Morphoceratidae Hyatt, 1900

Parent, Horacio & Zatoń, Michał, 2016, Sexual dimorphism in the Bathonian morphoceratid ammonite Polysphinctites tenuiplicatus, Acta Palaeontologica Polonica 61 (4), pp. 875-884 : 879-882

Family Morphoceratidae Hyatt, 1900 Genus Polysphinctites Buckman, 1922

Type species: Polysphinctites polysphinctus Buckman, 1922 , Dorset ( England), Lower Bathonian.

Polysphinctites tenuiplicatus ( Brauns, 1865)

[M and m]

Figs. 4–6.

1865 Ammonites tenuiplicatus sp. nov.; Brauns 1865: 135, pl. 25: 8–11.

1865 Ammonites tenuiplicatus Brauns ; Schloenbach 1865: 186, pl. 29: 3 (holotype, lost), 4.

1950 Grossouvria secunda sp. nov.; Wetzel 1950: 79.

1951 Siemiradzkia bajociformis sp. nov.; Arkell 1951: 13, pl. 3: 1.

1958 Asphinctites tenuiplicatus (Brauns) ; Westermann 1958: 86, pl. 45: 3 (neotype), pl. 46: 1, 2.

1958 Asphinctites gaertneri sp. nov.; Westermann 1958: 87, pl. 46: 3.

1958 Asphinctites bathonicus sp. nov.; Westermann 1958: 88, pl. 46: 4.

1970 Asphinctites tenuiplicatus (Brauns) ; Hahn 1970: 50, pl. 7: 1–5, pl. 8: 14, text-fig. 8.

1970 Polysphinctites secundus (Wetzel) ; Hahn 1970: 57, pl. 8: 11–13.

1987 Asphinctites aff. tenuiplicatus (Brauns) ; Torrens 1987: pl. 2: 7.

1987 Polysphinctes secundus Wetzel ; Torrens 1987: pl. 2: 8.

1997 Asphinctites tenuiplicatus (Brauns) ; Dietze et al. 1997: 12, pl. 2: 1–8, pl. 3: 1–3 (with synonymy).

1997 Asphinctites secundus (Wetzel) ; Dietze et al. 1997: 14, pl. 1: 5–11, pl. 3: 1.

1997 Asphinctites (Asphinctites) tenuiplicatus (Brauns) ; Mangold and Rioult 1997: pl. 16: 7.

1998 Asphinctites tenuiplicatus (Brauns) ; Kopik 1998: pl. 6: 5.

2000 Asphinctites tenuiplicatus (Brauns) ; Matyja and Wierzbowski 2000: 207, pl. 7: 4, 5.

2000 Polysphinctites secundus (Wetzel) ; Matyja and Wierzbowski 2000: 207, pl. 7: 3.

2001 Asphinctites tenuiplicatus (Brauns) ; Matyja and Wierzbowski 2001: pl. 1: 1–8, pl. 2: 4, 5, 9, 10.

2001 Polysphinctites secundus (Wetzel) ; Matyja and Wierzbowski 2001: pl. 2: 1–3, 6–8.

2010 Asphinctites tenuiplicatus (Brauns) [M and m]; Zatoń 2010b: 163, pl. 13E–I, text-figs. 11B, C, 15, 16A–E.

Material.— 37 macroconchs ( GIUS 8-2608 – 2609 , 2679 , 2687–2691 , 2694–2695 , 2698–2699 , 2702 , 2706 , 2713–2714 , 2716–2719 , 2728–2734 , 2736–2737 ; IGPUW /J/133, 139, 183, 200, W5–W8), 80 microconchs and seven juveniles ( GIUS 8-2583 – 2586 , 2590–2595 , 2597–2607 , 2611–2616 , 2620–2627 , 2629–2631 , 2635–2639 , 2641–2657 , 2664–2666 , 2670–2673 a-b; IGPUW /J/127, 138, 180, 186, 188–189, 192, 194, 197, 199, 201, 203, 207, 209–210, W1-W4) from the Lower Bathonian ( Polysphinctites tenuiplicatus Zone ) of Kawodrza Górna and Faustianka, Poland .

Description.— Macroconch/female: Inner whorls moderately evolute with oval, slightly higher than wide whorl section (W/H 1 = 0.9). Fine, prorsi- to rectiradiate ribs are visible from D ~ 8 mm. Middle whorls, from D ~ 25–30 mm, more involute with the narrowest relative umbilicus at about D = 50 mm where in most specimens begin the adult body chamber. The whorl section is high-oval with rounded flanks and venter, gradually becoming more compressed towards the adult stage. The primary ribs are recti- to prorsiradiate, sharper on the inner and middle whorls, and more rounded on the outer ones. Rib division is rather irregular, with primaries typically divided from about the mid-flank in 3–4 finer secondaries. Polyschizotomic divisions (furcation of secondaries) are frequent through the adult phragmocone and body chamber; some primaries bifurcate on the lowermost flank and each one divide again slightly above the mid-flank. Ventral ribs cross the venter unchanged. Some specimens show about three weak, prorsiradiate constrictions per whorl. Rib density increases gradually towards the adult peristome, passing from P = 12 (phragmocone) to P = 16–21 (body chamber). The body chamber is longidomic, ranging 360–440° in length. The aperture is simple and may be preceded by a shallow constriction. Adult size is very variable, ranging Dp = 70.0–109.2 mm. The septal suture line has the typical perisphinctoid design, in the adult phragmocone is finely frilled.

Microconch/male: Identical to the inner whorls of the macroconch up to D = 15–20 mm, although some few specimens can be differentiated from D ~ 11–12 mm by the whorl section being more compressed with lower flanks respect to the macroconchs at similar size. The whorl-section is subrectangular, higher than wide up to about D = 13 mm, then passing to high-oval with a narrowly rounded venter. Primary ribs are thin, recti- or slightly prorsiradiate, divided into 3–4 finer secondaries from the middle of the flank or slightly above. Primary ribs become thicker and convex adaperturally in the last half of the adult body chamber. At the peristome, large, well rounded spatulate lateral peristomatic lappets emerge. The length of the body chamber is significantly shorter than that in the macroconch, ranging 255–295°. Adult size ranges Dp = 21.4–36.7 mm. The septal suture line has the same structure and design of the macroconch, but less frilled.

Remarks.—The variability of shell-shape and sculpture in macro- and microconchs is low ( Table 1: CV), especially in the phragmocone, with respect to what is commonly observed in ammonites (e.g., Sturani 1971; Callomon 1985; Parent 1998; Kennedy 2013; De Baets et al. 2015). The variation in adult size is rather broad in both dimorphs ( Fig. 3B ). It is, however, virtually identical in the Polish material studied by Matyja and Wierzbowski (2001), slightly larger than the representatives in other areas of the Tethys.

The neotype, designated by Westermann (1958: pl. 45: 3), corresponds to a macroconch phragmocone which perfectly matches our specimens, at comparable size, in shell-shape and ribbing (cf. Figs. 4A–E, 5I, K, L ).

According to the synonymy considered above many of the nominal species of Asphinctites do not differ significantly from Polysphinctites tenuiplicatus [M]. Especially it concerns the species Asphinctites recinctus (Buckman, 1924) , which is the index-species of the Lower Bathonian Asphinctites recinctus Zone , corresponding to the Oxycerites yeovilensis Zone of the North-West European Province (Subboreal Province; see Mangold and Rioult 1997). Dietze et al. (1997) considered it as a synonym of Polysphinctites tenuiplicatus , as earlier suggested by Torrens (1987: 98). Indeed, although the holotype is very similar (see Buckman 1924; Arkell 1955), its stratigraphic position is not known accurately. Moreover, Asphinctites recinctus in Mangold and Rioult (1997: pl. 16: 9) does not resemble the holotype and may represent a different, probably undescribed species (Volker Dietze, written communication 2005).

According to Hahn (1970), the older macroconchiate form Asphinctites patrulii Hahn, 1970 , occurring in the Oxycerites yeovilensis to Polysphinctites tenuiplicatus zones ( Hahn 1970; Dietze et al. 1997), differs by its delicate and denser ribbing, as well as by the more involute inner whorls bearing deep constrictions. A. replictum (Buckman, 1922) is more involute not only in the inner whorls, but also in the outer ones. Unfortunately, its stratigraphic position „Zigzag Bed” according to Arkell 1955: 137) has not been established accurately. A. pinguis ( De Grossouvre, 1919) is characterized by its inflate whorl-section and involute inner whorls that rapidly become evolute. Its stratigraphic position, on the other hand, is confined to the Parkinsonia convergens – Oxycerites yeovilensis zones (see Torrens 1987; Mangold and Rioult 1997).

The microconch Polysphinctites polysphinctus Buckman, 1922 , on the other hand, differs from microconchs of P. tenuiplicatus by its distinct, deep and prorsiradiate constrictions (3 per whorl) starting from the inner whorls, as well as denser ribbing. Its stratigraphic position is limited to the P. convergens – O. yeovilensis zones (see Schairer 1994; Schlögl et al. 2005). At least a part of the specimens presented by Hahn (1970: pl. 8: 8–10) as P. polysphinctus and coming from the Polysphinctites tenuiplicatus Zone , because of the lack of morphological differences, should in fact belong to P. tenuiplicatus .

Dimorphism.—Soon after the papers by Makowski (1962) and Callomon (1963), many authors (e.g., Callomon 1969; Westermann 1969; Guex 1973) made efforts trying to find a general rule or protocol for taxonomic arrangement of sexual dimorphs which, in many cases have been, or remain assigned to different taxa (even family and superfamily in the worst cases, see e.g., Maeda 1993). Nevertheless, it has become clear that these taxonomic problems must be treated almost case by case after sexual dimorphic correspondences are demonstrated. The present material is virtually indistinguishable in significant features from the other material figured in the literature of both dimorphs. Thus, the indistinguishable morphology of the inner whorls as well as their co-occurrence in large numbers in the Polish Jura ( Matyja and Wierzbowski 2001; Zatoń 2010b) and southern Germany ( Hahn 1970; Dietze et al. 1997) leave no doubts that all specimens belong to a sexual dimorphic pair.

After the studies of Hahn (1970), Dietze et al. (1997), Matyja and Wierzbowski (2001), and the discussion above, it seems clear that the Polysphinctites – Asphinctites complex is composed by two species in direct phyletic succession: Asphinctites pinguis ( De Grossouvre, 1919) [M] / Polysphinctites polysphinctus Buckman, 1922 [m] ( Parkinsonia convergens – Oxycerites yeovilensis zones) followed by P. tenuiplicatus [M and m] ( Polysphinctites tenuiplicatus Zone ). A. pinguis / P. polysphinctus (including the many synonyms indicated in Matyja and Wierzbowski 2001) differs from P. tenuiplicatus by the consistent occurrence of strong constrictions, the more involute middle whorls of the macroconchs and the finer sculpture of the microconchs. These differences are not very strong and, in this sense, Westermann and Callomon (1988: 15) have even considered these forms as the end-members in the range of variability of Morphoceras .

According to the proposed composition of the lineage, including the several nominal species currently assigned to Asphinctites and/or Polysphinctites , the corresponding genus name should be Polysphinctites (= Asphinctites as a junior synonym).

Stratigraphic and geographic range.— Polysphinctites tenuiplicatus is the index species of the highest zone of the Early Bathonian and its stratigraphic range seems to be confined to its nominal chronostratigraphic zone (see e.g., Dietze et al. 1997). The geographic distribution of the species is relatively wide, a non-exhaustive list includes: Poland ( Kopik 1979; Matyja and Wierzbowski 2000, 2001), Germany ( Hahn 1970; Dietze et al. 1997), France (e.g., Mangold and Rioult 1997), England (Buckman 1924; Arkell 1951; see also Dietze et al. 1997).

Figures

Fig. 3. A. Dimensions considered for description of the ammonite morphology. D, diameter; H1, whorl height; H2, ventral (or apertural) whorl height; U, umbilical width; W, whorl width. B. Comparative representation of the ontogenies of the sexual dimorphs with indication of the developmental stages (K0, K1, and K2) considered for statistical comparison of the ontogenies (definitions in the text). All values representing the studied material.

Fig. 5. Morphoceratid ammonite Polysphinctites tenuiplicatus (Brauns, 1865) [M] and [m] showing successive growth stages, Lower Bathonian, Polysphinctites tenuiplicatus Zone, Polish Jura, Kawodrza Górna (A–N) and Faustianka (O). A. GIUS 8-2592, juvenile with beginning of body chamber; natural size (A1), enlargement (A2). B–H. Microconchs, males. B. GIUS 8-2590. C. GIUS 8-2594. D. GIUS 8-2611, lateral (D1) and apertural (D2) views. E. GIUS 8-2646. F. GIUS 8-2593. G. GIUS 8-2640. H. GIUS 8-2622. I–O. Macroconchs, females. I. GIUS 8-2608, lateral (I1) and apertural (I2) views. J. GIUS 8-2703. K. GIUS 8-2687. L. GIUS 8-2717. M. GIUS 8-2710. N. GIUS 8-2733. O. GIUS 8-2715. H, O, examples of complete adult specimens with peristome. The asterisks indicate the last septum. All natural size, except A2.