Discoscaphites sphaeroidalis Kennedy and Cobban, 2000

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 : 34-35

Discoscaphites sphaeroidalis Kennedy and Cobban, 2000

Figure 12O, P

Discoscaphites sp. Stephenson, 1955: 135, pl. 23, figs. 20–22.

Discoscaphites sphaeroidalis Kennedy and Cobban, 2000: 185 , pl. 1, figs. 1–11 , text-fig. 6

Discoscaphites sphaeroidalis Kennedy and Cobban, 2000 . Kennedy et al., 2001: 169, fig. 4b, c. Discoscaphites iris ( Conrad, 1858) . Landman et al., 2004b: 71, fig. 34A–D.

Discoscaphites sphaeroidalis Kennedy and Cobban, 2000 . Landman et al., 2007a: 92, figs. 47D, 48.

TYPE: The holotype is USNM 465615, a macroconch missing part of the hook. It is from the Owl Creek Formation at its type locality, 4 km northeast of Ripley , Tippah County, Mississippi .

MATERIAL: The material from Brazos consists of six specimens, three of which are identifiable as adult microconchs ( AMNH 108185, 111957, and 116345) and three of which are fragments of adult microconchs ( AMNH 108174, 108184, and 133094) from the top of the Corsicana Formation in Darting Minnow and Cottonmouth creeks ( AMNH locs. 3620 and 3621), Falls County, Texas.

MICROCONCH DESCRIPTION: The three nearly complete specimens range in size from 31.6 to 36.1 mm. The phragmocone extends slightly below the line of maximum length and the body chamber occupies approximately one-half whorl. As shown in AMNH 116345 (fig. 12P), the aperture is constricted with a weak ventral projection.

The ornamentation on the phragmocone is well preserved in AMNH 11957 (fig. 12O). The ribs are narrow, long, and uniformly spaced. They bear four rows of tubercles, of which the inner ventrolateral row is the most prominent. The ribs on the body chamber are weaker, but the tubercles are stronger than those on the phragmocone. The tubercles are widely spaced on the shaft and more closely spaced on the hook. Both rows of ventrolateral tubercles extend to the aperture.

REMARKS: Because these specimens are crushed, no information is available about the shape of the whorl section, which is important for identifying this species and distinguishing it from Discoscaphites iris . However, the ornamentation on the phragmocone is well preserved and consists of narrow, long ribs, one of the defining features of this species. It is curious that our collection consists only of microconchs whereas other collections of this species contain both dimorphs ( Kennedy and Cobban, 2000; Kennedy et al., 2001; Landman et al., 2007a). If macroconchs were present, they would be easily recognizable. Their absence is probably due to the small size of the collection.

OCCURRENCE: This species is known from the upper part of the Corsicana Formation along the Brazos River and its tributaries in Falls County, Texas. Kennedy et al. (2001) reported it as occurring throughout the top ~ 8 m of the Corsicana Formation. Elsewhere on the Gulf Coastal Plain, it occurs in the Owl Creek Formation in Mississippi and Missouri ( Stephenson, 1955; Kennedy and Cobban, 2000). On the Atlantic Coastal Plain, it occurs in the upper part of the Tinton Formation and as reworked material at the base of the Hornerstown Formation, central Monmouth County, and in the upper part of the New Egypt Formation and as reworked material at the base of the Hornerstown Formation in northeastern Monmouth County ( Landman et al., 2007a).

Figures

FIGURE 1. Locality map of the Brazos River area south of Texas Rt. 413 in southern Falls County, Texas, showing outcrops (AMNH locs. 3620, 3621, 3968, Brazos 1) and drill cores (Mullinax-1-3) mentioned in the text.

FIGURE 2.

FIGURE 3. Litho- and biostratigraphic sections. K-Pg boundary shown as dashed line connecting outcrops. Biostratigraphy is illustrated for each section based on (1) ammonites, (2) calcareous nannofossils, and (3) planktonic foraminifera. Cephalopod-occurrence color code: green = Cretaceous; white = incorporated into K-Pg event deposit; orange = Paleogene. Abbreviations: DSB, Dirty Sandstone Bed; LCH, Lower Concretion Horizon; MSB, Middle Sandstone Bed; va, volcanic ash horizon at AMNH loc. 3621; * single cephalopod occurrence from the MSB (Eutrephoceras sp.) collected at Darting Minnow Creek (AMNH loc. 3620), but illustrated on the stratigraphically continuous section at AMNH loc. 3698 from Hart et al. (2012) and Leighton et al. (2017). The DSB, LCH, and MSB concretionary marker beds can be found and correlated throughout the Brazos River area (e.g., Yancey, 1996; Hart et al., 2012).

FIGURE 4. Biostratigraphic framework for the Brazos River K-Pg boundary successions and correlation to magnetostratigraphy and international timescale. A. Calcareous nannofossils based on the schemes of Perch- Nielsen (1985) for the Cretaceous and Martini (1971) for the Paleogene (see Tantawy et al., 2011). M. prin = Micula prinsii; C. inter. = Cruciplacolithus intermedius. B. Planktonic foraminfera based on the CF scheme of Li and Keller (1998) for the Cretaceous and Berggren and Pearson (2005) for the Paleogene. P. hariaensis = Pseudoguembelina hariaensis; G. gansseri = Gannsserina gansseri; P. hantk. = Plummerita hantkeninoides; P. eugub. = Parvularugoglobigerina eugubina; S. triloc. = Subbotina triloculinoides. C. Dinoflagellate ranges based on Aydin (2013). P. grallator = Palynodinium grallator; T. pelagica = Thalassiphora pelagica; D. carposph. = Disphaerogena carposphaeropsis; C. corn. = Carpatella cornuta; D. calif. = Damassadinium californicum. D. Ammonites (this study). D. iris = Discoscaphites iris; D. minardi = Discoscaphites minardi. E. Lithostratigraphy (Smith and Pessagno, 1973; Yancey, 1996). Dashed lines indicate uncertainty in position/timing of boundary between range zone.

FIGURE 6. Occurrences and spatial distribution of cephalopod taxa reported from the upper Maastrichtian Discoscaphites iris range zone in the Atlantic and Gulf Coastal Plains of the United States (AK= Arkansas; AL = Alabama; MD = Maryland; MO = Missouri; MS = Mississippi; NJ = New Jersey; TX = Texas). Localities arranged on a general north-south transect from right to left. The? next to D. gulosus from Texas reflects that we have not recognized this taxon in our extensive collection of Discoscaphites from Brazos. Discoscaphites minardi is tentatively listed as occurring at Brazos (dashed line) after discovery of a poorly preserved specimen in the Mullinax-1 drill core. Similarly, Eubaculites latecarinatus is not found in the Corsicana Formation in our collections, but several specimens are found in the basal units of the K-Pg event deposit suggesting it inhabited this area during the late Maastrichtian. Data derived from a survey of AMNH Invertebrate Paleontology Collections, UNM Paleontology Collections (Landman et al., 2004a; 2004b; 2007; Larina et al., 2016; Witts et al., 2018; and this study).

FIGURE 7. Eutrephoceras sp., UNM 15495, Middle Sandstone Bed, AMNH loc. 3620, basal part of the Paleogene (Danian), Kincaid Formation, Falls County, Texas. A, left lateral; B, ventral; C, apertural.

FIGURE 8. Gaudryceras sp., AMNH 111956, AMNH loc. 3620, Corsicana Formation, 1 m below the K-Pg boundary, Darting Minnow Creek, Falls County, Texas. A, B. Part and counterpart.

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.

FIGURE 10.

FIGURE 11.