Sisonia ultimoi sp. nov., 2023

Kiel, Steffen, Sami, Marco & Taviani, Marco, 2023, Mollusks (Gastropoda, Bivalvia) from Miocene cold-seep deposits in northern Italy: revisions and additions, European Journal of Taxonomy 910, pp. 115-160 : 148-149

Sisonia ultimoi sp. nov.

urn:lsid:zoobank.org:act:C6300134-6227-42D0-94C9-EED1086B8634

Fig. 13

Diagnosis

Shell small for genus; only moderately inflated; posterior ridge present but not very distinct, centralposterior sulcus weakly developed; umbones in anterior third; regular, fine commarginal ribs.

Etymology

Named for Ultimo Bazzani (Castiglione dei Pepoli), who collected many of the specimens studied here.

Material examined

Holotype

ITALY – Tuscany • Ca’ Cavalmagra ; MGGC 22330 .

Paratypes ITALY – Tuscany • 10 specs; Ca’ Cavalmagra ; MSF 1265 (2 specs, MSF 1265 a, L = 9.3 mm, MSF 1265 b, L = 13.8 mm), MSF 1278 (26.3 mm), MSF 1280 (18.3 mm), MSF 1281 (19 mm), MSF 1235 (5 specs) .

Description

Shell small for genus, reaching 26.3 mm in length, trapezoid to cunaeiform, moderately inflated with maximum inflation mid-shell at the umbones; umbones in anterior third of shell, moderately elevated, pointed, orthogyrate to very slightly prosogyrate; blunt ridge running from umbo to posteroventral margin, with weak sulcus to its anterior; ventral margin convex to straight; no lunule, escutcheon lanceolate, short; shell surface covered by numerous weak but distinct and regularly spaced commarginal ribs; interior covered by fine, radial striations. The largest articulated specimen, with broken posterior margin, is 16 mm high and 11 mm wide.

Remarks

Compared to the Late Miocene Philippine type species Sisonia frijellanae ( Kiel et al. 2022) , Sisonia ultimoi sp. nov. is much smaller (max. 20 mm vs 68.7 mm in length), is posteriorly less inflated, and the median sulcus is weakly developed and does not always produce a concavity in the ventral margin.

Stratigraphic and geographic range

Middle Miocene (upper Langhian), northern Italy.

Figures

Fig. 13. The bivalve Sisonia ultimoi sp. nov. from the Middle Miocene Ca’ Cavalmagra seep deposit in northern Italy. A. Left valve (MSF 1280). B. Small right valve (MSF 1265). C. Left valve (MSF 2372). D. Large right valve with severe shell injury (MSF 1278). E. Right valve (MSF 1281). F–G. Articulated specimen with broken posterior end (MSF 2367). H–I. Holotype (MGGC 22330), articulated specimen showing outline of shell. J–K. Paratype, articulated specimen with deformed posterior side, showing the regular commarginal ornament; this specimen preserves the most realistic inflation of all available specimens (MSF 2368). L–M. Articulated specimen (MSF 2369). N–O. Articulated specimen showing fine internal striation (MSF 2370). P–Q. Deformed specimen showing the regular commarginal ornament (MSF 2371).