Micronautilus, Branger, 2023

Branger, Patrick, 2023, Micronautilus n. gen., a new dwarf Bathonian (Middle Jurassic) nautilid from Western France, Comptes Rendus Palevol 22 (22), pp. 479-490 : 480-482

Genus Micronautilus n. gen.

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DIAGNOSIS. — Small sized adult nautilids (less than 60 mm in diameter) with a generally compressed shell showing a quite deep and narrow hyponomic sinus. The whorl section may be oval on the phragmocone, becoming subrectangular to subquadrate, depending on the species, rarely depressed at adult stage. The umbilicus is open with an umbilical area up to 20% of shell diameter, boarded by a subvertical wall. Septa are relatively close, mostly on the typespecies; sutures always exhibit a shallow lateral lobe ( Fig. 3 A-C).

TYPE SPECIES. — Micronautilus evolutus n. sp. ( Figs 1-5; Appendix 1).

COMPOSITION OF THE GENUS. — Micronautilus evolutus n. gen., n. sp. (type) and Micronautilus minoti n. gen., n. sp.

ETYMOLOGY. — The generic name Micronautilus refers to the very small diameter of these nautilid shells.

REMARKS

On account of its small size and unique shell shape, Micronautilus n. gen. cannot be mistaken with any other contemporaneous Jurassic nautilids and its taxonomic validity is unquestionable. At the same stratigraphic level, two other nautilid genera exhibit a compressed shell form: the earliest representatives of Pseudaganides Spath, 1927 ( Figs 1-3; Appendix 2A) and Pictonautilus ( Figs 1-3; Appendix 2B). However, both these genera are typically slightly larger than Micronautilus n. gen., moreover Pseudaganides exhibits a closed umbilicus (U/D = 0). Furthermore, as the suture lines of Pseudaganides are much more sinuous than Micronautilus n. gen. ( Fig. 3E), these two taxa are not considered here to be closely related. Micronautilus n. gen. resembles Pictonautilus with regard to its shallow, weakly sinuous sutures ( Fig. 3D). The angular margins of some Micronautilus n. gen. shells are also reminiscent of Pictonautilus but the latter exhibits sharp ventral edges. Coeval Pictonautilus displays an occluded umbilicus and its inner whorls remain rounded in section. Pictonautilus clavifer (Tintant, 1994) , of early Bathonian age possesses a relatively stout, small-sized shell and an open umbilicus. This taxon may be close to an ancestral form of Micronautilus n. gen. that originated during Bajocian time through dwarf species like ‘ Cenoceras’ fuscum ( Crick, 1898).

The important intraspecific variation in shell size of modern Nautilus Linnaeus, 1758 is well known and, despite of these differences, some of them are considered a single species ( Saunders et al. 2017; Tajika et al. 2018). Nowadays the size variations seem to be linked to the geographical distribution of these animals. The fossils described within the present study are all coming from a restricted area, consequently such an intraspecific diversity is doubtful for this material. Moreover, the features of shells are unlike of coeval species and nowhere in the world any similar species have ever been described.

According to the recent classification chart for post-Triassic Nautilida ( King et al. 2016) , Micronautilus n. gen. would logically be included within the family Paracenoceratidae .

OCCURRENCE

Micronautilus n. gen. is only currently known from Bathonian limestones in Western France and ranges from the lower Bathonian Zigzag Zone to the upper Bathonian, Retrocostatum Zone.