Biwaella Morikawa and Isomi, 1960

Davydov, Vladimir I., 2011, Taxonomy, nomenclature, and evolution of the early schubertellid fusulinids, Acta Palaeontologica Polonica 56 (1), pp. 181-194 : 188-190

Genus Biwaella Morikawa and Isomi, 1960

Figs. 3–5 .

1960 Biwaella gen. nov.; Morikawa and Isomi 1960: 300–301. 1964 Biwaella Morikawa and Isomi ; Thompson in Loeblich and Tappan 1964: C418.

1965 Biwaella Morikawa and Isomi ; Skinner and Wilde 1965: 95.

1988 Biwaella Morikawa and Isomi ; Loeblich and Tappan1988: 280.

1996 Biwaella Morikawa and Isomi ; Chediya in Rauser−Chernousova et al. 1996: 114.

Type species: Biwaella omiensis Morikawa and Isomi, 1960 ; Minamitoba , near Lake Biwa, Shiga Prefecture, Japan;? Artinskian .

Description.—Test large for schubertellids, inflated fusiform to subcylindrical, with broadly rounded axial ends, usually exceeds 1mm in length. Proloculus is relatively small, its outside diameter is around 100–150 µm. Proloculus/test ratio is 1:8 to 1:15. The axis of initial subglobose tightly coiled volution is typically at a large angle to the axis of other volutions. Second volution is ovoid. Following volutions ex−

doi:10.4202/app.2010.0026

pand rapidly in length and height, especially starting from third volution. Form ratios in first volution are around 1.0, in third—2.5–4.0, in the final volution it varies from 3.0 to 4.5. Wall is thin in early volutions 10–15 µm. It increases in thickness rapidly and in the final volution it becomes very thick, up to 100 µm. Wall in first volutions consists of two layers: a thin, dark tectum and a thicker and lighter lower structureless tectorium. A rarely observed upper tectorium is not typical for the genus. Wall in the final volution perforated with coarse mural (simple, branchless) pores. The pores may reach a diameter of 10 µm. The porosity, however, does not develop into keriothecal type, i.e., pores are straight and never join each other as in true keriotheca (Davydov 2007). Therefore, no differentiation of lower and upper keriotheca can be observed ( Figs. 4B , 5B ).

Septa are widely spaced, nearly straight throughout the length of the test and slightly fluted in axial ends. Chomata are small to prominent in all volutions except for the final one.

Remarks.— Biwaella closely resembles elongate Schwageriniformis and Obsoletes , but differs from both of these genera in having a much smaller test, skewed initial volution and, most important, a wall with mural pores only in final volution as opposed to keriothecal wall with lower and upper keriotheca that are developed in all volutions in Schwageriniformis and Obsoletes . Davydov (1984) has shown that although Biwaella and its descendant genus Dutkevichites possess coarse porosity, these genera are schubertellids. Nevertheless they both are often included in the schwagerinids ( Loeblich and Tappan 1988; Rauser−Chernousova et al. 1996). Traditionally, a wall with coarse pores ( Figs. 3B, D , 4B , 5B ) is called keriotheca. It has been demonstrated ( Thompson 1964; Davydov and Krainer 1999; Forke 2002; Leppig et al. 2005; Davydov 2007) that there is a principal difference between a true keriothecal wall developed in the family Schwagerinidae and a wall with coarse mural pores. A keriothecal wall possesses two sets of “piped” pores that are joined with each other and form a lower and upper keriotheca. In the lower part of the keriotheca the “pipes” are coarser than in the upper part of the keriotheca ( Fig. 5E, G, F ). In paraxial sections of keriothecal wall, two sets of pores (or “pipes”) of different size are clearly seen ( Fig. 5F ). Pores in the Biwaella wall are uniform in diameter throughout the thickness of the wall and in oblique sections only uniform pores can be observed ( Fig. 5B ). Late Gzhelian Dutkevichites Davydov, 1984 , which probably evolved from Biwaella , differs from the latter in fluting of the septa developed throughout the length of the test.

Stratigraphic and geographic range.—Similar to the rest of the Schubertellidae described here this genus is distributed globally within tropics−subtropics. It appeared in the early Gzhelian and continued to develop throughout the Cisuralian.

Figures

Fig. 3. Biwaella zhikalyaki sp. nov. from Kalinovo section, P 2 Limestone, lower Gzhelian, Donets Basin, Ukraine. A. SUI 114228, sample A−3/31a−2; axial 2 section of paratype (A1), box indicates the enlarged part of final volution in A1, showing coarse mural pores (A2). B. SUI 114229, sample A−3/31a−14; axial section of holotype (B1) arrow pointed into septal pores, box indicates the enlarged part of final volution in B1 showing coarse mural pores (B2). C. SUI 114230, sample A−3/31a−93; axial section of paratype. D. SUI 114231, sample A−3/31a−79; axial section of paratype. E. SUI 114232, sample A−3/31a−97; axial section of paratype. F. SUI 114233, sample A−3/31a−21; axial section of paratype. G. SUI 114234, sample A−3/31a−77; axial section of paratype. H. SUI 114235, sample A−3/31a−84; axial section of paratype. I. SUI 114236, sample A−3/31a−88; axial section of paratype. J. SUI 114237, sample A−3/31a−101; axial section of paratype. Scale bars: A1, B1, C–J 1 mm, B2 0.1 mm.

Fig. 4. Type of Biwaella and Biwaella poletaevi sp. nov. A–H. Biwaella poletaevi sp. nov. A–G. Kalinovo section, P 2 Limestone, Donets Basin, Ukraine. 2 A. SUI 114238, sample A−3/31a−8, axial section of paratype (A1), box indicates the enlarged part of final volution in A1 showing coarse mural pores (A2). B. SUI 114239, sample A−3/31a−5, axial section of paratype. C. SUI 114240, sample A−3/31a−77; sagital section of paratype. D. SUI 114241, sample A−3/31a−83, axial section of holotype.E. SUI 114242, sample A−3/31a−84, axial section of paratype.F. SUI 114243, sample A−3/31a−96, axial section of paratype.G. SUI 114244, sample A−3/31a−8; sagital section of paratype. H. Biwaella omiensis Morikawa and Isomi, 1960, repository unknown, axial section of holotype, from Morikawa and Isomi (1960:pl. 54:1);?Artinskian; Minamitoba, near Lake Biwa, Shiga Prefecture, Japan.Scale bars:A1, D–F, H 1 mm, C 0.5 mm, A2 0.1 mm, G 0.1 mm.

Fig. 5. The types of porosity in keriothecal and non−keriothecal wall. All specimens from Kalinovo section, P 2 Limestone, lower Gzhelian, Donets Basin, 2 Ukraine. A. Biwaella poletaevi sp. nov., SUI 114245, axial section of paratype, sample A−3/31a−3. B. Biwaella sp., SUI 114246, oblique section, arrow points to coarse mural pores MP; the diameter of the pores is the same elsewhere within the wall; sample A−3/31a−86. C, D. Schubertella subkingi Putrja, 1939. C. SUI 114247, axial section, sample A−3/31a−9. D. SUI 114248, axial section, sample A−3/31a−73. E. Darvasoschwagerina archaica (Leven and Scherbovich, 1978), SUI 114249, axial section, sample A−3/31a−22. F. Darvasoschwagerina sp., SUI 114250, oblique section, arrows point to the two different types of pores: UK − fine pores in upper keriotheca, LK − coarse pores in lower keriotheca; sample A−3/31a−22. G. Darvasoschwagerina satoi (Ozawa, 1925), SUI 114251, axial section, sample A−3/31a−9. H–M. Nodosinelloides sp. H. SUI 114252, axial section, sample A−3/31a−16. I. SUI 114253, axial section, sample A−3/31a−84. J. SUI 114254, axial section, sample A−3/31a−4. K. SUI 114255, axial section, sample A−3/31a−5. L. SUI 114256, axial section, sample A−3/31a−5. M. SUI 114257, axial section, sample A−3/31a−104. Scale bars A, B, E, G 1 mm, C, D, F 0.5 mm, H–M 0.1 mm.