Sphenodiscus lobatus ( Tuomey, 1856 )

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 : 22

Sphenodiscus lobatus ( Tuomey, 1856)

Figure 9A

Ammonites lenticularis Owen, 1852: 579 , pl. 8, fig. 5.

Ammonites lobatus Tuomey, 1856: 168 .

Sphenodiscus pleurisepta (Tuomey, 1854) . Cobban and Kennedy, 1995: 61, fig. 3a, v.

Sphenodiscus lobatus ( Tuomey, 1856) . Cobban and Kennedy, 1995: 12, figs. 6.2, 6.3, 8.4, 8.6– 8.11, 12.18, 12.19, 16.16, 16.17 (with full synonymy).

Sphenodiscus lobatus ( Tuomey, 1856) . Kennedy and Cobban, 1996: 802, fig. 2.4–2.6, 2.13, 2.14, 2.19, 2.21 .

Sphenodiscus lobatus ( Tuomey, 1856) . Kennedy et al., 1997: 4, figs. 3–8, 9A–I, 10.

Sphenodiscus lobatus ( Tuomey, 1856) . Landman et al., 2004a: 28, fig. 12.

Sphenodiscus lobatus ( Tuomey, 1856) . Landman et al., 2004b: 51, figs. 23–25.

Sphenodiscus lobatus ( Tuomey, 1856) . Landman et al., 2007a: 58, figs. 26–28.

TYPE: The holotype, from Noxubee County, Mississippi, is lost (fide Stephenson, 1941: 434) .

MATERIAL: Three fragments AMNH 112039, 112070, and 112085, all from the upper 1 m of the Corsicana Formation just below the K-Pg boundary, and one fragment AMNH 135054 from the basal unit (mudstone-clast-bearing conglomerate) of the K-Pg event deposit itself.

DESCRIPTION: AMNH 112070 is a fragment of a phragmocone, AMNH 112085 is a fragment of the outer flanks of a body chamber, and AMNH 135054 is a single small chamber with part of the septum. All show the characteristic smooth flanks indicative of this species.

REMARKS: In contrast to the preservation of whole scaphites and baculitids, all the specimens of Sphenodiscus lobatus in our collection are fragmentary. This suggests that perhaps the shells floated into the area after death and may have lived closer to shore, which is consistent with isotopic evidence from shell samples of this species from the age-equivalent Owl Creek Formation in Mississippi ( Sessa et al., 2015).

OCCURRENCE: The few fragments are from the top 1 m of the Corsicana Formation (AMNH loc. 3620 and AMNH loc. 3621) and the basal unit of the overlying K-Pg event deposit at AMNH loc. 3620. Sphenodiscus lobatus 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. Elsewhere in Mexico, it occurs in the Maastrichtian Cerro del Pueblo Formation of the Difunta Group at Rincón Colorado, Coahuila ( Ifrim et al., 2004, 2005). On the eastern Gulf Coastal Plain, it occurs in the upper part of the Ripley Formation in Mississippi and the Prairie Bluff Chalk in Alabama and Mississippi ( Cobban and Kennedy, 1995). On the Atlantic Coastal Plain, it occurs in the top of the Tinton Formation, the upper part of the Navesink Formation, the lower part of the New Egypt Formation, and as reworked material at the base of the Hornerstown Formation in Monmouth County ( Weller, 1907; Reeside, 1962; Gallagher, 1993; Landman et al., 2004a, 2007a) and in the upper part of the Navesink Formation in Gloucester County, New Jersey ( Gallagher, 1993; Kennedy et al., 1995; Kennedy and Cobban, 1996); in the Providence Sand in the Chattahoochee River area, Alabama and Georgia; in the upper part of the Peedee Formation in North Carolina ( Landman et al., 2004a); and in the Severn Formation in Prince Georges County, Maryland ( Kennedy et al., 1997). In the Western Interior, this species occurs in the Hoploscaphites nicolletii and H. nebrascensis Zones of the Fox Hills Formation in north-central South Dakota ( Landman and Waage, 1993) and in the H. nebrascensis Zone of the Pierre Shale in southeastern South Dakota and northeastern Nebraska ( Kennedy et al., 1998).

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.