Sisonia ultimoi Kiel & Sami & Taviani 2023, sp. nov.

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.