Gzheloceratidae Ruzhencev & Shimansky 1954

Family Gzheloceratidae Ruzhencev & Shimansky, 1954 Fig. 25A Diagnosis Family of the superfamily Pleuronautiloidea with a rather small conch and a weakly depressed, elliptical or reniform whorl profile. Venter flattened or slightly convex; ventrolateral shoulder, flanks and umbilical margin usually broadly rounded. Sculpture with short ribs or transversely elongated nodes on the flank. Suture line with shallow lobes and low saddles. Internal lobe very shallow, without annular process (after Ruzhencev & Shimansky 1954). Included genera Parametacoceras Miller & Owen, 1934 (Bashkirian to Moscovian; 7 species). Gzheloceras Ruzhencev & Shimansky, 1954 (Gzhelian to Artinskian; 14 species). Heurekoceras Ruzhencev & Shimansky, 1954 (Artinskian; 1 species). Celox Shimansky, 1967 (Viséan to Bashkirian; 3 species). Pseudogzheloceras Dernov, 2021 (Bashkirian to Kasimovian; 12 species). Remarks The evolutionary history of the family Gzheloceratidae seems to have extended from the Viséan of the Early Carboniferous to the Early Permian, if not longer. During this long period, only few morphological changes occurred, both in the shape of the conch and in the sculpture. The species of the family show a very conservative shape of the whorl profile, which is either depressed elliptical or reniform. There is neither an angular umbilical margin nor a prominent ventrolateral shoulder developed. The sculpture consists of simple ribs or nodes on the flank (Ruzhencev & Shimansky 1954). The shape of the juvenile conch is similar to that of all other members of the superfamily Pleuronautiloidea, with the exception of the family Mosquoceratidae, which have a much larger and stouter initial conch. With an elliptical or strongly rounded trapezoidal whorl profile, the adult conch of the Mosquoceratidae has a different shape to the juvenile. Another difference from the other families is the almost straight suture line in the Gzheloceratidae.