962), SMNH X11321. H, I. Platysolenites antiquissimus Eichwald, 1860, SMNH X11322 and 11323, respectively. Scale bar: 250 µm (A) and 500 µm (B–I).