Specimen ZPAL Ga.9/174 (formerly described as Pusillina sp.) from the Oxfordian (Late Jurassic) of  Kłęby outcrop, in lateral and apical view.

 Specimen BGR X 12923 from the Oxfordian of  Kłęby outcrop in Lateral and Apical view.

Diagnoza

The shell shape varies from elongate to conical. The teleoconch starts with 13-14 straight axial ribs per whorl that run from suture to suture with equal prominence. The number of ribs decreases during ontogeny to 10-12 per whorl and their interspaces widen. The ribs are opisthocline and opisthocyrt. The axial ribs are more prominent on the adapical part and disappear gradually abapicalwards. This asymmetry varies from distinct to almost unrecognisable. The axial ribs fade out when entering the convex base. The wide-oval aperture is somewhat pointed posteriorly. The terminal thickening of the outer lip is not visible on the specimens examined.

Porównanie

This species is apparently the oldest known Rissoa-like species that differs from species of Palaeorissoina Gründel, 1999 in its shorter protoconch (only 2.5 whorls compared to 3.5-4 in Palaeorissoina), in lacking spiral lirae and axial ribs on the last part of the protoconch, and in the absence of spiral lirae on the teleoconch. Some specimens of R. valfini differ from the Recent species of Rissoa in possessing sturdy axial ribs that are pointed adapically; however, in some other specimens this character is absent. Moreover, the axial ribs are usually blunt in the Recent species and accompanied by spiral ornamentation.

Autekologia

Występowanie geograficzne

France; in Poland only in Kłęby (Western Pomerania)

Zasięg czasowy

mid to late Oxfordian

Materiały muzealne

Three specimens (BGR X 05189-91) in Schmidt (1905) collection and about 250 specimens (BGR, Buchholz, Beckert, GG, and ZPAL).

Literatura

Dmoch, I. 1971. Osady i fauna górnojurajska z Kłębów na Pomorzu Zachodnim. Studia Societatis Scientarum Torunensis. Sectio C (Geographia et Geologia), 7, 1‒60.

Gründel, J & Kaim, A. 2006. Shallow-water gastropods from Late Oxfordian sands in Kłęby (Pomerania, Poland). Acta Geologica Polonica 56, 2, 121‒157.

Guirand, E. & Ogérien, J. A. C. E.  1865. Quelques fossiles nouveaux du Corallien de Valfin. Mémoires de la Société d'émulation du Jura de l'année 1865, 367‒394

Kaim 2004. The evolution of conch ontogeny in Mesozoic open sea gastropods. Palaeontologia Polonica, 62, 3‒183.

Schmidt, M. 1905. Über Oberen Jura in Pommern. Abhandlungen der Königlich Preußsischen Geologischen Landesanstalt und Bergakademie, Neue Folge, 41, 1‒222.