Blinia, Anistratenko & Bandel & Anistratenko, 2006

Anistratenko, Olga Yu., Bandel, Klaus & Anistratenko, Vitaliy V., 2006, A new genus of patellogastropod with unusual protoconch from Miocene of Paratethys, Acta Palaeontologica Polonica 51 (1), pp. 155-164 : 157-160

Genus Blinia nov.

Type species: Helcion angulata d’Orbigny, 1844 in Hommaire de Hell (1844): 470, table 4, figs. 13–15. Middle Sarmatian of Volhyno−Podolia and Kerch Peninsula ( Kolesnikov 1935).

Derivation of the name: “Blin” is Russian for pancake, and the general shape of the protoconch of Blinia is like a pancake.

Diagnosis.—Conical, relatively small patellogastropods with the smooth or wrinkled “pancake”−like protoconch; ornament of axial ribs; aperture oval. The new genus differs from Tectura mainly in regard to the outer layer of the shell whereas the organization of the inner crossed lamellar layers in both taxa is more or less similar. The outer layer of the shell of Blinia is characterized by a simple type of aragonitic in construction crossed lamellar structure while Tectura has a calcitic one. Blinia differs from Helcion in the same characteristics of composition and structure of the outer layer as from Tectura , while the inner crossed lamellar layer is similar. From all other known patellogastropods Blinia differs by having a round “pancake”−like protoconch.

Description.—The shell is small to moderate in size, has a thin wall, and approximately conical shape without marginal slit, apical hole or internal septum. It measures up to 13.5 mm in length (exceptionally—up to 20 mm), up to 11 mm in width and up to 6.5 mm in height. The apex lies out of the center and is tilted forward or backward. The anterior end of the shell is narrower than its posterior end. The surface has numerous radiating ridges crossed by concentric lines, which lie usually parallel to the apertural plane.

The protoconch has round to oval “pancake”−like shape, is quite flat and measures from 0.13 mm to 0.16–0.18 mm in maximum diameter; the lesser diameter is up to 0.11–0.12 mm. The protoconch surface is smooth or wrinkled; sometimes with a small pit on top of embryonic shell. The transition from the embryonic shell to the teleoconch is usually clearly marked by a constriction or a rim and change in ornament.

Discussion.—The more or less bulbous embryonic shell of Blinia species indicates a lecithotrophic type of early development and the absence of any short free−swimming larval stage following the yolk−rich embryogenesis. The young started its independent life as a crawling animal. The shape and proportions of “pancake”− type of protoconch may suggest the brooding of young snails in the maternal adult individuals.

The new genus is established for at least two Sarmatian species, which have been usually attributed to Tectura or Acmaea ( Acmaeidae ). The teleoconch characteristics (rather than the shell structure, see below) support a relationship of Blinia with those two genera.

The shell shape and the muscle scars of Blinia resemble those known among patellogastropods ( Lindberg 1998). The imprints on the inner surface of the shell of Blinia , we interpret as the muscle scars, are horseshoe−like, gently expressive tracts ( Fig. 3 ). The impressions were not SEM−documented as they appear usually as regions of different shade that is not recognizable after coating of the specimens. In some cases even under the light microscope it is difficult to recognize whether the edge of these horseshoe−shaped scares is opened. At the same time their protoconch morphology indicates a clearly lecithotrophic embryonic development and suggest that they should be considered an independent genus. Moreover, the position of apex in some representatives of Blinia is quite distinctive. In contrast to a forward−tilted apex in Patella , Acmaea , Tectura , and majority of other patellids, some Blinia (e.g., Blinia pseudolaevigata ) have an apex that is tilted backwards, as in Propilidium ancyloides ( Forbes, 1840) ( Lepetidae ) and also in the slit bearing Emarginula Lamarck, 1801 ( Fissurellidae ). The family assignment of Blinia is uncertain. Comparison with other Patellogastropoda and Patella −like gastropods as a whole indicates that general shape and morphology of a protoconch may tell us practically nothing about systematic position. Ornamentation can rarely help in this respect, but shape and size of the protoconch can indicate the size of the eggs and the mode of embryonic development ( Bandel 1982).

The shell structure of Blinia is characterized by a composition predominated by a simple type of crossed lamellar structure ( Fig. 4A). Such aragonitic crossed lamellae are known from the oldest well preserved patellid species Scutellastraea costulata ( Münster, 1841) from the Triassic St Cassian Formation of the Alps and could also be found in a patellogastropod species of very similar shape from the Paleocene of Alabama ( Hedegaard et al. 1997; personal observations). The outer layer of the Blinia shell is thin and porous, and also aragonitic in construction. That distinguishes Blinia clearly from Patella and its relatives, which have a calcitic outer layer, usually quite thick and commonly with rather complex structure ( Bandel and Geldmacher 1996). In case of Blinia most of the shell is composed of one layer of crossed lamellae in which the needles of the two directions of lamellae of the first order commonly intersect, giving in perpendicular section an almost spherulitic appearance.

The shell structure (see Fig. 4) of Blinia pseudolaevigata is similar to that of “ Tectura” zboroviensis but differs substantially from that of Patella virginea Müller, 1776 (type species of Tectura ) and from Helcion pellucidus ( Linnaeus, 1758) of the family Patellidae . Tectura as well as Helcion have a calcitic outer layer of the shell of characteristic layered structure that is absent in Blinia and “ Tectura ” zboroviensis .

In Tectura virginea ( Müller, 1776) from the North Sea shell structure is quite different from that of Blinia . The outer shell layer is formed by a calcitic layer of coarse inclined prismatic appearance of its elements ( Fig. 4C). The inner crossed lamellar layer is well organized in lamellae of first, second, and third order, with needles of the first order usually not intersecting. The shell of Helcion pellucidus ( Linnaeus, 1758) from holdfasts on brown algae of the intertidal zone of the Bretagne ( France) also has a thick outer layer composed of inclined sheets, massive in the outermost layer and spherulitic below ( Fig. 4D). The crossed lamellar layer forming most of the inner shell is similar to that of Blinia with crossed lamellae intersecting the individual needles.

The combination of features of the shell layers in Blinia closely resembles that of a patellogastropod limpet of similar shell morphology from the Paleocene of Alabama (personal observations by KB), but that species has a typical patellid protoconch, resembling that of Tectura zboroviensis described here.

Descriptions and illustrations of all species studied viz. Blinia angulata (d’Orbigny, 1844) , Blinia pseudolaevigata ( Sinzov, 1892) , and Tectura zboroviensis (Friedberg, 1928) are presented below to demonstrate the differences between their protoconchs and corresponding inferred types of early ontogeny.

Blinia angulata (d’Orbigny in Hommaire de Hell, 1844) Fig. 5A .

1844 Helcion angulata sp. nov.; d’Orbigny in Hommaire de Hell 1844: 470, pl. 4: 13–15.

1935 Acmaea angulata (d’Orbigny, 1844) ; Kolesnikov 1935: 128–129, pl. 19: 12–15.

2000 Tectura (Tectura) angulata (d’Orbigny, 1844) ; Anistratenko 2000a: 33–39, pl. 2: 1a, 1b.

Type material: Repository unknown.

Material.— Twenty one specimens from middle Sarmatian of Letichev locality ( IGS NANU 1 /2004–17/2004), and lower Sarmatian of Shirokoe locality ( IGS NANU 1 /1999– 4/1999) ,

Ukraine. Description.—The shell is moderate in size, thin, low conical in lateral profile and oval to egg−shaped in dorsal view. The apex is subcentral, somewhat tilted forwards. The apical angle measured in anterior view varies from 120 ° to 140 °. Posterior and anterior slopes are straight or slightly convex. Sculpture consists of numerous, weak but clearly visible radial ribs crossed by concentric lines of growth. The outer, thin, sculptured layer of a shell is often peeled off, and in that case the surface of teleoconch appears to be totally smooth. The aperture is egg−shaped to broadly oval.

The protoconch is round to oval, pancake−like, almost flat and smooth. The greater diameter of the embryonic shell (i.e., “pancake”) measures about 0.16 mm; symmetrically located in the sagittal axis of the teleoconch. The protoconchteleoconch transition is usually sharp, marked by a clear constriction or rim.

Dimensions.—Length of shell (of material studied)—up to 13.5 mm; width of shell—up to 11.0 mm; height of shell—up to 6.5 mm.

Discussion.— Blinia angulata differs from Blinia pseudolaevigata by having a larger and relatively lower shell with a somewhat mammillated apex. Stratigraphic and geographic range.—This species represents the most abundant and widespread Blinia species in Sarmatian deposits of the Eastern Paratethys (d’Orbigny in Hommaire de Hell 1844; Sinzov 1892; Friedberg 1911 –1928; Kolesnikov 1935; Anistratenko 2000a).

Figures

Fig. 3. A sketch drawing of the inner shell surface of Blinia sp. with muscle scars indicated by arrows. Specimen (IGS NANU 88/2004) from the middle Sarmatian of Letichev. Anterior edge of a shell is on the top.

Fig. 5. Blinia gen. nov. and Tectura Gray, 1847 from Miocene of Ukraine. A. Blinia angulata (d’Orbigny, 1844). Specimen (IGS NANU 1/2004) from the middle Sarmatian of Letichev, apical view (A1). Lateral view of the same specimen (A2). B. Blinia pseudolaevigata (Sinzov, 1892). Specimen (IGS NANU 41/2004) from the middle Sarmatian of Letichev, apical view (B1). Lateral view of the same specimen (B2). Detailed apical view of the embryonic shell and juvenile teleoconch of the same specimen as in B1 (B3). The embryonic shell is ornamented by fine concentric wrinkles and pit on the apex. C. Tectura zboroviensis (Friedberg, 1928). Specimen (IGS NANU 18/2003) from the Chokrakian of Yurkino, apical view (C1). Lateral and frontal views of the same specimen (C2, C3). Detailed apical view of the embryonic shell and juvenile teleoconch of the same specimen as in C1 (C4). The onset of juvenile teleoconch is ornamented by fine spiral threads. D. Blinia sp. Specimen (IGS NANU 4/2004) from the middle Sarmatian of Letichev. Detailed apical view of the embryonic shell and juvenile teleoconch.