Scaphites (Scaphites) preventricosus Cobban, 1952
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publication ID |
https://doi.org/ 10.1206/0003-0090-414.1.1 |
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persistent identifier |
https://treatment.plazi.org/id/2520FD4B-5D39-FFA5-9BAF-FDFE72B9F951 |
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treatment provided by |
Felipe |
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scientific name |
Scaphites (Scaphites) preventricosus Cobban, 1952 |
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Scaphites (Scaphites) preventricosus Cobban, 1952 View in CoL
Figures 7–9A View FIG View FIG
1952. Scaphites preventricosus Cobban : 26, pl. 9, figs. 1–16.
1952. Scaphites preventricosus var. sweetgrassensis Cobban : 27, pl. 10, figs. 18–25.
non 1952. Scaphites preventricosus var. artilobus Cobban : 27, pl. 8, figs. 1–6 (= S. (S.) mariasensis Cobban, 1952).
1955. Scaphites preventricosus Cobban. Cobban : 201, pl. 1, fig. 9; pl. 2, fig. 5.
1965. Scaphites (Scaphites) preventricosus svartenhukensis Birkelund : 83, pl. 16, fig. 3; pl. 18, figs. 2, 3; pl. 19, fig. 1; text-figs. 75–77.
1968. Scaphites cf. preventricosus Cobban. Cobban : L3, pl. 1, fig. 15.
1970. Scaphites preventricosus Cobban. Casanova : fig. 79.
1976. Scaphites preventricosus Cobban. Cobban : 124, pl. 1, figs. 8, 9.
1977. Scaphites preventricosus Cobban. Kauffman : pl. 24, figs. 1, 2.
1983. Scaphites preventricosus Cobban. Cobban : 10, pl. 8, figs. 11–13.
1987. Scaphites preventricosus Cobban. Landman : 227.
1989. Scaphites preventricosus Cobban. Landman : fig. 1 (6a, b).
1991. Scaphites (Scaphites) preventricosus Cobban, 1952 . Kennedy and Cobban: 84, text-figs. 28C, D.
1994. Anascaphites preventricosus (Cobban) . Cooper: 176.
1994. Scaphites preventricosus Cobban. Braunberger : 107–110, pl. 1, figs. 9–12; pl. 2, figs. 1–5; pl. 3, figs. 1–6; pl. 4, figs. 1–9; pl. 5, figs. 1–5; pl. 6, figs. 1–3.
2001. Scaphites preventricosus Cobban, 1951 . Braunberger and Hall: 340–342, pl. 2, figs. 8–12; pl. 2, figs. 1–10.
DIAGNOSIS: Macroconchs large and stout with an oval outline in lateral view; body chamber loosely uncoiled with a reduced aperture; apertural angle approximately 100°; ornamented by fairly straight primary and secondary ribs that are uniformly spaced on the body chamber; microconchs smaller, more slender, with a more loosely uncoiled body chamber; suture complex with asymmetrically bifid lateral lobes.
TYPE: The holotype is USNM 106675 from a bed of calcareous concretions in the Kevin Member of the Marias River Shale, 514 to 525 feet below the top, in the north bank of the Marias River, 5.5 miles south of Shelby, Toole County, Montana.
MATERIAL: The collection consists of 15 specimens, all of which are incomplete. They are divided into nine macroconchs and six microconchs.
MACROCONCH DESCRIPTION: LMAX averages 70.1 mm (table 3). Adults are robust with an oval outline in side view. The exposed phragmocone occupies approximately one whorl and terminates below the line of maximum length. The septal angle averages 52°. The umbilical diameter of the phragmocone is small; it averages 4.8 mm (table 3). The body chamber consists of a shaft and recurved hook. The umbilical shoulder of the shaft is straight in side view. In TMP2016.041.0038, LMAX / HS and LMAX /HP equal 2.02 and 3.18, respectively. The body chamber is slightly uncoiled producing a small gap between the phragmocone and hook, with a constricted aperture. The apertural angle equals 104° in TMP2016.041.0038.
The whorl section of the phragmocone along the line of maximum length, as shown in TMP2016.041.0207, 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 equals 1.49 in this specimen. As the shell passes from the phragmocone into the body chamber in this specimen, both the whorl width and height increase slightly, and 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 equals 1.30 in TMP2016.041.0207. In contrast, in TMP2016.041.0038, the whorl section at midshaft is nearly equidimensional with broadly rounded flanks; WS/ HS equals 1.03. Adoral of the midshaft, as shown in this specimen, both the whorl width and especially the 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 equals 1.64 in TMP2016.041.0038. The shell culminates in a constricted aperture with a dorsal lappet.
The ornamentation is well preserved in forward and then backward again, before subdi- TMP2016.041.0038. On the phragmocone, pri- viding into three thin secondary ribs, with as mary ribs emerge at the umbilical seam and are many as four thin ribs in between. Ribs are unislightly rursiradiate on the umbilical wall and formly strong and closely spaced on the venter of shoulder. They develop into broad, elongate the shaft, with a rib density of 6 ribs/cm. They swellings that swing gently forward and then are equally closely spaced on the venter of the backward again before subdividing into three hook, but exhibit a stronger adoral projection. thin ribs, with another two thin ribs in between. The suture is complex with asymmetrically They are sharp and uniformly strong on the ven- bifid lateral lobes (fig. 9A). ter, which they cross with a slight adoral projec- MICROCONCH DESCRIPTION: Microconchs tion. The ribs are equally and closely spaced, are elongate in lateral view. Because all of our with a rib density of 6 ribs/cm on the adoral part specimens are incomplete, it is difficult to estiof the phragmocone. mate LMAX. However, the body chambers of all
The same pattern of ribbing persists onto the of the microconchs are smaller than those of the body chamber. Primary ribs develop into broad macroconchs. The body chambers are also more elongate swellings on the flanks that swing gently loosely uncoiled leaving a larger gap between the phragmocone and hook. In addition, the umbilical shoulder of the shaft is slightly more concave in microconchs than in macroconchs.
The whorl section at the base of the body chamber (we measured the whorl section at the base of the body chamber because the phragmocone was missing) is depressed and subovoid. WP /HP averages 1.17 and ranges from 1.09 to 1.24 (table 4). The umbilical wall is steep and nearly vertical, the flanks are broadly to 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 the point of recurvature. Whorl height, on the other hand, decreases such that, together, the whorl section at midshaft is much more depressed than that at the base of the body chamber. The umbilical wall slopes outward and the flanks are broadly rounded. WS/ HS averages 1.34 and ranges from 1.28 to 1.38. The whorl section at the point of recurvature is even more depressed than that at midshaft. WH/HH averages 1.62 and ranges from 1.49 to 1.73.
Primary ribs are prorsiradiate on the umbilical wall and shoulder of the shaft. They develop into broad straight or slightly concave swellings on the flanks, which reach their maximum strength at two-thirds whorl height. In TMP2016.041.0034, which is a coarsely ornamented specimen, each primary rib subdivides into two thin ribs, with another one or two thin ribs intercalating between them. In contrast, in TMP2016.041.0036, which is a more finely ornamented specimen, each primary rib subdivides into three thin ribs, with as many as four thin ribs intercalating between them. Ribs are sharp and uniformly strong on the venter of the shaft, which they cross with a slight adoral projection. The density of ribs on the venter of the shaft ranges from 4.5 to 6 ribs/cm among the specimens in our sample. Ribs are equally closely spaced on the venter of the hook, which they cross with a stronger adoral projection. The density of ribs on the venter of the hook ranges from 5 to 6 ribs/cm.
The suture of the microconchs is the same as that of the macroconchs.
REMARKS: Dimorphism is present in Scaphites (S.) preventricosus . Cobban (1952) initially segregated out microconchs as the variety sweetgrassensis . Macroconchs are larger and more robust, with a more closely coiled body chamber. In the present collection, all the microconchs are incomplete, but even so, the body chambers of the microconchs are smaller than those of the macroconchs.
Scaphites (S.) preventricosus can be distinguished from the overlying species S. (S.) ventricosus by its smaller size, more closely spaced ribbing, and more loosely uncoiled body chamber. The degree of uncoiling of the body chamber in macroconchs is expressed by the apertural angle. Based on our data, the apertural angle ranges from 92.0° to 104° in S. (S.) preventricosus whereas it ranges from 72° to 89° in S. (S.) ventricosus . The degree of uncoiling of the body chamber in macroconchs is also expressed by the ratio LMAX/HP. Based on our data, this value averages 3.07 in S. (S.) preventricosus whereas it averages 2.55 in S. (S.) ventricosus .
OCCURRENCE: In the Upper Cretaceous of the Western Interior of North America, this species demarcates the lower Coniacian Scaphites (S.) preventricosus Zone. In the study area, the lowest occurrence of this species is just above erosional surface E5.5, which marks the beginning of a major transgression just above the base of the lower Coniacian (Walaszczyk et al., 2014). It is present in the Cardium Formation at Highwood River (TMP2016.041.0136–.0138) and in the Wapiabi Formation at Mill Creek (TMP2016.041.0034,.0036–.0039), Cutpick Creek (TMP 2016.041.0207 and.0208), Oldfort Creek (TMP2016.041.0473), Wapiabi Creek (TMP2016.041.0085 and.0087), and Bighorn Dam (TMP2016.041.0365), Alberta. Elsewhere, this species is abundant in the Kevin Member of the Marias River Shale in north-central Montana and the uppermost part of the Frontier Formation in Wyoming. Outside North America, it has been reported from Umivik, Svartenhuk, Greenland (Birkelund, 1965).
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