Pseudaganides dimidiatus Marchand & Tintant, 1971

Branger, Patrick, 2024, Antaganides n. gen., a new genus of Middle Jurassic nautiloid, ancestor of Pseudaganides Spath, 1927, with discussion on the phyletic origin of the family Pseudonautilidae, Geodiversitas 46 (14), pp. 471-493 : 487

Pseudaganides dimidiatus Marchand & Tintant, 1971 (Fig. 14B1-B3)

Pseudaganides subbiangulatus subsp. dimidiatuo Marchand & Tintant, 1971: 150 , pl. II, fig. 1a-c, fig. III, pl. 6.

Nautilus subbiangulatus – Couffon 1919: pl. XVII, figs 7a-7b.

Pseudaganides dimidiatus – Tintant 1994: 36.

MEASUREMENTS. — See Table 9.

DESCRIPTION

According to Marchand &Tintant (1971), these are small sized individuals, less than 80 mm in diameter, with a compressed and high section. Flanks are flat, becoming slightly rounded on the body-chamber. Suture line with a wide lateral saddle, not as deep as the external one.

REMARKS

As pointed byMarchand & Tintant (1971), P.dimidiatus differs from P. subbiangulatus in several ways. Besides its smaller size, the existence of quite sharp ridges at an earlier stage proves that it is a different species. All the other characteristics of the shell are the same. According to these authors, the age of the two species were different. New in-situ collecting demonstrates that this point of view is probably erroneous. Those authors also assumed that, in the collections, P. dimidiatus was rather common beside the total lack of any other sample of P. subbiangulatus . This study does not confirm this opinion since, at the same level, we collected more P. subbiangulatus than P. dimidiatus . Therefore, another explanation which could be considered would be a possible sexual dimorphism. This phenomenon has been described by several authors in modern Nautilus . Willey (1902) was the first biologist who noticed that males are larger and have a broader aperture than females. More recent studies ( Saunders & Spinosa 1978; Saunders et al. 2017) lead to the same conclusions. Consequently, we suggest that P. dimidiatus , the smallest shell, could be the presumed female and P. subbiangulatus , the largest, could be the male of a single species.