Protacanthoplites abichi ( Anthula, 1900 )

Frau, Camille, 2023, The systematic affinities between the Lower Cretaceous Ammonoidea Protacanthoplites abichi (Anthula, 1900) and Acanthohoplites aschiltaensis (Anthula, 1900), European Journal of Taxonomy 888 (1), pp. 137-158 : 139-141

Protacanthoplites abichi ( Anthula, 1900)

Figs 2A–K , 3A, F

Conch shape and ontogeny

Lectotype IPUW 1900-3 a

Understanding of the lectotype was so far based on the hand-drawing of Anthula (1900: pl. IX(VIII) fig. 2a–c) (see Fig. 2A–B ). It is here properly illustrated for the first time ( Fig. 2C–G ). The specimen corresponds to a small-sized, 3D-preserved internal mould with a calcitic phragmocone and calcareous body chamber with trace of aragonitic shell. The body chamber occupies half of the last whorl, but the peristome is not preserved. Measurements are given in Table 1 . Shell shape of the lectotype is characterised by a discoidal (Ww/D ~ 0.41), weakly depressed (Ww/Wh ~ 1.08), very evolute (U/Wh ~ 1.18) subophiocone coiling (U/D ~ 0.45).

The embryonic (Ammonitella) stage is poorly preserved, and remains poorly characterised. The first visible whorls of the shell develop a reniform whorl section, crateriform umbilicus, spaced and flat-topped ribs angulate at shoulders followed by smooth interspaces. This corresponds to the Royerianum stage that is typical of the early ontogeny of the basal Acanthohoplitidae ( Frau et al. 2020).

Then, the whorl section progressively becomes depressed, sub-rounded with convex flanks. The umbilical wall is rounded. The ornamentation consists of an alternation of straight to convex, strong primary ribs and variable number, two to four, of sharp atuberculate secondaries, irregular in thickness and irregularly arranged. Primary ribs are gradually thickened on the flank. They develop strong, elongated tubercles, sometimes clavate, located high on the flank, in tight contact with the succeeding whorl. The tuberculate ribs get a distinct hexagonal cross section. The tubercles divide into two, rarely three, branches over the venter. They strengthen as fold-like ribs, notably the posterior branch, and generally join on the tubercles of the other side. A third discrete rib sometimes appears between two ventral ribs. The secondary ribs are simple, rarely bifurcate, and can be coalescent on the primaries forming a polygyrate pattern. Primary ribs can be followed by an approximated and enlarged secondary rib so that they mimic constrictions. By these features, the ontogeny closely resembles the Gargasense stage sensu Frau et al. (2020) of the Acanthohoplitidae Colombiceras crassicostatum but differences are seen in the quadratic whorl section with rounded venter, and distinctive strengthened ribbing and tubercles. This stage is here referred to as the Abichi stage.

As growth proceeds, the number of intercalatories decreases, being mostly one to two, and coalescent ribs become sporadic. Primary ribs are dominantly bifurcate, then simple. They thicken along the flank, especially at the umbilical margin, while the tubercles tend to disappear. Ribs distinctly bend forward on the venter. All ribs are flat-topped or cuneiform over the venter. This stage mimics the Crassicostatum stage sensu Frau et al. (2020) of Colombiceras crassicostatum , but differences are seen in the whorl section that is broadly rounded, with subhexagonal rib section with a convex venter lacking ventral attenuation. This sequence is here referred to as the Crassicostatum-like stage.

The ontogenetic sequence of Protacanthoplites abichi is illustrated on Fig. 3A .

Specimen IPUW 1900-3b

The second syntype of Anthula (1900) is here properly illustrated for the first time ( Fig. 2H–K ). It is a small, 3D-preserved internal mould with a calcitic phragmocone and calcareous body chamber with remains of aragonitic shell. It shows a slight distortion of the shell at the beginning of the body chamber. The latter occupies half of the last whorl. The peristome is not preserved. Measurements are given in Table 2 . Shell shape is characterised by a discoidal (Ww/D ~ 0.39), weakly depressed (Ww/Wh ~ 1.02), very evolute (U/Wh ~ 0,91) subvirgacone coiling (U/D ~ 0.35).

The first visible whorl of the shell develops the Royerianum stage, changing into the Abichi stage of the lectotype Protacanthoplites abichi . To the difference, the ribbing is more gracile, with discrete spiny tubercles. The venter is narrower. As growth proceeds, the Crassicostatum-like stage initiates on the body chamber, but it is also much more gracile and regular than the lectotype of Protacanthoplites abichi , and develops flexuous primary ribs. By these features, specimen IPUW 1900-3b poorly matches the lectotype.

Suture line

Anthula (1900: pl. IX (VIII) fig. 2c) provided a partial suture line drawing for Protacanthoplites abichi showing a quinquelobate pattern. The lectotype seems to be immature as it does not show any sign of suture approximation at the end of the phragmocone. The suture morphogenesis of that species has subsequently been examined by Mikhailova (1957: fig. 5; 1958: fig. 1) and Tovbina (1970: fig. 4). According to these authors, the umbilical lobe is tripartite and rather symmetrical. A wide umbilical saddle is divided into two parts, the outer of which in turn becomes bifid. The ventral and dorsal lobes are bifid and complicated by lateral teeth. The ventral lobe has a low median saddle.

Age

The litho- and biostratigraphy of the Akusha locality was documented by Mordvilko (1962) and Bogdanova & Mikhailova (2016), but none of these authors have reported the type stratum of Protacanthoplites abichi . The species is, however, known to occur in the ‘ Acanthoplites (sic) aschiltaensis – Acanthoplites uhligi Zone’ sensu Mordviko (1962) of Dagestan ( Bogdanova & Mikhailova 2016). Correlation of this zone with the SMAZ is still not obtained with certainty, but it may cover part of the Epicheloniceras martini and Parahoplites melchioris standard zones ( Luber et al. 2017; Frau, 2020a, 2020b).

Figures

Fig. 2. A–K. Parahoplites abichi Anthula, 1900. A–B. Reproduction of the original hand-drawing. C–G. Lectotype (IPUW 1900-3a). G. Focus on its inner whorls. H–K. Syntype (IPUW 1900-3b), which is here excluded from the species. – L–R. Parahoplites aschiltaensis Anthula, 1900.L–M. Reproduction of the original hand-drawing. N–R. Lectotype (PMU.24105). R. Focus on its inner whorls. Scale bar = 10 mm.

Fig. 3. Illustration of the ontogenetic stages observed after the ammonitella (not seen), including Royerianum (green), Abichi (yellow), Crassicostatum-like (orange), Tobleri (violet), and Aschiltaensis (blue). A. Protacanthoplites abichi (Anthula, 1900), lectotype (IPUW 1900-3a). B. Acanthohoplites aschiltaensis (Anthula, 1900), lectotype (PMU.24105). C. A. aschiltaensis, paralectotype (PMU.24107). D–J. Schematic evolution of the whorl section of the plexus P. abichi – A. aschiltaensis. D–E. After Bogdanova & Mikhailova (2016: fig. 57), not to scale. F. After IPUW 1900-3a, not to scale. G. After paralectotype (PMU.24108), at scale. H–J. After paralectotype (PMU.24107), at scale.

Tables

Table 1. Dimensions of Protacanthoplites abichi (Anthula, 1900), lectotype (IPUW 1900-3a). *measurements taken between ventral ribs. Abbreviations: see Material and methods.

Specimen	D	U	Wh*	Ww*
IPUW 1900-3a	33.9	15.2	c.12.9	c.14.0