Watsonella crosbyi Grabau, 1900

Kouchinsky, Artem, Bengtson, Stefan, Landing, Ed, Steiner, Michael, Vendrasco, Michael & Ziegler, Karen, 2017, Terreneuvian stratigraphy and faunas from the Anabar Uplift, Siberia, Acta Palaeontologica Polonica 62 (2), pp. 311-440 : 343-344

Watsonella crosbyi Grabau, 1900

Fig. 16 .

Material.—Ten phosphatic internal moulds (and fragments), including figured SMNH Mo160433, 182235–182239, from samples 1/29.2, 1/29.5, 1/6 (section 1, Fig. 4 ), K8 (section 3, Fig. 3), and M424/10 (section M424); Medvezhya and Kugda-Yuryakh formations, western Anabar Uplift, Siberian Platform, Russia. Correlated with the Fortunian–Cambrian Stage 2 transitional beds and lower part of Cambrian Stage 2.

→ Fig. 15. Mollusc Anabarella plana Vostokova, 1962 , internal moulds, from early Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, eastern Anabar Uplift, Siberia, Russia; sample 5a/1.2 from section 96-5a. A–I. SMNH Mo182226–182234, respectively. A 1 –D 1, E, F 1, G 1, H, I, lateral; A 2, B 4, C 3, D 2, F 2, upper; A 4, B 2, C 2, D 3, F 3, G 2, subapical; A 3, G 3, apertural views. Scale bar 500 μm (A 1) and 1 mm (A 2 –A 4, B–I).

Description.—Bilaterally symmetrical, laterally compressed shell, up 4 mm long. Aperture is very elongate oval in outline, with length/width ratio of ca. 3 in planar view, curved in sagittal plane. Apex blunt evenly expands into the main part of the shell. Subapical side very short. Supra-apical side (dorsum) long, broadly convex in lateral view, flattened to gently depressed in the middle. Subapical and supra-apical margins of aperture arched towards dorsum. Inner surface of the shell (replicated on internal moulds) smooth, with irregularly spaced, gentle comarginal folds.

Remarks.—The outline and width of these internal moulds vary from broad ( Fig. 16E ) to relatively slim with a slightly more pronounced apex ( Fig. 16A, B ). The variation agrees well with that observed by Li et al. (2011) and Devaere et al. (2013) and interpreted as intraspecific, but the presence of different species or sub-species of Watsonella cannot be excluded. A medial furrow interpreted as a reflection of a hinge-like structure was observed in the apical part of a single internal mould ( Dzik 1994: fig. 12G), but remains, however, questionable, because numerous other specimens from various localities display undivided juvenile shells (Vendrasco et al. 2011a, b; Li et al. 2011; Devaere et al. 2013). Its presence likely can only be confidently assessed in specimens with preserved conchs (e.g., Steiner et al. 2007: fig. 6A, D). A medial furrow along the dorsal surface of the shell was likely present (Landing et al. 1989: fig. 5.20; Steiner et al. 2007: fig. 6D; Li et al. 2011) and is reflected on the available internal moulds by a variably developed longitudinal dorsal depression ( Fig. 16A 3, B 3). The shell microstructure of Watsonella is similar to that in Anabarella plana (see Kouchinsky 1999 and Watsonella -like forms of A. plana in Fig. 14).

Watsonella crosbyi ( Grabau, 1900) occurs in the lower Emyaksin (Kengede River; Bokova 1992: 17, 46, fig. 14), Medvezhya and Kugda-Yuryakh formations of the Anabar Uplift, upper Kessyusa Group of the Olenyok Uplift, lower Pestrotsvet Formation of the Lena-Aldan and Uchur-Maya regions of the SE Siberian Platform, and lower Krasny Porog (Krasnoporog) Formation on the NW margin of the Siberian Platform ( Bokova 1992; Khomentovsky and Karlova 1993, 2005; Rowland et al. 1998; Kouchinsky 1999; Sipin 2001; and herein). Isotopic data from the Kengede section are unavailable, but the lower Emyaksin Formation at Bol’shaya Kuonamka, eastern flank of the Anabar Uplift, contains prominent positive excursions (see Kouchinsky et al. 2001 and Fig. 2 herein) thought to be characteristic of Cambrian Stage 2 around the world. Herein, Watsonella crosbyi is reported from the middle Medvezhya Formation of the Ary-Mas-Yuryakh section (section 1 this report; see also Landing and Kouchinsky 2016), just below the positive peak of the excursion Iʹ (Knoll et al. 1995a, b; Kaufman et al. 1996).

As in Siberia, Watsonella crosbyi is used as an index fossil in western Mongolia. It is known to occur there in the upper Bayan-Gol Formation, near the basal part of the δ 13 Ccarb excursion F ( Brasier et al. 1996). However, the species has a very limited stratigraphic and geographic distribution in western Mongolia ( Esakova and Zhegallo 1996), which limits its use as an index fossil ( Demidenko and Parkhaev 2014). A probably significantly truncated range is also known from the Heraultia Limestone , France, where a Watsonella crosbyi ‒ Oelandiella korobkovi Interval Zone was defined and referred to Cambrian Stage 2 ( Devaere et al. 2013). Although Watsonella crosbyi first occurs in significantly younger strata than those with the earliest shelly molluscs in South China, Siberia, and western Mongolia, the Watsonella crosbyi Zone of the Chapel Island Formation on eastern Newfoundland and New Brunswick contains the earliest molluscs in Avalonia, with forms such as W. crosbyi , Aldanella attleborensis (Shaler and Foerste, 1888) , and Helcionella sp. ( Bengtson and Fletcher 1983; Landing et al. 1989, 1998, 2013; Kouchinsky et al. 2012).

Stratigraphic and geographic range.— Probably the uppermost Fortunian (depending on stratigraphic correlation) and Cambrian Stage 2 of Siberia, Altai-Sayan Folded area, western Mongolia, East Gondwana (South China, Australia), West Gondwana (Armorican France) and Avalonia (Massachusetts, Cape Breton Island, eastern Newfoundland). The species reportedly ranges into the lower part of Cambrian Stage 3 in Avalonia (Landing et al. 1989, 2013) and South Australia ( Gravestock et al. 2001; but Jacquet et al. 2016).

Figures

Fig.16. Mollusc Watsonella crosbyi Grabau, 1900, internal moulds, from early Cambrian Medvezhya and Kugda-Yuryakh formations, Kotuj River, western Anabar Uplift, Siberia, Russia; samples 1/29.5 (A–C) and 1/29.2 (E), section 1; K2/60 (F), section 3; and M424/10 (D) from section M424. A–F. SMNH Mo182235–182237 (A–C, respectively), SMNH Mo160433 (D), SMNH Mo182238 (E), SMNH Mo182239 (F) (compare D with Kouchinsky 1999: fig. 3A–C). A1, B2, C1, D2, E1, F, lateral; A2, B1, C2, D1, E3, subapical; A3, B3, C3, D3, E2, upper views. Scale bar 1 mm.

Fig. 4. The δ13Ccurve for Kotuj River section 1and sampled horizons with fossiliferous samples (arrows with numbers). Shift in stratigraphic column carb at Nemakit-Daldyn–Medvezhya formations contact indicates position of sequence boundary. Stratigraphic ranges and first appearances of fossils (see SOM 1): 2, Oelandiella korobkovi; 3, Securiconus cf. simus; 4, Bemella jacutica?; 8, Anabarella plana; 9, Watsonella crosbyi; 10, Pseudoyangtzespira selindeica; 11, Aldanella crassa; 13, Aldanella attleborensis; 15, Philoxenella spiralis; 16, Barskovia hemisymmetrica; 18, Khairkhania cf. evoluta; 19, Ceratoconus striatus; 21, Salanyella cf. costulata; 23, Purella cristata; 25, Yunnanopleura sp.; 26, Obscurania auriculata; 29, Protoconus cf. crestatus; 32, Siphogonuchites triangularis; 34, Lopochites sp.; 37, first appearance of Halkieria sp.; 39, first appearance of Chancelloria sp.; 40, Tianzhushanella sp.; 42, Khetatheca cotuiensis; 46, Turcutheca sp.; 48, Orthothecid operculum type A; 52, Fomitchella infundibuliformis; 57, Hyolithellus sp.; 62, Selindeochrea missarzhevskyi; 63, Selindeochrea ternaria; 64, Anabarites trisulcatus; 67, Anabarites kelleri; 68, Anabarites biplicatus; 71, Anabarites hexasulcatus; 74, Tiksitheca licis; 75, Cambrotubulus decurvatus; 76, Platysolenites antiquissimus; 77, Blastulospongia sp.

Fig. 2. Composite δ13Ccurve for the Bol’shaya Kuonamka River sections 96-4, 5, and 5a and sampled horizons with fossiliferous samples (arrows with carb numbers). Arrow 4/4 indicates “marker bed with angustiochreids” (Val’kov 1975; Kouchinsky et al. 1999). For section 96-6 see Kouchinsky et al. (2001, 2015a). Shift in stratigraphic column at Manykay–Emyaksin formations contact indicates position of sequence boundary. Stratigraphic ranges and first appearances of fossils (see SOM 1): 1, first appearance of helcionellid molluscs; 2, Oelandiella korobkovi; 3, Securiconus cf. simus; 6, Obtusoconus vostokovae; 7, Obtusoconus cf. mirabilis; 8, Anabarella plana; 13, Aldanella attleborensis; 14, Aldanella cf. sibirica; 22, Purella antiqua; 24, Yunnanopleura accurata; 34, Lopochites sp.; 35, Tianzhushania longa; 37, first appearance of Halkieria sp.; 38, Chancelloria symmetrica; 42, Khetatheca cotuiensis; 44, Conotheca cf. mammilata; 46, Turcutheca sp.; 47, Cupitheca sp.; 50, Orthothecid operculum type C; 51, Protohertzina anabarica; 52, Fomitchella infundibuliformis; 53, Fomitchella aff. infundibuliformis; 54, Fomitchella acinaciformis; 55, Fomitchella aff. acinaciformis; 56, Hyolithellus vladimirovae; 57, Hyolithellus sp.; 60, Selindeochrea cf. tecta; 61, Selindeochrea tripartita; 62, Selindeochrea missarzhevskyi; 63, Selindeochrea ternaria; 64, Anabarites trisulcatus; 65, Anabarites latus; 66, Anabarites natellus; 67, Anabarites kelleri; 68, Anabarites biplicatus; 69, Anabarites tristichus; 70, Anabarites compositus; 71, Anabarites hexasulcatus; 72, Anabarites hariolus; 73, Aculeochrea rugosa; 74, Tiksitheca licis; 75, Cambrotubulus decurvatus; 76, Platysolenites antiquissimus.