Acanthohoplites aschiltaensis ( 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 : 142-147

Acanthohoplites aschiltaensis ( Anthula, 1900)

Figs 2L–R , 3B–C, F–J , 4–6

Conch shape and ontogeny

Lectotype PMU.24105

Understanding of the lectotype was so far based on the hand-drawing of Anthula (1900: pl. X(IX) fig. 2a–b) (see Fig. 2L–M ). It is here properly illustrated ( Fig. 2N–R ). The lectotype shows the same state of preservation as that of Protacanthoplites abichi described above. Peristome is not preserved. The body chamber occupies half of the outer whorl. Measurements are given in Table 3 . Its shell shape is characterised by a moderate size, discoidal (Ww/D ~ 0.35), weakly compressed (Ww/Wh ~ 0.82), subevolute (U/Wh ~ 0.76) and subvirgacone coiling (U/D ~ 0.33).

The ontogenetic sequence develops four stages following the worn embryonic whorls ( Fig. 3B ) that closely match those of Protacanthoplites abichi described above. The first visible whorls bear the latest part of a typical Royerianum stage that is well visible on the left face. It changes to an Abichi stage almost identical to that of Protacanthoplites abichi , except its more regular pattern of secondary ribs at the beginning. The Abichi stage occupies one whorl and a half. As growth proceeds, the primary ribs lose their tubercles, while secondaries become irregular, and start decreasing. Ribbing changes progressively on the late phragmocone into an alternation of simple, primary ribs, and generally one secondary, irregular, both in length and strength on the flank. The secondaries can be indistinctly branched on the primary ribs in the lower third of the flank or at mid-flank. All ribs are flat-topped or cuneiform over the venter. This is almost similar to the Crassicostatum-like stage of Protacanthoplites abichi , but differences are seen in the progressive change of the whorl section becoming weakly compressed, subrectangular, higher than wide.

The Crassicostatum-like stage changes again in the late phragmocone into a regular alternation of strong, slightly flexuous, primary ribs, with a slight retrocurvature at the umbilical margin, and a single secondary starting high on the flank. The maximum whorl width is reached in the lower third of the flank. Twenty-five ventral ribs are observed on the last half whorl. They are distinctly flat-topped over the venter. These features conform to the adult ontogeny of Colombiceras crassicostatum referred to as the Tobleri stage ( Frau et al. 2020). The Tobleri stage starts around D ~ 40 mm (estimation) and occupies the rest of the outer whorl.

Paralectotype PMU.24106

This paralectotype was not figured by Anthula (1900). It corresponds to a fragment of a large-sized phragmocone displaying the features of the Tobleri stage, including a rather regular alternation of strong, moderately flexuous primary ribs, with a slight retrocurvature at the umbilical margin, and a single secondary of variable height ( Fig. 4 ).

Paralectotype PMU.24107

Anthula (1900: pl. XI(X) fig. 1) provided a hand-drawing of this paralectotype. It is here properly illustrated for the first time ( Figs 5A–B , 6A–C ). It corresponds to a 3D-preserved internal mould but of larger size compared to the lectotype. Measurements are given in Table 3 . Estimated diameter is about 297 mm. Its phragmocone is broken, but the general shell shape is virgacone (U/D ~ 0.39) with extremely discoidal (Ww/D ~ 0.25), very evolute (U/Wh ~ 1.19) and weakly compressed coiling (Ww/ Wh ~ 0.77). Its last preserved sutures are approximated suggesting its maturity.

The first preserved whorls bear a late Abichi stage, consisting in strong primaries, simple or bifurcate, bearing tubercle-like thickenings at the point of furcation, and a variable number of strong secondaries. It gives way to a short Crassicostatum-like stage (estimation: D ~ 35 mm), made of strong primaries, simple or bifurcate, with decreasing thickenings on the flank, and generally one strong secondary. All ribs are flat-topped or cuneiform over the venter. Then, a Tobleri stage develops at D ~ 50 mm (estimation) bearing spaced, sharper primary ribs, being slightly flexuous with a slight retrocurvature at the umbilical margin. Secondary ribs are unique, high on the flank, so that they are almost covered by the succeeding whorl. Ribbing changes in the last two whorls into more spaced and enlarged, more flexuous, primaries with a maximum on the body chamber. Primary ribs start with a distinctive retrocurvature at the umbilical margin except approaching the aperture, and are simple, rarely bifurcate, over the flank. The secondaries are unique, and generally start in the middle, or upper flank. This last ontogenetic stage is here referred to as the Aschiltaensis stage ( Fig. 3C ). Number of ventral ribs cannot be estimated but it seems denser than during the Tobleri stage.

Paralectotype PMU.24108

Anthula (1900: pl. X(IX) fig. 1) only gave an idealised hand-drawing of the cross section of this paralectotype. A proper illustration is given here ( Fig. 6D–H ). It corresponds to a half of a moderately large phragmocone. Measurements are given in Table 3 . The broken part allows to observe the Abichi stage extending up to D ~ 18 mm (estimation), and then, a part of the Crassicostatum-like stage. Beyond, the ontogeny is rather similar to the lectotype although the Tobleri stage is more robust and shows great affinity with specimen PMU.24107. Indeed, the shell shape differs from the lectotype by a more discoidal (Ww/D ~ 0.33) and weakly compressed (Ww/Wh ~ 0.94) coiling, while the umbilicus is more open (U/Wh ~ 1.19). Specimen PMU.24108 further displays a lower number of ventral ribs (estimated at ~ 21) on the last half whorl.

Suture line

A partial suture line of Acanthohoplites aschiltaensis was drawn by Anthula (1900: pl. X(IX) fig. 3b) based on the lost syntype. It is of quinquelobate type and shows a deep inner lateral lobe with distinctive central and inner branches. This drawing was commented on by Stoyanow (1949, 1958), and his re-examination of the lectotype PMU.24105 highlighted a distinctive symmetrically trifid first lateral lobe, with some asymmetry in its inner branch, and bifid saddles. The lectotype seems to be mature as it shows suture approximation at the end of the phragmocone. The suture morphogenesis of that species has then been examined by Drushchits (1956), and Bogdanova & Mikhailova (2016) based on individuals from Dagestan. The suture line consists of a bifid ventral lobe with a low bifid median saddle; rather symmetrical umbilical lobe with distinctive lateral branches; symmetrical, short and trifid first umbilical lobe; deep inner lateral lobe with distinctive central and inner branches; bifid dorsal lobe with variable digits; broad and high, asymmetrically bifid external and umbilical saddles. The first umbilical saddle (U1/I) is indistinctly asymmetrically trifid. There is no difference between the suture lines of Protacanthoplites abichi and Acanthohoplites aschiltaensis .

Age

There is no comprehensive description of the Ashil’ta locality that yielded the type material of Acanthohoplites aschiltaensis . Nevertheless, the Acanthohoplites -bearing beds of Akusha consist of a ~ 25 m-thick interval assigned to the ‘ Acanthoplites aschiltaensis – Acanthoplites uhligi Zone’ ( Mordvilko 1962). This is the case in the reference Dargi River section ( Bogdanova & Mikhailova 2016: figs 10–11). Most of the taxa from the ‘ Acanthoplites aschiltaensis – Acanthoplites uhligi Zone’ of the Dargi River section derive from a siltstone horizon including fossiliferous phosphatic nodules. Acanthohoplites aschiltaensis co-occurs here with Parahoplites sjogreni Anthula, 1900 , Phylloceras (Hypophylloceras) velledae Michelin, 1834 , Aconeceras sp. , and the problematic Cheloniceras sp. ( Bogdanova & Mikhailova 2016). This horizon is of regional significance since it also crops out in the Rubas-chai section, in the vicinity of Khuchni ( Bogdanova & Mikhailova 2016: fig. 12). A younger Acanthohoplites -bearing horizon is documented in the upper part of the ‘ Acanthoplites aschiltaensis – Acanthoplites uhligi Zone’ in the Dargi River and Rubas-chai sections. Representatives assigned to Acanthohoplites ex gr. aschiltaensis would be still present in the corresponding horizon but the fauna lacks illustration for further confirmation.

Subspecies of Acanthohoplites aschiltaensis

Klein & Bogdanova (2013) listed three subspecies of Acanthohoplites aschiltaensis in the Fossilium Catalogus. Their validity is discussed below.

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.

Fig. 4. Acanthohoplites aschiltaensis (Anthula, 1900), paralectotype (PMU.24106). Scale bar = 10 mm.

Fig. 5. Acanthohoplites aschiltaensis (Anthula, 1900), paralectotype (PMU.24107). Scale bar = 10 mm.

Fig. 6. Acanthohoplites aschiltaensis (Anthula, 1900). A–C. Paralectotype (PMU.24107). D–H. Paralectotype (PMU.24108). Scale bars = 10 mm.