Acutimitoceratinae Korn, 1994

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Gattendorf (Devonian-Carboniferous boundary; Upper Franconia, Germany), European Journal of Taxonomy 883, pp. 1-61 : 13-14

Subfamily Acutimitoceratinae Korn, 1994

Diagnosis

Subfamily of the family Prionoceratidae with the sutural formula E A L U I; adventive lobe V-shaped or lanceolate, pointed. Conch in the juvenile stage subevolute or evolute; adult stage involute or subinvolute. Coiling rate usually moderately high or high (WER =1.75–2.25) and rarely very high (up to 2.35). Shell ornament with fine to coarse growth lines, usually without ribs.

Subfamily composition

The subfamily comprises six genera: Acutimitoceras Librovitch, 1957 (4 species); Costimitoceras Vöhringer, 1960 (3 species); Sulcimitoceras Kusina, 1985 (1 species); Nicimitoceras Korn, 1993 (7 species) and Stockumites Becker, 1996 (36 species).

Morphology

In the adult stage, species of the subfamily Acutimitoceratinae are distinguished from those of the Prionoceratinae almost exclusively by the higher whorl expansion rate of the former. In the Acutimitoceratinae it is almost always above a value of 1.75 and in the Prionoceratinae below this value. In Acutimitoceratinae , adult conchs are almost always completely involute and range from thinly discoidal to globular; in contrast to the mostly stout conchs of the Prionoceratinae , however, discoidal conchs are more common than pachyconic or globular conchs in the Acutimitoceratinae . Oxyconic conchs occur independently in several evolutionary lineages.

Juvenile conchs of the Acutimitoceratinae show a very wide variation in their morphology, ranging from subinvolute to very evolute. Likewise, the length of the more widely umbilicate juvenile stage varies markedly between species.

The shell ornament consists of simple growth lines in almost all species, these growth lines are usually convex across the flank and form a sinus on the venter. Only some species show growth lines with a biconvex course. Spiral lines occur in Costimitoceras and, together with the growth lines, form a reticulate ornament. Some species possess shell constrictions, in others only radial internal shell thickenings are present. Some species have neither shell constrictions nor internal shell thickenings.

The suture line is simple and consists of the elements E A L U I. The external lobe is usually lanceolate with parallel flanks; in some cases, however, the external lobe may be narrow V-shaped or very weakly pouched. The adventive lobe is often V-shaped and varies in shape from symmetrical to moderately asymmetrical.

Ontogeny

The ontogeny of the Acutimitoceratinae shows spectacular changes; differentiating it from all species of the Prionoceratinae . All acutimitoceratins show an early ontogenetic stage of variable length in which the juvenile whorls only slightly embrace the preceding one, resulting in a rather widely umbilicate or even serpenticonic juvenile conch shape. In the middle growth stage, the closure of the umbilicus begins by more or less wide overlap upon the preceding whorl. In most species, the umbilicus is closed by 10 mm conch diameter.

Ontogenetic changes in acutimitoceratin conch morphology can be illustrated in diagrams of ontogenetic trajectories ( Korn 2012), which show the ontogenetic trajectory of the ww/dm ratio follows a strikingly triphasic course. The amplitude of change depends on the length of the widely umbilicate juvenile stage and the width of the conch in the middle growth stage. Species with stout conchs tend to show a more pronounced triphasic ontogeny than discoidal forms.

Phylogeny

The Acutimitoceratinae are the dominant earliest Carboniferous ammonoid group immediately after the Hangenberg Event. The origin of the subfamily is most probably in the genus Mimimitoceras or related forms of which some developed rather evolute inner whorls already in the late Famennian ( Korn et al. 2015). Although specimens of the subfamily Acutimitoceratinae are very abundant in all earliest Carboniferous ammonoid occurrences, there is no undoubted record older than the Hangenberg Event. This means that the phylogenetic origin of the entire group, which most probably gave rise to all post- Hangenberg ammonoids (except for a few failed survivors such as some clymeniids and species of Mimimitoceras ), is still unknown.

The Acutimitoceratinae gave rise to at least two ammonoid clades, the subfamily Imitoceratinae and the family Gattendorfiidae . The first is characterised by a pouched external lobe and the second by an incompletely closed or open umbilicus in the adult stage. A possible third evolutionary lineage are the prolecanitids and with these all Mesozoic ammonoids.

Stratigraphic occurrence

Species of the subfamily Acutimitoceratinae are present already in the lowermost beds deposited directly after the Hangenberg Event with morphologically advanced species, i.e., species with a widely involute juvenile conch (e.g., Korn 1984; Price & House 1984; Kusina 1985; House 1996; Korn et al. 2004). Investigations in the Oberrödinghausen section show that most of the species have a very short stratigraphic range. A wide distribution across several ammonoid zones, as considered possible by Vöhringer (1960), could not be confirmed by our new study and revision.

In contrast to the good stratigraphic control on early Tournaisian species, little is known about the subfamily’s middle Tournaisian occurrences. Occurrences of possible members of the subfamily in Karaganda ( Librovitch 1940) and the American Midcontinent ( Miller & Collinson 1951) require confirmation.

Geographic occurrence

The subfamily Acutimitoceratinae has a very wide geographic range. In practically all assemblages of early Tournaisian ammonoids, the species of this subfamily are the dominant elements. The following list provides an overview on the occurrences (and selected references) of the Acutimitoceratinae : Rhenish Mountains ( Schmidt 1924, 1925; Vöhringer 1960; Korn 1981, 1984; Becker 1988; Korn 1992c, 1994; Korn et al. 1994; Becker 1996; Korn & Weyer 2003), Thuringia ( Schindewolf 1952; Weyer 1976, 1977; Bartzsch & Weyer 1982, 1986), Upper Franconia ( Schindewolf 1923; Korn 1994), Silesia ( Tietze 1869, 1870; Weyer 1965; Dzik 1997), Carnic Alps ( Korn 1992b), Montagne Noire ( Becker & Weyer 2004; Korn & Feist 2007), Anti-Atlas ( Korn 1999; Korn et al. 2004; Bockwinkel & Ebbighausen 2006; Ebbighausen & Korn 2007; Becker et al. 2013), Gourara in Algeria ( Ebbighausen et al. 2004), South Urals ( Balashova 1953; Barskov et al. 1984; Kusina 1985; Nikolaeva 2020), Karaganda ( Librovitch 1940), Guizhou ( Sun & Shen 1965; Ruan 1981; Sheng 1989), questionable also Missouri ( Furnish & Manger 1973) and Illinois ( Smith 1903; Miller & Collinson 1951).

Remarks

In the revision of the Treatise, Kullmann (2009) applied a very conservative concept with respect to the subdivision of the Early Carboniferous prionoceratid ammonoids; he did not accept the subfamilies Acutimitoceratinae , Imitoceratinae and Balviinae as valid. Instead, he merged the subfamily Acutimitoceratinae with the family Gattendorfiidae without accepting subfamilies. This means that two rather well-separable ammonoid groups were lumped: on one side the conservative clade with forms that close the umbilicus in the adult stage (e.g., Acutimitoceras , Stockumites , Nicimitoceras ) and on the other side those forms in which the umbilicus stays open in the adult stage (e.g., Gattendorfia , Weyerella ). Furthermore, Kullmann (2009) also put the genera Imitoceras and Irinoceras in the Prionoceratidae , despite their close morphological similarity, in conch shape, ontogenetic development and suture line, with Nicimitoceras , which suggests instead a rather close phylogenetic relationship with that genus. As Bockwinkel & Ebbighausen (2006) showed in their study of Early Tournaisian assemblages from Morocco, early representatives of Imitoceras can easily be related to genera of the Acutimitoceratinae (e.g., Nicimitoceras ). Thus, Kullman’s (2009) concept of the family Prionoceratidae is most probably of a polyphyletic taxon.