Orthothecida Marek, 1966

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 : 386-388

Order Orthothecida Marek, 1966

Family uncertain

Remarks.—Probably the oldest known orthothecid hyoliths in Siberia have been recovered ca. 60 m below the top of the Ust’-Yudoma Formation from the Mount Konus section on the Uchur River (Fig. 1). These specimens were assigned to Lophotheca socialis Val’kov, 1987, and Lophotheca sp. They derive from the Purella Zone of the Nemakit-Daldynian Stage (Khomentovsky et al. 1990: pl. 3: 6–8; Brasier et al. 1993). Hyoliths have not previously been conclusively identified from the upper Nemakit-Daldyn Formation on the western flank of the Anabar Uplift, although tubular remains referred to as orthothecid hyoliths have been reported there, but not illustrated. The supposed orthothecids include Ladatheca dorsocava (Sysoev, 1959a) (i.e., Bokova 1985; Missarzhevsky 1989), Loculitheca sp. and Spinulitheca sp. i.e., Bokova 1985), Circotheca kuteinikovi Missarzhevsky in Rozanov et al., 1969(i.e.,Meshkova 1974), and Conotheca and Circotheca (i.e., Shishkin 1974). However, abundant tubular remains with circular to rounded triangular cross-section from these beds were described as Anabarites spp. by Bokova (1985), which makes the earlier assignment of the tubes from the Nemakit-Daldyn Formation to the hyoliths questionable.

In western Mongolia, Pseudorthotheca bistriata Qian, 1978 , is found in the basal Purella Zone (Khomentovsky and Gibsher 1996; Brasier et al. 1996). From South China, Conotheca subcurvata (Yu, 1974) is reported from Meishucunian Assemblage Zone 1 (Qian and Bengtson 1989; Steiner et al. 2004a). Other reports of early orthothecids in Meishucunian Assemblage Zone 1 include Kunyangotheca ostiola Qian, 1978 ; Spinulitheca billingsi (Sysoev, 1962) and Turcutheca crasseocochlia (Sysoev, 1962) , in beds 3–4 of the Zhujiaqing Formation at the Meishucun section, eastern Yunnan (Luo et al. 1982) but are not well documented (Kouchinsky et al. 2012). Lophotheca cf. multicostata Qian, 1977 and Conotheca -like forms are also reported from the Lesser Himalaya in an interval correlated with Meishucunian Assemblage Zone 1 of China ( Brasier and Singh 1987; Hughes et al. 2005). “ Ladatheca ” cylindrica ( Grabau, 1900) from the upper Fortunian Stage (“ Ladatheca ” cylindrica Zone ) and higher strata is the oldest presumed orthothecid hyolith from Avalonia (Landing et al. 1988, 1989; Landing 1993). However, it is alternatively regarded as operculate tubular problematicum (Landing 1988; Landing et al. 1989) or an annelid (Landing 1993).