| Measurements | D mm | H/D | T/D | O/D | H/T |
|---|---|---|---|---|---|
| RJ-365 | 35.7 (32.4) | 0.39 | 0.47 | 0.36 | 0.83 |
| Lectotype | 40 | 0.40 | 0.45 | 0.32 | 0.89 |
| Kennedy & Klinger 2023 | 95 | 0.34 | 0.34 | 0.39 | 1.00 |
| Kennedy & Klinger 2023 | 135 | 0.37 | 0.32 | 0.41 | 1.14 |
| Age | Origin |
|---|---|
|
Phosphatic-glauconitic sand cristatum to inflatum zones Upper Albian condensed level |
Disused phosphate mine Valserhône (ex-Bellegarde) Ain, France |
Description. Glauconitic sandstone ammonite covered with a beige phosphate shell, without visible sutures. Whorls 20% covered, with a subcircular section, a flattened venter, and weak umbilical and ventrolateral shoulders. The umbilicus has a high wall, initially almost straight at 70°, then broadly rounded towards the flank. The ribs originating at the umbilical suture are very irregular. Some are slightly rursiradiate and concave, others folded forward near venter, others are both sigmoid and rursiradiate, resulting in unequal intercostal spaces. A few higher ribs bifurcate at varying heights in the middle third of the flank, with a small, prominent tubercle at the point of division. The highest bifurcating ribs also have a small umbilical bulla. Two ribs are even observed to merge. The ribs reach the base of the low, fine, and sharp ventral carina, forming a 110° chevron. There are approximately 40 ribs on venter. The venter is slightly grooved on each side of the carina.
Remarks. This ammonite, of highly variable thickness according to the table, defines the first zone of Lower Albian in the Paris Basin. It is also described in Matrion & Touch (1997) and Kennedy & Klinger (2023). See the entry for Dipoloceras bouchardianum for the differences between the two species. Our specimen has rather inconspicuous ornamentation: there are specimens of Dipoloceras cristatum with more pronounced hypertrophied ribs and a venter that is more deeply hollowed along the keel.