Micronautilus minoti, 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 : 484-486

Micronautilus minoti n. gen., n. sp.

( Figs 1-4; 7; Appendices 1F; 3A-D)

urn:lsid:zoobank.org:act:106A89E8-AF1A-459D-9847-F18DC295B3D9

DIAGNOSIS. — Small sized and rather compressed Micronautilus n. gen. with a maximum diameter reaching 60 mm. Shell section is oval on inner whorls becoming subrectangular to subquadrate on full grown individuals. Umbilicus is always open. Septa show a broad and shallow lateral lobe and a ventral lobe.

TYPE MATERIAL. — All the samples have been collected within the southern part of the Deux-Sèvres department ( Fig. 1). Holotype 2021.BR.N.150 is from les Lucs (46°21’27”N, 0°25’31”W), temporary exposure near Échiré, it is figured in Fig. 1; Appendix 3A. Eight paratypes were sampled from various localities. 2021.BR.N.042 ( Fig. 4; Appendix 3D), 2021.BR.N.152 ( Fig. 2; Appendix 3B) and 2021.BR.N.153 are from the old Trotte-Buie quarry, Mougon commune , 2021.BR.N.095 ( Fig. 3 A-C; Appendix 3C) was found in Souché (temporary roadworks near Niort), 2021.BR.N.135 ( Fig. 6; Appendix 1F) and 2021.BR.N.151 come from les Hauts-de-Rochefort (Sainte-Éanne quarry), 2021.BR.N.155 is from Saint-Gelais .

TYPE LOCALITY AND STRATUM TYPICUM. — Most of the specimens have been collected from the “Banc pourri” ( Welsch 1903), a stratigraphic level that extends all along the northern border of the Aquitaine basin . This level has yielded many ammonites, all of them have been reported from the lower Bathonian, Zigzag Zone ( Sauvaget 1906; Énay et al. 2012). A single example, 2021. BR.N.135, a little younger, middle Bathonian, Progracilis Zone, is assigned to the same species.

MEASUREMENTS. — See Table 2.

ETYMOLOGY. — The specific name minoti is dedicated to Jean-Michel Minot, an amateur paleontologist who published the first exhaustive monograph on brachiopods from Western France ( Minot & Branger 2007) and collected some interesting nautilids.

DESCRIPTION

Small sized nautilus, maximum diameter observed is about 60 mm. The section of the shell is compressed, oval on the phragmocone. It widens on the body chamber where the width can exceed slightly the height (W/H = 1.06 on 2021. BR.N.095), getting rectangular, sometimes quadrate. Venter is rounded on the inner whorls but flattens as the animal becomes adult. The umbilicus is always open (U/D = 0.15) but narrower than on Micronautilus evolutus n. gen., n. sp. with an umbilical wall that is subvertical at any stage. The suture line shows a broad and shallow lateral lobe whereas the ventral lobe is faint but clearly visible, at least from a diameter of 15 mm. Each half of a whorl shows nine chambers.

REMARK

Micronautilus minoti n. gen., n. sp. has not previously been figured and no former description corresponds to this new species. The shell of Cenoceras fuscus ( Crick, 1898) , upper Bajocian, is also rather small but larger than Micronautilus minoti n. gen., n. sp. with an open umbilicus. Micronautilus evolutus n. gen., n. sp. possesses a smaller, more compressed shell with a relatively wide umbilicus and more crowded septa. Moreover, its stratigraphical occurrence is younger, namely upper Bathonian. The final two septa in the adult shells of both species are typically approximated with an angular distance lower than 10° instead of 12° to 20° for previous ones.

DWARFISM

Dwarfism has been described from several Jurassic nautilids including Paracenoceras parvulum Tintant, 1984 and Paracenoceras dorsoexcavatum (Parona & Bonarelli, 1895) ( Tintant 1984b) . The description of the smallest taxon ( P. parvulum ) was based on one complete specimen and two body chambers. According to Tintant, this species was adult with a shell diameter of about 30 mm. According to the plates presented in Tintant’s paper, the approximation of the two last septa does not clearly appear. Paracenoceras parvulum has only previously been recorded from Burgundy but is now known to occur at the same stratigraphic level (upper Callovian) in Western France (personal data of the author). These small sized specimens are a little larger than those described by Tintant; one example in the author’s collection shows feature of an adult specimen at a diameter of 55 mm. Another interesting dwarf species is ‘ Cenoceras’ fuscum figured in Chirat (1997: pl. 5, fig. 5a-c), whose adult diameter is 48 mm.

Nevertheless, all these nautilids are larger at adult size than Micronautilus evolutus n. gen., n. sp. The origin of this unusual dwarfism among nautilids has already been discussed by Tintant (1984b). At the same stratigraphic levels, larger shells, up to diameter of 300 mm, are mixed with small to dwarf individuals. This is also true for the accompanying ammonite fauna and the benthic elements, bivalves and gastropods, that shows a mixing of larger and smaller individuals. As the limestone layers where the nautilids are preserved have been deposited within an open shelf environment, those populations were not isolated. Consequently, it seems impossible to explain this case of dwarfism as a result of an highly stressed environment, it has to be considered as a specific feature due to the evolution of an original lineage.

Another explanation which could be evoked would be a possible sexual dimorphism. This phenomenon has been proved by many 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. In comparison, the differences between Micronautilus n. gen. and other Bathonian nautiluses are disproportionate and cannot be the result of any sexual dimorphism.

ORIGIN AND EVOLUTION

As discussed above, dwarf nautiluses were already present during Bajocian times ( Chirat 1997). Micronautilus n. gen. could have evolved from these forms through species like ‘ Cenoceras’ fuscum. All these shells show similarities: small and compressed whorl section, subrectangular section whose venter is boarded by angular wedges. One lineage ( Fig. 7) would be constituted by the following species: ‘ Cenoceras’ fuscum, upper Bajocian, Pictonautilus clavifer (Tintant) , lower Bathonian, Pictonautilus verciacensis (Lissajous) , middle and upper Bathonian, and Pictonautilus sp. (lower Callovian). In the proposed phylogeny, Micronautilus n. gen. could have evolved from ‘ Cenoceras’ fuscum at the very end of the Bajocian and would have disappeared before the beginning of the Callovian stage without any descendants.