Sphaerirhynchia gibbosa ( Nikiforova, 1954 )

Baliński, Andrzej, 2012, The brachiopod succession through the Silurian-Devonian boundary beds at Dnistrove, Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 897-924 : 911-912

Sphaerirhynchia gibbosa ( Nikiforova, 1954)

Figs. 10 , 11 , 12A–E .

1929 Camarotoechia (Wilsonia) Wilsoni (Sowerby) ; Kozłowski 1929: 159, pl. 7: 27–33 (not 34 = Sphaerirhynchia wilsoni ?); fig. 50.

1954 Wilsonella wilsoni (Sowerby) var. gibbosa var. nov.; Nikiforova 1954: 110; pl. 11: 5.

1968 Sphaerirhynchia gibbosa (Nikiforova) ; Modzalevskaya 1968: pl. 27: 40–43.

1978 Sphaerirhynchia gibbosa (Nikiforova) ; Rubel and Teller 1978: 477; pl 2: 16–18.

1985 Sphaerirhynchia gibbosa ( Nikiforova, 1954) ; Nikiforova et. al. 1985: 37.

Material.—86 complete and 109 fragmentary or deformed shells. Statistical analysis of the main shell dimensions is given in Fig. 10 . Sphaerirhynchia gibbosa is one of the most characteristic species in the studied section at Dnistrove. The first specimens appear in the interval at 1.3–1.5 m above the S–D boundary (beds 5 and 7). Slightly higher in the section (5.5 m above the boundary; beds 47–48) it becomes one of the most numerous taxa in the assemblage. Sporadically it occurs in beds 92, 96, 102, 105, 110, and 115.

Description.—Shell attaining about 18 mm in width, strongly dorsibiconvex in profile, subelliptical to subpentagonal in outline, globose, slightly wider than long; commissure serrate, moderately to strongly uniplicate. Ventral valve regularly convex in both lateral and anterior profiles; umbo massive but low, beak incurved; sulcus poorly defined, shallow, developed in anterior half, anteriorly forms subtrapezoidal to subrectangular tongue. Dorsal valve regularly and strongly convex with greatest convexity at about midvalve or slightly posteriorly; fold very low, poorly defined in anterior half of the valve.

Ribs fine, simple, extending from beaks, those at the anterior margin medianly grooved; 6–8 ribs on fold, 5–7 in sulcus, one pair of parietal ribs frequently present.

Interior of ventral valve with short, thin subparallel dental plates. In dorsal valve thin median septum attains less than half of the valve length and supports V−shaped septalium.

Dental plates slightly laterally convex, thin and short; outer hinge plates flat and horizontal; V−shaped septalium and median septum short; crura fairly long, slightly divergent, curving ventrally, with anterior surface concave distally ( Fig. 11A, C ).

Remarks.— Kozłowski (1929) described Camarotoechia (Wilsonia) wilsoni (Sowerby, 1816) (= Sphaerirhynchia wilsoni ) from the Skala and Borshchiv horizons. Later, Nikifohttp://dx.doi.org/10.4202/app.2011.0138

rova (1954) also collected the species from the older Malinovtsy Horizon. She found morphological differences between specimens from the Malinovtsy and Borshchiv horizons, and named the younger form as a new variety gibbosa . Nikiforova et al. (1985) then confirmed that specimens of Sphaerirhynchia gibbosa from the Khudykivtsi Beds of the Borshchiv Horizon are separable from true S. wilsoni as recovered by Nikforova (1954) from the Malinovtsy horizon.

Stratigraphic and geographic range.—According to Nikiforova et al. (1985), this is a common species in the Khudykivtsi Beds. S. gibbosa was also reported by Johnson et al. (1973) from the lower Devonian of Nevada.

Figures

Fig. 10. Scatter diagrams of shell width (A) and shell thickness (B) to shell length ratios in Sphaerirhynchia gibbosa (Nikiforova, 1954). Dnistrove, beds 47–48.

Fig. 11. Transverse serial sections through three shells of Sphaerirhynchia gibbosa (Nikiforova, 1954). A. ZPAL Bp. 56/64−3. B. ZPAL Bp. 56/64−2. C. ZPAL Bp. 56/64−1. Numbers refer to distance in mm from ventral umbo. Dnistrove, beds 47–48.

Fig. 12. Rhynchonellide brachiopods from the Silurian–Devonian boundary beds at Dnistrove (Podolia, Ukraine). A–D. Sphaerirhynchia gibbosa (Nikiforova, 1954). A–C. Three complete shells ZPAL Bp. 56/59 (A), ZPAL Bp. 56/60 (B), and ZPAL Bp. 56/61 (C) in dorsal (A1, B1, C1), ventral (A2, B2, C2), lateral (A3, B3, C3), posterior (A4, B4, C4), and anterior (A5, B5, C5) views. D, E. Juvenile, ZPAL Bp. 56/62 (D) and gerontic, ZPAL Bp. 56/63 (E) shells in dorsal (D1, E1), lateral (D2, E2), and posterior (D3, E3) views. F–H. Lanceomyonia borealiformis (Siemiradzki, 1906), three well preserved shells ZPAL Bp. 56/65 (F), ZPAL Bp. 56/66 (G), and ZPAL Bp. 56/67 (H), in different individual age in dorsal (F1, G1, H1), ventral (F2, G2, H2), lateral (F3, G3, H3), posterior (F4, G4, H4), and anterior (F5, G5, H5) views; all from beds 47–48.