Wotte and Sundberg—Small shelly fossils from the Great Basin It should be kept in mind that individual complete plates of
Microdictyon could combine features (e.g., node morphology) diagnostic for a range of species (Chen et al., 1989; Topper et al., 2011). The simple, hump-like nodes of Microdictyon montezumaensis n. sp. are singular for this species and not known from other fragments and complete plates of Micro- dictyon. However, it couldn’t be excluded that the generally applied diagnostic characteristics result in a multitude of different species of Microdictyon, probably overestimating the real taxonomic diversity, and thus have to be critically revised. Most Microdictyon sclerites are found in lower Cambrian
successions worldwide. Few specimens are known from the middle Cambrian (Ptychagnostus gibbus Zone) of Utah and Bornholm, both probably representing reworked lower Cam- brian material (Bengtson et al., 1986; Berg-Madsen, 1981).
Microdictyon cuneum new species Figure 8.26, 8.27
Holotype.—Fragmented specimen FG 544/GB/M/5/C8-17 from sampleM5 from the upper Fallotaspis Zone of the Montenegro Member; middle Montezuman Stage; 5m below the 127m aluminum tag; Montezuma Range section.
Diagnosis.—Microdictyon with short, wedge-shaped nodes.
Description.—Fragment with fully preserved, dense crystalline capping and partly preserved, coarse crystalline framework. Thickness ~50 μm. Hole diameter nearly uniform, ~75 μm. Nodes wedge shaped, gradually develop from ridges that sur- round the holes. One side of the nodes forms an angle of ~30° with the ridge surface forming the wedge shape. Other side of nodes forms an overhang or an acute angle to the ridge. Wedge- shaped nodes are oriented into the same direction.
Etymology.—Latin cuneus, meaning wedge. Referring to the wedge-shaped appearance of the nodes.
Remarks.—Only two species of Microdictyon are characterized by spike-shaped nodes: Microdictyon robisoni Bengtson, Matthews, and Missarzhevsky, 1986 shows tall and narrow nodes ending in slight expansions. These expansions are clearly offset from the basal socket, which is not present in our speci- men. Nodes of M. sphaeroides Hinz, 1987 develop from a smaller base to a wide, flat rim, finally culminating in a sloped spine. Therefore, node morphology of both species is completely different compared to M. cuneum n. sp. Nodes of
897
M. cuneum n. sp. develop continuously from the rim of the capping showing no offset or rim. They are also shorter than the spiny nodes of M. robisoni.
Microdictyon rhomboidale Bengtson, Matthews, and Missarzhevsky, 1986 Figure 8.28, 8.29
1986 Microdictyon rhomboidale Bengtson, Matthews, and Missarzhevsky, p. 102, figs. 4–6.
2007 Microdictyon aff. rhomboidale; Zhang and Aldridge, p. 405, fig. 2N–2R.
2013 Microdictyon rhomboidale; Bengtson and Conway Morris, p. 461, fig. 2F.
2015 Microdictyon cf. rhomboidale; Kouchinsky et al., p. 481, fig. 55.
2015 Microdictyon sp.; Kouchinsky et al., p. 481, fig. 56.
Holotype.—Microdictyon rhomboidale Bengtson et al., 1986 (figs. 4–6); lower Cambrian (upper Atdabanian or lower Botoman stages; correlated with the middle to upper Cambrian Stage 3); north of Bograd village, Batney Hills, Kuznetskij Alatau Range, Republic of Khakassia, Russia (see Zador- ozhnaya et al., 1973).
Occurrence.—One fragment from sample M 5 from the upper Fallotaspis Zone of the Montenegro Member; middle Montezuman Stage; 5m below the 127m aluminum tag; Montezuma Range section.
Description.—Holes are circular to sub-circular, decreasing in size towards the rim, range of 115 μmto 14 μm near margin. Nodes slightly mushroom-shaped with distinct brim.
Remarks.—The capping of the fragment is completely pre- served, whereas the major part of the framework is corroded. The fragment represents the periphery of a complete sclerite with a steep rim (Fig. 8.28). The shape of the nodes is typical for M. rhomboidale Bengtson et al., 1986. The fragment compares well to Microdictyon n. sp. 1 of Bengtson et al. (1986) described from the region 42 km south of Goldfield (Esmeralda County, Nevada; Albers and Stewart, 1972). The material is derived from the lower Nevadella Zone and is thus stratigraphically slightly younger than the fragments described herein. According
Figure 8. Small shelly fossils from the Echo Shale, Pyramid Shale, Combined Metals, and Comet Shale members of the Montezuma Range, Oak Spring Summit, and Log Cabin Mine sections, as well as from the basal Emigrant Formation of Split Mountain; Montezuman–Delamaran stages. (1–8) indeterminate echinoderm ossicles; scale bar = 400μm: (1) FG 544/GB/E/12/B1-2; (2) FG 544/GB/E/15/B4-9; (3) FG 544/GB/E/15/B4-5; (4) FG 544/GB/E/12/B1-1; (5) FG 544/GB/E/15/ B4-10; (6) FG 544/GB/E/16/B4-2; (7) FG 544/GB/SM/15/B8-5; (8) FG 544/GB/E/12/B1-6. (9–11) Allonnia sp.; scale bar = 400μm: (9) FG 544/GB/OS/7/C11-13; (10) FG 544/GB/OS/7/C11-22; (11) FG 544/GB/OS/4/C2-13. (12–15) Chancelloria sp. 1; scale bar = 400μmexcept (14)(700μm): (12) FG 544/GB/LC/6/A8-3; (13) FG 544/GB/LC/6/A8-4; (14) FG 544/GB/OS/4/C2-12; (15) FG 544/GB/LC/6/A8-5. (16–22) Chancelloria sp. 2; scale bar = 300μm: (16) FG 544/GB/OS/7/ C11-11; (17) FG 544/GB/OS/7/C11-12; (18) FG 544/GB/OS/7/C11-10; (19) FG 544/GB/OS/7/C11-21; (20) FG 544/GB/OS/7/C11-18; (21) FG 544/GB/OS/7/C11- 15; (22) FG 544/GB/OS/7/C11-4. (23) Archiasterella cf. A. hirundo Bengtson in Bengtson et al., 1990; FG 544/GB/LC/1/B3-7; scale bar 300μm. (24, 25) Holotype of Microdictyon montezumaensis n. sp.; FG 544/GB/M/5/C8-8; scale bar = 100μm: (24) ventral view, (25) lateral view of (24). (26, 27) Holotype of Microdictyon cuneum n. sp.; FG 544/GB/M/5/C8-17: (26) ventral view; scale bar 100μm, (27)detailof (26); scale bar 50μm. (28, 29) Microdictyon rhomboidale Bengtson et al., 1986; scale bar = 100μm: (28) FG 544/GB/M/5/C8-9, (29)oblique lateral view of (28). (30–32) Microdictyon sp.; scale bar = 100μm: (30) ventral view; FG 544/ GB/M/5/C6-8; (31) ventral view; FG 544/GB/M/6/B10-3; (32) ventral view; FG 544/GB/M/6/B10-2. (33, 34) Archaeooides cf. A. granulatus Qian, 1977; scale bar = 100μm: (33) FG 544/GB/SM/15/B8-20; (34) FG 544/GB/SM/14/A1-5. (35–37) Pelagiella aff. P. subangulata (Tate, 1892); scale bar = 200μm: (35)FG 544/GB/M/5/C8-13, lateral view; (36) FG 544/GB/M/5/C8-12, lateral view; (37) FG 544/GB/M/5/C8-11, lateral view.
1987 Microdictyon sp.; Voronova et al., p. 56, pl. 24, figs. 6–7. 1992 Microdictyon rhomboidale; Bengtson and Conway Morris, p. 461, fig. 2F.
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