Thyasira becca Kauffman, 1967

Hryniewicz, Krzysztof, Amano, Kazutaka, Jenkins, Robert G. & Kiel, Steffen, 2017, Thyasirid bivalves from Cretaceous and Paleogene cold seeps, Acta Palaeontologica Polonica 62 (4), pp. 705-728 : 721

Thyasira becca Kauffman, 1967

Fig. 16 .

1967 Thyasira becca becca subsp. nov.; Kauffman 1967: 126, pl. 4:1– 21, pl. 5: 28, 33, 37, 38.

1967 Thyasira becca cobbani subsp. nov.; Kauffman 1967: 132, pl. 1: 28, pl. 5: 34–36.

Type material: Holotype: USNM 153512 ; paratypes listed by Kauffman (1967).

Type locality: USGS Mesozoic locality D1410, Fall River County, South Dakota .

Type horizon: Middle Baculites scotti Zone, Pierre Shale , upper Campanian, Upper Cretaceous .

Material.— Three illustrated specimens ( ZPAL L.16/18–20; Fig. 16 ) from Campanian seep carbonate at Warbonnet Battlefield Monument, Montrose, Nebraska.

Remarks.—The specimens from Montrose fit the description of Thyasira becca from the Campanian of the Western Interior Seaway ( Kauffman 1967). Thyasira becca was initially subdivided into two subspecies: the rounded T. becca becca , and the subrectangular T. becca cobbani ( Kauffman 1967: 126) ; however we hesitate to include our specimens in any of those subspecies due to the small number of available specimens. Thyasira becca is significantly larger and more rounded than T. tanabei from the Albian–Campanian cold seeps and sunken wood from Japan ( Kiel et al. 2008). Thyasira xylodia from Eocene–Miocene whale and wood falls in Washington State is, at a length of approximately 10 mm, more triangular and higher than the Montrose specimens, and it also differs by having a much lower posterior fold ( Kiel and Goedert 2007: fig. 7A).

Thyasirid species from the Western Interior Seaway belong to the Thyasira advena – Thyasira becca species complex, comprising large, rounded to oval species of Thyasira with protruding umbones. The group comprises T. advena sensu lato, T. becca sensu lato, and T. cantha ( Kauffman 1967) . The species complex is contrasted with the Thyasira rostrata complex, comprising smaller, thin-shelled and triangular species with weakly protruding umbones. This group is composed of T. beauchampi sensu lato, T. quadrula , T. rostrata , and T. triangulata . Shape and size-wise, the species from the T. advena – T. becca complex resemble Recent rounded species from the T. sarsi – T. southwardae – T. methanophila complex, comprising chemosymbiotic opportunists ( T. sarsi ) and seep-restricted specialists ( T. methanophila ) ( Taylor et al. 2007; Amano et al. 2015).

Stratigraphic and geographic range. — Campanian of the Western Interior Seaway, North America. The species has been recorded from cold seep carbonates known as “Tepee Buttes” ( Kauffman et al. 1996) and from non-seep localities in Colorado, South Dakota and potentially Canada ( Kauffman 1967). Here we report the species from a Campanian “Tepee Butte” seep carbonate from Warbonnet Battlefield Monument, Montrose, northwestern Nebraska.

Figures

Fig. 16. Thyasirid bivalve Thyasira becca Kauffman, 1967, from a Campanian seep carbonate at Warbonnet Battlefield Monument, Montrose, Nebraska, USA. A. ZPAL L.16/18, a partial internal mold in left (A1) and right (A2) lateral views, with fragments of the shell preserved and weak radial striation on the surface of the mold; dorsal view (A3) shows broad posterior sulcus; anterior view (A4) shows poorly preserved anterior adductor muscle scar. B. ZPAL L.16/19, a partial internal mold in left lateral view, showing broad posterior sulcus and posterior part of the posterior adductor muscle scar. C. ZPAL L.16/20, of a complete internal mold in right lateral view showing the shell outline.