Sphenodiscus pleurisepta ( Conrad, 1857 )

Landman, Neil H., 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 2. Northeastern Monmouth County, New Jersey, Bulletin of the American Museum of Natural History 2004 (287), pp. 1-107 : 50-54

Sphenodiscus pleurisepta ( Conrad, 1857) Figure 22

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.

TYPE: The holotype is USNM 9888, said

to be from ‘‘Jacun, 3 miles below Laredo’’, but probably from the Escondido Formation in the Rio Grande Region, Maverick County, Texas ( Stephenson, 1941, 1955).

MATERIAL: AMNH 47118 from the New Egypt /Hornerstown formational contact, AMNH loc. 3345, Parkers Creek, near Eatontown, Monmouth County.

DESCRIPTION: AMNH 47118 is a phragmocone 69.8 mm in diameter with part of the innermost flanks missing and most of the venter worn off (fig. 22). The specimen is involute with a very small umbilicus. The umbilical wall is steep and convex and the umbilical shoulder is sharply rounded on the adapical end of the specimen. The inner flanks are concave and the middle and outer flanks are broadly rounded on the adapical end of the specimen. Rectiradiate, straight, broad ribs are visible on the adapical onehalf of the specimen. There is a row of five radially elongated nodes on the midflanks with a distance of approximately 12 mm between the two most adoral nodes. Two nodes are preserved on the outer flanks on the right side.

DISCUSSION: The presence of two rows of nodes on the flanks distinguishes Sphenodiscus pleurisepta from the widely occurring Sphenodiscus lobatus . 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 possible that more than one species is masquerading under this name. It is probably a good idea to closely compare samples of this species from opposite ends of its stratigraphic range.

OCCURRENCE: New Egypt /Hornerstown formational contact, Parkers Creek, near Eatontown, Monmouth County. Sphenodiscus pleurisepta has not previously been reported from New Jersey. It also occurs in the Crosswicks Creek Drainage in southwestern Monmouth County (Landman et al., in prep. b). Elsewhere on the Gulf and Atlantic Coastal Plains, this species is known from the Owl Creek Formation in Missouri, Mississippi, and Tennessee ( Kennedy and Cobban, 2000), the Prairie Bluff Chalk in Mississippi ( Cobban and Kennedy, 1995), the Corsicana Formation in northeast Texas ( Kennedy and Cobban, 1993), and the Severn Formation, Prince Georges and Kent counties, Maryland ( Kennedy et al., 1997; Landman et al., 2004). It is known from the Escondido Formation in Trans­Pecos Texas and northern Mexico ( Stephenson, 1941, 1955). In the Western Interior, this species occurs in 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) Figures 23–25

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 are two specimens: MAPS A 2002a8 and A2002a9 from the Tinton Formation, AMNH loc. 3348, 0.2 km north of the intersection of Water Street and Tinton Avenue, Tinton Falls, Monmouth County, New Jersey.

DESCRIPTION: MAPS A 2002a9 is one­third whorl of a large phragmocone (not illustrat­ ed). The whorl width and height at the adoral end of the specimen are approximately 66.6 mm and 164.0 mm, respectively; the ratio of whorl width to height is 0.41. The flanks are very broadly rounded and converge to an acute venter. Part of the suture on the adapical end of the specimen is illustrated (fig. 25A).

MAPS A 2002a8 is a large specimen, 285 mm in diameter, with the adoral end slightly crushed in (figs. 23, 24). Most of the speci­ men is phragmocone with barely less than one­eighth whorl of body chamber attached. However, part of the rest of the body chamber was still present but was not collected in the field because it was too crushed. Three oval to circular depressions appear on the midflanks of the body chamber on the left side, the largest of which is approximately 16 mm in diameter. The inner flanks are nearly flat and slightly divergent, the midflanks are very broadly rounded and subparallel, and the outer flanks are nearly flat and

23). A, Apertural; B, ventral. Figure reduced X0.70.

converge to an acute venter. The whorl width and height near the base of the body chamber are approximately 35.2 mm and 170 mm, respectively, although the whorl width is underestimated due to crushing; the ratio of whorl width to height is 0.21. The flanks are smooth without any nodes. The suture has a very broad and shallow external lobe with a low, little incised median saddle (fig. 25B).

DISCUSSION: These specimens lack the two rows of nodes characteristic of Sphenodiscus pleurisepta and are referred to Sphenodiscus lobatus . Cobban and Kennedy (1995) fully discussed S. lobatus and the differences with congeneric species. The shallow circular depressions and crushed body chamber may indicate predation.

OCCURRENCE: Tinton Formation, near Tinton Falls, northeastern Monmouth County, New Jersey ( Weller, 1907; Reeside, 1962; see also Gallagher, 1993). Sphenodiscus lobatus has also been reported in New Jersey from the MFL at the Inversand Marl Pit, Gloucester County ( Gallagher, 1993; Kennedy et al., 1995; Kennedy and Cobban, 1996). Elsewhere on the Gulf and Atlantic Coastal Plains, this species is known from the Corsicana Formation in northeast Texas ( Kennedy and Cobban, 1993); the upper part of the Ripley Formation in Mississippi; the Prairie Bluff Chalk in Alabama and Mississippi ( Cobban and Kennedy, 1995); the Providence Sand in the Chattahoochee River area, Alabama and Georgia; the upper part of the Peedee Formation in North Carolina ( Landman et al., 2004); and the Severn Formation in Prince Georges County, Maryland ( Kennedy et al., 1997). It is known from the Escondido Formation in Trans­Pecos Texas and northern Mexico ( Stephenson, 1941, 1955). In the Western Interior, this species 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).

Figures

Fig. 22. Sphenodiscus pleurisepta (Conrad, 1857). AMNH 47118, uppermost New Egypt Formation and basal Hornerstown Formation, AMNH loc. 3345, northwest of Eatontown, Monmouth County. A, Right lateral; B, left lateral. All figures are X1.

Fig. 23. Sphenodiscus lobatus (Tuomey, 1856). MAPS A2002a8, Tinton Formation, AMNH loc. 3348, Tinton Falls, Monmouth County, New Jersey, left lateral. Note the circular depressions near the base of the body chamber on the adoral end of the specimen. Figure reduced X0.62.

Fig. 24. Sphenodiscus lobatus (Tuomey, 1856). A, B. MAPS A2002a8 (same specimen as in fig.

Fig. 25. Sphenodiscus lobatus (Tuomey, 1856), Tinton Formation, AMNH loc. 3348, Tinton Falls, Monmouth County, New Jersey. A. MAPS 2002a9, suture at the adapical end of the specimen at a whorl height of approximately 139 mm. B. MAPS A2002a8, suture at a whorl height of approximately 91 mm.