Gilbert et al.—Himalayan Cambrian microfossils
specimens as phosphatic internal molds (Porter, 2004; Qian and Bengtson, 1989) (Fig. 8.6, 8.8). These specimens preserve fewer features than the first mode, and so can be identified with less confidence. This second mode occurs in material from the PO3, PO9, PO15, PO21, PO24, PO25, and PV880 collections.
Remarks.—As individual chancelloriid scleritomes can contain sclerites with varied structure (Qian and Bengtson, 1989; Fernandez Remolar, 2001; Janussen et al., 2002; Randell et al., 2005), it is important to document the degree of variation among disarticulated sclerites collected from single beds before tax- onomoic determination. Our collections include several hundred sclerites that can be assigned to this genus. As there is no variation in ray formula or ray articulation facet condition among any of these sclerites, we conclude that all belong to a single species that was apparently invarient in these characters, both within and among the 8 horizons in the Parahio Formation from which this was collected. Unfortunately, many species of chancelloriid have been established based on a single, or very few, specimens, some of which are poorly preserved. Several such species are assigned to Archiasterella and share the 4+0 ray structure, and these are potentially senior synonyms of our new species. These are dis- cussed below. Sclerite morphology and terminology are given in Fig. 7.1–7.3. As mentioned above,many species of Archiasterella have ray
formulae other than 4+0. Archiasterella dhiraji n. sp. is invarient in ray formula, and the characters that distinguish it fromother species relate specifically to the intersection of the four rays, and so we restrict our comparision here to only those species that share the 4+0 condition. We consider that some specimens figures as Oneotodus from the Parahio Formation (Bhatt and Kumar, 1980) are detrached rays of A. dhiraji. Bengtson (in Bengtson et al., 1990, Fig. 29D, 29E) figured a single specimen with a similar ray configuration to that in A. hirundo (see below) but lacking the robust structure and flattened base, and with abapical and adapical rays that meet in a tranverse articulation facet. He referred this and similar specimens to
A.cf. hirundo. These individuals appear identical to A. dhiraji n. sp., so we place them within the new species. The two specimens attributed to Archiasterella sp. fromthe late early Cambrian Forteau Formation of western Newfoundland (Skovsted and Peel, 2007, fig. 6C, 6D) also share these characteristics, and so we also consider these to be conspecific. The geological implication of this synonymy is that the species has quite a long range, with a first known occurencewithin some of the earliest trilobite bearing beds inAustralia, likely approximately 520 million years old (Bengtson et al., 1990), and ranging into late in Cambrian Stage 5, likely approximately 505 million years old. Archiasterella dhiraji n. sp. resembles the well characterized species A. hirundo in that both have sclerites with four relatively
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slender rays, but A. hirundo has a broader basal surface and larger foramina than A. dhiraji n. sp. More importantly, the two species also differ in the articulation facets between the four rays. In A. hirundo the ascending horizontal rays meet at a sagittal articulation facet, whereas in A. dhiraji n. sp. (Figs. 6.1–6.3, 6.1–6.8) the abapical and adapical rays meet in a tranverse articulation facet and the ascending horizontal rays are isolated fromeach other, resulting in shorter sclerite length along the sagittal plane. In addition, A. hirundo sclerites are more robust and less recurved than those of A. dhiraji n. sp. and curvature of the short, barb-like abapical ray is particularly pronounced in A. hirundo. Duan (1984, pl. 4, figs. 3, 4) erected a new species,
Archiasterella tetractina, based on two illustrated specimens. Archiasterella tetractina has 4+0 rays per sclerite, but lacks a recurved adapical ray suggesting that it may not actually belong within Archiasterella (seeRandell et. al., 2005, p. 994). Moore and colleagues (2014, p. 26) pointed out the difficulty in assessing the morphology of this species given the quality of the figured material.While the broad structure of the rays and ray suture of this late earlyCambrian form do resemble A. dhiraji n. sp., it is difficult to determine if one ray projects upward from the plane of all the other rays, which is the defining characteristic of the genus. For these reasons we recommend isolating the name of A. tetractina Duan, 1984 to its type material, pending a more complete description of additional topotype material. Vasil’eva (in Vasil’eva and Sayutina, 1988) illustrated three
late early Cambrian specimens that were assigned to a new species Archiasterella tetractina (non Duan, 1984) that were later renamed A. tetraspina (Vasil’eva in Vasil’eva and Sayutina, 1993) on account of being a homonym of Duan’s species. In A. tetraspina, the individual rays appear to be equilateral and are not recurved, as in A. dhiraji n. sp. These three specimens, along with a specimen described as Onychia rossica (Sayutina in Vasil’eva and Sayutina, 1988) closely resemble both A. hirundo and A. dhiraji n. sp. In particular, the basal surface of O. rossica closely resembles that of A. dhiraji n. sp. both in ray articulation facet pattern and in possessing a rimmed foramen. However, it is unclear whether one ray curves upward at an angle distinct from those of the others, so assignment of any of this material to Archiasterella is insecure if Moore’s (2014, p. 858) criteria for recognizing the genus, are accepted. Likewise, the basal ray structure in the material described as A. tetraspina is unclear. For these reasons, we recommend isolating the names of A. tetraspina and O. rossica to the published material, pending better knowledge of topotype material. Lee (1988) erected a new early Cambrian species,
Archiasterella quadratina based upon a single incomplete sclerite with a 4+0 ray configuration diagnosed as four radiating nearly perpendicular rays within a plane, thus having a cruciform outline
Figure 6. Archiasterella dhiraji n. sp. from the Parahio Formation. All specimens coated with platinum/palladium prior to SEM imaging. (1–7, 9–12) Collected 74.11m above base of PU3 section (PI13), from Zanskar Valley, Parahio Formation. (1–3) Holotype, WIMF/A/3956, (1) oblique view; (2) oblique view; (3) vertical view; (4–12) paratypes, (4, 9, 10) WIMF/A/3957, (4) articulation facet between linear abapical ray and recurved adapical ray; (5) WIMF/A/ 3958, near vertical view showing articulation facet between linear abapical ray and recurved adapical ray and robust ascending horizontal rays and linear abapical ray in comparison to recurved adapical ray; (6) WIMF/A/3959, basal view showing foramen and articulation facet between linear abapical ray and recurved adapical ray; (7) WIMF/A/3960, vertical view; (8) WIMF/A/3961, from 880.93m above base of Parahio Valley section, Parahio Formation, vertical view of an infilled specimen showing articulation facet between linear abapical ray and recurved adapical; (9) horizontal view showing linear abapical ray and ascending horizontal rays residing in a single plane, with recurved adapical ray protruding from this plane; (10) horizontal view showing linear abapical ray and ascending horizontal rays reside in a single plane, with recurved adapical ray protruding from this plane; (11) WIMF/A/3963, basal view showing foramen with raised “rim” representing a restricted foramen; (12) WIMF/A/3964, vertical view showing angle variation between ascending horizontal rays and recurved adapical ray. Scale bars represent 500 µm(1-4, 7, 9, 10, 12) and 200 µm(5, 6, 8, 11).
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