Pachydiscus (Pachydiscus) duelmensis ( Schlüter, 1872 )

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 : 35-38

Pachydiscus (Pachydiscus) duelmensis ( Schlüter, 1872)

Figure 27A–B , Figure 28A–B , Figure 29A–B ,

Figure 30B–E , Figure 31A , Figure 32A–B , Table 12

1872 Ammonites dülmensis Schlüter. p. 52. plate 16. figure 1-2

1894 Pachydiscus dülmensis Schlüter sp.; de Grossouvre. p. 199. plate 20

1920 Pachydiscus dülmenensis Schlüter ; Köplitz. p. 69. plate 8. figure 25

1936 Pachydiscus (Parapachydiscus) jimenezi Renz. p. 3. plate 2. figure 4, 4a

1963 Eupachydiscus jimenezi Young. p. 59, plate 14, figures 1, 5; plate 16, figure 4; text-figure 10k

1963 Parapuzosia paulsoni Young. p. 53, plate 11. figures 3–5; plate 12. figures 1-4; plate 15. figure 10; plate 17. figure 9; plate 19. figures 3-4; text-figures 8a-b. 9g, j, r

1986 Pachydiscus (Pachydiscus) duelmensis (Schlüter. 1872) ; Kennedy. p. 35. text-figure 12

1989 Pachydiscus (Pachydiscus) duelmensis (Schlüter. 1872) ; Jagt. p. 7. plate 1-5. plate 6. figure 1

1992a Pachydiscus (Pachydiscus) paulsoni (Young. 1963) ; Cobban and Kennedy. p. 69, plate 1, figures 4-7; plate 4. figures 7- 9; plate 5, figures 7-8; text-figures 3a-c, 4b

1992b Pachydiscus cf. P. paulsoni (Young. 1963) ; Kennedy et al., p. 441

1993a Pachydiscus sp. ; Kennedy and Cobban.

p. 831., figures 3-4

1995 Pachydiscus (Pachydiscus) duelmenensis (Schlüter. 1872) ; Wippich, p. 52., plate 2. figures 1-2; plate 4. figures 1-2 1995 Pachydiscus (Pachydiscus) duelmensis (Schlüter. 1872) ; Kennedy and Kaplan, p. 27, plate 9. figures 1-2; plate 10, figures 1-2; plate 11; plate 12, figures 1, 2 (with synonymy)

1997 Pachydiscus cf. P. paulsoni ( Young, 1963) ; Kennedy et al., p. 9

2005 Pachydiscus (Pachydiscus) duelmenensis ( Schlüter, 1872) ; Kaplan et al., p. 62, plate 6, figures 6-7; plate 10

2006 Pachydiscus (Pachydiscus) duelmensis ( Schlüter, 1872) ; Kaplan et al., p. 34, plate 2, figures 1-3

Type. The lectotype is GPIB 47, the original of Schlüter (1872, pl. 16, figures 1-2) by subsequent designation by ( Jagt, 1989, p. 7). It is from the lower Campanian Dülmen Formation, Westphalia, Germany, and was re-figured by Kennedy (1986, figure 12) and Jagt (1989, pl. 5).

Material. Five collected specimens, CPC –2193, – 2195, –2197, –2545,and –2561 from bed TPY40 and upwards.

Description. Involute shell (U/D between 0.24 and 0.36) with compressed oval whorl section (WB/WH is around 1 but with variation). The shell is ornamented by narrow, sharp, shallow, moderately sinuous primary ribs separated by one or two long, narrow secondary ribs. The primaries show elongated bullae from the umbilical shoulder to the inner part of flank. All ribs cross the venter with forward arching. Distance of ribs is constant, with increasing D, so that the number of ribs increases linear with D.

Dimensions. Table 12.

Remarks. The specimens included here are characterized by numerous sharp, shallow primary and secondary ribs. These are also present on shells Pachydiscus (Pachydiscus) jimenezi ( Renz, 1936) , P. (P.) paulsoni Young, 1963 and Pachydiscus (Pachydiscus) duelmensis ( Schlüter, 1872) from the lower Campaniain of Mexico, Texas, and Germany.

Specimens from the Tepeyac-Jimenez area were first described by Renz (1936). The holotype of Pachydiscus (P.) jimenezi is the specimen of Renz (1936, p. 3, pl. 2, figure 4, 4a) by monotypy. Renz (op. cit.) states that it is specimen No. 1 in the Collection Böse-Staub, housed in the Geologi-

PALAEO- ELECTRONICA.ORG cal Institute of the University of Bern. This collection has been transferred to the Naturhistorisches Museum Bern, but the holotype is untraceable there and is here regarded as lost .

The morphology described here is also seen in Parapuzosia paulsoni Young, 1963 (p. 53ff). The holotype of Parapuzosia paulsoni is TMM 30625, from the “Gober chalk” in McCurtain County, Oklahoma. Specimens from Texas were described by Cobban and Kennedy (1992b), in addition to unfigured specimens from Texas Cobban and Kennedy (1992a) and Alabama ( Kennedy et al. (1997a). Cobban and Kennedy (1992a) shortly mention similarities of P. paulsoni Young, 1963 with Pachydiscus (Pachydiscus) duelmensis ( Schlüter, 1872, p. 52, P1. 16, figures 1-2; see Kennedy, 1986 a, text-figure 12). They suggested that the American species differed by weak bullae, a more compressed whorl section, and fewer, wider spaced ribs. However, these differences appear to be related to different ontogenetic stages. When plotting the morphometric data of all three species ( Figure 31A–B ), it becomes evident that no notable differences exist between the three closely related species regarding whorl dimensions and ribbing. Rather, each of the species is morphologically variable ( Cobban and Kennedy, 1992a; Kaplan et al., 2005; Kaplan et al., 2006; Kennedy, 1986; Kennedy et al., 1997a; Kennedy and Kaplan, 1995). In fact, the holotypes of Pachydiscus (P.) duelmensis ( Schlüter, 1872) , P. (P.) jimenezi ( Renz, 1936) , and P. (P.) paulsoni ( Young, 1963) plot together in all morphometric aspects ( Figure 31A–B ). As no other unequivocal characteristics exist between them, the three are considered to be conspecific, with Pachydiscus (P.) duelmensis as the homonym.

The low rib index in CPC–2193 is probably due to abrasion of the outer flanks and venter, also allowing for destruction of the secondaries. Specimen CPC–2545 shows a smooth adult body chamber, which exludes an assignation to the genus Eupachydiscus , because the latter presents prominent ribs on the terminal whorl.

Morphometric relations of Menuites gordoni Young, 1963 resemble those of Pachydiscus (P.) duelmensis . However, ribbing differs significantly

IFRIM & STINNESBECK: CAMPANIAN AMMONOID BIOZONATION from the conspicious ornament of P. (P.) duelmensis , and separation of the two taxa is here maintained.

Occurrence. Basal to middle lower Campanian of Germany, Belgium, Netherlands, southwestern France, Oklahoma, Texas, Alabama, and Mexico. In the Tepeyac section, the taxon occurs in bed TPY40 and above this layer, i.e., in the Menabites (Delawarella) tequesquitense , M. (D.) delawarensis and Baculites taylorensis zones, and thus throughout the lower Campanian.

Figures

FIGURE 27. Pachydiscus (Pachydiscus) duelmensis (Schlüter, 1972) from bed TPY40. A–B: CPC–2561. Scale: 50 mm.

FIGURE 28. Pachydiscus (Pachydiscus) duelmensis (Schlüter, 1972) from bed TPY50. A–B: CPC– 2195. Scale: 50 mm.

FIGURE 29. Pachydiscus (Pachydiscus) duelmensis (Schlüter, 1972) from bed TPY50. A–B: CPC–2545. Scale: 50 mm.

FIGURE 30. Pachydiscus (Pachydiscus) duelmensis (Schlüter, 1972) from the Tepeyac section. A: CPC–2545, imprint of inner whorl. B–E: CPC–2197, bed TPY76. Scale: 50 mm.

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.