Egoianiceras exiquecostatum ( EGOIAN , 1969)

Frau, Camille, 2023, Upper Aptian Ammonites From Roussillon, Southern France, Fossil Imprint 79 (1), pp. 89-101 : 97-98

? Egoianiceras exiquecostatum ( EGOIAN, 1969)

Text-fig. 4l

?1960 Acanthohoplites nolani (SEUNES) ; Kudryavtsev, pl. XIII, fig. 4, text-fig. 117.

non 1968 Acanthohoplites nolani (SEUNES) ; Wiedmann and Dieni, p. 88, pl. IX, figs 10a–c, 17a, b, text-fig. 63a–c.

1969 Acanthohoplites nolani exiquecostata EGOIAN , p. 160, pl. XI, fig. 2a–v.

non 1979 Acanthohoplites nolani exiquecostatus EGOIAN ; Vašíček, p. 331, pl. 1, fig. 1.

non 1983 Acanthohoplites nolani exiquecostatus EGOIAN ; Menčík, pl. XLIII, fig. 2 (= Vašíček, 1979, pl. 1, fig. 1).

?1990 Acanthohoplites nolani exiquecostatus EGOIAN ; Ivanov and Stoykova, pl. I, fig. I.

non 2002 Acanthohoplites nolani exiquecostatus EGOIAN ; Skupien and Vašíček, fig. 7.10 (= Vašíček, 1979, pl. 1, fig. 1).

T y p e. The holotype by monotypy is specimen MEA-243 (Egoian coll.) of Egoian (1969: 160 pl. XI, fig. 2a–v), from the upper Aptian of the Khokodz River Basin, Russia .

M a t e r i a l. A single specimen LGB-ROU.1 ( Text-fig. 4l ).

D e s c r i p t i o n. The specimen corresponds to a small cast of an almost complete acanthohoplitid ammonite. The body chamber occupies half of the outer whorl.The peristome is not preserved. Shell shape is characterised by a very evolute coiling. The first visible whorls develop an apparent compressed, subrectangular whorl section. The ornaments first consist of irregular, fine, simple primary and secondary ribs. Then, the primary ribs strengthen, thicken in the upper flank, and are separated by one or two secondary ribs of variable length and strength. The latter can be coalescent on the primaries at varying heights. This abruptly changes for an alternation of sharp, slightly flexuous primary ribs, and generally one fine secondary rib, starting at varying heights. Primary ribs develop a slight retrocurvature at the umbilical margin. Ribbing progressively changes into a Tobleri adult stage, comprising an alternation of spaced, slightly flexuous flat-topped primary rib, with a clear retrocurvature at the umbilical margin, and generally one secondary rib starting at a variable height on the flank.

D i s c u s s i o n. Based on the description of Egoian (1969), Acanthohoplites nolani exiquecostata EGOIAN, 1969 corresponds to a small acanthohoplitid ammonite with an extremely discoidal (Ww/D ~ 0.32), strongly depressed (Ww/Wh ~ 0.32), very evolute (U/Wh ~ 0.77) subdiscocone coiling (U/D ~ 0.31). The whorl section is compressed and subquadrate, with a flattened venter in the juvenile, becoming suboval with convex flanks in the adult, and a flattened venter. There is no evidence of a ventral furrow. Although crushed, the specimen at our disposal very closely matches Acanthohoplites nolani exiquecostata EGOIAN, 1969 . Other reports in the literature are either doubtful or outright incorrect, and need a revision.

The shell parameters and the character of the sculpture of Acanthohoplites nolani exiquecostata EGOIAN, 1969 strongly deviate from the type species Nolaniceras nolani SEUNES, 1887 as revised by Bulot et al. (2014). The species broadly conforms to the diagnosis of Egoianiceras given above, except that it has a continued flat venter over the outer whorl and a slender juvenile stage lacking strong primary ribs. Also, there is a lower number of secondary ribs than the type species Egoianiceras angulatum . Acanthohoplites nolani exiquecostata EGOIAN, 1969 is tentatively referred to as? Egoianiceras , pending more material to be collected. Finally, it should be noted that the subspecies Acanthohoplites nolani nolani EGOIAN, 1969 non Seunes, 1887, and Acanthohoplites nolani nodosa EGOIAN, 1969 are fairly close to? Egoianiceras exiquecostata ( EGOIAN, 1969) , differing only in a denser ribbing and strengthening of the peri-umbilical retrocurvatures of the primary ribs in the adult. A revision is needed.

O c c u r r e n c e. The species? Egoianiceras exiquecostaum ( EGOIAN, 1969) and the closely-allied forms from western Caucasus were reported from the Acanthohoplites nolani Zone of Egoian (1969). Correlation with the SMAZ is almost impossible, because the affinities and ranges of Caucasian “ Nolaniceras nolani ( SEUNES, 1887) ” remain largely unclear with their west-European counterpart ( Bulot et al. 2014).

Figures

Text-fig. 4. Roussillon’s specimens assigned to Egoianiceras angulatum (EGOIAN, 1969).