Discoscaphites mullinaxorum, Witts & Landman & Garb & Irizarry & Larina & Thibault & Razmjooei & Yancey & Myers, 2021

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 : 35-37

Discoscaphites mullinaxorum , new species

Figure 14

Discoscaphites iris Conrad, 1858 ). Kennedy and Cobban, 2000: pl. 3, fig. 21.

DIAGNOSIS: Small, closely coiled shell, with no gap between the phragmocone and body chamber; body chamber covered with thin, sharp, slightly flexuous lirae and four rows of tiny tubercles (umbilicolateral, flank, and two rows of ventrolateral tubercles); umbilicolateral and lateral tubercles are radially elongated.

ETYMOLOGY: This species is named after Ronnie and Jackie Mullinax, who have generously granted permission to scores of geologists and paleontologists to explore and collect fossils on their ranch and ensured the preservation of key outcrops for further study. Without their help, the K-Pg sections along the Brazos River would not be as well known worldwide as they are today.

TYPES: The holotype is AMNH 112086 , a crushed microconch, from AMNH loc. 3620, from the top of the Corsicana Formation, Darting Minnow Creek , Falls County, Texas. The paratypes are AMNH 108188 , 111958 , and 112024, from the same locality .

MATERIAL: A total of 10 specimens, all of which are crushed microconchs, from the top 1.5 m of the Corsicana Formation, AMNH loc. 3620 ( Darting Minnow Creek ), Falls County, Texas .

MICROCONCH DESCRIPTION: LMAX averages 22.4 mm and ranges from 17.5 to 29.1 mm. The shell is closely coiled with a large umbilicus. The body chamber occupies approximately one-half whorl and terminates in a constricted aperture (fig. 14P). The whorl height gradually increases in passing from the phragmocone to the body chamber. The umbilical shoulder of the body chamber follows the curvature of the venter.

The ornament on the phragmocone consists of broad indistinct ribs. The body chamber is covered with thin, sharp, slightly flexuous lirae, especially on the adoral one-half. The lirae become less flexuous, coarser, and more closely spaced near the aperture (fig. 14Q, R).

Four rows of tubercles are present on the adoral one-half of the body chamber (fig. 14A, B, G, H). In the holotype, the umbilicolateral tubercles are bullate and increase in size toward the aperture. They are perched on the umbilical shoulder (fig. 14G, H). The flank tubercles are tiny and radially elongate; they are generally not associated with the lirae. Because of crushing, the outer ventrolateral tubercles are not exposed in the holotype, but all four rows of tubercles are visible in AMNH 111958 (fig. 14A, B). The number of inner ventrolateral tubercles exceeds the number of flank tubercles .

REMARKS: This species differs from Discoscaphites iris in its smaller size, its flatter flanks, and more delicate ornament. In particular, the tubercles in D. mullinaxorum are thin and radially elongate, whereas they are conical and pointy in D. iris . Although our collection consists only of microconchs, Kennedy and Cobban (2000: pl. 3, fig. 21) illustrated a small macroconch of this species, which they described as D. iris , from the upper part of the Owl Creek Formation in northeastern Mississippi. This macroconch is approximately 1.3× the average size of the microconchs in our collection. Discoscaphites mullinaxorum also resembles D. minardi Landman et al., 2004a , with its subdued ornament. However, the tubercles on the flanks of the body chamber in D. mullinaxorum are thin and radially elongate whereas they are rare or absent in D. minardi . In its subdued ornament, D. mullinaxorum also resembles D. conradi , but there are only four rows of tubercles in D. mullinaxorum compared to as many as six rows in D. conradi , sometimes even including a midventral row.

OCCURRENCE: This species is rare in the upper part of the Corsicana Formation along the Brazos River and its tributaries in Falls County, Texas. Elsewhere on the Gulf Coastal Plain , it occurs in the Owl Creek Formation in Mississippi ( Kennedy and Cobban, 2000) .

Figures

FIGURE 14. Discoscaphites mullinaxorum n. sp. A–H, K–P. AMNH loc. 3620, Corsicana Formation, Falls County, Texas.I, J. AMNH loc. 3621, Corsicana Formation, Texas. A, B. AMNH 111958, paratype. C, D. AMNH 112028. E, F. AMNH 112024, paratype. G, H. AMNH 112086, holotype. I, J. AMNH 112097. K, L. AMNH 116358. M, N. AMNH 108171. O,P. AMNH 108188, paratype. Specimens in boxes are ×1, others are expanded.