Discoscaphites, Meek, 1871

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 : 37-39

Discoscaphites sp.

Figures 15 , 16

MATERIAL: All sites we studied at the Brazos River localities yield many small juveniles of Discoscaphites (fig. 15). Because of their small size, it is impossible to identify them to a species level.

DESCRIPTION: Juvenile specimens range in LMAX from 1.7 to 10.4 mm and retain part or all of their body chamber (fig. 15; table 1) Although the specimens preserve traces of the outer

shell wall, the ornamentation is subdued or absent. Several specimens show exquisite details of the microornamentation of the embryonic shell (ammonitella; fig. 16).

REMARKS: The presence of complete to nearly complete juveniles suggests that the animals lived at the site in which they are buried. All the specimens appear as isolated occurrences except for AMNH 63319, which occurs in a concentration of shell hash along with other juveniles and fish debris.

OCCURRENCE: All specimens are from the upper portion of the Corsicana Formation just below the K-Pg boundary at AMNH locs. 3620 and 3621, Darting Minnow and Cottonmouth creeks, respectively.

Lower jaws of Discoscaphites

Figure 11E–J

MATERIAL: Four isolated specimens ( AMNH 108284, 108179, 108282, 63317) and one preserved in situ ( AMNH 63312) from the top 1.25 m of the Corsicana Formation at AMNH loc. 3620 and AMNH loc. 3621, Darting Minnow and Cottonmouth creeks, Falls County, Texas. The specimens consist of the outer calcareous valve (aptychus), although three of them also retain traces of the inner black layer

DESCRIPTION: Each valve is roughly triangular in outline. The ratio of valve length to valve width ranges from 1.4 to 1.9 in our sample. The symphysal edge is straight and forms a flange that increases in height posteriorly. The anterior margin is nearly straight, the lateral margin is broadly rounded, and the posterior margin is sharply rounded. The ventral surface of each valve is covered with small folds that parallel the posterior margin.

Most of the jaws are preserved with only one valve exposed. However, in AMNH 63317 the two valves are folded folio style. As a result, the ventral surface of one valve and part of the dorsal surface of the other valve are exposed. AMNH 66312 occurs inside a very poorly preserved specimen of Discoscaphites iris surrounded by small oysters. The specimen of D. iris is 32.1 mm in maximum length, so that the ratio of LMAX to the length of the jaw is 2.3.

REMARKS: These specimens are similar to the lower jaws of scaphites illustrated from Maastrichtian deposits of South Dakota ( Landman and Waage, 1993: figs. 37–41, 167E–I) and northern Europe ( Birkelund, 1982: pl. 2, figs. 6, 7; Birkelund, 1993: pl. 17, figs. 2–4 ; Machalski, 2005b: fig. 26).

Figures

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 11.

FIGURE 15.

FIGURE 16. Discoscaphites sp., overview and close-up of ammonitella showing microornamentation, AMNH 112066, AMNH loc. 3620, Corsicana Formation, Falls County, Texas.