Overtoniini Muir-Wood and Cooper, 1960

Carniti, Alessandro P., Porta, Giovanna Della, Banks, Vanessa J., Stephenson, Michael H. & Angiolini, Lucia, 2022, Brachiopod fauna from uppermost Visean (Mississippian) mud mounds in Derbyshire, UK, Acta Palaeontologica Polonica 67 (4), pp. 865-915 : 881-882

Tribe Overtoniini Muir-Wood and Cooper, 1960 Genus Overtonia Thomas, 1914

Type species: Producta fimbriata Sowerby, 1824 , from the Mississippian of Derbyshire, England .

Remarks.— Overtonia differs from Avonia Thomas, 1914 , and other genera of the Overtoniinae in having prominent concentric rugae, each bearing a single row of spine bases.

Overtonia fimbriata (Sowerby, 1824) Fig. 8G, H .

1809 Conchyliolithus Anomites ( punctatus ); Martin 1809 (pars): pl. 37: 7–8, non 6.

1824 Producta fimbriata ; Sowerby 1824: 85, pl. 549: 1.

1862 Productus fimbriatus Sowerby, 1824 ; Davidson 1862 a: 171, pl. 33: 12–15; non pl. 44: 15. (cum syn.)

1914 Overtonia fimbriata (Sowerby, 1824) ; Thomas 1914: 259, 335, pl. 20: 12–17. (cum syn.)

1960 Overtonia fimbriata (Sowerby, 1824) ; Muir-Wood and Cooper 1960: 183, pl. 46: 10–20.

1965 Overtonia fimbriata (Sowerby, 1824) ; Muir-Wood 1965: fig. 339: 1: a–e.

1966 Overtonia fimbriata (Sowerby, 1824) ; Brunton 1966: 214–218, pl. 9: 1–13; pl. 10: 1–7; text-fig. 9.

1985 Overtonia fimbriata (Sowerby, 1824) ; Brunton 1985: fig. 9: a–d.

1991 Overtonia fimbriata (Sowerby 1824) ; Brunton and Tilsley 1991: 289.

2000 Overtonia fimbriata (Sowerby 1824) ; Brunton et al. 2000: fig. 283: 1a–g.

Material.—Two articulated specimens: MPUM 11870

RCK 15-1b); MPUM 11871 ( RCK 15-53) from mud mound complex of Ricklow Quarry, Monyash, Derbyshire, UK, Monsal Dale Limestone Formation, Brigantian, upper Visean.

Description.—Small, concavo-convex shell with subtriangular outline; cardinal margin wide, corresponding to the maximum width. Corpus cavity deep. Anterior commissure rectimarginate. Dorsal valve moderately concave. Fold-sulcus absent, trail very short.

Ornamentation consists of low, subrounded rugae with broad interspaces, becoming closer and lamellose on flanks. Rugae number three per 7.0 mm length. Rugae bearing regularly spaced, coarse, elongated spine bases. Dorsal valve with lamellae with regularly spaced dimples. Regularly concentric growth lines on both valves.

Cardinal process stout, lateral ridges diverging anteriorly at an angle of 20° from the cardinal margin.

Remarks.—The specimens belong to Overtonia fimbriata for the characteristic ornamentation and the disposition of the lateral ridges ( Muir-Wood and Cooper 1960; Brunton et al. 2000). They differ from O. laciniata ( M’Coy, 1844) , as the latter has a greater number of rugae and smaller, more numerous spine bases.

Stratigraphic and geographic range.— Overtonia fimbriata is recorded in several middle–upper Visean units in Great Britain and Ireland (e.g., Davidson 1862 a; Thomas 1914; Wolfenden 1958; Brunton 1966; Mitchell 1971; Brunton and Tilsley 1991; Dean 2017; this study), Belgium ( Davidson 1862 a), and Germany ( Winkler-Prins and Amler 2006).

Figures

Fig. 8. Productellidae from the Brigantian (uppermost Visean) of Ricklow Quarry, near Monyash (localities RCC-, RCK-; see Fig. 3) and Wensley Dale, near Matlock (locality WI1; see Fig. 2), Derbyshire, UK. A–E. Alitaria aff. triquetra (Muir-Wood, 1928). A. MPUM11860 (RCC2-13) in anterior view. B. MPUM11861 (RCC12-2) in anterior (B1) and lateral (B2) views. C. MPUM11862 (RCC60-2) in antero-ventral view. D. MPUM11864 (RCC2-40), ventral valve in antero-ventral view.E. MPUM11866 (RCC128-13), dorsal valve in anterior view.F. Eomarginiferina trispina Brunton, 1966, MPUM11869 (WI1- 31) in ventral (F1), dorsal (F2), anterior (F3), and lateral (F4) views. G, H. Overtonia fimbriata (Sowerby, 1824). G. MPUM11870 (RCK15-1b) in ventral G1), dorsal (G2), and lateral (G3) views. H. MPUM11871 (RCK15-53) in ventral (H1) and dorsal (H2) views. I. Avonia sp., MPUM11872 (RCC128-4a), ventral valve in anterior view. J–N. Quasiavonia aculeata (Sowerby, 1814). J. MPUM11873 (RCC32-28a) in anterior view. K. MPUM11874 (RCC49- 28) in anterior (K1), postero-ventral (K2), and lateral (K3) views. L. MPUM11877 (RCC49-24) dorsal valve interior and external cast. M. MPUM11875 RCK17-2) in ventral view. N. MPUM11878 (RCK15-51) dorsal valve interior. O.?Quasiavonia sp., MPUM11883 (RCC3-11); anterior (O1), posterior O2), and lateral (O3) views. P, Q. Krotovia spinulosa (Sowerby, 1814). P. MPUM11884 (RCC54-17b) in ventral view. Q. MPUM11885 (RCC68-2) in ventral (Q1), anterior (Q2), posterior (Q3), and lateral (Q4) views.