Menuites stephensoni Young, 1963

Ifrim, Christina & Stinnesbeck, Wolfgang, 2021, Ammonoids and their biozonation across the Santonian-Campanian boundary in north-eastern Coahuila, Mexico, Palaeontologia Electronica (a 34) 24 (3), pp. 1-62 : 40-43

Menuites stephensoni Young, 1963

Figure 31A–B ; Figure 32C–F ; Figure 33A–B , Table 13

1932 Menuites n. sp., Adkins, p. 467

1963 Menuites stephensoni , n. sp.; Young, p.

57, p1. 15, figures 1-2, text-figures 7o, 9n

1963 Menuites juv. sp., Young, p. 58, plate 15,

figures 6, 7, 9, 11, 12; plate 20, figures

11, 11; text-figure 9q

1999 Menuites sp. ; Kennedy and Cobban, p.

70

p.2005 Menuites portlocki portlocki (Sharpe,

1855); Kaplan et al, p. 76, plate 33, figure

3 only

p.2005 Pachydiscus pseudostobaei (Moberg

[sic], 1885); Kaplan, et al., p. 66, plate

31, figures 1-2 only

Type. The holotpe is WSA–69 by original designation by Young (1963, p. 57, plate 15, figures 1, 2; text-figures 7o, 9n). It is from a horizon with calcareous concretions in the upper Taylor Marl in Travis County, Texas .

Material. Four specimens, CPC –2194,–2349, – 2554, and –2562, from beds TPY17, 50, 52, and 62 .

Description. Involute (U/D with moderate growth rate). Juveniles with rounded whorl sections and 28 to 32 distant, sharp, straight to slightly prorosiradiate ribs. At D> 120 mm lateral and lateroventral rounded bullae develop and whorl section is gradually subrectangular. In large adults ribs become irregular and finally disappear. Suture line with a trifid L that is equal in depth to E.

Dimensions. Table 13.

Remarks. The taxon is characterized by high morphologic variability. Juveniles assigned to Menuites juv. sp. by ( Ifrim et al., 2013) may differ from Menuites stephensoni with respect to their U/D ratio and rib index ( Figure 31A–B ) and are kept separately until intermediate growth stages are known.

Specimen RE A 4214 from questionable lower Campanian sediments of Germany was described as Menuites portlocki portlocki (Sharpe, 1855) by Kaplan et al. (2005), but rather agrees with M. stephensoni than with the upper Campanian species (compare plate 32, op. cit.).

Specimen RE A 4193 from the lower Campanian of Germany described as Pachydiscus pseudostobaei (Möberg, 1885) by Kaplan et al. (2005) also rather shows characteristics of a juvenile Menuites stephensoni as understood here.

The species is closely related to, and most probably the ancestor, of the upper Campanian Menuites portlocki (Sharpe, 1855) known from North America. Menuites portlocki differs from M. stephensoni by a stronger ornamentation (compare e.g., Błaszkiewicz, 1980), although morphometry is similar between the two species.

Occurrence. Upper Santonian to lower Campanian of the Tepeyac section. Young (1963, p. 57) states that the holotype from Texas is upper Campanian in age, but without providing further details or age calibration. In central to western Europe the taxon is known from the lower Campanian. The record from Tepeyac is thus the oldest .

Figures

FIGURE 31. Morphometry of lower Campanian Menuites and Pachydiscus. A: WB/WH and U/D against D for Pachydiscus (P.) duelmensis from Tepeyac (red) and Germany (green), the holotype of Pachydiscus jimenezi Renz, 1936 (turquoise) and Pachydiscus paulsoni of Young (1963, grey) and Cobban and Kennedy (1992b, blue). B: WB/WH and U/D against D for Menuites stephensoni from Tepeyac (red) and western Coahuila (Ifrim et al., 2013, black). Morphometric data of Young (1963) are implausible and thus not shown.

FIGURE 32. Pachydiscids from the Tepeyac section. A–B: Pachydiscus (Pachydiscus) duelmensis (Schlüter, 1972), CPC–2193 from bed TPY40; C–F: Menuites stephensoni Young, 1963, CPC–2194 from bed TPY50. Scale: 50 mm.

FIGURE 33. Menuites stephensoni Young, 1963 from bed TPY62. A–B: CPC–2562. Scale: 50 mm.