Mesoschubertella Sakagami, 1957

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

Genus Mesoschubertella Sakagami in Kanuma and Sakagami, 1957

Type species: Mesoschubertella thompsoni Sakagami in Kanuma and Sakagami, 1957; found in limestone pebble in Tamanouchi Limestone conglomerate together with Yakhtashian (late Artinskian–Kungurian) fusulinids; Yagooki Valley , Tamanouchi , Hinode−rnura , Nishitama−gun, Tokyo−to, Japan .

Description.—Medium to large elongate−fusiform to inflated−fusiform schubertellids with more than 4–5 volutions. The test lengths is exceed 1.0– 1.5 mm. Proloculus/test ratio is 1:20 to 1:30 and is the greatest among the rest of genera discussed in this paper. Coiling is typical for schubertellids, i.e., the initial one or one and a half volutions are coiled at a large angle in respect to the following volutions. Wall is thick, with thin, dark tectum, well developed upper tectorium, lower tectorium and lighter layer between the tectum and lower tectorium (diaphanotheca). The latter layer often can be barely recognized due to poor preservation. Chomata are small to medium, always prominent. Septa straight, slightly fluted in the polar ends.

Remarks.—Thompson already noted the prominent features of Permian Schubertella that he called advanced ( Thompson 1948: 33), such as a relatively large size and a large number of volutions. At the same time, he stated that there was a single−layered wall. It seems that preservation severely affects schubertellid’s wall structure, and sometimes the wall may appear as a single structureless layer. However, in sufficiently well preserved forms ( Fig. 1L, M) four layers of the wall with diaphanotheca are commonly observed. Ueno (1996) call the light intermediate and less dense layer between dark tectum and dense lower tectorium, as protheca. He pointed that this layer is quite different from actual diaphanotheca of Fususlinella, Beedeina , and Yangchienia , but did not explain how exactly it is different. In my opinion the term diaphanotheca does not represent chemically or compositionally determined layer, but simply the descriptive term for the light and less dense layer between the two more dense layers ( Rauser−Chernousova and Gerke 1971).

Nevertheless, the wall structure is not the only feature that allows separation of Mesoschubertella from Schubertella . Mesoschubertella also differs from Schubertella in its greater size, generally exceeding 1.0 mm, and greater number of volutions (4–6 versus 3–4 in Schubertella ). The morphological features of Schubertella and Mesoschubertella overlap, as these genera are closely related to each other, and a taxonomic differentiation in some specimens could be difficult.

Stratigraphic and geographic range.— Mesoschubertella commonly is considered to be Tethyan form only, but as shown here it also occurs in Mexico and Nevada ( Fig. 1L). Therefore, the genus is global in distribution and ranges from the Cisuralian (possibly the late Gzhelian) to the Guadalupian.