Orthothecimorpha Sysoyev, 1968

Clausen, Sébastien & Álvaro, J. Javier, 2006, Skeletonized microfossils from the Lower-Middle Cambrian transition of the Cantabrian Mountains, northern Spain, Acta Palaeontologica Polonica 51 (2), pp. 223-238 : 230-232

Subclass Orthothecimorpha Sysoyev, 1968

Remarks.— Qian and Xiao (1995) have recently proposed a relatively comprehensive hyolith classification (see English translation in Kruse 2002). Due to the lack of conchs associated with opercula, detailed suprageneric−level groupings will not be used herein.

Operculum A

Fig. 5A–E.

Material.—About 20 phosphatized opercula from the “Beleño” facies, section Cr 4.

Description.—Operculum circular to slightly oval in outline, up to 0.6 mm in diameter; outer side strongly convex and smooth, with faint concentric growth lines, composed of a central, convex (sometimes almost hemispherical) area separated by a distinct depression from a peripheral band, corresponding to internal marginal zone and reaching ca. 1/10 of the diameter; well rounded apex (growth center) situated generally in the centre of the operculum or slightly excentric; inner (concave) side bearing a distinctly offset marginal zone, up to about 1/10 of the operculum diameter in width; paired cardinal processes prominent, crescent−shaped, symmetrically bounding internally the offset marginal zone, diverging at 30–40 ° at their most prominent edge, and reaching 4/5 of the diameter; clavicles absent.

Remarks.—The Cantabrian operculum differs from Conotheca australiensis Bengtson (in Bengtson et al. 1990) and C. laurentiensis Landing and Bartowski, 1996 (respectively from the Lower Cambrian of Australia and Laurentia), in the absence of clavicle−like tubules. It differs also from Turcutheca ? sp. A ( Hinz 1987), from the Lower Cambrian of Shropshire ( UK), in the crescent shape of the cardinal processes; from Sysoieva exilis Marek, Malinky and Geyer, 1997, from the Middle Cambrian Micmacca Breccia , in the shape of the cardinal processes (like wings of butterfields) and central termination of their lateral edges, attached to the interior surface and terminating in a central point; and from Neogloborilus hymenodes (Duan and Xiao in Qian et al. 2000), from the Lower Cambrian Yuertus Formation, Xinjiang ( China), in the presence of a central elevated area (ca. 1/3 of the diameter) lacking a central apex on the convex side, and of paired cardinal processes bearing transverse faint constrictions on the concave side. As Marek (1963) pointed out that hyolith taxa should not be erected without complete information from both conch and operculum morphology, the present operculum is taxonomically unassigned.