Imitoceratinae Ruzhencev, 1950

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Oberrödinghausen (Early Carboniferous; Rhenish Mountains, Germany), European Journal of Taxonomy 882, pp. 1-230 : 107

Subfamily Imitoceratinae Ruzhencev, 1950

Diagnosis

Subfamily of the family Prionoceratidae with the sutural formula E A L U I; external lobe pouched, usually asymmetric adventive lobe V-shaped, pointed. Conch in the juvenile stage subinvolute; adult stage involute. Coiling rate rarely low, usually high to very high (WER = 2.00–2.50). Shell ornament with fine to very coarse growth lines, without ribs.

Subfamily composition

The subfamily comprises three genera: Imitoceras Schindewolf, 1923 (17 or 18 species); Irinoceras Ruzhencev, 1947 (11 species); Triimitoceras Korn, Bockwinkel, Ebbighausen & Klug, 2003 (3 species).

Remarks

The subfamily Imitoceratinae was not excluded as valid in the revision of the Treatise by Kullmann (2009), but was included in the synonymy of the Prionoceratinae instead. Following this view would mean that the Prionoceratinae are most probably a polyphyletic taxon with quite heterogeneous content.

Morphology

Two of the three genera of the Imitoceratinae , namely Imitoceras and Irinoceras , show a similar morphology with thickly discoidal, fully involute conch with a high coiling rate. This morphology did not change throughout evolutionary history; the clade can be considered morphologically very conservative. Only the third genus Triimitoceras , which appears briefly in the late Tournaisian, deviates from this ( Korn et al. 2003a, 2010a). It is characterised by small conchs with a low coiling rate, but shows the suture line of Imitoceras . They are probably dwarf forms that may represent a progenetic side branch.

Ontogeny

The ontogeny is best known from specimens of the genus Imitoceras from localities in Morocco and Algeria ( Bockwinkel & Ebbighausen 2006; Korn et al. 2010a). It differs only slightly from discoidal species of Stockumites and Nicimitoceras . General ontogenetic trends are the closure of the umbilicus already in the juvenile stage, the continuously more slender conch and the increase in the coiling rate.

Phylogeny

Bockwinkel & Ebbighausen (2006) illustrated the transition between Nicimitoceras and Imitoceras using co-occurring species from the Anti-Atlas of Morocco. Their newly established species I. oxydentale already shows all the typical characteristics of Imitoceras (short, pouched external lobe, strongly convergent flanks), but at the same time a conch ontogeny that corresponds to that of Nicimitoceras (e.g., N. heterolobatum ). The derivation of Imitoceras from Nicimitoceras can therefore be considered very likely.

Stratigraphic occurrence

The oldest representatives of the subfamily Imitoceratinae are known to occur in the younger part of the early Tournaisian. During the middle and late Tournaisian, it is still present in many ammonoid assemblages, but the subfamily is quite irregularly distributed in Viséan and Serpukhovian strata. Irinoceras is a rather important component of the ammonoid assemblages of the late Viséan and Serpukhovian in the South Urals ( Ruzhencev & Bogoslovskaya 1971).

Geographic occurrence

The subfamily is widely distributed and mainly known from Ireland (de Koninck 1882), Central Europe (de Koninck 1844; Holzapfel 1889; Nicolaus 1963; Korn 2006), the Cantabrian Mountains ( Kullmann 1963), Serbia ( Stevanović & Kullmann 1962; Korn & Sudar 2016), North Africa ( Korn et al. 2003a, 2010a, 2010b; Bockwinkel & Ebbighausen 2006), the South Urals ( Ruzhencev 1947; Ruzhencev & Bogoslovskaya 1971), Tajikistan ( Nikolaeva 1994, 1995), Xinjiang ( Wang 1983), Tibet ( Liang 1976), Eastern Australia ( Campbell & Engel 1963; Campbell et al. 1983), British Columbia ( Work et al. 2000) and the American Midcontinent ( Hall 1860; Miller & Gurley 1896; Smith 1903; Miller & Collinson 1951; Miller & Garner 1955; Gordon 1965)