Hoploscaphites peterseni, Landman, Neil H., Kennedy, W. James, Grier, Joyce, Larson, Neal L., Grier, James W., Linn, Thomas, Tackett, Lydia & Jicha, Brian R., 2020

Landman, Neil H., Kennedy, W. James, Grier, Joyce, Larson, Neal L., Grier, James W., Linn, Thomas, Tackett, Lydia & Jicha, Brian R., 2020, LARGE SCAPHITID AMMONITES (HOPLOSCAPHITES) FROM THE UPPER CRETACEOUS (UPPER CAMPANIAN – LOWER MAASTRICHTIAN) OF NORTH AMERICA: ENDLESS VARIATION ON A SINGLE THEME, Bulletin of the American Museum of Natural History 2020 (441), pp. 1-131 : 1-131

Hoploscaphites peterseni , n. sp.

Figures 10A, 32E , 59–78

Macroconch Synonymy

1997. Jeletzkytes plenus . Larson et al., p. 81, unnumbered fig.

2019. Hoploscaphites sp. Landman et al., figs. 11E, F, 23F.

DIAGNOSIS: Macroconchs medium to large in size, nearly circular in outline; whorl cross section of shaft compressed subovoid with broadly rounded flanks and venter; width of venter approximately 60% whorl height; small umbilicus with umbilical bulge; apertural angle averaging 56°; long, fine, straight, closely spaced ribs on adoral part of phragmocone, with little branching or intercalation, and weak adoral projection on venter; long, fine, weakly concave, slightly more widely spaced ribs on shaft, with moderately strong adoral projection on venter; umbilicolateral tubercles absent or small and closely spaced on body chamber, forming a semicircle; ventrolateral tubercles small and closely spaced at ⅞ whorl height, usually persisting to aperture. Microconchs medium to large in size, and more loosely uncoiled than macroconchs; umbilical wall of shaft broad and outwardly sloping; pattern of ornament similar to that on macroconchs, with relatively more prominent umbilicolateral tubercles. Suture deeply incised with broad-stemmed, asymmetrically bifid first lateral saddle.

ETYMOLOGY: This species is named in honor of Jack G. Petersen (Waterloo, Iowa) who, for over 25 years, collected and skillfully prepared hundreds of specimens of Eutrephoceras and Hoploscaphites from the Cedar Creek Anticline in east-central Montana. He has also generously donated many of these specimens to the AMNH for scientific research and display. It is fair to say that without Jack’s contributions to this study, we would never have succeeded in conveying the richness and variety of the ammonites from the Cedar Creek Anticline.

TYPES: Following the traditional practice in scaphite systematics (Landman and Waage, 1993), we designate a macroconch rather than a microconch, as the holotype of this species. It is AMNH 71848 (fig. 60) from the Baculites baculus Zone or lower part of the B. grandis Zone of the Pierre Shale on the Cedar Creek Anticline in east-central Montana. It is a steinkern that retains some of its shell and is 88.7 mm in diameter with a weak umbilical bulge. It exhibits a repaired injury on the right side of the phragmocone near the point of exposure, which is manifested by an interruption in the ribbing. The macroconch paratypes are AMNH 41294 (fig. 61) and 105906 (fig. 70) from the B. baculus Zone from the same general site and USNM 723217 (fig. 67) from the B. eliasi Zone of the Bearpaw Shale, Valley County, Montana. The microconch paratypes are AMNH 76400 (fig. 77G–I) and 105901 (fig. 75A–D) from the B. baculus Zone or lower part of the B. grandis Zone , and the upper part of the B. baculus Zone , respectively, from the Pierre Shale on the Cedar Creek Anticline in east-central Montana.

MATERIAL: The collection consists of approximately 70 complete or nearly complete specimens of which 36 macroconchs and 26 microconchs comprise the measured set. Those for which we have detailed stratigraphic information are mostly from the lower part of the B. baculus Zone of the Pierre Shale on the Cedar Creek Anticline, east-central Montana, and the B. eliasi Zone of the Bearpaw Shale, Valley County, Montana.

MACROCONCH DESCRIPTION: Macroconchs are medium to large in size and robust. LMAX averages 87.8 mm and ranges from 69.5 to 114.6 mm (table 5). The holotype is in the middle of the size range ( LMAX = 88.7 mm). The ratio of the size of the largest specimen to that of the smallest is 1.65. The size distribution is bimodal with a primary peak at 75-80 mm and a secondary peak at 100–105 mm (fig. 59).

All specimens are circular in side view. LMAX /HS averages 2.04, indicating that LMAX is approximately 2× the whorl height at midshaft, conforming nearly perfectly to the proportions of a circle. In addition, all specimens are tightly coiled with hardly any gap between the phragmocone and hook. LMAX /HP 2 averages 2.77 and ranges from 2.46 to 3.11 (3.11 in the holotype).

As in other species of Hoploscaphites , the phragmocone of the adult shell is relatively large and represents approximately 60% of the shell length. The phragmocone usually terminates adoral of the line of maximum length. The apertural angle is relatively high; it averages 56.3° and ranges from 45° to 67°. The apertural lip is flexuous with a deep constriction and accompanying varix. The dorsal margin of the aperture is bordered by an elongate, broadly rounded projection.

The umbilicus is small and deep. The umbilical diameter averages 4.2 mm and ranges from 2.6 to 5.6 mm (table 5). UD / LMAX averages 0.05 and ranges from 0.04 to 0.06. In many specimens, the umbilicus is partially occluded by an umbilical bulge on the umbilical shoulder of the shaft. Due to the presence of the bulge, the outline of the umbilical shoulder is convex in lateral view. The umbilical bulge is prominent in AMNH 41294 (fig. 61) but not as well developed in the holotype (fig. 60).

The whorl section at the point of exposure is subovoid and nearly equidimensional, with maximum whorl width at ¼ whorl height. WP 1 /HP 1 averages 1.03 and ranges from 0.73 to 1.24 (1.12 in the holotype). The umbilical wall is steep and convex and the umbilical shoulder is sharply rounded. The flanks are well rounded and gently converge toward the venter. The ventrolateral shoulder is sharply rounded and the venter is broadly rounded.

In passing from the adapical to the adoral part of the phragmocone, both whorl width and whorl height increase, so that the shape of the whorl section of the phragmocone along the line of maximum length is approximately the same as that at the point of exposure. WP 2 /HP 2 averages 1.04 and ranges from 0.78 to 1.24 (1.24 in the holotype). The cross section is subovoid with maximum whorl width at ⅓ whorl height. The umbilical wall is steep and subvertical and the umbilical shoulder is sharply rounded. The flanks are broadly rounded and gently converge toward the venter.

Whorl width increases markedly and reaches its maximum value on the adoral part of the shaft, whereas whorl height reaches its maximum value at midshaft. WS/HS averages 0.99 and ranges from 0.80 to 1.16 (1.08 in the holotype). The whorl section at midshaft is compressed subovoid, as in the holotype (fig. 60), or reniform, as in USNM 723217 (fig. 67), with maximum whorl width at ⅓ whorl height. The umbilical wall is steep and weakly convex and the umbilicolateral shoulder is sharply rounded. The inner flanks are well rounded and the outer flanks are broadly rounded and converge steeply toward the venter. The ventrolateral shoulder is sharply rounded and the venter is broadly rounded. VS/HS averages 0.58 and ranges from 0.46 to 0.73 (0.71 in the holotype), indicating that the width of the venter is, on average, slightly more than one-half the height of the whorl.

In passing from the shaft to the hook, the whorl width remains nearly the same. In contrast, the whorl height decreases markedly. As a result, the whorl section is more depressed at the point of recurvature than at midshaft. WH/HH averages 1.16 and ranges from 0.93 to 1.37 (1.19 in the holotype). The umbilical shoulder is sharply rounded and the flanks are broadly rounded. The opening at the aperture is reduced in size relative to the whorl section at midshaft. It is subovoid and slightly depressed in the holotype (fig. 60).

At the point of exposure, ribs are straight and rectiradiate on the umbilical wall. They bend slightly backward on the inner flanks and pass between or merge with the umbilicolateral tubercles. In the holotype (fig. 60), one rib joins an umbilicolateral tubercle dorsally and three ribs branch from it ventrally, with one or two ribs intercalating between tubercles. The ribs bend forward on the outer flanks and form a broad convexity. Intercalation and branching occur on the outer margins of the outer flanks and at the ventrolateral tubercles. In the holotype, one or two ribs join a ventrolateral tubercle dorsally and two or three ribs branch from it ventrally, with an equal number of ribs intercalating between tubercles. The ribs that branch from tubercles on one side of the venter loop to tubercles on the other side of the venter. Ribs are uniformly strong and evenly spaced on the venter, and show a slight adoral projection. The rib density on the venter of the adapical end of the phragmocone in the holotype is 6 ribs/cm.

On the adoral part of the phragmocone, ribs bend slightly backward on the umbilical wall. They are straight on the inner flanks, and bend forward on the midflanks and backward on the outer flanks, forming a broad convexity. Ribs are long, narrow, and closely spaced on the outer flanks, a pattern similar to that in Hoploscaphites crassus and H. plenus . Intercalation and branching occur on the outer flanks and at the umbilicolateral and ventrolateral tubercles. In the holotype, one rib joins an umbilicolateral tubercle dorsally and two ribs branch from it ventrally, with two ribs intercalating between tubercles. One or two ribs join a ventrolateral tubercle dorsally and groups of two or three ribs branch from it ventrally, which loop to paired tubercles on the opposite side of the venter, with one or two ribs intercalating between groups. Ribs are closely and evenly spaced on the venter, which they cross with a slight adoral projection. The rib density is higher on the adoral than on the adapical part of the phragmocone (7 vs. 6 ribs/cm in the holotype).

The shaft is covered with thin, closely spaced ribs. They bend slightly backward on the umbilical wall and shoulder. In the holotype, 10 ribs/ cm are present on the umbilical wall. Ribs bend strongly forward on the inner flanks and strongly backward on the outer flanks, forming a broad convexity. They are rectiradiate on the adapical end of the shaft and become progressively more prorsiradiate toward the adoral end of the shaft. As on the phragmocone, intercalation and branching occur on the outer flanks and at the sites of the umbilicolateral and ventrolateral tubercles. As a result, the midflanks are covered with a broad area of nonbifurcating ribs. In the holotype, one or two ribs join an umbilicolateral tubercle dorsally and two or three ribs branch from it ventrally, with as many as five ribs intercalating between tubercles. Two or three ribs join a ventrolateral tubercle dorsally and three or four ribs branch from it ventrally, with up to five ribs intercalating between tubercles. Because the ventrolateral tubercles are paired on opposite sides of the venter in the holotype, ribs that branch from a tubercle on one side of the venter loop to the tubercle on the other side of the venter. Ribs cross the venter with a moderately strong adoral projection. The rib density on the shaft of the holotype is the same as that on the adoral part of the phragmocone (7 ribs/cm).

As on the shaft, ribs on the flanks of the hook are narrow, prorsiradiate, and nearly straight. Intercalation and branching occur on the outer margins of the outer flanks and at the umbilicolateral and ventrolateral tubercles. Ribs cross the venter with a moderately strong adoral projection and become progressively more closely spaced toward the aperture, culminating in a rib density of 10 ribs/cm on the venter of the holotype.

Umbilicolateral tubercles are present in only one-half of the specimens in our collection. For example, they are present in the holotype starting at the point of exposure (fig. 60), but are absent in AMNH 41294 (fig. 61). The tubercles occur at ⅓ whorl height and are evenly spaced, becoming slightly more widely spaced toward the adoral end of the phragmocone. In the holotype, the distance between the two most adoral tubercles on the phragmocone is 5 mm. The tubercles are small with a maximum height of 1 mm and are elongated radially. A total of seven tubercles are present on the phragmocone of the holotype. Some of these tubercles do not occur on the ribs themselves but rather in the interspaces between them.

In those specimens in which umbilicolateral tubercles are present on the phragmocone, the tubercles continue onto the body chamber, usually persisting to the aperture. They occur at ⅓ whorl height and are arranged in a semicircle paralleling the outline of the venter in side view. They gradually become more widely spaced and attain their maximum spacing at midshaft, after which they become more closely spaced. The maximum distance between tubercles in the holotype is 9 mm. As on the phragmocone, the umbilicolateral tubercles on the body chamber are small, with a maximum height of 1 mm. They are bullate and follow the curvature of the ribs, so that the tubercles on the midshaft and hook are elongated in a prorsiradiate direction. A total of 6 umbilicolateral tubercles are present on the body chamber of the holotype, so that the total number of tubercles on the exposed shell of this specimen is 11.

All specimens bear ventrolateral tubercles starting at the point of exposure. They occur at ⅞ whorl height and are more or less evenly spaced, with maximum spacing at the adoral end of the phragmocone. However, in many specimens, the tubercles are grouped in clusters. For example, in the holotype (fig. 60), the distance between consecutive tubercles on the exposed phragmocone on the left side, starting at the point of exposure, is 8, 4, 6.5, 11, 6.5, 7.5, 6, 7.5, 8.5, and 7.5 mm. A total of 11 ventrolateral tubercles are present on the phragmocone of the holotype and are paired on opposite sides of the venter. In contrast, in SDSM 149991 (fig. 64), the ventrolateral tubercles are offset from one side of the venter to the other. Tubercles are conical in shape and slightly elongated radially, with a maximum height of 3 mm.

Ventrolateral tubercles extend onto the body chamber and, almost always, persist to the aperture. They are moderately widely spaced and nearly evenly distributed, forming a pattern similar to that in Hoploscaphites crassus . The tubercles attain their maximum spacing on the adoral one-third of the shaft (11.5 mm in the holotype), after which they become more closely spaced. Similarly, the tubercles attain their maximum size on the adoral one-third of the shaft, after which they decrease in size. In many specimens, such as SDSM 149990 (fig. 71), the tubercles develop into clavi with a maximum height of 3 mm. A total of 14 ventrolateral tubercles occurs on the body chamber of the holotype, so that the total number of ventrolateral tubercles on the exposed shell of this specimen is 25.

Lateral tubercles are very rare. In the holotype (fig. 60), one bullate tubercle occurs on the adapical end of the phragmocone on the left side of the specimen just dorsal of the ventrolateral tubercle. Similarly, in SDSM 149991 (fig. 64), two bullate tubercles are present on the adapical end of the phragmocone on the right side of the specimen just dorsal of the ventrolateral tubercles.

The suture is deeply incised with a broad stemmed and asymmetrically bifid first lateral saddle (E/L) and a narrow, symmetrically bifid first lateral lobe (L) that is nearly as deep as the ventral lobe (fig. 32E).

MICROCONCH DESCRIPTION: Microconchs are smaller and more loosely uncoiled than macroconchs, but exhibit a similar ornamentation and shape of the whorl section. LMAX averages 58.1 mm and ranges from 44.2 to 79.1 mm (table 6). The size distribution is unimodal with a peak at 50–55 mm (fig. 59). The ratio of the average size of microconchs to that of macroconchs is 0.66. The umbilical shoulder of the shaft in microconchs is concave in lateral view and parallels the curvature of the venter. UD averages 3.5 mm and ranges from 2.6 to 4.5 mm (table 6). UD / LMAX averages 0.06 and ranges from 0.05 to 0.07. In AMNH 76311 (fig. 78E-H), the umbilical diameter on the right side is much larger than that on the left side due to a pathology. As noted, microconchs are more loosely uncoiled than macroconchs. This difference is expressed by the ratio LMAX /HP 2, which averages 2.98 in microconchs versus 2.77 in macroconchs.

The whorl section at the adapical end of the phragmocone is compressed subquadrate with maximum whorl width at ¼ whorl height. WP 1 / HP 1 averages 0.94 and ranges from 0.74 to 1.09 (table 6). The umbilical wall is steep and subvertical and the umbilical shoulder is sharply rounded. The flanks are broadly rounded and the ventrolateral shoulder is sharply rounded. The whorl section at the adoral end of the phragmocone is more depressed than that at the adapical end because of a larger increase in whorl width than whorl height. WP 2 /HP 2 averages 1.05 and ranges from 0.93 to 1.22. The whorl section is compressed subovoid with maximum whorl width at ⅓ whorl height. The inner flanks are well rounded, and the outer flanks are more broadly rounded and gently converge toward the venter.

Whorl width and height both increase in passing into the shaft. Whorl width attains its maximum value at the point of recurvature whereas whorl height attains its maximum value at midshaft. The whorl section at midshaft is subquadrate to reniform with maximum whorl width at ¼ whorl height, coincident with the position of the umbilicolateral tubercles. WS/HS averages 1.03 and ranges from 0.78 to 1.18. The umbilical wall is broad and slopes outward, and the umbilical shoulder is sharply rounded. The flanks are broadly rounded to nearly flat and gently converge toward the venter. The ventrolateral shoulder is sharply rounded and the venter is broadly rounded. VS/HS averages 0.67 and ranges from 0.55 to 0.80, indicating that, on average, the venter is slightly more than one-half as wide as the whorl is high. The whorl section at the point of recurvature is more depressed; WH/HH averages 1.14 and ranges from 0.82 to 1.32.

On the exposed phragmocone, ribs arise at the umbilical seam and are straight to slightly rursiradiate on the umbilical wall. They are

LMAX (mm)

FIGURE. 59. Size frequency histogram of Hoploscaphites peterseni , based on the samples in tables 5 and 6.

weakly flexuous on the flanks, swinging slightly backward on the inner flanks, slightly forward on the midflanks, and slightly backward again on the outer flanks. On the adoral end of the phragmocone, where umbilicolateral tubercles appear, one rib usually joins an umbilicolateral tubercle dorsally and three ribs branch from it ventrally, with two ribs intercalating between tubercles. Additional branching and intercalation occur on the outer flanks and at the ventrolateral tubercles. One rib usually joins a ventrolateral tubercle dorsally and two ribs branch from it ventrally, with an equal number of ribs intercalating between tubercles. Ribs are uniformly and evenly spaced on the venter, which they cross with a slight adoral projection. The rib density on the venter on the adoral part of the phragmocone is 7 or 8 ribs/cm.

Ribs are closely spaced and slightly rursiradiate on the umbilical wall of the shaft. They are moderately widely spaced and prorsiradiate on the flanks. They swing forward on the inner flanks and backward on the outer flanks, forming a broad convexity. Intercalation and branching occur on the outer flanks and at the umbilicolateral and ventrolateral tubercles. One rib usually joins an umbilicolateral tubercle dorsally and three ribs branch from it ventrally, with up to three ribs intercalating between tubercles. In AMNH 105901 (fig. 75A-D), three or four ribs join a ventrolateral tubercle dorsally and five to seven ribs branch from it ventrally, with one to three ribs intercalating between tubercles. The result of so much branching at the ventrolateral tubercles is that the venter is covered with fine, closely spaced ribs. The rib density on the venter at midshaft in this specimen is 9 ribs/cm. Ribs become progressively more prorsiradiate and closely spaced toward the aperture.

Umbilicolateral tubercles appear on the adoral end of the exposed phragmocone. They strengthen on the shaft and usually persist to the aperture. They are perched on the umbilical shoulder at ⅓ whorl height. The umbilicolateral tubercles in microconchs are relatively stronger than they are in macroconchs. They are bullate and become progressively more prorsiradiate toward the aperture. In many specimens, they develop into clavi as much as 2 mm high, with steep adapical faces and gently sloping adoral faces. They are evenly and moderately widely spaced; the maximum distance between tubercles usually occurs at midshaft, e.g., 8.5 mm in AMNH 76400 (fig. 77G–I). The number of umbilicolateral tubercles on the exposed shell ranges from 4 to 6.

Figures

FIGURE 32. Sutures of Hoploscaphites crassus (Coryell and Salmon, 1934), H. plenus (Meek and Hayden 1860), and H. peterseni. A. Hoploscaphites crassus, AMNH 76167, microconch, second from last suture, H = 22.1 mm, AMNH loc. 3921, lower Baculites baculus Zone, Pierre Shale, Cedar Creek Anticline, Montana. B. Hoploscaphites crassus, AMNH 76166, microconch, fourth from last suture, H = 21.5 mm, AMNH loc. 3921, lower Baculites baculus Zone, Pierre Shale, Cedar Creek Anticline, Montana. C. Hoploscaphites plenus, macroconch, BHI 4130, second from last suture, AMNH loc. 3921, Baculites baculus or lower B. grandis Zone, Pierre Shale, Cedar Creek Anticline, Montana. D. Hoploscaphites plenus, microconch, YPM 35597, second from last suture, W = 18.4 mm, Baculites baculus or lower B. grandis Zone, Pierre Shale, Wibaux (= Mingusville), Montana. E. Hoploscaphites peterseni, macroconch, AMNH 76187, last suture, AMNH loc. 3921, lower Baculites baculus Zone, Pierre Shale, Cedar Creek Anticline, Montana.