Freeman et al.—Upper Cambrian linguliform Brachiopods from Texas
trilobite Zone and the Symphysurina trilobite Zone in the Lava Dam Member of the Notch Peak Formation and the lower part of the Barn Canyon Member of the House Limestone in Utah (Popov et al., 2002; Miller et al., 2011). Type material from Malyi Karatau Range, southern Kazakhstan Euloma- Leiostegium trilobite beds (Koneva and Popov, 1988), and Cordylodus proavus conodont Zone (Holmer et al., 2001). In Iran in the latest Cambrian Shirgesht Formation, slightly below strata bearing the conodont Cordylodus angulatus (Popov et al., 2009a).
Description.—Subconical ventral valve and gently inflated dorsal valve. Shape of outline of shell not known. Larval shell ~150 μm in diameter with prominent, slightly raised margin. Larval shell ornamented with micropits, circular in outline, with rounded bottoms, from ~3 μm in diameter to <1 μm. Dorsal larval shell with prominent thickened rim at margin
(Fig. 4.4). Dorsal valve with occasional small disruptions of growth exhibited as ‘nickpoints,’ especially in posterolateral area of valve. Slightly anacline pseudointerarea divided by wide, triangular median groove. Median buttress prominent, transversely subrectangular. Low median ridge arises just anterior to median buttress and extends anteriorly for an undetermined distance (Fig. 4.6, 4.7). Cardinal muscle scars suboval, closely spaced, and located slightly lateral to the median buttress. Ventral pseudointerarea apparently catacline to slightly
procline, divided by narrow interridge (Fig. 4.1, 4.2). Posterior margin convex in outline, with lateral edges of pseudointerarea poorly defined. External foramen contained within larval shell, located at apex. Internal pedicle tube. All other features of ventral interior exfoliated or obscured by sediment.
Materials.—Eighty-seven dorsal valves and 77 ventral valves (Supplemental Data Tables 1, 2), including illustrated dorsal valves NPL 45472 and NPL 45473, and ventral valve NPL 45471.
Remarks.—The interridge on these fragmental, poorly pre- served ventral valves is a useful character for identification, because most species of Eurytreta have an intertrough or an undivided pseudointerarea. Eurytreta discors Popov in Koneva
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and Popov, 1988 has an interridge, but differs in being procline. Although no complete dorsal valves were recovered, a low median septum is present and is a characteristic of E. sublata. The dorsal cardinal muscle scars of Eurytreta sublata are raised and surrounded by a high rim, which is a feature not clearly seen on this abraded and exfoliated material. Streng et al. (2011) rediagnosed the genus Eurytreta using
key characteristics that include a catacline to apsacline ventral pseudointerarea, hemispherical (round-bottomed) pits on larval growth, and a baculate mantle canal system. This rediagnosis questionably included the species E. sublata in the genus because of its catacline to weakly procline ventral pseudointer- area and because of lack of knowledge of larval shell pitting. This Texas material reveals the type of larval pitting assignable to the genus Eurytreta, but it is too poorly preserved to allow determination of the mantle canal system. The specimens identified as Conotreta sp. by Winston and
Nicholls (1967) from coeval strata in central Texas demonstrate the distinctive characteristics ofE. sublata, including a pronounced ventral interridge and a low, narrow but elongate dorsal septum that does not form a triangular tip. Winston and Nicholls (1967) described dorsal specimens with a small but deep median groove, whereas Popov et al. (2002) described the median groove of the species as shallow. Winston and Nicholls’ illustrated specimen (1967, pl. 12, fig. 27) does not appear to differ in this regard from previously illustrated specimens of E. sublata (Koneva and Popov, 1988; Holmer et al., 2001; Popov et al., 2002).
Eurytreta sp. 1 Figure 4.8–4.14
Occurrence.—In the Cordylodus angulatus conodont Zone, in the lowest part of the Threadgill Member of the Tanyard Formation.
Description.—High, subconical ventral valve with foramen placed at apex, within larval shell. Ventral valve transversely suboval in outline. Steeply procline to catacline pseudinterarea with broad, poorly defined intertrough on some specimens. Growth lines especially prominent in central area of ventral pseudointerarea (Fig. 4.10–4.12). Larval shell of ventral valve ~150 μm in diameter, with microornamentation of
Figure 3. Lingulella sp., from the Cordylodus angulatus conodont Zone, in the Threadgill Member of the Tanyard Formation, Texas; scale bar is 500 μm: (1–3) ventral exterior and interior valve from LR-1465.5, detail of hinge (NPL 45434); (4, 5), ventral valve fragment exterior, interior from LR-1465.5 (NPL 45435). Stittia ornata from lower Saukia trilobite Zone and the Proconodontus tenuiserratus conodont Zone, in the San Saba Member of the Wilberns Formation, Texas; scale bar is 1mm (6, 7), 500 μm(8): (6, 7), dorsal exterior fragment from TC-1197.5, detail of larval shell and postlarval ornament, (NPL 45436); (8) ventral interior fragment from TC-1197.5 (NPL 45438). Stittia?, from the Cambrooistodus minutus Subzone of the Eoconodontus notchpeakensis conodont Zone and the Prosaukia serotina Subzone of the Saukia trilobite Zone, in the San Saba Member of the Wilberns Formation, Texas; scale bar is 1mm: (9) fragment from LR-21, exterior showing larval and postlarval ornamentation (NPL 45439). Stittia sp., from the Eoconodontus notchpeakensis Subzone of the Eoconodontus conodont Zone and the Prosaukia serotina Subzone of the Saukia trilobite Zone, in the San Saba Member of the Wilberns Formation, Texas; scale bar equals 500 μm: (10, 11) dorsal fragment exterior and interior from LR-13.5 (NPL 45441). Stittia cf. S. notchensis (Walcott, 1908) from the Cambrooistodus minutus Subzone of the Eoconodontus notchpeakensis conodont Zone and the Prosaukia serotina Subzone of the Saukia trilobite Zone, in the San Saba Member of the Wilberns Formation, Texas; scale bar is 1mm (12, 13), 500 μm(14): dorsal (?) fragment interior and exterior from LR-24 (NPL 45442); (14) fragment showing larval and postlarval growth from LR-24 (NPL 45443). Wahwahlingula sp. from the Proconodontus posterocostatus conodont Zone and the Rasettia magna Subzone of the Saukia trilobite Zone of the San Saba Member of the Wilberns Formation, Texas; scale bar is 300 μm unless indicated otherwise: (15–18) ventral exterior and interior, detail of ornament on postlarval shell at position indicated by arrow on 15, detail of ornament on larval shell at position indicated by arrow on 15 (NPL 85570). Zhanatella utahensis Popov, Holmer, and Miller 2002, from Proconodontus posterocostatus conodont Zone and the Rasettia magna trilobite Subzone of the Saukia Zone in the San Saba Member of the Wilberns Formation, Texas; scale bar is 300 μm unless indicated otherwise: (19, 24–26) ventral valve exterior, detail of larval shell, ornament of early larval shell growth, ornament of later larval shell growth, from TC-1245 (NPL 45448). Zhanatella utahensis Koneva, 1986, from the Proconodontus tenuiserratus conodont Zone and the Saukia trilobite Zone of the San Saba Member of the Wilberns Formation, Texas; scale bar is 200 μm unless indicated otherwise: (20, 21) dorsal interior, detail of ornament on proparea (NPL 85571); (22, 23) dorsal exterior, detail of ornament at position indicated by arrow on 22 (NPL 8557).
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