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 : 6-38

Genus Didymoceras Hyatt, 1894 (= Emperoceras Hyatt, 1894: 575 ; Didymoceratoides Kennedy and Cobban,

1993a: 90)

TYPE SPECIES: Ancyloceras ? nebrascense Meek and Hayden, 1856a: 71 , by original designation by Hyatt (1894: 574).

DIAGNOSIS: Juvenile stage of either loose helical coils or loose straight limbs connect- ed by elbows followed by a middle growth stage of helical whorls that may or may not be in contact; body chamber in a retroversal whorl.

DISCUSSION: Hyatt (1894: 573) proposed this genus for ‘‘... a series of forms having loose helicoid spirals, two rows of more or less irregular ventral tubercles and irregularly bifurcated costae, which also have, or appear to have, a gerontic stage with a retroversal volution, as in Nostoceras . These are all larg- er shells and are separable by the helicoceran mode of growth in the ephebic stage.’’ He assigned to his new genus the species Ancyloceras ? nebrascense Meek and Hayden (1856a: 71) , Turrilites (Helicoceras) cochleatus Meek and Hayden (1858: 55) , and Helicoceras tortum Meek and Hayden (1858: 54) . Hyatt also noted that Heteroceras newtoni Whitfield (1877: 40) and Ancyloceras tricostatus Whitfield (1877: 43) probably belonged to Didymoceras .

Hyatt (1894: 575) also proposed the new genus Emperoceras with Emperoceras beecheri as the type species, from the Pierre Shale of the same general locality as his specimen of Didymoceras nebrascense . The types of E. beecheri consist of two magnificent specimens that have early hamitid whorls in a plane followed by a spire of loose whorls not in contact with one another. Whitfield (1902: 68, pls. 23–27), apparently unaware of Hyatt’s work, described and figured several excellent specimens of E. beecheri under the name of Heteroceras simplicostatum Whitfield (1880) , all from the same general area as Hyatt’s E. beecheri and D. nebrascense . As described below, all are synonyms of Didymoceras nebrascense ; Emperoceras is thus a junior synonym of Didymoceras .

Kennedy and Cobban (1993a: 90) diagnosed their new genus Didymoceratoides (type species Didymoceras binodosum Kennedy and Cobban, 1993a: 92 , figs. 8.1, 8.2, 8.5, 8.6, 8.13–8.15, 8.22–8.24, 8.28, 8.29, 8.32, 8.33, 8.35–8.39, 9.1–9.5, 12.1) as follows: ‘‘Early low open helix followed by loose elliptical criocone, ornamented by sharp rursiradiate ribs that weaken on dorsum; all ribs bear ventral spines during early and middle growth; nontuberculate ribs in- tercalate on adult body chamber; constrictions and flared ribs may be present; sutures deeply and intricately incised with bifid lobes and saddles.’’ They regarded Didymoceratoides as a paedomorphic offshoot of Didymoceras . Subsequent work has shown that the symmetrical, planispiral body chambers present among the type series are body chambers of juvenile, rather than adult individuals. Adult D. binodosum have helical whorls and a recurved body chamber, as in typical Didymoceras , of which Didymoceratoides is thus a synonym. Larson et al. (1997: 53, unnumbered fig.) have illustrated a reconstruction of the species.

The following Western Interior and Gulf Coast nominal species, with their original references and spellings, are here referred to Didymoceras ; many are synonyms, as described below. The list is in aphabetical order by genus and then by species.

Ancyloceras cheyenensis Meek and Hayden, 1856a: 71 .

Ancyloceras ? nebrascense Meek and Hayden, 1856a: 71 .

Ancyloceras ? nicolletii Hall and Meek, 1856: 397, pl. 4, fig. 4.

Ancyloceras tricostatus Whitfield, 1877: 43 , pl. 15, figs. 7, 8.

Ancyloceras (Hamites) uncus Meek and Hayden, 1858: 56 .

Didymoceratoides binodosum Kennedy and Cobban, 1993a: 92 , figs. 8.1, 8.2, 8.5, 8.6, 8.13– 8.15, 8.22–8.24, 8.28, 8.29, 8.32, 8.33, 8.35– 8.39, 9.1–9.5, 12.1.

Emperoceras beecheri Hyatt, 1894: 575 , pl. 14, figs. 15–17.

Helicoceras angulatum Meek and Hayden, 1860a: 176 .

Helicoceras mortoni var. tenuicostatum Meek, 1876: 487 , pl. 22, fig. 3a–c.

Helicoceras navarroensis Shumard, 1861: 190 .

Helicoceras stevensoni Whitfield, 1877: 39 .

Helicoceras tenuicostatus Meek and Hayden, 1858: 56 .

Helicoceras tortus Meek and Hayden, 1858: 54 .

Helicoceras vespertinus Conrad, 1874: 455 .

Heteroceras newtoni Whitfield, 1877: 40 .

Nostoceras platycostatum Kennedy and Cobban, 1993b: 131 , pl. 2, figs. 16, 17; pl. 4, figs. 1–13, 33, 34; text-fig. 7a.

Turrilites (Helicoceras) cochleatus Meek and Hayden, 1858: 55 .

Turrilites ? umbilicatus Meek and Hayden, 1858: 56.

OCCURRENCE: Didymoceras is a common heteromorph in the zones of Baculites scotti , Didymoceras nebrascense , Didymoceras stevensoni , and Didymoceras cheyennense in the Bearpaw and Pierre shales in Montana; in the same zones as well as those of Baculites gregoryensis and B. scotti in the Pierre Shale in Wyoming and South Dakota; in the D. nebrascense and D. stevensoni zones in the Mesaverde Formation in Wyoming; in the Baculites reduncus and D. stevensoni zones in the Rock River Formation in Wyoming; in the D. nebrascense zone in the Pierre Shale in Kansas; in the B. gregoryensis , B. scotti , D. nebrascense , D. stevensoni , D. cheyennense , and Baculites reesidei zones in the Pierre Shale in Colorado; in the B. gregoryensis , B. scotti , and D. stevensoni zones in the Mancos Shale in Colorado; in the D. cheyennense zone in the Williams Fork Formation in Colorado; in the B. scotti zone in the Sego Sandstone in Utah; in the B. gregoryensis , D. nebrascense , and D. stevensoni zones in the Pierre Shale in New Mexico; and in the B. scotti , D. nebrascense , and D. cheyennense zones in the Lewis Shale in New Mexico. Didymoceras is also found in the Bergstrom Formation in Texas, Ozan Formation and Annona Chalk in Arkansas, Mount Laurel Sand in Delaware, and Wenonah Formation in New Jersey. Outside the United States, Didymoceras is known from Canada, Mexico, France (Kennedy and Bilotte, 1995), Spain (Martinez, 1982), Poland (Blaszkiewicz, 1980), Israel (Lewy, 1969), Egypt (Luger and Gröschke, 1989), Madagascar (Collignon, 1971), Angola (Howarth, 1965), and Nigeria (Zaborski, 1985).

Didymoceras nebrascense (Meek and Hayden, 1856a) Figures 3 , 4 , 6 , 7 , 8C, D , 9–13 , 62

Ancyloceras ? Nebrascensis Meek and Hayden, 1856a: 71.

Turrilites Nebrascensis Meek and Hayden. Meek and Hayden, 1856b: 280 .

Turrilites Nebrascensis Meek and Hayden. Hayden, 1859: 684 .

Helicoceras Nebrascense Meek and Hayden. Meek and Hayden, 1860a: 185 .

Helicoceras Nebrascense Meek and Hayden. Meek and Hayden, 1860b: 421 .

Heteroceras ? Nebrascense Meek and Hayden. Meek, 1876: 480 , pl. 22, fig. 1a–c ( cheyennense in explanation of plate).

non Heteroceras ? nebrascense Meek and Hayden. Whitfield, 1880: 451 , pl. 14, fig. 9; pl. 15, fig. 6 (= Helicoceras Stevensoni Whitfield, 1877 ).

Heteroceras nebrascense Meek and Hayden. Stanton, 1888: 186 .

Heteroceras ? nebrascense (Meek and Hayden) . Boyle, 1893: 148.

Didymoceras nebrascense Meek. Hyatt, 1894: 574 , pl. 14, figs. 13, 14.

Emperoceras Beecheri Hyatt, 1894: 575 , pl. 14, figs. 15–17 (subjective synonym).

non Heteroceras nebrascense . Gilbert, 1896: pl. 64 (= Helicoceras Stevensoni Whitfield, 1877 ).

Heteroceras simplicostatum Whitfield, 1902: 68 , pls. 23–27 (subjective synonym).

Didymoceras nebrascense Meek sp. Spath, 1921: 253.

Hamitoceras simplicostatum Whitfield. Abel, 1924 : text fig. 295c.

Turrilites (Heteroceras) simplicicostatum (sic) Whitfield. Diener, 1925: 91.

Turrilites (Heteroceras) nebrascense Meek and Hayden. Diener, 1925: 91 .

Cirroceras nebraskense (sic) (Hyatt). Wright, 1957: L224, fig. 251.3.

Emperoceras simplicicostatum (sic) (Whitfield). Wright, 1957: L224, fig. 251.5.

Didymoceras nebrascense (Meek and Hayden) . Gill and Cobban, 1966: A31.

Didymoceras nebrascense (Meek and Hayden) . Scott, 1969: pl. 2 (unnumbered fig.).

Didymoceras nebrascense (Meek and Hayden) . Gill and Cobban, 1973: 7, text fig. 5a.

Didymoceras nebrascense (Meek and Hayden) . Scott and Cobban, 1975: unnumbered fig.

Didymoceras nebrascense (Meek and Hayden) . Hirsch, 1975: text fig. 7a.

Didymoceras nebrascense (Meek and Hayden) . Kennedy and Cobban, 1976: text fig. 7.1.

Cirroceras (Didymoceras) sp. Case, 1982: fig. 12.36.

Didymoceras nebrascense (Meek and Hayden) . Scott and Cobban, 1986a: unnumbered fig.

Didymoceras nebrascense (Meek and Hayden) . Scott and Cobban, 1986b: unnumbered fig.

Didymoceras nebrascense (Meek and Hayden) . Jagt, 1987: fig. 4.

Didymoceras navarroense (Shumard, 1861) . Emerson et al., 1994: 313, part of unnumbered fig.

Didymoceras nebrascense (Meek and Hayden, 1856) . Kennedy and Bilotte, 1995: fig. 3A.

Nostoceras (Didymoceras) nebrascense (Meek and Hayden) . Wright, 1996: 247, fig. 191, 5.

Didymoceras nebrascense (Meek and Hayden, 1856) . Larson et al., 1997: 54, 3 unnumbered figs.

Didymoceras nebrascense (Meek and Hayden) . Sealey and Lucas, 1997: 235, fig. 4A–E.

TYPES: The holotype, by monotypy, is USNM 469, from ‘‘Iquor creek, north fork, Cheyenne’’ (Meek and Hayden, 1856a: 71). The north fork of the Cheyenne River is now called the Belle Fourche River. Figured specimens are USNM 450333 and 482417– 482435.

MATERIAL: About 1000 fragments from

482428, USGS Mesozoic locality D1422 (fig. 2, loc. 13). Figures are ×1.

100 localities. Most specimens are undeformed; nearly all are from limestone concretions. The largest collections consist of about 300 specimens (fragments) from USGS Mesozoic locality D2629 (fig. 2, loc. 3) and about 120 fragments from USGS Mesozoic locality D1421 (fig. 2, loc. 12) .

DIAGNOSIS: Earliest whorls are straight limbs connected by elbows and followed by a gently curved limb all in a loose planispire. Middle growth stage is a loose helix of several whorls that are usually not in contact. Last part of body chamber bent in a U-shape with the aperture facing upward. Ornament consists of dense ribs and two rows of tubercles (fig. 3).

DESCRIPTION: The holotype, about a quarter of a poorly preserved septate, sinistral helical whorl, is about 50 mm long and has a circular cross section 23.5 mm in diameter at the smaller end. Twenty-one rounded ribs, narrower than the interspaces, cross the venter. The specimen is limonitic and contains much brown barite.

An ammonitella is not preserved in the collections. The smallest specimen observed (USNM 482417, not illustrated) from USGS Mesozoic locality 23381 (fig. 2, loc. 1) is a straight limb 28.3 mm long with end diameters of 1.0 mm and 2.7 mm. A furrow on the following elliptical whorl reveals that the smaller part of the straight limb extended at least 7 mm beyond the preserved part. Ornament consists of single, prorsiradiate ribs that bear small, nodate tubercles at the edge of the venter; the rib index is 4. Other small, straight limbs (USNM 482418, not illustrat- ed) from USGS Mesozoic locality 23381 reveal small, sharp spines rising from the tubercles and constrictions at about every sixth rib. Constrictions bordered by high adapical ribs and spaced at every fifth or sixth rib are present on some of the small, straight limbs (USNM 482419, 482420, not illustrated).

The shell curves into a narrow elbow at the larger end of the small, straight limb and then continues on as a second but larger straight limb that may parallel the smaller one, or form an acute angle of 10 to 30°. Whorl height of the larger limb increases more rapidly than width, producing an elliptical cross section. A constriction may be present on the elbow. Ribbing becomes rectiradiate on the elbow and then changes to rursiradiate on the larger limb. Ribs tend to be of irregular height on the larger limb. Some specimens (for example, USNM 482421, not illustrated, from USGS Mesozoic locality D2629) have tuberculate ribs that alternate in height with smaller nontuberculate ribs. Other specimens (for example, USNM 482422, not illustrated), may have every fourth or fifth rib flared on the early part of the limb.

The larger straight limb merges into a somewhat broadly curved elbow that leads to a gently curved limb. The cross section of the elbow and the curved limb is usually an inflated ellipse that is a little higher than wide. Ribbing on the elbow and curved limb is slightly rursiradiate to rectiradiate, and every second, third, or fourth rib bears a small nodate or flat-topped tubercle that supports a small, sharp spine (fig. 11C). The rib index is 8–10; ribs cross the venter transversely. The larger end of the curved limb leads to a broadly curved elbow that merges into the helical spire (figs. 8D, 9A). Occasional individuals have an abbreviated planispiral stage of only one straight limb and one gently curved limb that merges into the helical stage.

The middle or helical growth stage (which can begin at quite small sizes: figs. 11A, B) consists of about three loosely coiled whorls that are either not in contact or just barely touch (figs. 3, 9B). Whorl sections are circular. Ribs are usually wirelike, narrower than the interspaces, closely spaced, and mostly single. The rib index is 10–14. Ribs are rursiradiate on the lower and outer whorl faces, but curve forward on the upper face and cross the dorsum in a slight convexity. Every few ribs, a rib bears small, nodate, bullate, or flat-topped tubercles in two rows of about equal size (fig. 3). Tubercles are generally matched, but they may alternate for short distances (fig. 9B). Occasional specimens that have flat-topped tubercles may also have flat-topped ribs (USNM 482423– 482426). These flat-topped ribs and tubercles are on an inner layer of shell material and form the bases of an outer layer of hollow rounded ribs and nodate to bullate tubercles (fig. 9B). Tubercles usually disappear on the last half of the final helical whorl, which also includes the first part of the body chamber (figs. 6A, 9B, 10). The body chamber is long; it includes half to nearly all of the last whorl of the helix as well as a retroversal whorl that forms a U and curves back toward the helix (fig. 3). The intercostal section is subcircular and slightly higher than wide. Oblique ribbing, characteristic of the helix, continues on the older, twisted part of the body chamber. Ribbing becomes rectiradiate on the younger third of the body chamber, where the U-shaped whorl becomes planispiral (figs. 7, 13). Here ribs are transverse on the venter, where the rib index is 6–11. Tubercles are rejuvenated at the beginning of the uncoiled part of the body chamber; at first small and bullate, they rapidly enlarge into spine-bearing tubercles that are mostly nodate with some bullate and a few clavate (figs. 10, 13). They may be opposite on part of the body chamber and alternate on the remainder (fig. 7). Tubercles weaken and disappear rapidly near the adult aperture, where the last 6 to 11 ribs may be nontuberculate. The aperture is normal and follows the form of the ribbing (figs. 10, 12A, 13A).

Coiling is both dextral and sinistral (figs. 6, 8C, 9B). The collection from USGS Mesozoic locality D1421 (fig. 2, loc. 12) has 118 fragments of helical whorls, 62 dextral, 56 sinistral. USGS Mesozoic locality D1422 (fig. 2, loc. 13) has 70 helical fragments, 32 dextral, 38 sinistral. USGS Mesozoic locality D2629 includes 103 helical fragments, 54 dextral, 49 sinistral. The largest collection, 591 specimens of one-third of a whorl or more, collected by Steve Jorgensen (Omaha, NB) and members of the Black Hills Museum of Natural History, from the Pierre Shale at localities 11–13 (fig. 2) has 304 sinistral and 287 dextral specimens (N. Larson, written commun., 1994).

Didymoceras nebrascense is dimorphic. Adults in the USGS collections are as small as 180 mm in height and as large as 270 mm in height, but the number of adults at hand is too few to determine the size ranges of microconchs and macroconchs. A microconch in the BHMNH collections has a body chamber 100 mm high; this incomplete individual probably had an original height of 140 mm (N. Larson, written commun., 1994).

The suture has expanded lateral and umbilical lobes (fig. 4). The saddles between the external and lateral lobes and between the lateral and umbilical lobes are deeply bifid.

DISCUSSION: Didymoceras nebrascense is easily identified by its densely ribbed, loosely coiled helix of 3 ½ whorls followed by a large U-shaped body chamber. The poor and incomplete preservation of the holotype (less than half a whorl) led some previous workers to introduce several names for fragments of other ontogenetic stages and more complete individuals, that we regard as synonyms: Emperoceras beecheri Hyatt (1894: 575 , pl. 14, figs. 15–17; herein, figs. 8D, 9A, B) is based on the helical growth stage and the earlier hamitid whorls (parallel or subparallel limbs connected by elbows all in a plane) of very well-preserved specimens from the Pierre Shale near Buffalo Gap, South Dakota.

Whitfield (1880: pl. 14, fig. 4 and footnote to explanation) briefly described the new heteromorph species Helicoceras ( Heteroceras ?) simplicostatum from ‘‘ferruginous sandstone, East fork of Beaver Creek, Black Hills. Fort Benton Group.’’ According to Whitfield’s illustration, the specimen is a complete dextral whorl of a densely ribbed helical ammonite that has periodic constrictions bordered by high thin ribs. The specimen could have come only from rocks now assigned to the upper Turonian Turner Sandy Member of the Carlile Shale somewhere along the Beaver Creek drainage near Newcastle, Weston County, Wyoming (west flank of the Black Hills uplift). Whitfield’s illustration suggests an Eubostrychoceras . Later Whitfield (1902: pl. 68, pls. 23–27) described and illustrated, as Heteroceras simplicostatum , several well-preserved hetero- morph ammonites from the Pierre Shale near Buffalo Gap, South Dakota. Whitfield’s specimens are very much like those of Hyatt’s from the Buffalo Gap area. All are excellent examples of Didymoceras nebrascense .

Healed injuries are scarce in the USGS specimens, and only three or four were observed in all the specimens examined. The injuries consist of a dent in the shell or some sort of a shallow groove that interrupts the ribbing for a few centimeters. The large BHMNH collection of 591 specimens has ‘‘44 injured conchs with most injuries present on the body chamber. Most injuries are tubercle and rib displacements of which seven specimens have a single row of tubercles along the venter. Fifteen specimens have dents, and four of these show healed ruptures or bites’’ (N. Larson, written commun., 1994).

OCCURRENCE: Didymoceras nebrascense has been found at many localities along the western part of the Late Cretaceous seaway from northeastern Montana southward into northern New Mexico (fig. 5). Lewy (1969: 116, pl. 1, fig. 2a, b) recorded Didymoceras cf. D. nebrascense from the Mishash Formation of Israel. The Mishash specimen has hamitid whorls like those of D. nebrascense , but the ribbing is denser than that of D. nebrascense .

Didymoceras stevensoni (Whitfield, 1877) Figures 8A, B , 14–17 , 19D–I , 20–26

Helicoceras Stevensoni Whitfield, 1877: 39 .

Helicoceras Stevensoni Whitfield. Whitfield, 1880: 447 , pl. 14, figs. 5–8.

Heteroceras ? nebrascense (Meek and Hayden) . Whitfield, 1880: 451, pl. 14, fig. 9; pl. 15, fig. 6.

Helicoceras stevensoni Whitfield. Boyle, 1893: 146 .

Helicoceras Stevensoni (Whitfield) . Hyatt, 1894: 568.

Heteroceras nebrascense . Gilbert, 1896: pl. 64.

Helicoceras stevensoni Whitfield. Whitfield, 1901: 219 , pls. 29, 30.

Helicoceras stevensoni Whitfield. Chamberlin and Salisbury, 1906 : fig. 417e, f.

Heteroceras stevensoni (Whitfield) . Scott, 1907: pl. 15, fig. 17.

Helicoceras stevensoni Whitfield. Grabau and Shimer, 1910: 203 , figs. 1470, 1471.

Helicoceras stephensoni (sic) Whitfield. Chamberlin and Salisbury, 1914: fig. 470b, c.

by John R. Stacy. Figure is reduced ×0.90.

×1.

Helicoceras stevensoni Whitfield. Grabau, 1921 : fig. 1754d.

Didymoceras ? stevensoni (Whitfield) . Spath, 1921: 250.

Turrilites (Helicoceras) Stevensoni (Whitfield) . Diener, 1925: 89.

Heteroceras sp. Schuchert and Dunbar, 1933: pl. 33, fig. 9.

Helicoceras stevensoni Whitfield. Stovall and Brown, 1954: 375 , fig. 5.

Helicoceras stevensi (sic) (Whitfield). Fenton and Fenton, 1958: 197, unnumbered fig.

Didymoceras stevensoni (Whitfield) . Scott and Cobban, 1965: unnumbered fig.

Didymoceras stevensoni (Whitfield) . Gill and Cobban, 1966: A31, A32.

Didymoceras stevensoni (Whitfield) . Cobban, 1970: D72, figs. 1a–f, m, 2.

Didymoceras stevensoni (Whitfield) . Gill and Cobban, 1973: 7, fig. 5b.

Didymoceras stevensoni (Whitfield) . Scott and Cobban, 1975: unnumbered fig.

Didymoceras stevensoni (Whitfield) . Hirsch, 1975: figs. 6b, 7b.

Didymoceras stevensoni (Whitfield) . Scott and Cobban, 1986a: unnumbered fig.

Didymoceras stevensoni (Whitfield) . Scott and Cobban, 1986b: unnumbered fig.

Nostoceras (Nostoceras) sp. Luger and Gröschke, 1989: 401, pl. 49, figs. 5, 9, 10.

Didymoceras sp. Eldredge, 1991 : pl. 112.

Didymoceras stevensoni (Whitfield) . Kennedy and Bilotte, 1995: pl. 49, fig. 1; pl. 51, figs. 1, 2; text fig. 3B.

Didymoceras stevensoni (Whitfield, 1877) . Kennedy et al., 1995: pl. 3, figs. 12, 13, 18, 19.

Didymoceras stevensoni (Whitfield, 1877) . Kennedy and Cobban, 1997: 69, figs. 5.6–5.8, 5.13– 5.15, 5.18, 5.19, 8.4–8.11.

Didymoceras stevensoni (Whitfield, 1877) . Larson et al., 1997: 55, unnumbered fig.

TYPES: The holotype, USNM 12307, came from ‘‘limestone of the Fort Pierre group, on the west side of Beaver Creek, southwest Black Hills’’ (Whitfield, 1877: 40). The specimen came from a limestone concretion in rocks now assigned to the upper part of the Pierre Shale below the Monument Hill Bentonitic Member in the Beaver Creek drainage northwest, west, or southwest of present day Newcastle, Weston County, Wyoming (fig. 2).

MATERIAL: About 550 specimens, mostly less than half a whorl, from about 70 local-

TABLE 1 Dimensions and Rib Counts of Didymoceras stevensoni (Whitfield, 1877) a Ribs per USNM D U Wb Wh Wb:Wh whorl

482456a 41.1 13.5 (0.33) — — — 22 482456b 42.2 10.8 (0.26) 19.1 17.3 1.10 21 482456c 47.0 — 21.0 17.5 1.20 23 482456d 48.2 11.5 (0.24) 23.8 21.0 1.13 24 482456 e 49.4 13.5 (0.27) 22.4 19.3 1.16 25 482456f 61.3 18.7 (0.31) — — — 27 482456g 62.0 — — — — 25 482456h 62.1 14.9 (0.24) 31.4 26.1 1.20 30 482456i 64.6 19.3 (0.30) 29.1 25.5 1.14 27 482456j 66.0 18.0 (0.27) — — — 22 482456k 66.3 20.5 (0.31) 29.0 25.2 1.15 22 482456l 68.0 19.3 (0.28) 29.9 27.1 1.19 22 482456m 69.0 20.3 (0.29) 29.6 26.8 1.10 22 482456 n 70.2 14.8 (0.21) 31.6 29.6 1.07 27 482456 o 71.6 16.2 (0.23) 32.0 28.2 1.13 30 482456p 73.0 18.3 (0.25) — — — 23 482456q 78.4 17.4 (0.22) 36.2 32.8 1.10 23 482456r 80.6 20.4 (0.25) 36.0 30.2 1.19 23 482456 s 90.0 21.0 (0.23) 41.8 35.1 1.19 21

a All specimens are from the Pierre Shale at USGS Mesozoic locality D454 near Newcastle, Wyoming. Measurements in millimeters. Abbreviations : D, shell diameter ; U, umbilical diameter; Wb, whorl breadth at larger end; Wh, whorl height at larger end; Wb:Wh, ratio of whorl breadth to whorl height. Figures in parentheses are dimensions expressed as a ratio of shell diameter; dashes (—) indicate no measurement.

ities chiefly in Montana, Wyoming, Colorado, and New Mexico. Most specimens are from limestone or ironstone concretions. The largest collections include over 100 fragments from USGS Mesozoic locality D1234 near Pueblo, Colorado (fig. 2, loc. 54), and about 70 fragments from USGS Mesozoic locality D454 near Newcastle, Wyoming (fig. 2, loc. 17).

DIAGNOSIS: High-spired, with variable loosely coiled early whorls, later whorls in contact, final uncoiled stage in form of recurved hook-like body chamber with aperture facing upward. Ornament consists of strong, narrow, bituberculate ribs, often separated by nontuberculate ribs (fig. 14).

DESCRIPTION: The holotype (figs. 15, 20C) is an uncrushed, dextrally coiled spire of three stout whorls of which the lower two are in contact. The spire has a height of 82 mm, a breadth of 58 mm, and an apical angle of about 31°. The ratio of the umbilical diameter to the shell diameter at the larger end is 0.26. All but the last 15 mm of the spire is septate. All whorls have broadly rounded flanks and narrowly rounded shoulders. Whorl cross sections are wider than high. Ribs arise on the umbilical wall, where they are curved and prorsiradiate. They are narrow, strong, and rursiradiate on the whorl flank. Thirtyseven ribs are present on the last whorl, and 35 on the penultimate. On much of the specimen, every other rib is bituberculate with about equally sized, fairly small tubercles. One row of tubercles is at midflank, the other near the base of the whorl. Tubercles are bullate to nodate and of irregular height; most are sharp, a few flat-topped. Occasional tubercles of the lower row give rise to paired ribs. The complex suture of the holotype was illustrated by Whitfield (1880: pl. 14, fig. 8).

An adult specimen consisting of a complete body chamber and three whorls of the spire was well illustrated but briefly described by Whitfield (1901). According to Whitfield’s illustrations, the specimen has a length of about 162 mm. Whitfield observed that the body chamber included the last half of the final whorl of the spire and the uncoiled part of the shell that bent down away from the spire and then curved upward toward it. The aperture is normal and follows the course of the ribbing.

The large USGS collections of Didymoceras stevensoni reveal three growth stages: a highly variable juvenile stage of loose coils or loose limbs, a middle helicoid stage in which some whorls are in contact, and a late uncoiled stage in which the recurved hooklike body chamber ends with the aperture facing upward (fig. 14). Most of the specimens at hand represent the middle helicoid stage.

The juvenile stage is quite variable; all specimens are uncoiled to some degree (fig. 16). The smallest individual available begins as an open planispiral coil followed by elliptical coiling (fig. 19G). Terminal whorl heights are 1.3 and 3.5 mm. Ornament is of single, straight, prorsiradiate ribs; the rib index is 3. The early whorls of another specimen (fig. 21A) consist of two parallel limbs followed by an elliptical whorl that merges into the main spire (fig. 16). Some specimens have a lengthy (figs. 19E, F, 20A, B, 23B– D), others a much shorter uncoiled stage (fig. 20E). Ribs on these early whorls are usually single and rectiradiate (figs. 19E, 20A). Each rib supports a small, spinose tubercle (fig. 19F) that borders the slightly flattened venter. Opposite tubercles are connected across the venter by low transverse ribs; the rib index is 3–5.

The middle helicoid stage consists of three or four whorls of which two or three are in contact (fig. 20C, E). An impressed area is present on the upper whorl face of the younger of the two whorls in contact. Whorls are stout and have subcircular cross sections. Nineteen well-preserved whorls from USGS Mesozoic locality D454 near Newcastle, Wyoming, have ratios of whorl breadth to whorl height of 1.07–1.20 and ratios of umbilical diameter to shell diameter of 0.21–0.33 (ta- ble 1). The dorsum is either smooth or has very weak transverse ribs or striae of irregular height. Rectiradiate to rursiradiate ribs strengthen rapidly on the lower whorl face, and are rursiradiate on the flank. They bear two rows of tubercles. The ribs curve forward and then weaken greatly or disappear at the juncture of the flank and upper whorl face. Ribs may be single and connect opposite tubercles before continuing onto the upper whorl face, or may split into two ribs at the lower tubercle, connect the lower and upper tubercles as looped ribs, and then proceed on across the upper whorl face as a single rib or as paired ribs. On an occasional specimen, a rib crosses the lower whorl face to a lower tubercle and then splits into two ribs without intercepting an upper tubercle. Nontuberculate ribs are fairly common and may alternate with tuberculate ribs, as in the holotype (fig. 15). Nontuberculate ribs may extend across the entire flank or may be present only on the lower or upper part of the flank. The density of ribs of the helicoid stage is 21–30 per half whorl (table 1). Tubercles are rather small, bullate (fig. 22A, B), nodate (fig. 23C, D), flat-topped (figs. 19D, 23A), and rarely clavate. All apparently supported small, sharp spines (fig. 23C, D).

As noted by Whitfield (1901), the adult body chamber includes the last half of the final helicoid whorl of the spire followed by a loose retroversal whorl (fig. 14). The gap between the body chamber hook and the base of the helicoid spire is small. The cross section of the hook becomes circular, and the ribbing rectiradiate. Ribs become more closely spaced near the aperture (figs. 21B, 26D). Tubercles at first increase in size on the hook and then decrease greatly near the aperture, which follows the trend of the ribs.

Didymoceras stevensoni is coiled dextrally and sinistrally. A large collection from the Pierre Shale at USGS Mesozoic locality D1234 near Pueblo , Colorado (fig. 2, loc. 54), contains 145 fragments of helicoid spires ; 77 are sinistral, 68 dextral. Of this large collection, only two individuals are pathologic. Neal Larson (written commun., 1994) noted that a collection made by Howard Ehrle , Miles City , Montana, from the Pierre Shale of Carter County, Montana, consists of 119 specimens of half a whorl or more, of which 62 are sinistral, and 57 dextral. Only one specimen in this large collection has an injury ; it consists of a large groove along the venter accompanied by displaced ribs and tubercles.

The species is dimorphic. Microconchs are about one-half as large as macroconchs.

The suture is highly digitate (fig. 17). Lobes are greatly expanded and have narrow

1856a (from Meek, 1876: pl. 21, fig. 2a, b).

bases. Parts of the suture were illustrated by Whitfield (1880: pl. 14, fig. 8) and by Cobban (1970: fig. 2).

DISCUSSION: Didymoceras stevensoni differs from Didymoceras nebrascense chiefly in having the larger helical whorls in contact, and in having coarser and fewer ribs, and a body chamber in which the adult aperture is closer to the helix. The early whorls of D. stevensoni are also more helical.

OCCURRENCE: Didymoceras stevensoni has been found at many localities in the Western Interior fairly close to the western shore of the Late Cretaceous Seaway (fig. 18). Phosphatic fragments, formerly thought to be from the Marshalltown Formation in Delaware (Cobban, 1970), are now believed to be from the base of the Mount Laurel Sand (Kennedy and Cobban, 1997). Kennedy and Bilotte (1995) recorded D. stevensoni from France. The Nostoceras (Nostoceras) sp. of Luger and Gröschke (1989) from the upper Campanian of Egypt is probably D. stevensoni .

Didymoceras cheyennense (Meek and Hayden, 1856a) Figures 27–31 , 33–38

Ancyloceras ? Cheyenensis (sic) Meek and Hayden, 1856a: 71.

Turrilites Cheyennensis Meek and Hayden. Meek and Hayden, 1856b: 280 .

Ancyloceras (Hamites) uncus Meek and Hayden, 1858: 56 (subjective synonym).

Turrilites Cheyennensis Meek and Hayden. Hayden, 1859: 684 .

Ancyloceras (Hamites) uncus Meek and Hayden. Hayden, 1859: 685 .

Helicoceras angulatum Meek and Hayden, 1860a: 176 (subjective synonym).

Helicoceras Cheyennense (Meek and Hayden). Meek and Hayden, 1860a: 185.

Helicoceras Cheyennense (Meek and Hayden). Meek and Hayden, 1860b: 421.

Helicoceras angulatum Meek and Hayden. Meek and Hayden, 1860b: 421 .

Ancyloceras ? uncus Meek and Hayden. Meek and Hayden, 1860b: 420 .

Heteroceras ? angulatum (Meek and Hayden) . Meek, 1864: 25.

Heteroceras ? Cheyennense (Meek and Hayden) . Meek, 1876: 483, pl. 21, fig. 2a, b.

Ancyloceras ? uncum Meek and Hayden. Meek, 1876: 409 , pl. 21, fig. 1a, b.

Ancyloceras ? cheyennensis Meek and Hayden. Boyle, 1893: 40.

Ancyloceras ? uncum Meek and Hayden. Boyle, 1893: 41 .

Helicoceras angulatum Meek and Hayden. Boyle, 1893: 146 .

Exiteloceras (Heteroceras) Cheyennense (Meek and Hayden). Hyatt, 1894: 577.

Exiteloceras (Ancyloceras) uncum Meek. Hyatt, 1894: 577 .

Exiteloceras (Heteroceras) angulatum (Meek and Hayden) . Hyatt, 1894: 577.

? Heteroceras angulatum (Meek and Hayden) . Logan, 1898: 514, pl. 108, fig. 2.

Turrilites (Heteroceras) angulatum Meek and Hayden. Diener, 1925: 90 .

Turrilites (Heteroceras) cheyennense Meek and Hayden. Diener, 1925: 90 .

Hamites uncus Meek and Hayden. Diener, 1925: 70 .

non Exiteloceras angulatum Meek. Adkins, 1928: 212 .

Didymoceras cheyennense (Meek and Hayden) . Gill and Cobban, 1973: 10, fig. 5c.

Didymoceras cheyennense (Meek and Hayden) . Scott and Cobban, 1975: unnumbered fig.

Didymoceras cheyennense (Meek and Hayden) . Hirsch, 1975: fig. 7c.

Didymoceras cheyennense (Meek and Hayden) . Kennedy and Cobban, 1976: fig. 7.

Didymoceras cheyennense (Meek and Hayden) . Scott and Cobban, 1986a: unnumbered fig.

Didymoceras cheyennense (Meek and Hayden) . Scott and Cobban, 1986b: unnumbered fig.

Didymoceras cheyennense (Meek and Hayden) . Jagt, 1987: fig. 4.

Didymoceras cheyennense (Meek and Hayden) . Kennedy and Cobban, 1994: 1290, figs. 7.1– 7.18, 8.4, 15.1–15.4.

Exiteloceras angulatum (Meek and Hayden, 1876) . Emerson et al., 1994: 310, 311, unnumbered figs.

Didymoceras cheyennense (Meek and Hayden) . Kennedy and Bilotte, 1995: fig. 3C.

Didymoceras cheyennense (Meek and Hayden, 1856) . Kennedy et al., 1995: pl. 5, figs. 1–3, 11–14, 20, 21.

Figures

Fig. 3. Restoration of Didymoceras nebrascense (Meek and Hayden, 1856a). Stipple board drawing by John R. Stacy. Figure is reduced ×0.90.

Fig. 4. Part of the external suture of Didymoceras nebrascense (Meek and Hayden, 1856a). E is the external lobe and L is the lateral lobe. The heavy, straight line marks the middle of the venter. USNM 482435, USGS Mesozoic locality 21574 (fig. 2, loc. 2).

Fig. 6. Didymoceras nebrascense (Meek and Hayden, 1856a). A. USNM 482427, USGS Mesozoic locality D1421 (fig. 2, loc. 12). B. USNM

Fig. 7. A, B. Didymoceras nebrascense (Meek and Hayden, 1856a). USNM 482429, USGS locality D1422 (fig. 2, loc. 13). Figures are ×1.

Fig. 8. A, B. Didymoceras stevensoni (Whitfield, 1877). BHMNH 4039, collected by Howard Ehrle, Pierre Shale of Carter County, Mont. C, D. Didymoceras nebrascense (Meek and Hayden, 1856a). C. USNM 482430, USGS Mesozoic locality D85 (fig. 2, loc. 23). D. YPM 6131B, plaster cast of one of two specimens described by Hyatt (1894) as the new genus and species Emperoceras beecheri, Pierre Shale near Buffalo Gap, S. Dak. Figures are ×1.

Fig. 9. A, B. Didymoceras nebrascense (Meek and Hayden, 1856a). YPM 6131A, plaster cast of Hyatt’s (1894) second specimen described as Emperoceras beecheri, Pierre Shale near Buffalo Gap, S. Dak. Figures are ×1.

Fig. 10. Didymoceras nebrascense (Meek and Hayden, 1856a). USNM 482431, USGS Mesozoic locality D85 (fig. 2, loc. 23). Figure is ×1.

Fig. 11. Didymoceras nebrascense (Meek and Hayden, 1856a). A, B. USNM 482432, USGS Mesozoic locality D2148 (fig. 2, loc. 10). C. USNM 450333, USGS Mesozoic locality D1422 (fig. 2, loc. 13). Figures are ×1.

Fig. 12. A, B. Didymoceras nebrascense (Meek and Hayden, 1856a). USNM 482433, USGS Mesozoic locality D1422 (fig. 2, loc. 13). Figures are ×1.

Fig. 13. A, B. Didymoceras nebrascense (Meek and Hayden, 1856a). USNM 482434, USGS Mesozoic locality D1422 (fig. 2, loc. 13). Figures are ×1.

Fig. 62. Didymoceras nebrascense (Meek and Hayden, 1856a) and Solenoceras bearpawense, n. sp. BHMNH 4040, collected by Neal L. Larson, Pierre Shale in the SE1/4 sec. 12, T. 7 S., R. 7 E., Fall River County, S. Dak. Figure is reduced ×0.90.

Fig. 14. Restoration of most of a Didymoceras stevensoni (Whitfield, 1877). Stipple board drawing

Fig. 15. A–C. Three views of the holotype of Helicoceras stevensoni Whitfield, 1877 (from Whitfield, 1880: pl. 14, figs. 5–7). Figures are

Fig. 16. A C. Restorations of parts of the juvenile whorls of Didymoceras stevensoni (Whitfield, 1877). Stipple board drawing by John R. Stacy. Figures are ×1.

Fig. 17. Part of the external suture of Didymoceras stevensoni (Whitfield, 1877). E is the external lobe and L is the lateral lobe. The heavy, straight line marks the middle of the venter. USNM 482436, USGS Mesozoic locality D4446 (fig. 2, loc. 47).

Fig. 19. A C. Nostoceras monotuberculatum Kennedy and Cobban, 1993a. A, B. USNM 482470, USGS Mesozoic locality D7028 (fig. 2, loc. 46). C. BHMNH 2093, collected by N.L. Larson from the Pierre Shale in the N½ sec. 13, T. 7 S., R. 7 E., Fall River County, S. Dak. D I. Didymoceras stevensoni (Whitfield, 1877). D. USNM 482437, USGS Mesozoic locality 23528 (fig. 2, loc. 7). E, F. USNM 482438, USGS Mesozoic locality D454 (fig. 2, loc. 17). G. USNM 482439, USGS Mesozoic locality 22935 (fig. 2, loc. 29). H, I. USNM 482440, USGS Mesozoic locality D1387 (fig. 2, loc. 27). Figures are ×1.

Fig. 27. A, B. Two views of the holotype of Ancyloceras? cheyenensis (sic) Meek and Hayden,

Fig. 28. Side view and cross section of the holotype of Ancyloceras (Hamites) uncus Meek and Hayden, 1858 (from Meek, 1876: pl. 21, fig. 1a, b). Figure is ×1.

Fig. 29. End (A), dorsal (B), and ventral (C) views of the holotype of Helicoceras angulatum Meek and Hayden, 1860b (from Meek, 1876: pl. 21, fig. 3a–c). Figures are ×1.

Fig. 30. Restoration of Didymoceras cheyennense (Meek and Hayden, 1856a). Stipple board drawing

Fig. 31. Part of the external suture of Didymoceras cheyennense (Meek and Hayden, 1856a). E is the external lobe, L is the lateral lobe, and U is the umbilical lobe. The heavy, straight line marks the middle of the venter. USNM 482461, USGS Mesozoic locality 23056 (fig. 2, loc. 6).

Fig. 33. Didymoceras cheyennense (Meek and Hayden, 1856a). A. USNM 482463, USGS Mesozoic locality D1794 (fig. 2, loc. 37). B, C. USNM 482464, USGS Mesozoic locality D2740 (fig. 2, loc. 39). D, E. USNM 482458, USGS Mesozoic locality D804 (fig. 2, loc. 45). All figures are ×1.

Fig. 34. A–C. Didymoceras cheyennense (Meek and Hayden, 1856a). BHMNH 2100, collected by Neal L. Larson, Pierre Shale near Red Shirt, Shannon County, S. Dak. Figures are reduced ×0.88.

Fig. 35. Didymoceras cheyennense (Meek and Hayden, 1856a). A. USNM 482465, USGS Mesozoic locality D1794 (fig. 2, loc. 37). B, C. USNM 482466, USGS Mesozoic locality D1689 (fig. 2, loc. 51). D. USNM 482460a, USGS Mesozoic locality D2740 (fig. 2, loc. 39). Figures are ×1.

Fig. 36. Didymoceras cheyennense (Meek and Hayden, 1856a). A. USNM 482467, USGS Mesozoic locality D221 (fig. 2, loc. 14). B. USNM

Fig. 37. A, B. Didymoceras cheyennense (Meek and Hayden, 1856a). BHMNH 2139, collected by Neal Larson, Pierre Shale near Red Shirt, Shannon County, S. Dak. Figures are reduced ×0.83.

Fig. 38. Didymoceras cheyennense (Meek and Hayden, 1856a). A, B. USNM 482462, USGS Mesozoic locality D2740 (fig. 2, loc. 39). C, D. USNM 445099, USGS Mesozoic locality D221 (fig. 2, loc. 14). Figures are ×1.