Egoianiceras AVRAM , 1974

Frau, Camille, 2023, Upper Aptian Ammonites From Roussillon, Southern France, Fossil Imprint 79 (1), pp. 89-101 : 92-94

Genus Egoianiceras AVRAM, 1974

T y p e s p e c i e s. Colombiceras angulatum EGOIAN,

1969, by original designation.

E m e n d e d d i a g n o s i s. Diminutive acanthohoplitids, very small to moderate size, up to D ~ 45 mm: E. angulatum ( EGOIAN, 1969) , or possibly 60 mm: E. multicostatum ( AVRAM, 1974) ]. Shell shape discoidal (Ww/D ~ 0,36–0,37), strongly compressed (Ww/Wh ~ 0,36–0.37), very evolute (U/Wh ~ 0,75–1), subvirgacone coiling (U/D ~ 0,33–0,37). Whorl section compressed, subquadrate, sometimes with flattened venter in juvenile, generally becoming suboval with rounded venter in adult. No evidence of ventral furrow at any growth stage. Four ornamental stages resembling those of Colombiceras crassicostatum (D’ ORBIGNY, 1841) . Embryonic (Ammonitella) stage poorly characterised, followed by short Royerianum stage, with reniform whorl section, crateriform umbilicus, spaced and flat-topped ribs angulate at shoulders or bearing small tubercles, smooth interspaces. Gargasense juvenile stage variable; sometimes long and gracile, made of dense, sharp, rather uniform, simple, bifurcate and intercalate ribs lacking tubercles, otherwise short and robust, made of irregular alternation of primary and secondary ribs. In that case, primary ribs are simple, bifurcate, rarely trifurcate, sometimes inflated, with or without tubercle-like thickenings at point of furcation. Secondaries variable in number, generally one to four, atuberculate, sometimes bifurcate or coalescent on primaries. All ribs and branches tend to be flat-topped over venter. Progressive change toward short Crassicostatum sub-adult stage made of enlarged simple and bifurcate, rarely trifurcate primary ribs, separated by one or two, rarely three secondaries. No elongated tubercles at point of furcation of primaries. Ribbing changes into Tobleri adult stage, sometimes irregularly disposed but generally alternation of spaced, slightly flexuous, primary rib, with or without slight retrocurvature at umbilical margin, generally one secondary rib starting at variable height on flank. Ribs become sharper, but often cuneiform and flat-topped over venter. Peristome marked by smooth interspace, followed by discrete simple rib. Suture line quinquelobate. Ventral lobe has bifid median saddle. Umbilical lobe deeper than ventral lobe, rather symmetrical outlines with long central branch. First umbilical, inner, and dorsal lobes remain poorly characterised.

R e m a r k s. With the introduction of the subgenus Egoianiceras, AVRAM (1974) separated the group of Colombiceras crassicostatum angulata EGOIAN, 1969 supposedly lacking lateral tubercles on the primary ribs of the early whorls. This species comes from mid-late Aptian aged outcrops of the Khokodz River Basin in southwestern Russia ( Egoian 1969), but occurrences of the species were later reported from Romania ( Avram 1974), Colombia ( Sharikadze et al. 2004), SE France ( Dauphin 2002), and northern Spain ( Frau 2021a, b). Avram (1974) also erected the species Colombiceras (Egoianiceras) multicostatum AVRAM, 1974 , based on Romanian Colombiceras , but the author failed to give a clear diagnosis for separating his taxon from Colombiceras (Egoianiceras) angulatum EGOIAN, 1969 . Regarding this, Sharikadze et al. (2004) identified the lack of tubercles as a non-reliable diagnostic feature, because some of the type specimens of Colombiceras (Egoianiceras) angulatum EGOIAN, 1969 bear sort of tubercle-like thickenings on the early whorls, a detail previously noted by Egoian (1969: 163). On that basis, Egoianiceras has been discarded, or considered doubtfully necessary by subsequent authors ( Sharikadze et al. 2004, Wright et al. 1996, Bogdanova and Mikhailova 2016).

The fact is that a distinction can be made between Egoianiceras and Colombiceras , thanks to the recent account of Frau et al. (2020) revising the type species Colombiceras crassicostatum (D’ ORBIGNY, 1841) . Egoianiceras is of smaller adult size, and possesses a more compressed subquadrate whorl section, with indistinct flattened venter in the early whorls. There is never a ventral furrow as in the juveniles of Colombiceras crassicostatum (D’ ORBIGNY, 1841) . Moreover, Egoianiceras more quickly reaches its adult shell morphology made of a suboval whorl section with a rounded venter. Regarding its ontogeny, Egoianiceras shares the post-embryonic Royerianum stage of Colombiceras crassicostatum (D’ ORBIGNY, 1841) , but the following Gargasense and Crassicostatum stages are reduced, attenuated or substantially modified, and never develop strong lateral tubercles on the primary ribs. However, some sort of thickenings can occur. Ribbing changes in the adult whorl into the Tobleri adult stage of Colombiceras crassicostatum (D’ ORBIGNY, 1841) , but the ribbing is rather irregular, due to more common simple ribs, variable lengths of secondaries, the presence of additional primaries, and sharper ventral ribs over the venter. On the whole, we suggest keeping Colombiceras and Egoianiceras separate, pending a better understanding of their representatives and clarification of their phyletic relationships. This agrees with the earlier suggestions of Dauphin (2002), and Frau et al. (2020).

S p e c i f i c c o n t e n t. Here we group into Egoianiceras the species Egoianiceras angulatum ( EGOIAN, 1969) , Egoianiceras lautum ( GLAZUNOVA, 1953) , and doubtfully, Egoianiceras multicostatum ( AVRAM, 1974) , pending revision. We provisionally refer to ‘ Egoianiceras ’ the three subspecies Acanthohoplites nolani exiquecostatus EGOIAN, 1969 , Acanthohoplites nolani nolani EGOIAN, 1969 , and Acanthohoplites nolani nodosus EGOIAN, 1969 , pending more material to be studied.

Through most of their distribution area, E. angulatum ( EGOIAN, 1969) , and E. lautum ( GLAZUNOVA, 1953) co-occur, and seem to be linked by specimens intermediate in ribbing (see discussion below). Such specimens call for attention, as it has been proven that brady- versus tachymorphic ontogenetic development is the dominant controlling factor in the variability of basal acanthohoplitids ( Frau et al. 2020). Re-examination of in situ Trans-Caspian material is needed to solve the taxonomy of Egoianiceras .

By comparison with the type species, Egoianiceras multicostatum ( AVRAM, 1974) reaches a greater adult size (D = 60 mm), has almost twice as many ribs on the last whorl, and all of them are distinctly flexuous on the flank, becoming flat-topped and cuneiform over the venter. Bifurcate primaries are frequent through most of the ontogeny. These features bring it closer to the group of Colombiceras caucasicum ( LUPPOV, 1949) according to Frau et al. (2020). Pending revision, the affinities between Egoianiceras multicostatum and Colombiceras caucasicum remains in doubt, particularly as Luppov (1949: 230) mentioned a short stage with distinct tubercles in the early whorls of the latter taxon.