Egoianiceras angulatum ( EGOIAN , 1969)

Frau, Camille, 2023, Upper Aptian Ammonites From Roussillon, Southern France, Fossil Imprint 79 (1), pp. 89-101 : 94-96

Egoianiceras angulatum ( EGOIAN, 1969)

Text-fig. 4a–f

1969 Colombiceras crassicostatum angulata EGOIAN , p. 163, pl. XII, figs. 6a, b, v, 7a, b, v, 7a’, 8a, b, v, 8a’, 9a, b, pl. XXII, fig. 34.

non 1974 Colombiceras (Egoianiceras) cf. angulatum EGOIAN ; Avram, p. 5(3), pl. I, figs 3, 4, text-fig. 1 (=? Egoianiceras multicostatum ).

non 1976 Colombiceras (Egoianiceras) cf. angulatum EGOIAN ; Avram, p. 53(49), pl. VI, figs 2, 3, text-fig. 17 (= Avram 1974, pl. I, figs 3, 4, text-fig. 1).

non 2004 Colombiceras cf. angulatum EGOIAN ; Sharikadze et al., p. 389, pl. 50, fig. 2a–c, pl. 54, fig. 2a–c (= Colombiceras gr. formosum ).

T y p e. Holotype designated by Egoian (1969: 163, pl. XII, fig. 7а–v) is specimen MEA-157 (coll. Egoian) from upper Aptian of Khokodz River Basin, Russia. Material deposited at All Russia Oil and Gas Scientific-Research Institute.

a: Specimen LGB-ROU.5a. b: Specimen LGB-ROU.5b. c: Specimen LGB-ROU.6. d: Specimen LGB-ROU.11a. e: Specimen LGB-ROU.14. f: Specimen LGB-ROU.4; Egoianiceras lautum ( GLAZUNOVA, 1953) . g: Specimen LGB-ROU.13. h: Specimen LGB-ROU.11b. i: Specimen LGB-ROU.10b. j: Specimen LGB-ROU.3. k: Specimen LGB-ROU.12a.? Egoianiceras exiquecostata ( EGOIAN, 1969) . l: Specimen LGB-ROU.1; Acanthohoplites sp. m: Specimen LGB-ROU.11d; Parahoplites sp. n: Specimen LGB-ROU.7. o: Specimen LGB-ROU.8. Scale bar 10 mm.

M a t e r i a l. Ten specimens LGB-ROU.2, LGB-ROU.5a ( Text-fig. 4a ), LGB-ROU.5b ( Text-fig. 4b ), LGB-ROU.6 ( Text-fig. 4c ), LGB-ROU.11a ( Text-fig. 4d ), LGB-ROU.14 ( Text-fig. 4e ), LGB-ROU.4 ( Text-fig. 4f ), LGB-ROU.11e, and doubtfully LGB-ROU.10a and LGB-ROU.11c.

D e s c r i p t i o n. Specimens LGB-ROU.4, LGB-ROU.5a, LGB-ROU.5b, LGB-ROU.11e, and LGB-ROU.14 are small, nearly complete, evolute ammonites. The body chamber occupies almost half of the outer whorl. The first visible whorls of specimen LGB-ROU.5b develop the ammonitella, and then a short Royerianum stage. This is followed by a densely ribbed Gargasense juvenile stage over one and a half whorls in other specimens. This progressively changes into a short Crassicostatum sub-adult stage over one whorl, with the rapid decrease of atuberculate secondaries between enlarged primaries. As growth increases, the ribs become much more spaced and cuneiform on the body chamber (= Tobleri stage), somewhat flat topped over the venter in specimen LGB-ROU.5a.

Specimen LGB.ROU.11a is a moderate-sized, evolute ammonite with an incomplete body chamber occupying half of the last whorl. The first visible whorls of the shell develop the Gargasense juvenile stage along nearly one and half whorls, changing into the Crassicostatum sub-adult stage over one whorl. Primary ribs are distinctly inflated, dominantly bifurcate, bearing strong thickenings on the posterior branches. Secondary ribs pass from four to two as growth increases. Ribbing changes abruptly into the Tobleri adult stage over three-quarters of the outer whorl. The peristome is not preserved. Specimens LGB.ROU.6 and LGB.ROU.12b are similar, but incomplete or badly crushed.

Specimen LGB-ROU.11c is a body chamber fragment of moderate size, showing a Tobleri stage, marked by the alternation of strong, slightly flexuous, primary ribs, with a slight retrocurvature at the umbilical margin and a single secondary starting at mid-flank. All ribs seem to be flat-topped over the venter.

D i s c u s s i o n. The type series of Egoianiceras angulatum ( EGOIAN, 1969) shows a limited range of variability affecting the adult size, the robustness of the primary ribs during the Gargasense juvenile stage, and the strength of thickening at the point of furcation ( Egoian 1969). Ventral ribs are about 35 over the last whorl, and this does not vary much in the type series. Most of the material from Roussillon falls within the range of variability of Egoianiceras angulatum ( EGOIAN, 1969) , with the exception of specimens LGB-ROU.6, 11a, and 12a. These three specimens are fairly close to the paratype MEA-310 figured by Egoian (1969: pl. XII, fig. 6), but they have inflated primary ribs during the Crassicostatum stage, bearing strong thickenings on the posterior branches. Then, the later growth stages conform with typical Egoianiceras angulatum ( EGOIAN, 1969) . These three specimens likely represent robust representatives of the variation series, and they should not be kept separate in a palaeobiological perspective. These strongly ribbed Egoianiceras angulatum ( EGOIAN, 1969) seem to also be present in the Parahoplites melchioris Zone of the Kopet Dag (e.g., Glazunova 1953: pl. VIII, fig. 3a–b) and Turkmenia (e.g., Tovbina 1982: pl. I, fig. 3a, b), where they have been mixed up with Acanthohoplites aschiltaensis

( ANTHULA, 1900) and its junior synonym Protacanthoplites abichi ( ANTHULA, 1900) . A revision is needed.

O c c u r r e n c e. The type series of Egoianiceras angulatum ( EGOIAN, 1969) comes from the ‘zone with Parahoplites melchioris and Colombiceras tobleri ’ of western Caucasus ( Egoian 1969). These two latter taxa are widespread in the Mediterranean Tethys, and usually found in a rock interval between the upper Epicheloniceras martini Zone and the lower Parahoplites melchioris Zone of the SMAZ ( Dauphin 2002). Elsewhere, Egoianiceras angulatum ( EGOIAN, 1969) , or allied forms, come from the Parahoplites melchioris Zone of the former soviet authors ( Glazunova 1953, Tovbina 1982). Our record from Roussillon is the first reliable occurrence of Egoianiceras angulatum ( EGOIAN, 1969) outside the Trans-Caspian regions, as the Romanian specimens better correspond to juveniles of Egoianiceras multicostatum ( AVRAM, 1974) , while those from Colombia better match the group of Colombiceras formosum SHARIKADZE, KAKABADZE et HOEDEMAEKER, 2004 . In the Vocontian Basin of SE France, Dauphin (2002) reported the occurrence of Egoianiceras angulatum ( EGOIAN, 1969) below the first black shale horizon (FA1) of the Fallot episodes, which is a marker of the base of her Colombiceras tobleri Zone. Unfortunately , the material remains unfigured for confirmation.

Figures

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