Clioscaphites saxitonianus (McLearn, 1929)
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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-5DDE-FF73-9B90-FB92722CFBEE |
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treatment provided by |
Felipe |
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scientific name |
Clioscaphites saxitonianus (McLearn, 1929) |
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Clioscaphites saxitonianus (McLearn, 1929) View in CoL
Figures 9F, G, 33–36 View FIG View FIG View FIG View FIG
1929. Scaphites ventricosus Meek and Hayden var. saxitonianus McLearn : 77, pl. 18, figs. 1–3; pl. 19, figs. 1, 2.
1952. Clioscaphites saxitonianus (McLearn) . Cobban: 36, pl. 13, figs. 1–10.
1952. Clioscaphites saxitonianus (McLearn) var. keytei Cobban : 37, pl. 20, figs. 5–7.
1965. Clioscaphites saxitonianus (McLearn, 1929) var. septentrionalis Birkelund : 132, pl. 45, figs. 2–5; pl. 46, figs. 1–3; text-figs. 115, 116.
1965. Clioscaphites sp. aff. saxitonianus (McLearn, 1929) . Birkelund: 135, pl. 46, figs. 4–7; pl. 47, figs. 1, 2; text-fig. 117.
1977. Clioscaphites saxitonianus (McLearn) . Kauffman: pl. 24, figs. 5, 6.
1994. Billcobbanoceras saxitonianum (McLearn) .
Cooper: 179.
DIAGNOSIS: Macroconchs large and stout with closely coiled shell, with a reduced aperture; apertural angle averages 66.5°; whorl cross section of the shaft depressed with nearly flat flanks and broadly curved venter; ribs finely and closely spaced on the exposed phragmocone, coarser and more widely spaced on the shaft, and finer and more closely spaced again on the hook; primary ribs strong on the shaft attaining their maximum height as incipient nodes at the ventrolateral shoulder; microconchs smaller and more slender; suture moderately complex with asymmetrically bifid first lateral lobes.
TYPES: The holotype is NMC 9041a from the Alberta Shale of the Crowsnest River area of southwestern Alberta. The paratype is NMC 9041a ; plesiotype is USNM 106739a, b. The holotype of the subspecies keytei, which is synonymized here with the typical form, is USNM 106727 from a calcareous concretion in the Apishapa Shale , 16 miles east of Trinidad, in sec. 1, T. 32 S., R. 62 W., Las Animas County, Colorado .
MATERIAL: The collection consists of 16 specimens, all of which are adult, comprising 13 macroconchs and 3 microconchs.
MACROCONCH DESCRIPTION: In the measured sample, LMAX averages 83.0 mm and ranges from 79.8 to 88.8 mm (table 9). The ratio of the size of the largest specimen to that of the smallest is 1.11. Adults are massive with a nearly circular outline in side view. The exposed phragmocone occupies approximately one whorl and terminates slightly above or below the line of maximum length. The umbilical diameter of the phragmocone is tiny; it averages 3.7 mm and ranges from 2.7 to 4.6 mm (table 9). The body chamber consists of a shaft and recurved hook. In side view, the umbilical shoulder of the shaft is straight and the venter of the shaft is broadly curved. LMAX / HS averages 2.38 and ranges from 2.37 to 2.38. The shell is tightly coiled. LMAX /HP equals 2.84 in TMP2016.041.0229. The aperture is reduced in size relative to that at midshaft. The apertural angle equals 66.5° and ranges from 55.5° in TMP2016.041.0279 to 84.0° in TMP2016.041.0229.
The whorl section of the phragmocone along the line of maximum length is depressed and subovoid with maximum whorl width at one-half whorl height. The umbilical wall is steep and subvertical; the flanks are broadly rounded and slope outward; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WP / HP equals 1.61 in TMP2016.041.0279. As the shell passes from the phragmocone into the body chamber, the whorl width remains nearly the same but the whorl height increases slightly, so that the whorl section at midshaft is slightly less depressed than that along the line of maximum length. The umbilical wall is steep and subvertical; the flanks are broadly rounded and slope outward; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WS/ HS averages 1.41 and ranges from 1.39 to 1.43. 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 broadly rounded; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. WH/HH averages 1.65 and ranges from 1.56 to 1.75. The shell culminates in a constricted aperture with a dorsal lappet.
On the exposed phragmocone, primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoulder. They develop into massive, elongate swellings that reach their maximum strength at the ventrolateral shoulder. On the adapical end of the phragmocone, the primary ribs split into bundles of two or three thinner ribs, with two or three ribs intercalating between them. The ribs are closely spaced on the venter, with a rib density of 5 ribs/cm in TMP2016.041.0017. They are sharp and uniformly strong on the venter, which they cross with a slight adoral projection. The primary ribs become more prominent and widely spaced on the adoral part of the phragmocone. They split into bundles of two or three thinner ribs with one rib intercalating between them. They are widely spaced on the venter with a rib density of 2.5–4 ribs/cm.
The rib pattern on the adoral part of the phragmocone becomes even more pronounced on the shaft. The primary ribs emerge at the umbilical seam and swing slightly forward and then backward again before developing into straight, massive, elongate swellings that attain their maximum strength at the ventrolateral shoulder in the form of incipient nodes. The swellings are widely and equally spaced. At the ventrolateral shoulder, they split into bundles of two secondary ribs with another secondary rib intercalating between them. Ribs are widely and equally spaced on the venter, with a rib density of 1.25–4 ribs/cm. They are uniformly strong and cross the venter with at most a slight adoral projection. The primary ribs become weaker and more closely spaced on the hook. Each rib subdivides into two or three secondary ribs, with another secondary rib intercalating between them. Ribs cross the venter of the hook with a slight adoral projection. They are closely and equally spaced, with a rib density of 4–6 ribs/cm.
The sutures are not generally preserved. However, in TMP2016.041.0229, the first lateral lobe is slightly asymmetrically bifid (fig. 9G).
MICROCONCH DESCRIPTION: Microconchs are smaller and more slender than macroconchs. In addition, the umbilical shoulder of the shaft is concave in microconchs whereas it is straight in macroconchs. LMAX equals 58.2 mm in TPM2016.041.0230 (table 10).
The whorl section of the phragmocone along the line of maximum length is partly visible in TPM2016.041.0230. It is depressed and subovoid with maximum whorl width at one-half whorl height. The flanks are broadly rounded and slope outward; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. As the shell passes from the phragmocone into the body chamber, both the whorl width and height increase slightly, so that the whorl section at midshaft is nearly the same as that of the phragmocone along the line of maximum length. The inner flanks of the phragmocone are broadly rounded and slope outward; the outer flanks are nearly flat; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. Adoral of the midshaft, both the whorl width and height 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 broadly rounded; the ventrolateral shoulder is sharply rounded; and the venter is broadly rounded. The shell culminates in a constricted aperture with a dorsal lappet.
At the base of the body chamber, primary ribs emerge at the umbilical seam and are straight and rectiradiate on the umbilical wall and shoul- der. They develop into massive, elongate swellings that reach their maximum strength at the ventrolateral shoulder. The primary ribs split into bundles of two thinner ribs, with one rib intercalating between them. They are sharp and uniformly strong on the venter, which they cross with a slight adoral projection.
The ribbing pattern is similar on the shaft. The primary ribs emerge at the umbilical seam and develop into straight, massive, elongate swellings that attain their maximum strength at the ventrolateral shoulder. The swellings are widely and equally spaced. At the ventrolateral shoulder, they split into bundles of two secondary ribs with another one or two secondary ribs intercalating between them. The ribs are widely and equally spaced on the venter, with a rib density of 3.75–4 ribs/cm. They are uniformly strong and cross the venter with at most a slight adoral projection. The primary ribs become weaker and more closely spaced on the hook. Each rib subdivides into two secondary ribs, with another one or two secondary ribs intercalating between them. Ribs cross the venter of the hook with a slight adoral projection. They are closely and equally spaced, with a rib density of 4.25 ribs/cm in TMP2016.041.0226.
The suture is not preserved in any of our specimens.
REMARKS: Dimorphism is present in Clioscaphites saxitonianus . The microconch was initially designated by Cobban (1952) as the variety keytei . Microconchs are smaller and more slender than macroconchs. In addition, the umbilical shoulder of the shaft is concave in microconchs whereas it is straight in macroconchs.
Clioscaphites saxitonianus is distinguished from the underlying species Scaphites (S.) depressus by its less globose shape, flatter flanks, and coarser ornamentation on the body chamber. It is distinguished from the overlying species Clioscaphites vermiformis (Meek and Hayden, 1862) by having incipient nodes rather than pointed tubercles on the body chamber and rarely having the first lateral lobe of the suture trifid.
Cooper (1994) established the genus Billcobbanoceras and included Clioscaphites saxitonianus as one of its species. While subsequent workers have acknowledged this reassignment (e.g., Cobban et al., 2006), none of them has ever followed it. We continue to assign this species to Clioscaphites as originally described by Cobban (1952), in anticipation of a thorough taxonomic revision of these Coniacian and Santonian scaphites in the future.
OCCURRENCE: In the Upper Cretaceous of the Western Interior of North America, this species demarcates the lower Santonian Clioscaphites saxitonianus Zone (Scott and Cobban, 1962). In the study area, the lowest occurrence of this species is at the base of the Santonian (surface SS0), coinciding with a major transgression and a marked change in facies to deeper-water, more offshore mudstone. This species is present in the Wapiabi Formation, Alberta, at James River (TMP2016.041.0157), West Thistle Creek (TMP2016.041.0222,.0224, and.0226–.0232), Cardinal River (TMP2016.041.0279), Cripple Creek (TMP2016.041.0148 and.0149), Lynx Creek (TMP2016.041.0339), and above the measured section at Ram River (TMP2016.041.0017 and.0018). In the U.S., this species is present in the Apishapa Shale of southeastern Colorado and in the Kevin Member of the Marias River Shale on the east flank of the Sweetgrass Arch of north-central Montana. Outside North America, it has been reported from west Greenland (Birkelund, 1965).
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