Sphenodiscus pleurisepta ( Conrad, 1857 )

Witts, James D., Landman, Neil H., Garb, Matthew P., Irizarry, Kayla M., Larina, Ekaterina, Thibault, Nicolas, Razmjooei, Mohammad J., Yancey, Thomas E. & Myers, Corinne E., 2021, Cephalopods from the Cretaceous-Paleogene (K-Pg) boundary interval on the Brazos River, Texas, and extinction of the ammonites, American Museum Novitates 2021 (3964), pp. 1-52 : 23-24

Sphenodiscus pleurisepta ( Conrad, 1857)

Figure 9B, C

Ammonites pleurisepta Conrad, 1857: 159 , pl. 15, fig. 1.

Sphenodiscus lenticularis (Owen) . Kellum, 1962: 68, pl. 4, figs. 3, 4; pl. 5, fig. 1; pl. 6, figs. 1, 2 . Sphenodiscus pleurisepta ( Conrad, 1857) . Cobban and Kennedy, 1993: 58, figs. 1, 3t.

Sphenodiscus pleurisepta ( Conrad, 1857) . Cobban and Kennedy, 1995: 12, fig. 8.5 (with full synonymy).

Sphenodiscus pleurisepta ( Conrad, 1857) . Kennedy et al., 1996: 11, figs. 4A, 5–12.

Sphenodiscus pleurisepta ( Conrad, 1857) . Kennedy et al., 1997: 9, figs. 9J, 11–14.

Sphenodiscus pleurisepta . Larson et al., 1997: 91.

Sphenodiscus pleurisepta ( Conrad, 1857) . Landman and Cobban, 2003: 17, figs. 12–15.

TYPE: The holotype is USNM 9888, said to be from “Jacun, 3 miles below Laredo,” but is probably from Maverick County, Texas, in the Escondido Formation of the Rio Grande Region ( Stephenson, 1941, 1955).

MATERIAL: One nearly complete specimen when found, now fragmentary ( AMNH 116378), from the Corsicana Formation 1.25 m below the K-Pg boundary at AMNH loc. 3620, Darting Minnow Creek, Falls County, Texas, and one fragmentary specimen ( UNM 15489) from the basal unit (mudstone-clast-bearing conglomerate) of the K-Pg event deposit also at AMNH loc. 3620.

DESCRIPTION: AMNH 116378 is a specimen with the body chamber preserved, ~ 12 cm in diameter. The body chamber bears a hole 12.1 mm in diameter. UNM 15489 is a fragment of the umbilical portion of a body chamber. Both show the characteristic rows of nodes on the smooth flanks that distinguish this species from S. lobatus .

REMARKS: Like Spenodiscus lobatus , S. pleurisepta is rare at Brazos, although the presence of a complete specimen suggests that it may not have experienced postmortem floating.

OCCURRENCE: The two specimens of Sphenodiscus pleurisepta come from the top 1.25 m of the Corsicana Formation and basal ~ 20 cm of the Kincaid Formation, Falls County, Texas. This species has also been reported from the Corsicana Formation in Navarro County, Texas ( Kennedy and Cobban, 1993a; see also Stephenson, 1941, 1955). It occurs in the Escondido Formation in Texas ( Böse, 1928) and northern Mexico. It also occurs in the Maastrichtian Cerro del Pueblo Formation of the Difunta Group at Rincón Colorado, Coahuila ( Ifrim et al., 2004; Ifrim et al., 2005). On the eastern Gulf Coastal Plain, it occurs in the upper part of the Owl Creek Formation in Missouri, Mississippi, and Tennessee and in the Prairie Bluff Chalk in Alabama and Mississippi ( Cobban and Kennedy, 1995). On the Atlantic Coastal Plain, it occurs in the top of the New Egypt Formation and as reworked material at the base of the Hornerstown Formation in Monmouth County, New Jersey ( Weller, 1907; Reeside, 1962; Gallagher, 1993; Landman et al., 2004a, 2004b) and in the Severn Formation in Prince Georges and Kent Counties, Maryland ( Kennedy et al., 1997). In the Western Interior, this species occurs in the Hoploscaphites birklundae Zone of the Pierre Shale in Meade and Pennington counties, South Dakota (Kennedy et al., 1996) and in the upper part of the Pierre Shale and Fox Hills Formation in Weld County, Colorado ( Landman and Cobban, 2003).

Despite possible postmortem drift of Sphenodiscus lobatus specimens, these new data suggest that both species of sphenodiscid cooccur in the Corsicana Formation of the Brazos River area immediately below the K-Pg boundary. Poorly preserved examples of both S. lobatus and S. pleurisepta are also found in the basal unit of the K-Pg event deposit itself. As noted above, this is different from the Maastrichtian deposits of the U.S. Western Interior, with S. pleurisepta present in the Hoploscaphites birklundae Zone , being replaced by S. lobatus in the overlying Hoploscaphites nicolletii Zone. A similar situation was noted by Ifrim et al. (2005), and Ifrim and Stinnesbeck (2010) in their studies of Maastrichtian ammonite faunas from Mexico. These authors suggested that the two sphenodiscid species may have inhabited slightly different environments, explaining why they are rarely found together in the same outcrop or bed. In the U.S. Gulf Coastal Plain, both species range to the top of the Maastrichtian ( Landman et al., 2015), and appear to cooccur at three localities in the Discoscaphites iris Range Zone (fig. 6): in the Owl Creek Formation in Mississippi (AMNH loc. 3460) and Missouri (AMNH loc. 3458) ( Larina et al., 2016) and the Corsicana Formation at Brazos.

Figures

FIGURE 2.

FIGURE 9. A. Sphenodiscus lobatus (Toumey, 1856), AMNH 112085, AMNH loc. 3620, Corsicana Formation, Falls County, Texas. B, C. Sphenodiscus pleurisepta (Conrad, 1857), AMNH loc. 3620, Falls County, Texas. B. UNM 15489, K-Pg event deposit at the base of the Kincaid Formation. C. AMNH 116378, Corsicana Formation, 1.25 m below the K-Pg boundary.