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Journal of Paleontology 90(1):10–30
size). Transverse rib profiles triangular to wedge-shaped with angularity ranging from sharp to well rounded, depending on preservation. All specimens are preserved in calcium phosphate. These
specimens thus display no preserved original surficial details or microstructure of the original organism.
Remarks.—The Parahio Formation specimens appear to be slightly tectonically deformed, displaying varied degrees of lateral compression likely due to variation in their orientation with respect to the principal extension direction. However, some variation within the sample may be biological in origin. In fact, Igorella maidipingensis morphology is known to be quite variable, particularly concerning shell proportions and extent of apical bending (Parkhaev and Demindenko, 2010). Despite being mildly deformed internal molds, our shells are sufficiently well preserved to permit evaluation at a low taxonomic level. Material described herein is comparable to Igorella
maidipingensis in several ways, such as the possession of a moderately high, moderately laterally compressed shell, an apex that is projected posteriorly over the posterior of the apertual margin at a distance of roughly one eighth of the shell length (Figs. 9.2, 9.5, 9.7, 9.9, 9.11, 9.14), an aperture that is oval to elliptical (Figs. 9.1, 9.4), the convex anterior field, and concave posterior field, the lateral fields that are straight to slightly concave (Figs. 9.1–9.14), and that the exterior ornamentation displays concentric comarginal ribs that are evenly spaced and are triangular in profile. Although our specimens display fewer ribs than I. maidingensis, (between 11 and 13, becoming less and less robust until smoothing out entirely as they approach the apex), this is possibly a result of preservation and does not justify exclusion of our specimens from the species. However, our specimens do lack a well-preserved apex and protoconch, thus limiting our confidence in taxonomic assignment to I. maidipingensis. Igorella cf. maidipingensis differs from I. ungulata in that it
has a lower shell, lacks radial striations, and displays sharper concentric ribs (Missarzhevsky in Rozanov et al., 1969). The presence of sharp concentric ribs also distinguishes
I.cf. maidipingensis from other congeneric species such as I. monstrosa, I. sanxiaensis, I. hamata, I. levis, I. talassica, I. durara,and I. arta (Parkhaev andDemidenko, 2010;Missarzhevsky in Rozanov et al., 1969; Yu, 1979; Esakova and Zhegallo, 1996; Missarzhevsky in Missarzhevsky and Mambetov, 1981; Kruse, 1991). Our specimens also exhibit comarginal ribs that are denser than those in I. minor and I. emeiensis in the lower section of the shell (Chen and Zhang, 1980; Yu, 1987). This species has a documented biostratigraphic range from
the Nemakit-Daldynian Stage to the Tommotian stages of the Siberian Platform, on the Yangtze platform, in France, and in Iran (Devaere et al., 2013; Parkhaev and Demidenko, 2010). If our material is confirmed in belonging to this species, it will extend the range of Igorella maidipingensis into informal Stage 5 of the Cambrian System.
Occurrence.—New material from Parahio Formation carbonates collected at 776m (PO25, Paramecephalus defossus Zone) above the base of the Parahio Valley section on the north side of the Parahio River, Spiti region, Parahio Formation,
informal global Stage 5 of the Cambrian. Approximately 15 conchs inspected.
Phylum Arthropoda von Siebold, 1848 Class Trilobita Walch, 1771
Order Ptychopariida Swinnerton, 1915 Indeterminate Ptychopariid Figure 10.1–10.5
Material.—WIMF/A/3982-3986.
of axial length. Glabellar medial portion less inflated than fixigenae. Fixigenae smooth, anterior and lateral margin defined by sharp break of slope with weakly incised border furrow of modestly inflated border. Anterior border long (exsag.), approximately 22% of cranial sagittal length at longest, contiguous with narrow (tr.) lateral border that widens posteriorly into base of stubby spine that extends rearward and outward from a location within posterior border. Additional specimens include fragments of the posterior lateral margin of several free cheeks, some of which bear marginal tubercles.
Description.—Cranidium subquadrate: sagittal length 240µm, maximum width 500µm. Glabella occupies entire axis, expanding transversely slightly where intersecting anterior border. Posterior margin of occipital ring strongly arched, occipital furrow weakly incised, occipital ring about one fifth
Remarks.—The apparently phosphatized cranidium WIMF/A/ 3982 shows mild tectonic shear. The overall form of this small, evidently early meraspid cranidium is broadly similar to that seen among “ptychopariid” trilobites (e.g. Lee and Chatterton, 2005a, 2005b; Cederström et al., 2011; Laibl, Fatka, Cronier, and Budil, 2014) in that the glabella expands anteriorly, the glabella lacks axial carination, and the occipital lobe is inflated. Notable differences are the wide anterior border and the stubby spines on the posterior border neither of which are typical for a ptychoparioid meraspid, and look more like the form seen in the protolenid Ichangia inchangensis (see Zhang and Pratt, 1999). This lone specimen might belong to any of the five trilobite species described from this locality (Peng et al., 2009), the most common of which is Gunnia smithi, or to other species as-yet unrecorded.
Occurrence.—All specimens are from Parahio Formation carbonates collected at 776m(PO25, Paramecephalus defossus Zone) above the base of the Parahio Valley section on the north side of the Parahio River, Spiti region, Parahio Formation, informal global Stage 5 of the Cambrian. Five specimens available.
Phylum Incertae Sedis Hyolith indet. Figure 11.1–11.5
Material.—WIMF/A/3987-3990.
Description.—Elongated, slightly tapered, small conchs decreasing evenly in diameter toward the apex. Subtriangular in cross-section, with slightly convex venter, and moderately
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