| Measurements | D mm | H/D | T/D | O/D | H/T |
|---|---|---|---|---|---|
| CP-339 | 29.7 | 0.48 | 0.43 | 0.27 | 1.13 |
| Collet 1907 | 33.0 | 0.45 | 0.36 | 0.33 | 1.25 |
| Collet 1907 | 54.0 | 0.38 | 0.30 | 0.31 | 1.31 |
| Casey 1965 | 72.0 | 0.38 | 0.31 | 0.34 | 1.22 |
| Age | Origin |
|---|---|
|
Quarry of grey clay schrammeni zone Late Aptian-Basal Albian |
Vöhrum Lower Saxony Germany |
Description. A beautiful, small, septate specimen with a pearly test. The coiling is somewhat evolute, with a whorl overlap of 40%. The slightly compressed section resembles a rectangle with slightly divergent flanks and truncated upper corners. The maximum thickness is at mid-flank. At the beginning of last whorl, the strong, somewhat flexuous ribs include primaries arising at the umbilical suture and bifurcating on an umbilical bulla, or at mid-flank on a pointed lateral tubercle. These are separated by untuberculate secondaries arising near the edge of umbilicus. Some secondaries arise very close to a primary, simulating trifurcated ribs. On the last quarter of whorl, the primaries become simple and alternate with one (sometimes two) secondary ribs, arising in the inner half of the flanks. In total, 38 ribs cross the venter in a straight line, with a perfectly horizontal crest and slight swellings on the shoulders. The lateral tubercles disappear on the last two primaries.
Remarks. This species differs from other Hypacanthoplites from Vöhrum by stronger ribs, which quickly develop an alternation between long and short. It thins with age and loses its lateral tubercles around 30 mm, the umbilical tubercles around 35 mm, and the ventral nodes around 40–45 mm (Casey, 1965). It resembles H. jacobi (Collet, 1907), which is a bit more compressed, with slightly stronger ribs and lateral tubercles vanishing earlier. Casey (1965) and Mutterlose et al. (2003) maintain the two species. Here we follow Kennedy (2000), who considers H. jacobi as a junior synonym of H. plesiotypicus because of intermediate forms. The name jacobi, however, is retained as the name of the last ammonite zone of German Aptian. We include H. plesiotypicus among our Albian ammonites because it is also found in the Proleymeriella schrammeni zone, which marks the beginning of the Albian at Vöhrum (Mutterlose et al., 2003). But we do not know the exact collection level of our specimen.