Pachydiscidae Spath, 1922

Wondrejz, Constanze, Nagm, Emad & Wilmsen, Markus, 2023, Dimorphism in Late Cretaceous ammonites- evidence from early Turonian ammonite faunas of the Briessnitz Formation in Saxony, Germany, Acta Palaeontologica Polonica 68 (4), pp. 639-657 : 643-646

Family Pachydiscidae Spath, 1922 Genus Lewesiceras Spath, 1939

Type species: Ammonites peramplus Mantell, 1822; Turonian of Sussex, southern England, UK; by original designation .

Lewesiceras peramplum ( Mantell, 1822) Figs. 3 , 4 .

1822 Ammonites peramplus; Mantell 1822: 200.

1981 Lewesiceras peramplum ( Mantell, 1822) ; Wright and Kennedy 1981: 29, pl. 2: 1–3; pl. 3; text-figs. 9, 12. [with synonymy]

2013 Lewesiceras peramplum ( Mantell, 1822) ; Wilmsen and Nagm 2013: 650, figs. 3–5.

2014 Lewesiceras peramplum ( Mantell, 1822) ; Wilmsen and Nagm 2014: 204, fig. 3a, b.

2014 Lewesiceras peramplum ( Mantell, 1822) ; Amédro and Delvaque 2014: 133, pl. 12; pl. 16: 2.

2018 Lewesiceras peramplum ( Mantell, 1822) ; Matrion 2018: 192, figs. 128A, 131A.

2019 Lewesiceras peramplum ( Mantell, 1822) ; Kennedy and Kaplan 2019: 35, pl. 7: 3, 9, 10, 17; pls. 8–11, 13; text-figs. 15, 16.

Material.— In total, 83 specimens from the lower Turonian (Cretaceous), Briessnitz Formation, Germany, Dresden, locality: Briessnitz (one specimen), Cotta (two specimens), Kauscha (two specimens), Leubnitz (28 specimens), Leutewitz (two specimens), and Lockwitz (48 specimens) were systematically studied. Due to the preservation, only 63 specimens were used for statistical analysis. Repository numbers are provided in SOM 3 .

Measurements.—See SOM 3.

Description.—The assemblage consists of large individuals, up to 228 mm in diameter (the average diameter of the Saxonian specimens is 117.5 mm while the largest specimens from the literature measure up to 900 mm), but also small individuals with D ≤ 60 mm occur. The specimens have a planulate shell at the boundary from moderately involute to moderately evolute (UD ≈30%) with rounded venter. Wb/Wh is approximately 0.58, and the aperture has a high oval to rectangular form. The ornamentation with the tubercles and ribbing changes during ontogeny, declines at maturity and may disappear completely. The inner whorls are characterised by five to six slightly projecting umbilical tubercles per whorl, giving rise to rather narrow, irregular, and curved primary ribs and two to four short secondary ribs. Later, the umbilical tubercles become weaker and bullate, the primary ribs become lower, and the secondary ribs increase in number and weaken. At a diameter of approximately 100 mm, the secondary ribs have normally disappeared, and the primary ribs are weak or absent on the venter. All ribs are separated from each other through intercostal sections, which have the same width as the ribs. The umbilicus is shallow to moderately deep, and the umbilical border is rounded.

Stratigraphic and geographic range.— Lewesiceras peramplum occurs abundantly in the lower and middle Turonian of Germany, the Czech Republic, Poland, France, Tunisia, and Morocco ( Wright and Kennedy 1981). Wilmsen and Nagm 2013) suggested that the species may have originated already in the latest Cenomanian and may range into the early late Turonian based on records from Saxony.

Statistical analyses.—Classical clustering and linear discriminant analysis, considering eight morphological variables, were executed for all 63 relevant specimens (see Fig. 4 and SOM). In the classical clustering diagram, two large groups inferred micro- and macroconchs) emerged, which differ primarily in the maximum diameter, similarly to the situation in M. nodosoides (see below). The Cophenetic distance at the last joint cluster (bottom of the diagram) is 38, and therefore the two large groups are more similar than those of M. nodosoides , probably due to the lower sample number. The LDA of L. peramlum shows again two large groups, which are broadly distributed but also in total farther away from each other. Nevertheless, they slightly overlap in the lower part of the diagram.

Figures

Fig.3. Photographic illustration of typical large (A) and small (B) specimens of the pachydiscid ammonoid Lewesiceras peramplum (Mantell, 1822) from the lower Turonian of the Briessnitz Formation, Dresden, Germany. A. MMG: SaK 5163 from Leubnitz, in lateral (A1) and ventral (A2) views. B. MMG: SaK 5343 from Leutewitz, in lateral (B1, B4), apertural (B2), and ventral (B3) views.

Fig. 4. Statistical test of potential dimorphism in Lewesiceras peramplum (Mantell, 1822) based on 63 specimens. The repository number of each specimen is given on top of the diagram. A. Classical clustering; the red color characterises particularly small individuals with D <77 mm in between the group of potential microconchs. B. Linear discriminant analysis (LDA); the color code follows the assignment of the classical clustering above. The dark blue lines are biplots of all variables, an overlaying of a score plot and a loadings plot in a single graph, which enables to visualise high-dimensional data by using a two-dimensional graph.