Discoscaphites iris ( Conrad, 1858 )

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 : 29-33

Discoscaphites iris ( Conrad, 1858)

Figure 12A–N

Scaphites iris Conrad, 1858: 335 , pl. 35, fig. 23.

Scaphites iris Conrad. Whitfield, 1892: 265 , pl. 44, figs. 4–7.

Discoscaphites iris (Conrad) . Stephenson, 1955: 134, pl. 23, figs. 23–30.

Discoscaphites iris ( Conrad, 1858) . Kennedy and Cobban, 2000: 183, fig. 5; pl. 3, figs. 3–35.

Discoscaphites iris ( Conrad, 1858) . Landman et al., 2004a: 39 , figs. 15A, B, G–O, 17A–G, 18R.

Discoscaphites iris ( Conrad, 1858) . Landman et al., 2004b: 71 , figs. 34E–W (non A–D = Discoscaphites sphaeroidalis Kennedy and Cobban, 2000 ), 35, 36A–H, K–Q, S–Z, l–p, 37A–l, 38, 39A–P, 41A–D.

Discoscaphites iris ( Conrad, 1858) . Landman et al., 2007a: 82, figs. 40-46, 47A–C.

Discoscaphites iris . Keller et al., 2011: 85, fig. 3E.

Discoscaphites iris ( Conrad, 1858) . Machalski et al., 2009: 375, fig. 2 .

Discoscaphites iris . Sessa et al., 2015: 15563, fig. 1A–C.

Discoscaphites iris . Larina et al., 2016: 132, fig. 2B ;145, fig. 14.1-26

Discoscaphites iris . Witts et al., 2018: 150, fig. 2C ; 156, fig. 7A–U.

Discoscaphites iris . Ferguson et al., 2019: 321.

TYPE: The holotype is the original illustrated in Conrad, 1858 (335, pl. 35, fig. 23), labeled ANSP 50989, from the bluffs of Owl Creek, Tippah County, Mississippi. See Landman et al. (2004b) for a more complete description of this specimen.

MATERIAL: A total of 27 specimens, mostly consisting of the body chamber or parts of the phragmocone and body chamber, plus numerous fragments, in the AMNH and UNM collections. The specimens are nearly equally divided between 13 microconchs and 14 macroconchs. All the specimens are crushed but retain their original aragonitic shell. The specimens are primarily derived from the top 1.5 m of the Corsicana Formation at AMNH locs. 3620, 3621, and 3968, but two ( AMNH 111961 and AMNH 112037 ) also occur in the first (mudstoneclast-bearing) and second unit (ejecta-spherule-rich coarse sandstone) of the K-Pg event deposit at AMNH loc. 3620. D. iris is the most common scaphitid at Brazos, and as such, many of the abundant juvenile scaphitids present in these sections likely belong to this species, but because of their small size, they cannot be identified to the species level .

MACROCONCH DESCRIPTION: Although the specimens are crushed, it is possible to measure the maximum length ( LMAX). They range from 37.2 to 61.6 mm with most specimens falling between 50 and 55 mm (fig. 13). AMNH 112082 and 108182 are examples of small and large specimens, respectively. The ratio of the size of the largest specimen to that of the smallest is 1.7. Specimens are tightly coiled with a small umbilicus. The body chamber occupies approximately one-half whorl. In passing from the phragmocone to the shaft, the whorl height increases slightly, and then decreases again at the aperture. As in other scaphitid macroconchs, the umbilical shoulder of the body chamber is straight and occasionally shows a slight bulge. The aperture is constricted and the angle of the aperture averages 30°.

The spire is covered by prorsiradiate ribs. They are broad and straight in AMNH 111959 and thin and slightly sinuous in AMNH 108178. Intercalation and branching occur at onethird and two-thirds whorl height. The ribs become broader and more widely spaced toward the adoral part of the spire. Two rows of ventrolateral tubercles are visible on the adapical part of the spire, although the outer row is difficult to discern because of crushing. An additional two rows of tubercles appear on the flanks soon thereafter.

The ornament on the body chamber consists of four rows of tubercles—umbilicolateral, flank, and inner and outer ventrolateral tubercles. The tubercles occur on broad, low convex ribs that become more prominent on the hook. All the tubercles end in sharp points. The most prominent tubercles are the two umbilicolateral tubercles on the midshaft just below the umbilical margin (e.g., AMNH 198178). The flank tubercles are slightly smaller than the umbilicolateral tubercles and occur midway between the umbilicolateral and inner ventrolateral tubercles.

MICROCONCH DESCRIPTION: Microconchs are, on average, smaller than macroconchs. Microconchs range in LMAX from 25.1 to 42.6 mm with most specimens falling between 30 and 35 mm (fig. 13). AMNH 111963 (fig. 10J) and 108186 (fig. 10N) are examples of small and large specimens, respectively. The ratio of the size of the largest specimen to that of the smallest is 1.7. In passing from the spire into the body chamber, the whorl height increases slightly. As a result, the umbilical seam follows the curvature of the venter. The body chamber is slightly uncoiled and occupies approximately one-half whorl.

The ornamentation on the phragmocone consists of thin, slightly flexuous prorsiradiate ribs and two rows of ventrolateral tubercles. The prorsiradiate ribs on the body chamber are more poorly defined. They bear four rows of tubercles, of which the umbilicolateral tubercles are the most prominent. They are perched on the umbilical shoulder and attain their greatest height just adoral of midshaft.

REMARKS: In comparison to specimens of Discoscaphites iris from New Jersey and the eastern Gulf Coastal Plain, the specimens from the Brazos River localities are larger (fig. 13). For example, LMAX of the largest macroconch from the Brazos River locality is 61.6 mm whereas it is 54.2 mm from New Jersey ( Landman et al., 2007a). In contrast, the specimen from Libya is still larger, with an estimated diameter of 80 mm ( Machalski et al., 2009). At least one specimen associated with oysters represents postmortem encrustations (fig. 5C, D).

OCCURRENCE: This species is known from the upper part of the Corsicana Formation and the base of the Kincaid Formation along the Brazos River and its tributaries in Falls County, Texas. It has also been reported in the Mullinax-1 and Mullinax-3 cores from the Corsicana Formation in the same area (Keller et al., 2011). Elsewhere on the Gulf Coastal Plain, it occurs in the Owl Creek Formation in Mississippi, Tennessee, and Missouri ( Stephenson, 1955; Sohl, 1960, 1964; 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; the upper part of the New Egypt Formation and as reworked material at the base of the Hornerstown Formation in northeastern and southwestern Monmouth County ( Landman et al., 2007a); and the upper part of the Severn Formation in Kent and Anne Arundel counties, Maryland ( Landman et al. 2004a). It is the name bearer of the Discoscaphites iris Zone in the Gulf and Atlantic Coastal Plains, where it represents the upper part of the upper Maastrichtian, corresponding to the upper part of calcareous nannofossil Subzone CC26b ( Landman et al., 2004a; 2004b; 2007a; Larina et al., 2016). This species has also been reported from the upper Maastrichtian of northwest Libya ( Machalski et al., 2009).

Figures

FIGURE 2.

FIGURE 12. Discoscaphites iris (Conrad, 1858) (A–N) and Discoscaphites sphaeroidalis Kennedy and Cobban, 2000 (O, P), macroconch (A–I) and microconch (J–P). A, B, D, E, G–M, O, P. AMNH loc. 3620, Corsicana Formation, Falls County, Texas. C, F, N. AMNH loc. 3621, Corsicana Formation, Texas. A. AMNH 111962. B. AMNH 111959. C. AMNH 108177. D. AMNH 112075. E. AMNH 108175. F. AMNH 108188. G. AMNH 108178. H. AMNH 116348. I. AMNH 116346. J. AMNH 111963. K. AMNH 116351. L. AMNH 116347. M. AMNH 112038. N. AMNH 108186. O. AMNH 111957. P. AMNH 116345.