Description. Type species Ammonites tardefurcatus d’Orbigny, 1841. According to Wright (1996), this genus comprises small, compressed, and evolute ammonites, showing a tendency to uncoil with growth. The ribs are proverse, concave, or slightly sigmoid, generally simple, flattened, or grooved, but branched in some species. They may or may not be interrupted on venter. Umbilical, lateral, or ventrolateral tubercles may be present. The suture line is rather simple: S, S1, L, and L1 are high, with parallel edges; the saddles are bifid; L and L1 are deep. Lower Albian of Europe, Transcaspia, Iran, Spitsbergen, and Japan.
Subgenera. The subgenus Leymeriella sensu stricto Jacob, 1907, has a high trapezoidal whorl section and simple ribs that do not extend down the umbilical wall. These ribs are proverse, concave, and hollowed by a groove at the top of the flanks, like the beginning of a bifurcation. They terminate in oblique clavi on the ventrolateral shoulders. Hence the ribs do not cross the venter, which is flat or concave. Umbilical tubercles, lateral tubercles, and constrictions are absent.
The subgenus Neoleymeriella Saveliev, 1973, has umbilical or lateral tubercles, at least on the initial whorls, and shallow constrictions on the outer whorls. Its ribs extend down the umbilical wall. Proleymeriella was first created as a separate genus by Breistroffer (1947), for forms with an oval whorl section and simple, strong ribs crossing venter with a pronounced anterior sinus. Only the oldest forms have constrictions. As in Leymeriella, the ribs do not extend down the umbilical wall. The photograph shows Proleymeriella schrammeni (Jacob, 1907). As Brinkmann (1937) demonstrated a rapid and continuous transition from Proleymeriella to Leymeriella, Mutterlose et al. (2003) reclassified the former as a subgenus of the latter. Their view is followed here.
| Leymeriella (1) | (Proleymeriella) schrammeni |