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Journal of Paleontology 90(1):10–30
molds of the lumen, the walls between adjacent cavities were perhaps thin enough for rays to adhere together even in acid-etched residues, thus leaving sclerites intact (Qian and Bengtson, 1989).
Remarks.—Various sclerite characteristics have been used to diagnose chancelloriid species. One is the number of rays- per-sclerite (Jiang in Luo et al., 1982). However, this number can vary within a single chancelloriid scleritome, and so ray number alone is not appropriate for designating species (Qian and Bengtson, 1989; Fernández Remolar, 2001; Janussen et al., 2002; Randell et al., 2005). Sclerites within our collections vary in possessing from four to seven lateral rays, which is compatible with that reported by Janussen and colleagues (2002), which showed ray configurations of 4+0, 5 +1, 6 +1, 7+1, and 8 +1 within a single articulated scleritome of C. eros. Similarly, disarticulated sclerites of C. maroccana (Sdzuy, 1969) are considered to span the range from 4 +1 to 7 +1, as in our material. Individual collections within this study do display ray configurations that vary from 4+1 to 7+1 (i.e., PO24). However, sample sizes within other individual collections are often too small to confidently suggest that the full range of variation is expressed within each collection. Accordingly, there is no reason to consider that Chancelloria sclerites in our sample belong to more than a single species. The earliest recorded occurrence of Chancelloria consists
Haydenaspis parvatya level (Singh et al., 2015, fig 3.1) appears to show five or more rays radiating in a plane, and might be a Chancelloria.
of articulated sclerites from the Purella antiqua Zone of the Nemakit-Daldynian Stage, Siberia (Khomentovsky et al., 1990; Maloof et al., 2010; Kouchinsky et al., 2012), and Chancelloria is known to endure until the Prochuangia Zone, low in the upper Cambrian, based on biostratigraphically correlated trilobites reported from the Mila Formation, Alborz Mountains, Iran (Mostler and Mosleh-Yazdi, 1976; Hamdi et al., 1995; Peng et al., 1999). While the FAD of Chancelloria is considered to be of biostratigraphic potential (Brasier, 1989; Qian and Bengtson, 1989; Janussen et al., 2002) the end of its range is less well constrained. A single specimen reportedly from ~20m above the
Remarks.—Placement of Himalayan material into the class Helcionelloida Peel, 1991a is based on specimens being untorted mollusks that are endogastrically coiled with the apex located posteriorly (Peel, 1991a, 1991b; Geyer, 1994; Gubanov
and Peel, 2000). We cannot be certain that the new material figured herein is untorted because no apices are sufficiently well preserved to permit assessment. However, as all other features of our shells compare closely with previously described helcionelloid material, we are confident in our assignment to Helcionelloida.
Order Helcionellida Geyer, 1994 Family Helcionellidae Wenz, 1938
Genus Igorella, Missarzhevsky in Rozanov et al., 1969
Type species.—Igorella ungulata Missarzhevsky in Rozanov et al., 1969, p. 141, lowerCambrian, Tommotian (Nochoroicyathus sunnaginicus Zone), West Anabar, and Uchur-Maya regions, Siberian Platform, Russia.
Diagnosis.—See Devaere et al. (2013, pg. 19-20).
Remarks.—The figured material is assigned to this genus because these coiled shells are moderately high and cap-shaped, and are also moderately laterally compressed. The apices are inclined and are significantly displaced posteriorly, projecting over the posterior apertual margin. In addition, the apertures are oval to elliptical and simple in form. Shells display external ornamentation represented by concentric comarginal ribs (rugae). All these features are consistent with placement in Igorella. These Himalayan shells can be excluded from the genus Oelandiella (Vostokova, 1962), to which they are similar in several ways, because they are much more loosely coiled (cyrtoconic), which gives them their cap-shaped appearance.
Igorella cf. maidipingensis Yu, 1974 Figure 9.1–9.14
Material.—WIMF/A/3972-3981.
Occurrence.—New material from Parahio Formation carbonates collected at 78.07m (PO3, Haydenaspis parvatya level), 439.44m (PO15, Kaotaia prachina Zone), 775.41m(PO24, Paramecephalus defossus Zone), 836.36m (PO31, Orytocephalus salteri Zone), 880.96m (PV880.96, O.
salteri Zone), and 1242.4m
(P09 unzoned 5) above the base of the Parahio Valley section on the north side of the Parahio River, Spiti region, Himachal Pradesh. The PO3 occurrence is from the top of informal global Stage 4 of the Cambrian System; all others are from informal global Stage 5 of the Cambrian. Approximately 30 spicules inspected.
Phylum Mollusca Cuvier, 1797 Class Helcionelloida Peel, 1991a.
Diagnosis.—See Devaere et al., (2013, pg. 6).
Description.—Univalves moderately laterally compressed and cap-shaped, slightly cyrtoconic, and loosely coiled to about one-half whorl. Several specimens display a rapidly expanding conch, flaring and widening slightly upon approaching the aperture. Aperture is elongated and elliptical, with the greatest length along anterior-posterior axis. Apertual margin is perpendicular to sagittal plane. Apex is posteriorly displaced over the posterior of aperturual margin by a distance of approximately one eighth of the total shell length, and is not
preserved.Umbilicus forms an even, convex curve. The outer surfaces of these internal molds display comarginal ribs that always cross the dorsum. Ribs straight along margin from the dorsum to umbilicus. Ribs symmetrical on both margins and most prominent on dorsal surface tapering and ultimately fading approaching umbilical area. Most specimens contain 11–13 moderately prominent ribs, with decreasing robustness along dorsum until obsolete near apex. Specimens display primary ribs only (apparently invariant in size, shape, and number among individual specimens of similar
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