Manzonia (Alvinia) miocrassicosta (Sacco, 1895)

Kowalke, Thorsten & Harzhauser, Mathias, 2004, Early ontogeny and palaeoecology of the Mid-Miocene rissoid gastropods of the Central Paratethys, Acta Palaeontologica Polonica 49 (1), pp. 111-134 : 124

Manzonia (Alvinia) miocrassicosta (Sacco, 1895)

Fig. 8A .

Rissoa zetlandica Montagu ; Hörnes 1856: 566, pl. 48: 11.

Manzonia zetlandica var. miocrassicosta Sacco 1895: 30 .

Manzonia (Taramellia) zetlandica Montagu ; Meznerics 1933: 332.

Flemingia zetlandica var. miocrassicosta Sacco ; Ferrero Mortara et al. 1984: pl. 39: 6.

Material.— Fifty−three specimens from the Badenian of

Steinebrunn, Lower Austria, NHMW 2002z0030/0016.

Description.—The small, thick, egg−shaped shell has 5–6

slightly rounded whorls measuring up to 3 mm in height. The last whorl forms about 70% of total shell height. The teleoconch sculpture consists of 12 strongly developed straight axial ribs, crossed by 4 slightly weaker spiral keels. The points of intersection of the axial and spiral sculpture are slightly thickened. The base is sculptured by two prominent spiral keels. The thickened, holostomatous aperture is rounded to egg shaped. It is characterised by a slightly pointed posterior portion and by a prominent circular pad.

The protoconch comprises 2.4 slightly rounded whorls measuring 0.3 mm in height and 0.36 mm in maximum diameter. The embryonic shell comprises nearly one inflated whorl measuring 0.06–0.07 mm in the width of the initial cap and 0.16–0.17 mm in maximum diameter. Sculpture consists of 6 tuberculate spiral threads. The embryonic shell is well demarcated from the subsequent larval shell by a thickened rim. The larval shell is sculptured by remains of tuberculate spiral threads. The protoconch is terminated by a short sinusigera notch. The onset of the teleoconch is indicated by the formation of the regularly reticulate adult sculpture.

Remarks.—The protoconch indicates indirect development including a free planktotrophic larval stage. The inflated embryonic shell reflects a yolk−rich embryogenesis. The protoconch is very similar to that of Recent Alvania spp. , especially to A. (A.) punctura with similar dimensions, which differs by weaker−developed embryonic sculpture, but stronger−developed larval sculpture (see Kowalke 1998: pl. 9: 9). The type species Manzonia (Alvinia) weinkauffi differs by its direct mode of early ontogenetic development and by its teleoconch sculpture of more but weaker−developed axial ribs ( Ponder 1985: fig. 102A, B). Manzonia (Alvinia) miocrassicosta differs from Recent M. (A.) zetlandica by having less but slightly stronger−developed axial ribs (see Ponder 1985: fig. 100D). The shell shape is additionally less acute than that of M. (A.) zetlandica . A species with very similar protoconch, described as Flemingia zetlandica var. perraricincta Sacco, 1895 from the Early Pliocene of Italy, differs by having fewer but more strongly developed axial ribs; they form short spines in the points of intersection with the spiral sculpture ( Ferrero Mortara et al. 1984: pl. 39: 7a, b).

Manzonia (Alvinia) miocrassicosta is known so far from the Central Paratethys only from the Vienna Basin ( Austria, Czech Republic) and Romania. Specimens from the Tortonian of the Mediterranean as described by Ferrero Mortara et al. (1984) could not be differentiated from M. (A.) miocrassicosta based on conchological considerations.

Figures

Fig. 8. Two species of Manzonia (Alvinia) Monterosato, 1884 and two of Manzonia (Manzonia) Brusina, 1870. A. Manzonia (Alvinia) miocrassicosta (Sacco, 1895). Specimen (NHMW 2002z0030/0016) from the Badenian of Steinebrunn (A1). Lateral and apical view of the protoconch of the same specimen (A2, A3). The protoconch is terminated by a short sinusigera notch. Detailed view of the initial whorl of the same protoconch as in A3 (A4). The embryonic shell is demarcated from the subsequent larval shell by a well developed thickened rim. B. Manzonia (Alvinia) partschi (Hörnes, 1856). Specimen (NHMW 2002z0030/0017) from the Badenian of Baden/Sooss, Lower Austria (B1). Lateral and apical view of the protoconch of the same specimen as in B1 (B2, B3). The protoconch is terminated by a short, slightly thickened sinusigera notch. Detailed view of the initial whorl of the same protoconch as in B3 (B4). The embryonic shell is strongly sculptured and terminated by a slightly sinuous rim. C. Manzonia (Manzonia) crassa (Kanmacher in J. Adams, 1798), specimen (NHMW 2002z0030/0018) from the Badenian of Niederleis, Lower Austria. D. Manzonia (Manzonia) scalaris (Dubois, 1831). Specimen (NHMW 2002z0030/0019) from the Badenian of Steinebrunn, Lower Austria (D1). Lateral and apical view of the protoconch of the same specimen as in D1 (D2, D3). The protoconch is terminated by a well developed sinusigera notch. Detailed view of the initial whorl of the same protoconch as in D3 (D4). The transition to the larval shell is indicated by a thickened rim.