Didymoceras, Hyatt, 1894

KENNEDY, W. J., LANDMAN, N. H., COBBAN, W. A. & SCOTT, G. R., 2000, Late Campanian (Cretaceous) Heteromorph Ammonites From The Western Interior Of The United States, Bulletin of the American Museum of Natural History 2000 (251), pp. 1-88 : 38-50

Didymoceras cf. D. cheyennense . Ward and Orr,

1997: figs. 5.7–5.9, 6.6, 6.7.

Didymoceras cheyennense (Meek and Hayden) .

Larson et al., 1997: 56, 2 unnumbered figs.

TYPES: The holotype, by monotypy, is USNM 470, from ‘‘Mouth Cheyenne river. No. 4 of series.’’ (Meek and Hayden, 1856a: 72). The specimen probably came from rocks now assigned to the DeGrey Member of the Pierre Shale in north-central South Dakota.

MATERIAL: About 170 specimens, mostly half a whorl or less. Most are from limestone concretions and are usually uncrushed. The largest collections contain about 35 fragments from the Pierre Shale at USGS Mesozoic locality D1794 near Fort Collins, Colorado, and about 30 fragments from the Pierre Shale at USGS Mesozoic locality D8784 near Pueblo, Colorado (fig. 2, locs. 37, 56, respectively).

DIAGNOSIS: Juvenile growth stage consists of loose hamitid limbs coiled in a plane; middle growth stage is a loose helix; last growth stage is a long retroversal whorl in which aperture faces helix.

DESCRIPTION: The holotype of Didymoceras cheyennense (fig. 27) is part of an adult body chamber about 70 mm long; the costal whorl height near the larger end is about 38.5 mm, the costal whorl breadth is about 38.8 mm (ratio of whorl breadth to whorl height is 1.01; measurements from a plaster cast). The fragment has a slight twist that indicates a position not far from the end of a dextrally coiled helix. Ribs are narrow, oblique, and widely spaced. Most are single; all cross the dorsum, flanks, and venter. Nodate to bullate tubercles occur on the margin of the venter on every rib except one on one side of the specimen; tubercles arise from single or paired ribs on the other side. Opposite tubercles are connected by low, broad, transverse ribs; the rib index is 5.

The holotype of Ancyloceras (Hamites) uncus Meek and Hayden, 1858 (fig. 28), first illustrated by Meek (1876: pl. 21, fig. 1a, b), is a somewhat crushed adult body chamber in the form of a hook. The whorl height at the larger end is 41 mm. Flank ribs are single, narrow, prorsiradiate, and widely spaced on the older part, rursiradiate and more closely spaced on the younger part. Each rib bears a small nodate or bullate tubercle that borders the venter. Tubercles are reduced in size toward the aperture. Opposite tubercles are connected across the venter by a transverse rib.

The holotype of Helicoceras angulatum Meek and Hayden, 1860a (fig. 29), first illustrated by Meek (1876: pl. 21, fig. 3a–c), resembles the holotype of Didymoceras cheyennense in size and form. It is a 72.5 mm long fragment of the older part of an adult body chamber with a costal whorl breadth of 41.0 mm and a costal whorl height of 36.0 mm (ratio of whorl breadth to whorl height is 1.14; measurements from a plaster cast). The specimen has a slight twist that indicates it is an early part of the body chamber. Ribs are narrow, sharp, oblique, prorsiradiate, and widely spaced. Each rib bears a small, somewhat bullate tubercle on one side. Ribs on the other side bear slightly larger nodate tubercles. Opposite tubercles are connected across the venter by low transverse ribs. One pair of opposite tubercles is connected by looped ribs. There are four tubercles in a distance equal to the whorl height.

The USGS collections of Didymoceras cheyennense reveal three growth stages (fig. 30) somewhat similar to those of Didymoceras stevensoni . A very loose helicoid stage that includes some straight limbs characterizes the juvenile whorls (figs. 33B–E, 34). Middle growth is characterized by a loose helix that includes whorls not in contact (fig. 34). The final stage is a retroversal hook (figs. 30, 34, 37).

The earliest whorls observed consist of straight divergent limbs connected by a narrowly bent elbow. USNM 482457 (not illustrated), has two straight limbs meeting at an angle of 48°. The cross section is subcircular. The smaller limb has a whorl height of 3.5 mm and ornament of single, prorsiradiate, tuberculate ribs; the rib index is 4. Single, rursiradiate, tuberculate ribs (rib index of 4) characterize the younger limb, which has a whorl height of 4.4 mm. USNM 482458, from the same locality, is a larger hookshaped, twisted limb that has terminal whorl heights of 7.8 and 13.6 mm (fig. 33D, E). Ribs are rectiradiate on the straighter part and rursiradiate on the curved part of the specimen. About every other rib bears small, sharp, bullate tubercles. Most juveniles in other collections, however, have loose helicoid coils that begin at smaller diameters. USNM 482459 (unfigured), from USGS Mesozoic locality D68 (fig. 2, loc. 43), has an elliptically coiled whorl that passes into a regular helix at a helical diameter of 30.7

482468, USGS Mesozoic locality D1794 (fig. 2, loc. 37). Figures are ×1.

mm; the ratio of umbilical diameter to shell diameter is 0.62.

Loose, but evenly coiled, helicoid whorls characterize the middle growth stage (figs. 30, 33A, 35A, D). Four helices, from USGS Mesozoic locality D2740 (fig. 2, loc. 39), have diameters of 19.0– 24.2 mm and ratios of umbilical diameter to shell diameter of 0.36–0.40 (USNM 482460a–d). Ratios of whorl breadth to whorl height are 1.08–1.18. The oblique ribs are rursiradiate on the outer whorl flank (venter) and lower whorl face and prorsiradiate on the upper whorl face and dorsum.

Ribs and tubercles are variable in middle growth. Tubercles of the lower row usually arise from single ribs, but tubercles of the upper row may give rise to paired ribs that cross the upper part of the flank and continue on across the upper whorl face (fig. 33A). An occasional rib extends from a lower tubercle and crosses the rest of the flank without giving rise to an upper tubercle, or a single rib may pass between two lower tubercles and support an upper tubercle before continuing across the upper whorl face. Rarely, three ribs arise from a lower tubercle, and one may connect to an upper tubercle. More common are nontuberculate ribs separating some of the tuberculate ones (fig. 38C). Looped ribs that connect opposite tubercles may be present here and there. Tubercles are bullate, nodate, or flat-topped. Three or four tubercles are present in a distance equal to the whorl height; the rib index is 7–10 on the upper whorl face.

The third growth stage is marked by a final whorl that separates away from the spire and bends downward for a considerable distance before recurving to form a long hook with the aperture facing upward toward the spire (fig. 37). As a result, the body chamber begins at the end of the last whorl of the spire or beyond. The adult whorl is twisted somewhat as it separates from the spire; this condition results in oblique ribbing on about one-third of the uncoiled whorl (fig. 34). Beyond the initial twisted area, the body chamber becomes swollen, and ribbing becomes transverse and coarse. Ribs on the body chamber are single, prorsiradiate on the older part, rectiradiate on the curve of the hook, and rursiradiate on the rest of the whorl (fig. 36B). Tubercles are mostly nodate and largest around the hook; they decline in size toward the aperture but generally persist to it. Opposite tubercles are usually matched across the slightly flattened venter and are connected by very low, broad ribs, but occasional tubercles may alternate on the hook. Four or five tubercles are present in a distance equal to the whorl height. The aperture follows the course of the ribbing, and is usually preceded by a few closely spaced ribs or by ribs of irregular height. The suture is complexly digitate and much like that of Didymoceras stevensoni (compare figs. 17 and 31).

The species is dimorphic. Microconchs (fig. 34) are about one-half as large as macroconchs (fig. 37).

Didymoceras cheyennense is coiled dextrally and sinistrally. Of 12 specimens from USGS Mesozoic locality D2740 (fig. 2, loc. 39), seven are dextral and five sinistral. Of 19 specimens from USGS Mesozoic locality D1794 (fig. 2, loc. 37), 7 are dextral, 12 sinistral. A large collection of 326 specimens a quarter whorl or more made by Steve Jorgensen and Neal Larson from the Pierre Shale southeast of Rapid City, South Dakota (fig. 32), includes 158 sinistral, and 168 dextral specimens (N. Larson, written commun., 1994). Thirty-seven have tubercle injuries that result in a single row of tubercles or in a displacement of the tubercles. Twenty-one specimens have constrictions, dents, or healed injuries due to bites.

DISCUSSION: Ancyloceras (Hamites) uncus Meek and Hayden (1858) is based on a crushed hook of a small macroconch of Didymoceras cheyennense ; Helicoceras angulatum Meek and Hayden (1860a) is based on the early part of a body chamber of a macroconch; both came from the Pierre Shale in the Cheyenne River valley east of the Black Hills in western South Dakota.

Didymoceras cheyennense typically consists of 2.5–3 narrow, loosely coiled whorls and a very long retroversal body chamber. The species differs from Didymoceras stevensoni in its longer retroversal whorl, fewer and looser helical whorls, and finer ribbing. D. cheyennense is not as densely ribbed as Didymoceras nebrascense .

OCCURRENCE: Didymoceras cheyennense has been found at many localities from central Montana southward into northern New Mexico. These localities are in the western part of the Late Cretaceous seaway (fig. 32). The species is also present in the basal part of the Mount Laurel Sand in Delaware. Ward and Orr (1997) recorded the species from Tercis (Landes) in southwest France.

Genus Nostoceras Hyatt, 1894

TYPE SPECIES: Nostoceras stantoni Hyatt, 1894: 570 , by original designation (= Ancyloceras approximans Conrad, 1855 ).

Hyatt designated Nostoceras stantoni as the ‘‘genotype’’ without a description, but he did describe, without illustrations, three varieties: retrosum, prematurum, and aberrans. Stephenson (1941: 407, pl. 80) treated the variety retrosum as N. stantoni sensu stricto, the type species. Hyatt had two specimens of his variety retrosum , and these were redescribed and illustrated by Stephenson (1941: 408, pl. 80, figs. 2–5) as cotypes of N. stantoni ( USNM 23278a, 23278b). One cotype ( USNM 23278a) is a nearly complete helical spire, and the other ( USNM 23278b) is a similar spire plus a complete retroversal body chamber.

DIAGNOSIS: Hyatt (1894: 569) proposed this genus for species that have ‘‘a closecoiled unsymmetrical shell during the ephebic stage and are true turrilites. There are two rows of ventral tubercles, which become more or less deflected during development towards the lower side (whether this be the left or right side) of the whorls. There is a contact furrow which is maintained as long as the whorls are sufficiently close coiled and then disappears on the free gerontic volution. This volution is excentric and then recurved as in all retroversal living chambers... the nepionic and perhaps earlier neanic substages... is not a normal ammonoidal spiral, but an open, whorled, irregular shell of some kind... the gerontic volution is apt to have tubercles even when they are absent in the ephebic stage.’’

DISCUSSION: Hyatt (1894: 573) also assigned to Nostoceras , Turrilites helicinus Shumard (1861: 191) . Although Hyatt lacked the very earliest growth stages of Nostoceras stantoni and Nostoceras helicinum , he had indications of an open whorl or two in as much as the smallest whorl on some specimens lacked a contact furrow. Specimens of N. stantoni and N. helicinum in the collections at hand also lack the earliest whorl or two, but Nostoceras alternatum (Tuomey, 1854) , from slightly younger rocks, clearly has a minute loosely coiled apex (Cobban, 1974: fig. 5).

Hyatt did not record constrictions in his descriptions of the varieties of Nostoceras stantoni and Nostoceras helicinum , but they are clearly visible in the specimens illustrated by Stephenson (1941: pl. 80). The constrictions are deep, parallel to the ribs, and number three to five per whorl. A high rib may border a constriction on one side or the other, or high ribs may bound both sides.

Species of Nostoceras show marked sized dimorphism. Nostoceras is thus a heteromorphic ammonite characterized by a dextral or sinistral helical spire of whorls mostly in tight contact and a partly to wholly uncoiled body chamber. Ornament consists of ribs, two rows of tubercles, and constrictions. The type species, Nostoceras stantoni , comes

from close to the Campanian-Maastrictian

boundary, and all other species that seem as-

signable to Nostoceras by the present authors

are of Campanian or Maastrictian age. An-

cyloceras? approximans Conrad (1855: 266,

1860: pl. 47, fig. 4), from the Saratoga Chalk

of Arkansas, is a microconch of N. stantoni

Hyatt, 1894, and approximans is thus the pri-

or name for the species.

The following Western Interior, Gulf

Coast, and Atlantic Seaboard species, with

their original references and spellings, are

considered Nostoceras by the present au-

thors. The list is in alphabetical order by ge-

nus and then by species.

Ancyloceras? approximans Conrad, 1855: 266.

Nostoceras arkansanum Kennedy and Cobban, 1993 b: 130, pl. 4, figs. 17–24.

Nostoceras colubriformis Stephenson, 1941: 412, pl. 81, figs. 1–3.

Nostoceras danei Kennedy and Cobban, 1993: 90, figs. 7.2, 7.3, 7.6–7.20, 7.23–7.32, 12.6.

Nostoceras draconis Stephenson, 1941: 413, pl. 82, figs. 5–9.

Nostoceras helicinum crassum Stephenson, 1941: 412, pl. 81, figs. 7, 8.

Nostoceras helicinum humile Stephenson, 1941: 412, pl. 81, figs. 4–6.

Nostoceras hyatti Stephenson, 1941: 410, pl. 81, figs. 9–12.

Nostoceras mendryki Cobban, 1974: 13, pl. 10, figs. 1–17, text-fig. 11.

Nostoceras monotuberculatum Kennedy and Cobban, 1993 a: 86, figs. 6.14–6.18, 6.22–6.24, 6.26, 6.27, 7.1, 7.4, 7.5, 7.21, 7.22, 7.33–7.35, 12.4.

Nostoceras pleurocostatum Kennedy and Cobban, 1993 a: 87, figs 6.19–6.21, 6.25, 6.28–6.39.

Nostoceras pulcher Kennedy and Cobban, 1993 a: 85, figs. 6.1–6.13, 12.2, 12.3.

Nostoceras stantoni Hyatt, 1894: 570 (= N. approximans).

Nostoceras stantoni aberrans Hyatt, 1894: 572 (= N. approximans).

Nostoceras stantoni prematurum Hyatt, 1894: 572 (= N. approximans).

Turrilites alternatus Tuomey, 1854: 168.

Turrilites helicinus Shumard, 1861: 191.

Turrilites pauper Whitfield, 1892: 268, pl. 45, figs. 1–5.

Turrilites saundersorum Stephenson, 1941: 416, pl. 83, figs. 6–8.

Turrilites splendidus Shumard, 1861: 191 .

OCCURRENCE: Nostoceras occurs in the

Western Interior in the lower Maastrictian

zone of Baculites eliasi in the Pierre Shale

in Colorado, in the upper Campanian zones of Baculites reesidei and Baculites jenseni in Colorado, in the B. reesidei zone in the Lewis Shale in Wyoming, in the upper Campanian zone of Didymoceras nebrascense in the Pierre Shale in South Dakota, and in the upper Campanian zone of Didymoceras stevensoni in the Mancos Shale in Colorado. The genus is also found in the Neylandville Marl and Nacatoch Sand in Texas; in the Ozan Formation, Annona Chalk, and Saratoga Chalk in Arkansas; in the Ripley Formation in Tennessee, Mississippi, Alabama, and Georgia; in the Prairie Bluff Chalk in Mississippi; in the Mount Laurel Sand in Delaware; and in the Navesink Formation in New Jersey.

Outside the United States, Nostoceras occurs in Columbia (Kennedy, 1992), The Netherlands (Kennedy, 1987), Belgium (Kennedy, 1993), France (Kennedy, 1986), Spain (Martinez, 1982; Ward and Kennedy, 1993), Austria (Kennedy and Summesberger, 1984), Bulgaria (Tzankov, 1964), Poland (Blaszkiewicz, 1980), Russia (Mikhailov, 1951), Israel (Lewy, 1967, 1969), Libya (Lefeld and Uberna, 1991), Madagascar (Collignon, 1971), Angola (Howarth, 1965), Zululand (Spath, 1921), Australia (Henderson et al., 1992), and Japan (Matsumoto, 1977).