Micronautilus evolutus, 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 : 482-484

Micronautilus evolutus n. gen., n. sp.

( Figs 1-5; Appendix 1 A-E)

urn:lsid:zoobank.org:act:FA6CB531-2520-41D7-B6B9-758B95FB3984

? Pictonautilus verciacensis (Lissajous) – Branger 2004: 143, pl. 1, fig. 3.

Pictonautilus sp. ? – Grulke 2016: 141.

DIAGNOSIS. — Small sized nautilid (less than 45 mm in diameter) with a compressed and subrectangular section of the shell. The umbilical area is always open and quite wide, up to 20% of shell diameter. Septa are relatively closely spaced and sutures exhibit a shallow lateral lobe.

TYPE MATERIAL. — Holotype N103 from Buffevent quarry, Niort , Deux-Sèvres, France. It is figured in Fig. 1; Appendix 1A.

Eight paratype specimens (figured in Fig. 2; Appendix 1 B-E) obtained from several localities ( Fig. 1) have also been studied: 2021. BR.N.091 , 2021.BR.N.103 , 2021.BR.N.136 , 2021. BR.N.137, 2021. BR.N.138 are from Buffevent quarry near Niort , 2021. BR.N.139 is from Jard-sur-Mer (Vendée ) and 2021.BR.N.066 from Saint-Maixent-l’École .

TYPE LOCALITY AND STRATUM TYPICUM. — Micronautilus evolutus

n. gen., n. sp. is relatively common at the top of the Calcaire à silex formation ( Welsch 1903) of late Bathonian (Retrocostatum Zone) age in Western France, especially in the vicinity of Niort. The taxon has not yet been described from any other area.

MEASUREMENTS. — see Table 1.

ETYMOLOGY. — The specific name evolutus relates to the relatively wide umbilicus of the shell.

DESCRIPTION Micronautilus evolutus n. gen., n. sp. is a very distinctive species characterized by a very small size at adult stage, ranging from 30 mm (estimated diameter) for 2021.BR.N.138 ( Fig. 3 A-C; Appendix 1C) up to 44 mm in 2021.BR.N.136 ( Fig. 5A, B; Appendix 1E). The individuals are relatively evolute for Jurassic nautilids. The umbilicus is widely open since U/D can reach 20% of the diameter on full grown shells. At the very beginning of the embryonic shell, the whorl section is rounded and, until a diameter of about 1 cm is reached, the shell displays reticulate ornamentation. This diameter seems to be approximately the hatching size of the animal according to the weak depression, corresponding to the nepionic constriction, observed on inner whorls of the holotype (2021.BR.N.103). Further adoral, the whorl section becomes sub-rectangular to sub-trapezoidal, compressed with a maximum width located close to the umbilicus. Flanks are flat, sub-parallel, boarded by angular but never sharp ventral edges; the wall of the umbilicus is sub-vertical. The venter is very slightly rounded on the inner whorls and flattens before the beginning of the body chamber, tending to be a little concave on stratigraphically younger specimens. The shell is narrowly camerate, with twelve to thirteen chambers occurring in each half of a whorl when adult. The suture lines are partly hidden on the holotype (2021.BR.N.103) but, on the paratypes, exhibit wide and shallow lateral and ventral lobes ( Fig. 3). The hyponomic sinus of the shell is quite deep and narrow. The position of the siphuncle has not been observed.

REMARKS

As discussed above, apart from the two specimens figured under the name of? Pictonautilus verciacensis ( Branger, 2004) and Pictonautilus sp. ( Grulke 2016: 141), Micronautilus evolutus n. gen., n. sp. has not previously been illustrated. At the same stratigraphic level, the closest small sized nautilid with a compressed shell belongs to the earliest representatives of Pseudaganides (ongoing studies from personal data). This genus is not considered here to be closely related due to its closed umbilicus and suture lines that are much more sinuous than Micronautilus n. gen.

Despite of their small size, the specimens of M. evolutus n. gen., n. sp. are regarded as mature shells because of two reasons. The first one is related to the crowding of the last septum at the end of the phragmocone. The septal spacing has been measured on four specimens, 2021.BR.N.066, 2021. BR.N.091, 2021.BR.N.136 and 2021.BR.N.138 ( Figs 4; 5) according the method used by Kraft et al. (2008). As inner whorls are small and hidden by overlapping whorls, they cannot be observed clearly and the measurements have been only done from 180° to 20° before the last septum. All of them show an angular distance that varies from 12 to 20° ( Fig. 4). The two last one are approximated like on the modern nautiluses, the angular distance falls under 10°. The second indication of maturity is shown by the increase in the relative umbilical width close to adulthood size when U/D reaches 20% on the largest specimens (Table 1; Fig. 6).