Scaphites (Scaphites) ventricosus Meek and Hayden, 1862

Landman, Neil H., Plint, A. Guy & Walaszczyk, Ireneusz, 2017, Allostratigraphy And Biostratigraphy Of The Upper Cretaceous (Coniacian-Santonian) Western Canada Foreland Basin, Bulletin of the American Museum of Natural History 2017 (414), pp. 1-173 : 127-138

Scaphites (Scaphites) ventricosus Meek and Hayden, 1862

Figures 9B, C, 10–16

1862. Scaphites ventricosus Meek and Hayden, 1862: 22 .

1876. Scaphites (Scaphites) ventricosus Meek and Hayden. Meek : 425, pl. 6, figs. 7, 8.

1894. Scaphites ventricosus Meek and Hayden. Stanton : 44, figs. 8, 9; pl. 43, fig. 1; non pl. 44, fig. 10.

1898. Scaphites ventricosus Meek and Hayden. Logan : 476, pl. 104, figs. 8, 9; pl. 105, fig. 1; non pl.104, fig.10.

1900. Scaphites ventricosus Meek and Hayden. Herrick and Johnson : pl. 45, figs. 8–10; pl.46, fig.1.

1927a. Scaphites ventricosus Meek and Hayden. Reeside : 6, pl. 3, figs. 11–18; pl. 4, figs. 1–4.

non 1927a. Scaphites ventricosus var. depressus . Reeside: 7, pl. 5, figs. 6–10 [= S. (S.) depressus ].

non 1927a. Scaphites ventricosus var. interjectus . Reeside: 7, pl. 5, figs. 1–5 [= Clioscaphites interjectus ].

non 1927a. Scaphites ventricosus var. oregonensis . Reeside: 7, pl. 6, figs. 11–15 [= S. (S.) depressus ].

non 1927a. Scaphites ventricosus var. stantoni . Reeside: 7, pl. 3, figs. 19, 20; pl. 4, figs. 5–10 [= S. (S.) depressus ].

1927b. Scaphites ventricosus Meek and Hayden. Reeside : 35, pl. 10, figs 1, 2.

non 1929. Scaphites ventricosus var. saxitonianus . McLearn: 77, pl. 18, figs. 1–3; pl. 19, figs. 1, 2 [= Clioscaphites saxitonianus ].

1942. Scaphites cf. ventricosus Meek and Hayden. Rosenkrantz : 38, pl. 1.

1952. Scaphites ventricosus Meek and Hayden. Cobban : 31, pl. 12, figs. 1–10; pl. 13, figs. 11–13.

1952. Scaphites tetonensis Cobban, 1952: 31 , pl. 14, figs. 1–10.

1952. Scaphites ventricosus Meek and Hayden. Basse : 607, fig. 14.

1955. Scaphites ventricosus Meek and Hayden. Cobban : 201, pl. 1, fig. 6.

1960. Scaphites ventricosus Meek and Hayden. Easton : fig. 11.28–3a, b.

1965. Scaphites (Scaphites) ventricosus Meek and Hayden. Birkelund : 87, pl. 19, figs. 2, 3; text-fig. 78.

1976. Scaphites ventricosus Meek and Hayden. Kennedy and Cobban : text-fig. 17 (part).

1977. Scaphites ventricosus Meek and Hayden. Kauffman : pl. 24, figs. 3, 4.

1989. Scaphites ventricosus Meek and Hayden. Landman : fig. 1 (7a, b).

1991. Scaphites (Scaphites) ventricosus Meek and Hayden, 1862 . Kennedy and Cobban: 85, text-fig. 28A, B.

1991. Scaphites (Scaphites) tetonensis Cobban, 1952 . Kennedy and Cobban: 87, text-fig. 30A–C.

1994. Scaphites ventricosus Meek and Hayden. Braunberger : 115–117, pl. 13, figs. 1–4.

1994. Anascaphites ventricosus (Meek and Hayden) . Cooper: 176, fig. 1A, B.

DIAGNOSIS: Macroconchs large and stout with slightly uncoiled body chamber producing a small gap between the phragmocone and hook; cross section of shaft depressed and subovoid; apertural angle averaging 82.5°; ribbing coarse and widely spaced; microconchs smaller with more loosely uncoiled body chamber; suture complex with asymmetrically bifid lateral lobes.

TYPES: The holotype is USNM 1903 from the upper part of the Colorado Shale, about 20 miles northeast of Fort Benton, Montana.

MATERIAL: A total of 24 specimens, all of which are incomplete. They consist mostly of the body chamber without the phragmocone. All of them are adult, comprising 16 macroconchs and 8 microconchs. The collection also contains fragments of body chambers and phragmocones, but they are difficult to identify to species level.

MACROCONCH DESCRIPTION: In the measured sample, LMAX averages 85.4 mm and ranges from 70.4 to 101.1 mm (table 5). The ratio of the size of the largest specimen to that of the smallest is 1.44. Adults are robust with an oval outline in side view. The exposed phragmocone occupies approximately one whorl and terminates slightly below the line of maximum length. The septal angle averages 12.0°. The umbilical diameter of the phragmocone is small and averages 4.3 mm (table 5). The body chamber consists of a shaft and recurved hook. The umbilical shoulder of the shaft is straight or slightly concave in side view. LMAX / HS averages 2.16 and ranges from 1.97 to 2.34. The body chamber is slightly uncoiled. LMAX /HP averages 2.55 and ranges from 2.26 to 2.95. As a result, a small gap appears between the phragmocone and hook, and is usually filled with sediment. The apertural angle averages 82.5° and ranges from to 72.0° to 89.0°.

The whorl section of the phragmocone along the line of maximum length is depressed and subovoid with maximum whorl width at one-third whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. WP /HP averages 1.34 and ranges from 1.18 to 1.46. As the shell passes from the phragmocone into the body chamber, both the whorl width and height increase slightly, and the shape of the whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed and subovoid with maximum whorl width at one-quarter whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. WS/ HS averages 1.31 and ranges from 1.11 to 1.51. Adoral of the midshaft, both the whorl width and especially whorl height abruptly decrease. As a result, the whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded. WH/HH averages 1.64 and ranges from 1.45 to 1.80. The shell culminates in a constricted aperture with a dorsal lappet.

Because none of our specimens preserves the adapical part of the phragmocone, our observations are restricted to the adoral part. Primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into broad elongate swellings that reach their maximum strength at one-half whorl height, approximately coinciding with the ventrolateral shoulder. They each subdivide into two thin ribs, with another one or two thin ribs intercalated between them. Ribs are sharp and uniformly strong on the venter, which they cross with a slight adapical or adoral projection. They are equally and widely spaced. Rib density ranges from 2 to 4.25 ribs/ cm among the specimens in our sample.

The same pattern of ribbing persists onto the body chamber. Primary ribs are rectiradiate on the umbilical wall and shoulder. They develop into broad elongate swellings that reach their maximum strength at one-half whorl height. They are prominent, rectiradiate, and equally spaced on the shaft, becoming weaker, prorsiradiate, and closely spaced on the hook. Each rib subdivides into two thin secondary ribs, with one or two thin ribs intercalating between them. Ribs are uniformly strong and widely spaced on the venter of the shaft, which they cross with a slight adoral projection. The density of ribs on the shaft ranges from 2.75 to 4 ribs/cm among the specimens in our sample. Ribs are more closely spaced on the venter of the hook, which they cross with a strong adoral projection. For example, in TMP2016.041.0379, the rib density is 2.75 ribs/cm at midshaft versus 4 ribs/cm on the hook.

A suture is not well enough preserved in any of the specimens in our study, but according to Cobban (1952), it is complex with asymmetrically bifid lateral lobes (fig. 9B, C).

MICROCONCH DESCRIPTION: Microconchs are elongate in lateral view. The most notable features of the microconchs relative to the macroconchs are their smaller size and more loosely uncoiled body chamber, leaving a larger gap between the phragmocone and hook. In addition, the umbilical shoulder of the shaft is concave in microconchs whereas it is straight in macroconchs.

LMAX averages 55.2 mm and ranges from 41.8 to 67.1 mm (table 6). The whorl section of the phragmocone along the line of maximum length is depressed and subovoid. WP/ HP averages 1.44 and ranges from 1.38 to 1.49. The umbilical wall is steep and nearly vertical, the flanks are sharply rounded, and the venter is broadly rounded. Whorl width increases gradually from the phragmocone into the body chamber and reaches its maximum value at midshaft. Whorl height also increases gradually from the phragmocone into the body chamber and attains its maximum value at the point of recurvature. The whorl section at midshaft is nearly the same as that along the line of maximum length. It is depressed and subovoid with maximum whorl width at one-half whorl height. The umbilical wall is steep and subvertical, the flanks are sharply rounded, and the venter is broadly rounded. The whorl section at the point of recurvature is more depressed than that at midshaft. The umbilical wall is flat and slopes outward, the flanks are sharply rounded, and the venter is broadly rounded.

At the base of the body chamber, primary ribs emerge at the umbilical seam and are rectiradiate on the umbilical wall and shoulder. They develop into broad, straight or slightly concave swellings on the flanks, which reach their maximum strength at one-half whorl height. They each subdivide into two thin ribs, with another one or two thin ribs intercalating between them. Ribs are sharp and uniformly strong on the venter, which they cross with a slight adoral projection. The ribs are equally and widely spaced, with, for example, 5 ribs/cm on the adoral part of the phragmocone in TMP2016.041.0161.

The same pattern of ribbing persists onto the body chamber. Primary ribs are prominent and equally spaced on the shaft, becoming weaker and more closely spaced on the hook. They are straight or 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. Each rib subdivides into two thin secondary ribs, with one or two thin ribs intercalating between them. Ribs are uniformly strong and widely spaced on the venter of the shaft, with, for example, 4.5 ribs/cm in TMP2016.041.0372. Ribs are more closely spaced on the venter of the hook, which they cross with a stronger adoral projection. The rib density ranges from 3.75 to 9 ribs/cm.

The suture of the microconchs is the same as that of the macroconchs.

REMARKS: Although Scaphites (S.) ventricosus is well established in the literature, it is rare to find complete specimens. Most specimens lack the phragmocone although, interestingly, the holotype retains the phragmocone but not the hook. The specimens in our collection closely match those from the U.S. Western Interior. For example, YPM 26721 is a macroconch illustrated by Reeside (1927b: pl. 4, fig. 1–4) from the Cody Shale of Wyoming. It is approximately the same size as the macroconchs in our collection (LMAX = 83 mm). It also displays the same pattern of ribs, with more widely spaced ribs on the midshaft than on the phragmocone (5 ribs/cm on the phragmocone versus 3 ribs/cm on the midshaft).

Dimorphs in this species are distinguished on the basis of size. The larger microconchs correspond to the macroconchs in our collection. However, two smaller microconchs (TMP2016.041.0161 and.0163) would previously have been referred to as Scaphites (Scaphites) tetonensis . This form occurs in the same beds as S. (S.) ventricosus , and we argue that it simply represents a small microconch of this species. Other than size, dimorphs are distinguished by the outline of the umbilical shoul- der of the shaft in side view. It is straight or slightly concave in macroconchs whereas it is markedly concave in microconchs. This is related to the fact that the body chamber is slightly more tightly coiled in macroconchs than in microconchs, although a small gap is present between the phragmocone and hook in both dimorphs.

Several closely related scaphite species occur in the Coniacian of the Western Interior of North America. Scaphites (S.) ventricosus is distinguished from the underlying species S. (S.) preventricosus by its larger size, more tightly coiled shell, and more widely spaced ribs. It is distinguished from the overlying species S. (S.) depressus by its less tightly coiled shell and more widely spaced ribs.

OCCURRENCE: In the Upper Cretaceous of the Western Interior of North America, this species demarcates the upper lower and middle Coniacian Scaphites (S.) ventricosus Zone. In the study area, the lowest occurrence of this species is immediately above surface CS 2 in allomember CA3, just below an interpreted highstand and prior to a major regression that culminates at surface CS4, which marks the boundary between the lower and middle Coniacian. It is present in the Wapiabi Formation at Ram River (TMP2016.041.0021), East Thistle Creek (TMP2016.041.0066 and.0067), James River (TMP2016.041.0155), Blackstone River (TMP2016.041.0106), Chungo Creek (TMP2016.041.0161–.0168), Sheep River (TMP2016.041.0296), Bighorn Dam (TMP2016.041.0366–.0368,.0370–.0374, and.0379), Mill Creek (TMP2016.041.0035), and Bighorn River (TMP2016.041.0349), Alberta. Elsewhere, it is abundant in the Kevin Member of the Marias River Shale in north-central Montana, the Cody Shale in western Wyoming, and the Mancos Shale in New Mexico. Outside North America, it has been reported from Alianaitsúnguaq, Nûgssuaq, Greenland (Birkelund, 1965).

Figures

FIG. 10. Cremnoceramus crassus inconstans (Woods, 1912), TMP 2016.041.0101, LV, Wapiabi Formation, Wapiabi Creek, 7.9 m, A, anterior view, B, oblique, posterodorsal view, C, lateral view of the juvenile stage. All photographs are ×1.

FIG. 11. Volviceramus koeneni (Müller, 1888), TMP 2016.041.0184, double-valve specimen, Wapiabi Formation, Chungo Creek, 45.5 m, A, dorsal view, B, lateral view of the LV, C, anterior view, D, lateral view of the RV. All photographs are ×0.95.

FIG. 12. Volviceramus involutus (Sowerby, 1828), TMP 2016.041.0243, LV, Wapiabi Formation, West Thistle Creek, 72.5 m, A, lateral view of the adult stage, B, anterior view, C, oblique, posterodorsal view, D, dorsal view, the photographs are ×1.

FIG. 13. Volviceramus exogyroides (Meek and Hayden, 1862), TMP 2016.041.0415, LV, Wapiabi Formation, Bighorn Dam, interval 50–60 m, A, lateral view of the juvenile stage, B, lateral view of the adult stage, C, anterior view. All photographs are ×1.0.

FIG. 14. A portion of the Muskiki Member exposed in Sheep River (fig. 11), illustrating typical sandier-upward successions (arrows) bounded by flooding surfaces. The regionally mappable surface CS3 is indicated.

FIG. 15. A. Overview of surface CS1 exposed at river level at the mouth of Oldfort Creek (fig. 10). At right, a coarse-grained wave ripple composed of granules and small pebbles of chert and quartz rests sharply on regional erosion surface CS1. Traced laterally over only 3 m, the coarse lag pinches out and surface CS1 is manifest simply as a sharp mud-on-mud contact; scale bar = 20 cm. B. Detail of the coarse-grained wave ripple.

FIG. 16. Intraclastic pebble lag resting on surface SS0 at Thistle Creek west (fig. 5). A. General view of siderite pebbles seen from above; scale bar in each image = 20 cm. B. Detail of panel A showing pitted, bioeroded surface of a siderite pebble. C. Side view of conglomerate showing clasts filling a shallow erosional trough (arrows).