Menabites (Delawarella) mariscalense ( 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 : 20-23

Menabites (Delawarella) mariscalense ( Young, 1963)

Figure 17A–K , Table 8

1963 Submortoniceras mariscalense , n. sp.; Young, p. 104, plate 59, figure 3; plate 60, figures 1, 4-6; text-figure 14b, f

Type. The holotype is BEG –20478 by original designation by Young (1963, p. 104, plate 59, figure 3; plate 60, figures 1, 4-6; text-figure 14b, f). It is from “the concretion horizon on the west side of Mariscal Mountain, Big Bend National Park, Brewster County,” Texas .

Material. Five specimens, CPC –2380, –2401, – 2553, –2565, and –2566 .

Description. Moderately compressed whorl with involute coiling, U/D is 0.16 to 0.19, WB/WH is 0.43 to 0.58, descreasing throughout ontogeny. The ornament is vanuxemi -like, with fine ribs rising from small umbilical tubercles, but ribs efface on the

PALAEO- ELECTRONICA.ORG flanks so that the outer whorl is almost smooth. At D> 50 mm, flanks and venter are smooth, and the venter is moderately wide and rounded. Only umbilical tubercles persist into this largest stage.

Dimensions. Table 8.

Remarks. Submortoniceras mariscalense Young, 1963 (p. 104, plate 59, figure 3, p1. 60, figures 1, 4- 6, text-figures 14b, f) is based on a single specimen to which five more are added here. It is is more evolute and smoother than all other species of the discoidal Menabites (Delawarella) group.

The species was hitherto included into Submortoniceras , but is here interpeted to have evolved from Menabites (Delawarella) vanuxemi . It is therefore here assigned to this genus. However, both are also close to compressed species from Madagascar, potentially indicating another relationship.

Occurrence. This species is endemic to the lower Campanian of western Texas and across the bor- der into northeastern Mexico.

Figures

FIGURE 17. Menabites (Delawarella) mariscalense (Young, 1963) from the Tepeyac section. A–D: CPC–2402, from bed TPY74, E–G: CPC– 2401 from bed TPY75, H–K: CPC–2380 from bed TPY33. Scale: 50 mm.