Aktubonautilidae Ruzhencev & Shimansky 1954

Family Aktubonautilidae Ruzhencev & Shimansky, 1954 Fig. 25B Diagnosis Family of the superfamily Pleuronautiloidea with a large, stout first whorl. Whorl profile weakly depressed, semicircular or reniform. Venter, flanks and umbilical margin broadly rounded. Sculpture with lateral, transversely elongated nodes. Suture line with a shallow external, a very shallow lateral and a rather deep internal lobe; without annular process (after Ruzhencev & Shimansky 1954). Included genera Aktubonautilus Ruzhencev & Shimansky, 1954 (Artinskian; 1 species). Basleonautilus Ruzhencev & Shimansky, 1954 (Roadian; 1 species). Remarks The family Aktubonautilidae is closest to the Rhiphaeoceratidae; there are similarities in the shape of the conch, the course of the suture line and partly in the sculpture. However, both families differ significantly in the morphology of the juvenile conch. The Aktubonautilidae are characterised by a stout first volution, whereas the Rhiphaeoceratidae have a slender, worm-like first volution. In this respect, the Aktubonautilidae are more similar to the Mosquoceratidae; however, this family differs greatly in the type of sculpture and suture line. The sculpture is formed by lateral transverse tubercles or ribs in the Aktubonautilidae but by longitudinal oval tubercles along the ventrolateral shoulder in Mosquoceratidae. In contrast to the Mosquoceratidae, the suture line in the Aktubonautilidae has only very shallow lateral lobes.