898
Journal of Paleontology 91(5):883–901
to Bengtson et al. (1986) their Nevadan material could be most probably referred toM. cf. rhomboidale. Due to the stratigraphic and regional closeness of both settings, an affiliation of our fragment to M. rhomboidale is most probable.
Microdictyon sp. Figure 8.30–8.32
Occurrence.—Several fragments from samples M 5 and M 6 from the upper Fallotaspis Zone of the Montenegro Member; middle Montezuman Stage; M 5 and M6 are located 5m and 6m, respectively, below the 127m aluminum tag; Montezuma Range section.
Description.—Thin phosphatic plate fragments with hexagonal meshwork. The plates represent only the upper capping. Hole diameter ranges from 85 μm and 180 μm on the slightly convex surface and decreases to <9 μm towards the periphery. Fragments strongly corroded, obliterating any prominent sur- face and probably enlarging hole diameters.
Remarks.—Due to the insufficient preservation of our fragments no exact determination is possible.
Genus Archaeooides Qian, 1977
Type species.—Archaeooides granulatus Qian, 1977 (pl. 2, fig. 21); Meishucunian Stage (correlated with the Cambrian Stage 2); central and southwest China.
Archaeooides cf. A. granulatus Qian, 1977 Figure 8.33, 8.34
Occurrence.—Two specimens from samples SM 14 and SM 15 from the lowermost Emigrant Formation of the Split Mountain section; Samples SM 14 and SM 15 are derived 1.0 m respec- tively 0.5 m above the base of the Emigrant Formation; Dyeran Stage.
Description.—Well-rounded to flattened on one side, hollow, ranging from 250 μm (Fig. 8.33) to 365 μm (Fig. 8.34) in dia- meter. Surface with crystalline texture and covered with circular to oval pits 15–23 μm in diameter.
Remarks.—The spherical specimen from sample SM 15 (Fig. 8.33) is similar to the ‘perforated sphere’ published by Skovsted (2006a, fig. 4C) from the same locality but from a slightly higher stratigraphic position (~1.4m above the base of the Emigrant Formation). The absence of a flattened area suggesting an encrusting lifestyle of the organism excludes an affiliation of both spheres to Aetholicopalla Conway-Morris in Bengtson et al., 1990. There is also no indication for a double-walled surface, even if an erosion of the outer wall could not be excluded. The spheres show clear similarities to Archaeooides granulatus Qian, 1977, which are single-walled. The surface of A. granulatus and related synonyms (e.g., A. kuanchuanpuensis Qian, 1977, A. acuspinatus Qian, 1977, Gaparella porosa Missarzhevsky in Missarzhevsky and Mambetov, 1981) is characterized by porous tubercles with pore diameters ranging
from 10 μmto30 μm (Missarzhevsky and Mambetov, 1981; Missarzhevsky, 1989; Parkhaev and Demidenko, 2010). Sur-
face ornamentations of the specimen of sample SM 15 and that figured by Skovsted (2006a) are probably corroded. The specimen of sampleSM14 (Fig. 8.34) shows a slightly
convex area, which could be interpreted as a zone of attachment of the organism on the substrate, typical for Aetolicopalla granulata Conway Morris in Bengtson et al., 1990. However, the occurrence of pores, even on this area, points against an encrusting life mode of the hemisphere. There is further no indication for a double-wall that necessary for defining this subsphere to Aetolicopalla. The porous structure of the surface indicates an association to Archaeooides granultus, even if a prominent sculpture/ornamentation does not occur. Archae- ooides granulatus is characterized by a wide morphology,
ranging from spheres, ellipsoids, and hemispheres to spheres flattened on two opposite sides (see Parkhaev and Demidenko, 2010). The specimen of sample SM 15 fits into this morphological range. The pores of the Laurentian specimens are fewer than known from Archaeooides. However, based on their (hemi)spheroidal morphology and the single wall, the Laurentian organisms from samples SM 14 and SM 15 and the specimen of Skovsted (2006a) are referred to Archaeooides cf. A. granulatus Qian, 1977. The general stratigraphic occurrence of Archaeooides
and Aetholicopalla is the Tommotian–Botoman interval of the Siberian nomenclature, which is the Meishucunian–Nangaoan stages of the Chinese nomenclature. The record of the Laurentian Archaeooides slightly below the Dyeran- Delamaran boundary most probably represents the youngest occurrence of these organisms worldwide.
Discussion
Fossil distribution patterns are most probably an artifact of the chemical preparation that eliminated portions of the calcareous microfossils. Helcionelloid molluscs, hyoliths, and hyolithelminths occur in almost all sections investigated for the inner, middle, and outer shelf environments of Nevada and California. Sclerites of sponges and chancelloriids are almost absent at Grassy Spring section (inner shelf), whereas echinoderm ossicles only occur at Split Mountain (outer shelf), Echo Canyon, and Log Cabin Mine sections (both inner shelf; Figs. 3, 4). Occurrences of Pelagiella aff. P. subangulata and several
species of Microdictyon in the lower part of the Montezuman Stage in the Montezuman Range section are most probably important for biostratigraphic correlation.Taxa such as Pelagiella subangulata, Microdictyon effusum, and the tooth-like sclerite Rhombocorniculum cancellatum (Cobbold, 1921) are character- ized by an almost worldwide distribution and are thus useful for correlation of Cambrian Series 2/Stage 3 (Li et al., 2003; Steiner et al., 2007; Rozanov et al., 2008). Well-established biozonations based on SSF assemblages including these taxa were used in Siberia (the so-called Tommotian fauna; e.g., Khomentovsky and Karlova, 1993), Australia (e.g., Gravestock et al., 2001; Jago et al., 2002, 2006), and South China (e.g., Qian, 1999). Steiner et al. (2007) using the P. subangulata and the R. cancellatum taxon-range zones for the base of Cambrian Series 2 have provided detailed correlation between several regions of the
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