MUNIERICERATIDAE Wright, 1952

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 : 28-34

Family MUNIERICERATIDAE Wright, 1952

Genus and subgenus PSEUDOSCHLOENBACHIA

Spath, 1921

Pseudoschloenbachia (Pseudoschloenbachia) mexicana Renz, 1936

Figures 24–26 , Table 11

1936 Schloenbachia bertrandi de Grossouvre var. mexicana Renz , p. 6, plate 1, figure 2; plate 3, figure 1

1963 Pseudoschloenbachia mexicana Renz ; Young, p. 121, plate 29, figures 3-4; plate 30, figures 1-7; plate 31, figures 1, 3-9;

PALAEO- ELECTRONICA.ORG

D [mm]

Tepeyac

Kennedy et al, 1997b

WB/WH

U/D

plate 32, figures 1-6; plate 33, figures 1- 3, 5-7, plate 44, figure 1; text-figures 13e, 28d, 29b, d

1963 Pseudoschloenbachia sp. juv. cf. P. mexicana Renz ; Young, plate 30, figure 9; plate 31, figure 2; plate 33, figure 4

1967 Pseudoschloenbachia mexicana Renz ; Wolleben, p. 1157, plate 149, figures 4-5

1991 Pseudoschloenbachia (Pseudoschloenbachia) mexicana Renz ; Kennedy and Cobban, p. 172, figures 6.1-2, 7.5-39, 10.17-19, 10.22-23, 12.4

2013 Pseudoschloenbachia (Pseudoschloenbachia) mexicana ( Renz, 1936) ; Ifrim et al., p. 885, figures 4.9–4.17, 5.1–5.11, 6.1, 7

Types. Lectotype is NMBE 5024066, designated as holotype by Young (1963, p. 121). It is the original of Renz (1936, pl. 3, figure 1). The paralectotype, which corresponds to the original of Renz (1936, pl. 1, figure 2), was lost (U. Menkveld, NMBE, personal comm. 2018). The type specimens are from the Arroyo Tecolote and Arroyo del Fresno, i.e., a few to a maximum of 10 km north of our working area in northeastern Coahuila, Mexico.

Material. Ten internal moulds, CPC –2334 to –2339 from beds TPY13 to TPY53; CPC –2447, –2450, and –2451 collected earlier from the working area.

Description. Oxyxcone, moderately involute. U is mostly covered with sediment, U/D is about 0.25, except for the largest specimen for which it is> 670 mm. WB/WH ratio ranges from 0.4 to 0.9. The umbilical wall is short and steep. It bends towards the flanks in a narrow arch in juveniles and in a wide arch in adults. Greatest whorl width is at mid-flank in juveniles and in the umbilical half in adults. Flanks bend towards the acute venter ventrolaterally in a moderately wide arch in juveniles, in a wide arch in late juveniles, and they are flat and convergent in adults. Juvenile ornament consists of an umbilical row of distant tubercles that give rise to seven straight primary ribs per half whorl. In between, secondary ribs are intercalated rising at mid-flank. Tubercles along the ventrolateral shoul- der accentuate this narrow edge in jveniles. Ventral to this row of tubercles, all ribs show equal strength and are prorsiradiate and straight, fading towards the venter. The narrow ventrolateral shoulder and the tubercles disappear at larger D, and ribs bifurcate; in adults the ribs are simple, broad,and shallow, with a broad tubercle in the center. The suture line shows an asymmetrically trifid L which exceeds E in depth. Auxiliary lobes are also asymmetrically trifid.

Dimensions. Table 11.

Remarks. An exceedingly wide morphological variability of Pseudoschloenbachia (P.) mexicana ( Renz, 1936) was documented by Ifrim et al. (2013) based on abundant pyritized moulds from the Union y Progreso site in western Coahuila. This full range of morphological variability is not identified in the Tepeyac material; while CPC–2339 is a rather stout specimen, most other individuals conform discoidal end members. The differences in the morphological habitus may relate to the larger sizes of Tepeyac specimens in comparison to the juveniles documented by Ifrim et al. (2013).

Occurrence. Pseudoschloenbachia (P.) mexicana Renz is restricted to the upper Santonian and lower Campanian of Texas, Chihuahua ( Renz, 1936; Young, 1963; Wolleben, 1967; Kennedy and Cobban, 1991; Becker et al., 1998), western ( Ifrim et al., 2013) and north-eastern Coahuila.

Figures

FIGURE 24. Pseudoschloenbachia (P.) mexicana (Renz, 1936) from the lower and middle Tepeyac section. A–C: CPC–2339, TPY13. D–F: CPC–2334, TPY48-50. Scale: 50 mm.

FIGURE 25. Pseudoschloenbachia (P.) mexicana (Renz, 1936) from bed TPY48 of the Tepeyac section. A–D: CPC– 2337. Scale: 50 mm.

FIGURE 26. Pseudoschloenbachia (P.) mexicana (Renz, 1936) from the Jiménez region. A–C: CPC–2351. Scale: 50 mm.