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Nostoceras (Nostoceras) rugosum Cobban & Kennedy 1991 from: Ifrim, C..Stinnesbeck, W..López-Oliva, J.G.. (2004): Maastrichtian cephalopods from Cerralvo, north-eastern Mexico . Palaeontology Vol. 47(6) p. 1575–1627
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Species Nostoceras (Nostoceras) rugosum Cobban & Kennedy 1991

[1]
[2]
[3]
[1] pl. 4, figs. 5-6 Ifrim et al. (2004) [2] pl. 4, fig. 7 Ifrim et al. (2004) [3] text-fig. 14d Ifrim et al. (2004)

Alternative name:
Diagnosis / Definition:
Ifrim et al. (2004):
Description: The low expansion rate results in a highly trochispiral cone with an apical angle of 40 degrees. The sense of coiling is both dextral and sinistral, but sinistral phragmocones predominate. Whorls are evolute to serpenticone. Inner walls create a spindle. This species displays dimorphism. The whorl section of microconchs (PI. 4, fig. 6) is rounded to slightly tearshaped, caused by a concave groove on the dorsum which accommodates the base of the previous whorl. External and internal whorls are widely arched. WH/WB is c. 1·1. Ornament in microconchs is similar to macroconchs but smoother. In macroconchs (PI. 4, figs 5, 7) there are c. 50 weak ribs per whorl on the lower flank and contact to the succeeding whorl. They are slightly flexuous, in later juvenile growth stages prorsiradiate. Ribs fork into pairs at bullae at or directly above the whorl suture. This ornament of macroconchs results in a rounded to square-shaped whorl section. Where test is preserved there is a tubercle on each end of a bulla. Simple ribs are present irregularly in early growth stages, whereas trifid ribs occur in later growth stages. A second row of bullae is present on the inner whorl suture. Both rows of bullae are linked by straight prorsiradiate ribs which are located on the contact between whorls. Up to three constrictions are present per whorl and flanked by collared ribs. Ribs on microconchs are similar, but ridges are smaller and much weaker on the internal moulds. Our material includes a single initial whorl of Nostoceras (Nostoceras) rugosum which is uncoiled at an undeterminable angle. Ornament is already present on the second whorl but too faint to be determined exactly. Only short parts of the suture line are preserved. It is asymmetrical, moderately incised and similar to that of Nostoceras (Nostoceras) colubriformis and N. (N.) alternatum.
Discussion / Comments:
Ifrim et al. (2004):
Remarks: Cobban and Kennedy (l99la) described dimorphism for the species based on uncoiling of the adult hook at different diameters. No hooks are preserved in the Cerralvo material and dimorphism can only be observed based on ornament. Microconchs strongly resemble N (N) colubriformis, but differences exist regarding rib index, apical angle, expansion rate, and WHlWD ratio. The sharp bullae on the lower outside of internal moulds also characterize Nostoceras (Nostoceras) rugosum but are absent in N (N) colubriformis. The initial whorl of the two species differ in angle of uncoiling. Macroconchs resemble N (N) alternatum in ornament but differ by a lower expansion rate and a lower apical angle. The high spire and sub quadrate whorl section separate this species from all other Nostoceras except for N (N) pauper (Whitfield, 1892), which has a much lower apical angle, stronger ribs and more tuberc1es. Occurrence: In the Nacatotch Sand of Arkansas N. (N.) rugosum marks a biozone below the N. (N.) alternatum biozone (Cobban and Kennedy 1991a, b). It characterises a local assemblage zone above the Upper Campanian N. (N.) hyatti Zone and below the N. (N.) alternatum assemblage zone (Cobban and Kennedy 1991b). The Mexican specimens presented here are the first recorded outside Hempstedt County, Arkansas.
Synonym list:
Ifrim et al. (2004):
1991 Nostoceras (Nostoceras) rugosum Cobban & Kennedy. - Cobban & Kennedy : C2 pl. 1, figs. 13-34
2004 Nostoceras (Nostoceras) rugosum Cobban & Kennedy. - Ifrim et al. : 39, 35 pl. 4, figs. 5-7; text-fig. 14d
Stratigraphy - relative ages:
upper Campanian: Ifrim et al. (2004)
References:

Cobban,W.A.. and Kennedy,W.J.. (1991):
Upper Cretaceous ammonites from the Nacatotch Sand of Hempstead County, Arkansas . United States Geological Survey Bulletin Vol. 1985 p. C1-C5

Ifrim,C..; Stinnesbeck,W.. and López-Oliva,J.G.. (2004):
Maastrichtian cephalopods from Cerralvo, north-eastern Mexico . Palaeontology Vol. 47(6) p. 1575–1627

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