Plesiotexanites shiloensis 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 : 4-6

Plesiotexanites shiloensis Young, 1963

Figure 3A–G , Table 1

1952 Texanites internodosus Young and Marks , p. 477 ff [non plate 1, figure 4 = Texanites internodosus Renz ]

1963 Texanites shiloensis n. sp.; Young, p. 89, plate 46, figures 1-4; plate 54, figures 4- 7; plate 70, figures 5, 6, 8; text-figure 24d 1970 Texanites sp. cf. Texanites (Plesiotexanites) shiloensis Young ; Matsumoto, p.

296, plate 47[51], figure 1; text-figure 26 [100]

1980 Plesiotexanites shiloensis (Young) ; Klinger and Kennedy, p. 65

1991 Texanites (Plesiotexanites) shiloensis Young, 1963 ; Kennedy and Cobban, p.

179, figure 8.30

1997 Texanites (Plesiotexanites) shiloensis Young, 1963 ; Kennedy and Cobban, p.

18, figure 13

Type. The holotype is TMM 1986 by original designation by Young (1963, p. 89, plate 46, figures 1–4, plate 70, figure 5; text-figure 24d). It is from the Dessau Chalk on Brushy Creek , 23.2 km south of Hutto , Williamson County, Texas .

Material. CPC –2363, –2371, –2376, –2378, – 2384, –2385, –2543, IGM 12795, and uncollected specimens in beds TPY 23 to TPY 46.

Description. Evolute, with U ranging from 0.31 to 0.44. Whorl section moderately compressed, with WB/WH of between 0.55 and 0.75, increasing with D. The umbilical wall is undercut and bends in a narrow arch into flattened flanks. Rib index is 27 to 32 at D> 200 mm. Ribs strengthen across the umbilical wall into sharp, closely spaced umbilical bullae that project into the umbilicus. Blunt, straight, slightly prorsiradiate ribs arise from these bullae. A row of weak, rounded tubercles is located at midflank, while a submarginal row of weaker tubercles is close to a marginal row of somewhat stronger clavate tubercles, each of which is linked by a low, broad rib to a strong, external clavus. A broad, smooth zone separates the external clavi from a strong siphonal keel.

Dimensions. Table 1.

Remarks. The species was included into Plesiotexanites by Matsumoto (1970), a view accepted by Kennedy et al. (1997a) and here, based on tuberculation, i.e., the presence of umbilical, lateral, submarginal, marginal, and external tubercles. The compressed, dense, and finely ribbed juvenile stage of the species (e.g., Figure 3B ) distinguish P. shiloensis from all other North American Plesiotexanites . Texanites s.s. is distinguished by tuberculation and details in ribbing.

Occurrence. Upper Santonian to lower Campanian of the Jiménez area of Coahuila, Mexico , the central Texas and Big Bend areas of Texas as well as the state of Mississippi in the US , and Hokkaido, Japan .

Figures

FIGURE 3. Plesiotexanites shiloensis Young, 1963 from the Tepeyac section. A–C: IGM 12795, bed TPY 14; D–F: CPC–2378, bed TPY 33; G: bed TPY 38, not collected. Scale: 50 mm.