Gilbert et al.—Himalayan Cambrian microfossils
17
Figure 5. Sponge spicules from the Parahio Formation, Parahio Valley, Spiti region. All specimens coated with platinum/palladium before SEM imaging. Scale bar represents 200 µm. (1, 3–6) From 775.41m (PO24) above base of the section. (2) From 880.93m (PV880) above base of the section. (1–3) Tetract. (1, 2) With blade-like rays. (1) WIMF/A/3951; (2) WIMF/A/3952; (3) with cylindrical rays, WIMF/A/3953; (4–6) pentact. (4) WIMF/A/3954; (5) WIMF/A/ 3955; (6) WIMF/A/3954.
Remarks.—Archiasterella has been usually diagnosed as having a 5+0 or 4+0 sclerite ray structure, although 2+0 ray sclerites are also known (Sdzuy, 1969; Qian and Bengtson 1989; Bengtson et al. 1990;Randell et al. 2005).Anew 3+0 sclerite, belonging to Archiasterella charma, has recently been described (Moore et al., 2014).Due to this, aswell as the fact that sclerite ray numbers can vary within an articulated scleritome (Doré and Reid, 1965; Sdzuy 1969;Randell et al., 2005;Zhao et al., 2011), the number of horizontal rays is not useful in diagnosing the genus. Moore and colleagues (2014) distinguished Archiasterella
from Allonnia by the arrangement of horizontal rays with the respect to the basal surface, making assignment of previous illustrated material such as Archiasterella tetractina Duan, 1984; Archiasterella tetraspina Vasil’eva in Vasil’eva and Sayutina, 1988; and Archiasterella quadratina Lee, 1988 difficult to determine because the basal structures of these species are too poorly known to permit confident taxonomic determination. We include then in the listed species above, but acknowledge the difficulty in placing them securely.
Archiasterella dhiraji new species Figures 6.1–6.12, 7.1–7.3
1980, Oneotodus sp. Bhatt and Kumar, p. 357, pl. 1, figs. 1,3 only.
1990, Archiasterella cf. A. hirundo Bengtson in Bengtson et al., p. 55, pl. 29, figs. D,E.
2015, Archiasterella sp. Singh et al., p. 2193, fig. 3.2 only.
2007, Archiasterella sp. Skovsted and Peel, p. 741, pl. 6, figs. C,D.
Etymology.—In honor of Prof. Dhiraj M. Banerjee of the University of Delhi for his many contributions to the late Neoprotoerozoic and Cambrian geology of India.
Holotype.—WIMF/A/3956. Other material.—WIMF/A/3957-3964.
Diagnosis.—Archiasterella with 4 +0 sclerites consisting of one recurved abapical ray, one linear adapical ray, and two ascending horizontal recurved rays. Linear adapical ray and two ascending horizontal recurved rays extend within a single plane. Recurved abapical ray projects abaxially from the basal plane and recurves adapically along the sagittal plane. Basal surface is slightly convex with restricted, rimmed, oval foramina. Trans- verse articulation facet connects bases of recurved abapical ray and linear adapical ray, ascending horizontal ray bases separated.
Description.—Sclerites bilaterally symmetrical about sagittal plane. Two rays are aligned along the sagittal plane, an abapical ray, which recurves upwards away from the basal plane at angles of 65°–105°, with angle varying among sclerites, and a linear adapical ray, which resides within the basal plane. Two ascending horizontal rays occupy the basal plane and are recurved distally toward the linear adapical ray. Sclerites can be up to 2–3mm long as measured along the sagittal plane. Specimens are isolated sclerites preserved in calcium
phosphate. Two modes of preservation occur in our material. In one, the surface of the sclerite was replaced and is therefore visible. Porter (2004) inferred that in such cases the originally aragonitic skeleton was secondarily replaced by calcium phosphate. This mode of preservation is seen in samples from PI13 of Zanskar Valley (Fig. 6.1–6.7, 6.9–6.12), including the holotype. A second mode of preservation occurs where the internal void within the sclerite, or lumen, was diagenetically infilled with calcium phosphate, thus preserving several
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