Egoianiceras lautum ( GLAZUNOVA , 1953)

Frau, Camille, 2023, Upper Aptian Ammonites From Roussillon, Southern France, Fossil Imprint 79 (1), pp. 89-101 : 96-97

Egoianiceras lautum ( GLAZUNOVA, 1953)

Text-fig. 4g –k

1913 Acanthohoplites aschiltaensis (ANTHULA) var. aplanata SINZOW ; Sinzow, p. 111, pl. 6, fig. 4.

1953 Acanthoplites lautus GLAZUNOVA , p. 37, pl. V, figs 9a, b, 10, 11a, b, text-fig. 13.

1953 Acanthoplites lautum var. laxa GLAZUNOVA , p. 38, pl. V, figs 12a–c, 13, 14, 15, text-fig. 14.

1953 Acanthoplites bigoti var. incivilis var. nov. GLAZUNOVA, p. 36, pl. V, fig. 7a–v.

1953 Acanthoplites bigoti SEUNES ; Glazunova, p. 35, pl. V, figs 3, 4, 5a, b, 6, text-fig. 11.

1967 Gargasiceras lautum lautum (GLAZUNOVA) ; Dimitrova, p. 189, pl. LXXXIX, fig. 10.

1967 Gargasiceras lautum laxa (GLAZUNOVA) ; Dimitrova, p. 189, pl. XC, fig. 9.

?1967 Gargasiceras aptiense (ROCH) ; Dimitrova, p. 189, pl. 90, fig. 10.

?1987 Acanthohoplites lautus (GLAZUNOVA) ; Leshchukh, p. 129, pl. 15, figs 6, 7.

2008 “ Protacanthoplites ” aff. lautus (GLAZUNOVA) ; Moreno-Bedmar et al., fig. 1.

Ty p e. There is no typification made in the literature, but Dimitrova (1967) and Leshchukh (1987) made repeated reference to Glazunova’s specimen ( Glazunova 1953: pl. V, fig. 9a–v) from the upper Aptian of the western Kopet Dag, Turkmenistan. It is designated here as a lectotype, in agreement with Article 72 of the ICZN.

M a t e r i a l. Five specimens LGB-ROU.13 ( Text-fig. 4g ), LGB-ROU.11b ( Text-fig. 4h ), LGB-ROU.10b ( Text-fig. 4i ), LGB-ROU.3 ( Text-fig. 4j ), and LGB-ROU.12a ( Text-fig. 4k ).

D e s c r i p t i o n. Specimens LGB-ROU.3 and LGB-ROU.12a are small, nearly complete, evolute ammonites. The body chamber occupies half of the outer whorl in the former specimen. The ontogeny starts with a densely ribbed Gargasensis juvenile stage over one whorl. The transition with the Crassicostatum sub-adult stage is worn, followed by a typical Tobleri stage on the body chamber in specimen LGB-ROU.3.

Specimen LGB-ROU.10b is a very small, complete, evolute ammonite, with a body chamber occupying half of the outer whorl. The inner whorls are worn, followed by moderately dense Gargasensis juvenile stage over one and a half whorls. This changes progressively into a short Crassicostatum sub-adult. Bifurcate primary ribs bear discrete points of furcation high on the flank. The transition toward the Tobleri stage is worn. The latter is somewhat erratic, marked by dominant simple ribs, irregular in thickness, and irregularly disposed. The peristome seems to be present and composed of two smooth interspaces separated by distant single atuberculate ribs.

Specimen LGB-ROU.11b is a slightly distorted ammonite showing a dense Tobleri stage over its body chamber, occupying half of the outer whorl.

D i s c u s s i o n. The plexus Gargasiceras lautum lautum ( GLAZUNOVA, 1953) and Gargasiceras lautum laxa ( GLAZUNOVA, 1953) from the upper Aptian of Mangyshlak were tentatively re-assigned to Egoianiceras by Frau et al. (2020). Indeed, they show close ornamental affinities with the juveniles of Egoianiceras angulatum ( EGOIAN, 1969) (compare with Glazunova 1953: pl. V, fig. 13 for example), but differ in a smaller adult size (D ~ 30 mm), an early acquisition of a rounded whorl section, and a more densely ribbed Gargasensis and Crassicostatum stage, in which primary ribs are discrete or erased. Differences between these two subspecies are found in rib density and its extent in the adult. Since both subspecies co-occur in Kopet Dag ( Glazunova 1953), Bulgaria ( Dimitrova 1967), SE France (this work), and possibly in Spain ( Moreno-Bedmar et al. 2008), it is tempting to consider that they belong to the same variation series, and are here gathered into Egoianiceras lautum ( GLAZUNOVA, 1953) . The Tobleri stage is more often irregular than in E. angulatum . Here we identify a few specimens from the Kopet Dag (e.g., Glazunova 1953: pl. V, fig. 7) that are intermediate in ribbing between Egoianiceras lautum ( GLAZUNOVA, 1953) and Egoianiceras angulatum ( EGOIAN, 1969) . Similar forms are found at Roussillon (e.g., Text-fig. 4g ), reinforcing the view that both taxa may correspond to two extreme morphotypes of the same species.

O c c u r r e n c e. The type material of Egoianiceras lautum ( GLAZUNOVA, 1953) comes from the Parahoplites melchioris Subzone of western Kopet Dag ( Glazunova 1953), and was later reported from the same zone in Bulgaria ( Dimitrova 1967), SE Spain ( Moreno-Bedmar et al. 2008), and doubtfully in Crimea ( Leshchukh 1987).

Figures

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