Wotte and Sundberg—Small shelly fossils from the Great Basin
morphology and ornamentation is large, as it is for P. sub- angulata (Tate, 1892) (Parkhaev in Gravestock et al., 2001; Skovsted, 2004). According to Parkhaev in Gravestock et al. (2001), it is most probable that P. subangulata and P. media- nensis (Zhou and Xiao, 1984) represent morphologic variations of the same species. However, differences between both species are often only observable from adult forms with well-preserved shell material. According to Parkhaev in Gravestock et al. (2001), P. medianensis is regarded as the junior synonym of P. adunca (He and Pei in He et al., 1984), which thus replaces the former species name. On the other hand, it seems that P. subangulata continuously shifts morphologically into P. adunca, thus suggesting both species represent a morpholo- gical continuumwithin a species. Therefore, P. medianensis and P. adunca have to be revised, critically. Shell ornamentation of P. subangulata and P. primaeva (Billings, 1872 [1871]) shows comparable V-shaped ridges on the shell periphery (Runnegar in Bengtson et al., 1990; Landing and Bartowski, 1996; Landing et al., 2002). Thus, P. primaeva needs a careful revision as well (Skovsted, 2004). Differences between Pelagiella and Costipelagiella are
subtle and only visible on shell morphology and ornamentation. Costipelagiella nevadense Skovsted, 2006a originally derives from the basal Emigrant Formation of the Split Mountain section (Skovsted, 2006a), and occurs in our samplesSM14 and SM 15 as well (Fig. 5.14–5.16). Without preserved shell material, an affiliation of our internal molds to C. nevadense could not be excluded with certainty. Considering the poor preservation of our material and the
taxonomic discrepancies mentioned above, we interpret our specimens as having an affinity to P. subangulata, characterized by a wide range of variability. However, it should be kept in mind that species identification is questionable when exclu- sively based on internal molds (Skovsted, 2004; Topper et al., 2009).
Cambrian strata worldwide. Its first occurrence was recently discussed as a potential marker for defining the base of the Cambrian Series2/Stage 3 (e.g., Steiner et al., 2007).
Pelagiella subangulata is known from lower and middle
Helcionellid gen. and sp. indet. 1 Figure 7.7–7.12
Occurrence.—Three specimens from the middle part of the Combined Metals Member (Dyeran Stage) of Oak Spring Summit section (OS 3).
Description.—Large, weakly cyrtoconic, cap-shaped shells. Apex blunt and rounded, probably located in a central position; exact position uncertain due to the complete disappearance of the aperture. Ventral cross-section of incomplete specimen OS 3/B2-3 (Fig. 7.7, 7.8) sub-circular to elliptical. Width and length ~1.3mm and ~2.6mm, respectively. More-complete specimens OS 3/B2-4 (Fig. 7.11, 7.12) and OS 3/B2-5 (Fig. 7.9, 7.10) with a long, slightly concave posterior field. Approximate length of these specimens is 2.2mmand 1.2mm, respectively. Maximum height ~1.3mm. Anterior field gently convex. Surface with distinct radial lirae (Fig. 7.7–7.12).
889
Remarks.—Specimens show similarities to several helcio- nelloid molluscs, such as Trenella Parkhaev, 2001, Mellopegma Runnegar and Jell, 1976, Stenotheca Salter in Hicks, 1872, and Helcionella Grabau and Shimer, 1909. The long posterior field of our specimens is similar to Trenella or Mellopegma. According to Parkhaev (2001) the posterior field of T. bifrons Parkhaev, 2001 is rather short and continues into a well- developed parietal train. The posterior field of the figured holotype (Parkhaev, 2001, pl. 3, fig. 1a–1c) seems to be more concave than that of our specimens. In addition, the shell of Trenella shows a significant lateral compression without radial lirae and the apex is more spoon-shaped. Species of Mello-
pegma are characterized by a long, slightly concave posterior field without parietal train. The elongated shell has faint comarginal rugae. The apex is blunt to slightly hooked (see Vendrasco et al., 2011). However, in comparison to our speci- mens, Mellopegma shows a significant lateral compression. There are also similarities of our material to Stenotheca pojetai Runnegar and Jell, 1976, showing a blunt apex and fine radial lirae. The convexity of the anterior side is similar to that of our material, but the posterior side of S. pojetai is more steep and short. Species of Stenotheca are also characterized by a strong lateral compression. A morphological similarity to Helcionella is given by the radial lirae and the oval cross section of the apertural region. Even if species of Helcionella show a wide morphological range (Jacquet and Brock, 2016), the absence of large concentric rugae and a blunter apex in our material make an affiliation to this genus questionable. Considering the sub- circular cross-section, similarities to Miroconulus Parkhaev in Gravestock et al. (2001) or Anuliconus Parkhaev in Gravestock et al. (2001) are probable. However, both taxa are characterized by concentric rugae, the apex of Miroconulus is slightly dis- placed and hooked posteriorly, and Anuliconus is highly conical with a posteriorly hooked apex. Therefore, it seems most prob- able that our Helcionellid gen. and sp. indet. 1 represents either a new species of Helcionella or a new genus of helcionelloid molluscs. However, the incompleteness of the material hinders a certain taxonomic affiliation.
Helcionellid gen. and sp. indet. 2 Figure 7.13–7.16
Occurrence.—Two specimens from the middle part of the Combined Metals Member (Dyeran Stage) of Oak Spring Summit section (OS 3).
Description.—Large, weakly cyrtoconic, cap-shaped shells with well-developed concentric rugae (Fig. 7.13, 7.15). Apex blunt and rounded; however, specimen OS 3/B2-8 slightly hooked (Fig. 7.13). Aperture is absent, but specimens show a sub- circular to elliptical outline. Height and length of the broken specimens OS 3/B2-8 and OS 3/B2-9 are ~ 0.9mm and ~ 1.9mm, and ~1.5mm and ~2.9mm, respectively.
Remarks.—Specimens are similar to Helcionellid gen. and sp. indet. 1, but with well-developed concentric rugae and without radial lirae. Rugae, the sub-circular cross-section and the slightly hooked apex indicate an affiliation to Helcionella, but our material shows a blunter apex and the concentric rugae are
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