Sphenodiscus Meek, 1871

LANDMAN, NEIL H., JOHNSON, RALPH O. & EDWARDS, LUCY E., 2004, Cephalopods from the Cretaceous / Tertiary Boundary Interval on the Atlantic Coastal Plain, with a Description of the Highest Ammonite Zones in North America. Part 1. Maryland and North Carolina, American Museum Novitates 3454 (1), pp. 1-64 : 26-29

Genus Sphenodiscus Meek, 1871

TYPE SPECIES: Ammonites lenticularis Owen, 1852: 579 (non Young and Bird, 1828: 269, fig. 5), by original designation, = Ammonites lobata Tuomey, 1856: 168.

Sphenodiscus pleurisepta ( Conrad, 1857)

Figures 10A–J

Ammonites pleurisepta Conrad, 1857: 159 , pl. 15, fig. 1.

Sphenodiscus lenticularis (Owen) . Kellum, 1962: 68, pl. 4, figs. 3, 4; pl. 5, fig. 1; pl. 6, figs. 1, 2.

Sphenodiscus pleurisepta ( Conrad, 1857) . Cobban and Kennedy, 1995: 12, fig. 8.5 (with full synonymy).

Sphenodiscus pleurisepta ( Conrad, 1857) . Kennedy et al., 1996: 11, figs. 4A, 5–12.

Sphenodiscus pleurisepta ( Conrad, 1857) . Kennedy et al., 1997: 9, figs. 9J, 11–14.

Sphenodiscus pleurisepta . Larson et al., 1997: 91.

Sphenodiscus pleurisepta ( Conrad, 1857) . Landman and Cobban, 2003: 17, figs. 12–15.

TYPE: The holotype is USNM 9888, said to be from ‘‘Jacun, 3 miles below Laredo’’, but probably from the Escondido Formation of the Rio Grande Region, probably in Maverick County, Texas ( Stephenson, 1941, 1955).

MATERIAL: There are four specimens: AMNH 49412 , 49417 , 49420 , and 49421 from AMNH loc. 3252 from the Severn Formation , Kent County, Maryland .

DESCRIPTION: AMNH 49412 is a fragment of a phragmocone slightly more than onehalf whorl long (fig. 10A–C). It is missing most of its venter on the adoral one­third whorl. The whorl width and height at the adapical end of the specimen are 7.8 mm and 25.7 mm, respectively; the ratio of whorl width to height is 0.30. The umbilicus is small but obscured by matrix. The whorl section is oxyconic. The inner flanks are slightly concave, the midflanks are broadly rounded, and the outer flanks gently slope to an acute venter. The ornament on the left side is better preserved. There are four radial swellings on the outer flanks, which become more widely spaced adorally. The distance between the two most adoral swellings is 8 mm. It is possible that there are two swellings on the midflanks but the specimen is not well enough preserved to determine this. The suture has narrow­stemmed saddles with phylloidlike terminations (fig. 11).

There are three other specimens. AMNH 49421 is a small fragment of a phragmocone consisting of part of the outer flanks and acute venter (fig. 10F–H). Two radial swellings are present on the outer flanks with a distance of approximately 9 mm between them. AMNH 49420 is a worn piece of a phragmocone with a maximum preserved whorl height of 41.6 mm (fig. 10D, E). It shows faint swellings on the inner and middle flanks. AMNH 49417 is a small piece of a body chamber with an acute venter and weak swellings on the outer flanks (fig. 10I, J).

DISCUSSION: Although fragmentary, these specimens all show swellings on the outer flanks and, in a couple of instances, on the midflanks as well. This suggests that these specimens are Sphenodiscus pleurisepta rath­ er than Sphenodiscus lobatus ( Tuomey, 1856) . Cobban and Kennedy (1995) discussed the synonymy of S. pleurisepta and its differences from congeneric species. Because this species has an unusually long range (the entire upper Maastrichtian), it is probably worthwhile in the future to more closely examine specimens of this species from the Gulf and Atlantic Coastal Plain and those from the Western Interior to confirm that they are all conspecific (N.L. Larson, personal commun., 2004).

OCCURRENCE: Severn Formation, Kent County, Maryland; Severn Formation, Prince Georges County, Maryland ( Kennedy et al., 1997). This species occurs in New Jersey in the top of the New Egypt Formation and as reworked material in the base of the Hornerstown Formation, northeastern Monmouth County, and in the upper part of the New Egypt Formation, southwestern Monmouth County (Landman et al., in prep. a, c). It is known on the Gulf Coastal Plain from the Corsicana Formation in northeastern Texas ( Kennedy and Cobban, 1993), the Owl Creek Formation in Missouri, Mississippi, and Tennessee ( Kennedy and Cobban, 2000), and the Prairie Bluff Chalk in Mississippi ( Cobban and Kennedy, 1995). It is also known from the Escondido Formation in Trans­Pecos Texas and northern Mexico ( Stephenson, 1941, 1955). In the Western Interior, this species is reported from the Hoploscaphites birkelundae Zone of the Pierre Shale in Meade and Pennington Counties, South Dakota (Kennedy et al., 1996; Larson et al., 1997), the Fox Hills Formation in Niobrara County, Wyoming (Kennedy et al., 1996), and the upper part of the Pierre Shale and Fox Hills Formation in Weld County, Colorado (Kennedy et al., 1996; Landman and Cobban, 2003).

Sphenodiscus lobatus ( Tuomey, 1856)

Figure 12A, B

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

Ammonites lobatus Tuomey, 1856: 168 .

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.

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

MATERIAL: There is one specimen ( AMNH 48554 ) from the Peedee Formation , Brunswick County, North Carolina .

DESCRIPTION: AMNH 48554 is an internal mold 81.6 mm in diameter (fig. 12A, B). It is part of the phragmocone of a much larger specimen. A portion of the ventrolateral surface on the right side is covered by a bryozoan colony. There are two large circular depressions near the adoral end. The whorl section is oxyconic; the whorl width and height near the aperture are 14.9 and 47.2 mm, respectively; the ratio of whorl width to height is 0.32. The umbilicus is small and bordered on the right side by the umbilical wall of the next whorl, most of which is missing. The inner flanks are slightly concave, the midflanks are broadly convex, and the outer flanks are nearly flat and converge to an acute venter. The surface of the shell is smooth without any trace of ornament.

OCCURRENCE: Peedee Formation, Brunswick County, North Carolina; Tinton Formation, near Tinton Falls, northeastern Monmouth County, New Jersey ( Gallagher, 1993); Tinton Formation, near Freehold, central Monmouth County, New Jersey (Landman et al., in prep. b); upper part of the Navesink Formation and lower part of the New Egypt Formation, southwestern Monmouth County, New Jersey (Landman et al., in prep. c); base of the Hornerstown Formation, Gloucester County, New Jersey ( Gallagher, 1993; Kennedy et al., 1995; Kennedy and Cobban, 1996); Severn Formation, Prince Georges County, Maryland ( Kennedy et al., 1997); Corsicana Formation, northeast Texas ( Kennedy and Cobban, 1993); upper part of the Ripley Formation, Mississippi; Prairie Bluff Chalk, Alabama, and Mississippi ( Cobban and Kennedy, 1995); and Providence Sand, Chattahoochee River area, Alabama and Georgia. This species is also known from the Escondido Formation in Trans­Pecos Texas and northern Mexico. In the Western Interior, it occurs in the Hoploscaphites nicolletii and Jeletzkytes nebrascensis Zones of the Fox Hills Formation in north­central South Dakota ( Landman and Waage, 1993) and in the J. nebrascensis Zone of the Pierre Shale in southeastern South Dakota and northeastern Nebraska ( Kennedy et al., 1998).