Hoploscaphites macer, Landman & Kennedy & Larson & Grier & Grier & Linn, 2019

Landman, Neil H., Kennedy, W. James, Larson, Neal L., Grier, Joyce C., Grier, James W. & Linn, Tom, 2019, Description Of Two Species Of Hoploscaphites (Ammonoidea: Ancyloceratina) From The Upper Cretaceous (Lower Maastrichtian) Of The U. S. Western Interior, Bulletin of the American Museum of Natural History 2019 (427), pp. 1-72 : 22-23

Hoploscaphites macer , n. sp.

Figures 8A, B , 11C, D, 13–22, 23A–C, 24, 25

DIAGNOSIS: Macroconchs oval to nearly circular in lateral view; cross section of shaft compressed subovoid with high whorls consisting of broadly rounded flanks and nearly flat venter; maximum width at adoral part of shaft; small umbilicus commonly with umbilical bulge; ribs coarse on phragmocone at the point of exposure, subsequently becoming finer; small, moderately closely spaced ventrolateral tubercles; small umbilicolateral tubercles on body chamber; occasional lateral tubercles on adapical part of phragmocone; apertural angle averaging 56°; microconchs oval in lateral view; cross section of shaft subovoid with broadly rounded flanks converging toward ventrolateral shoulder; large umbilicus exposing earlier whorls; closely spaced ventrolateral tubercles; relatively prominent umbilicolateral tubercles on body chamber; suture with broad, asymmetrically bifid first lateral saddle and narrow, symmetrically to asymmetrically bifid first lateral lobe.

ETYMOLOGY: The name is derived from the Latin word macer , “thin,” which is one’s first impression of this species relative to other species in the same biostratigraphic zones. Upon inspection, however, the shell is slightly inflated. Nevertheless, this name has been used informally for the last 20 years, and for the sake of stability, we decided to retain the name for this species.

TYPES: The holotype is AMNH 71839 (figs. 19E–H, 23A), a macroconch, from the lower part of the B. grandis Zone of the Pierre Shale on the Cedar Creek Anticline, Montana. It is a steinkern with some attached shell 54.2 mm in diameter with a small umbilicus. The macroconch paratypes are AMNH 113323 (fig. 20D–F) and 113324 (fig. 20G–I), both from the upper part of the B. baculus Zone or the lower part of the B. grandis Zone of the Pierre Shale on the Cedar Creek Anticline, Montana, and USNM 713612 (fig. 17A–C) from the B. grandis Zone of the Bearpaw Shale in Roosevelt County, Montana. The microconch paratypes are AMNH 72634 ( fig. 24 M–P) and 108314 ( fig. 24 E–H) from the upper part of the B. baculus Zone or the lower part of the B. grandis Zone of the Pierre Shale on the Cedar Creek Anticline, Montana.

MATERIAL: The collection consists of approximately 100 specimens of which 75 comprise the measured set (40 macroconchs and 35 microconchs) (tables 1, 2). All the specimens in our collection are from the upper part of the Baculites baculus Zone or lower part of the B. grandis Zone of the Pierre Shale in Wyoming, Colorado, and Montana and the Bearpaw Shale in Montana.

MACROCONCH DESCRIPTION: In the measured sample, LMAX averages 61.4 mm and ranges from 45.2 to 76.0 mm (table 1). The ratio of the size of the largest specimen to that of the smallest is 1.68. The specimens form a broad size distribution, with a peak at 50–55 mm (fig. 13). Adults are slender with an oval to circular outline in side view. LMAX /HS averages 2.05 and ranges from 1.86 to 2.25 (1.94 in the holotype). BHI 4346 (fig. 14A–D) is an example of a specimen with an oval outline ( LMAX /HS = 2.17) and AMNH 108487 (fig. 19A–D) is an example of a specimen with a more rounded, nearly perfectly circular outline ( LMAX /HS = 1.98).

The umbilicus is small and deep. The umbilical diameter averages 3.8 mm and ranges from 1.9 to 6.5 mm. UD/ LMAX averages 0.06 and ranges from 0.04 to 0.12. In lateral view, the umbilical wall is straight and usually exhibits a pronounced umbilical bulge, so that part of the umbilicus of the phragmocone is concealed. All specimens are tightly coiled with little or no gap between the phragmocone and hook. LMAX /HP 2 averages 2.78 and ranges from 2.55 to 2.98 (2.70 in the holotype). AMNH 76388 (fig. 18A–C) is an example of a tightly coiled specimen ( LMAX /HP 2 = 2.75) and BHI 4346 (fig. 14A–D) is an example of a more loosely coiled specimen ( LMAX /HP 2 = 2.89).

Figures

FIGURE 8. A, B. Unpaired ventral muscle scar (arrow) just adoral of the last suture, Hoploscaphites macer, microconch, AMNH 69736, AMNH loc. 3921, upper Baculites baculus or lower B. grandis Zone, Pierre Shale, Cedar Creek Anticline, east-central Montana. C. Apertural view of dorsal projection (arrow), Hoploscaphites criptonodosus (Riccardi, 1983), AMNH 108459, AMNH loc. 3278, Baculites grandis Zone, Pierre Shale, Weston County, Wyoming. The hook of the body chamber has broken off exposing the dorsal projection, which is plastered against the venter of the phragmocone. D. Kidney-shaped apertural opening showing the dorsal projection, Hoploscaphites criptonodosus (Riccardi, 1983), macroconch, AMNH 71830, AMNH loc. 3921, upper Baculites baculus or lower B. grandis Zone, Pierre Shale, Cedar Creek Anticline, east-central Montana.

FIGURE 24. Hoploscaphites macer, microconchs. A-D. YPM 35657, YPM loc. A4745, Baculites baculus or B. grandis Zone, Pierre Shale, Niobrara County, Wyoming. A, Right lateral; B, apertural; C, ventral; D, left lateral. E–H. AMNH 108314, paratype, AMNH loc. 3244, lower B. grandis Zone, Pierre Shale, Dawson County, Montana. E, Right lateral; F, apertural; G, ventral; H, left lateral. I–L. AMNH 76383, AMNH loc. 3921, upper Baculites baculus or lower B. grandis Zone, Pierre Shale, Cedar Creek Anticline, east-central Montana. I, Right lateral; J, apertural; K, ventral; L, left lateral. M–P. AMNH 72634, paratype, AMNH loc. 3921, upper Baculites baculus or lower B. grandis Zone, Pierre Shale, Cedar Creek Anticline, east-central Montana. M, Right lateral; N, apertural; O, ventral; P, left lateral. Arrows indicate the base of the body chamber. Specimens ×1.