416
Journal of Paleontology 92(3):412–431
Table 1. Dimensions (in millimeters) in D. graftonense (Meek and Worthen). N: specimen RM Mo number; SD: maximal shell diameter; WH: whorl height; WW: whorl width; WWI: whorl width index.
N
59467 59696 59698 59701 59706 59709 59712 59714 59718 59775 59782 59801 59803 59839a 59839b
28
SD WW WH WWI WER RI —
57.6 55.4 37.5 24.2 24.2 62 69 45
65.4 39.4 57.5 42.3 60
66.7
12.5 18
18.8 15.6 11.1 11.1 —
26 23
— —
—
16.5 20 22
7.5
16.8 18.5 11
8.8 7.4
19.5 15
20
16.7 11.2 15
12.3 16.9 20.7
1.67 1.07 1.02 1.42 1.26 1.5 —
1.33 —
1.38 — —
1.34 1.18 1.06
1.87 1.99 2.25 2.00 2.47 2.08 2.18 1.94 2.25 1.80 1.95 1.83 1.99 1.94 2.10
0.53 0.50 0.50 0.50 0.50 0.48 0.46 0.53 0.45 0.50 0.54 0.41 0.47 0.69
beds, unit g; Othem, Samsuguns 1: Slite beds, unit g. Bohemia; Llandovery, Aeronian, Demirastrites triangulatus Biozone, Solopisky locality, Demirastrites simulans Biozone, Koněprusy locality (exact site unknown); Wenlock, Sheinwoodian, Cyrtograptus murchisoni Biozone, Praha, Malá Chuchle- Vyskočilka. USA (Foerste, 1925); Australia (Etheridge, 1904; Furnish and Glenister, 1964); and Inner Mongolia (Zou, 1983).
Description.—Shell tightly coiled, exogastric, slightly expanding at most with six whorls; WER: min.=1.8, med.= 1.99, max=2.48, N=15, σ=0.18 (Fig. 10). RI: min.=0.41, med.=0.5, max.=0.69, N=14, σ=0.06. Umbilical perforation very small, drop shaped, with length ~0.6mm. Cross section slightly depressed, in later growth stage subcircular or almost subquadrate (WWI: min.=1.02, med. = 1.33, max.=1.67, N =11, σ=0.19, Fig. 10), impressed zone shallow. Length of body chamber about two-thirds of whorl. Aperture open, or may be slightly contracted in fully grown specimens with uncoiled adapertural part of the shell. Hyponomic sinus V-shaped (HW/HH: min.=1.54, med.= 1.79, max.=2.5, N=6, σ=0.35; Fig. 10). Phragmocone chambers relatively long (WH/CL: min.=1.7, med.=2.71, max.=4.3,N=36, σ=0.69; Fig. 10). First six chambers longer than those succeeding; first chamber cup-like (CL=2.7mm, WH=2.9 mm), length of following chambers measured as abscises along the ventral side (Fig. 1): 3.7mm (second), 3.6mm (third to sixth), and 3.1mm (seventh and following). Depth of septa during shell growth increases from about one- third of chamber length (first to fifth chamber) to about one-half of phragmocone chamber length, and later becomes slightly shorter. Suture oblique, with shallow lateral lobe, ventrally straight. Caecum central, with diameter 0.5mm, thick-walled; in second phragmocone chamber siphuncle shifted dorsally and in third chamber reached dorsal side. Diameter of siphuncle in
apical shell increased to reach constant relative thickness one- fifth of IWH in third chamber. Septal necks hemichoanitic. Connecting rings relatively thick, tubular or slightly vaulted. Sculpture consists of growth lines and transversal growth ridges. Early shell with fine regularly arranged growth lines, densely packed and appear when shell reached one-fifth to one-third of whorl. In following growth stages, growth lines run obliquely to shell axis and form deepening V-shaped hyponomic sinus; middle part of hyponomic sinus rounded. Growth lines in later
shell growth stages irregularly arranged with one or two more distinct growth lines between ridges. Growth ridges distinct laterally as well as ventrally, parallel with growth lines; internal mold usually smooth. Crests of growth ridges sharp. Lateral furrow, observed in a single specimen, is very shallow, ~2mm wide and accentuated by distinct lighter-colored band on shell. Maximum shell thickness 2mm. Three flattened specimens of D. graftonense preserved in
shale are known from Bohemia. Specimen NM L 42237 (Fig. 2.5) is a damaged, moderately expanding shell (SD=56 mm, three and one-quarter whorls); adapertural part of the shell missing. Shell tightly coiled, impressed zone very shallow, umbilical perforation small. Sculpture consists of prominent growth ridges and fine growth lines. Growth ridges, discernible from the beginning of the second whorl, appear as curved narrow elevations running obliquely to the shell axis. They are densely spaced, moderately vaulted to the aperture laterally, indicating a deep hyponomic sinus ventrally. Growth lines are prominent, regularly arranged, and parallel with growth ridges; 7–10 lines between crests of adjacent growth ridges. The second specimen of D. graftonense (NM L 4223) is a flattened internal mold of a body chamber (Fig. 2.1). The third specimen (NM L 46556; not figured) is a flattened fragment of the ventral side showing ribs forming a deep hyponomic sinus.
Materials.—17 specimens from Gotland: RM Mo 59467, 59505, 59696, 59698, 59701, 59706, 59709, 59712, 59714, 59718, 59775, 59782, 59801, 59803, 59806, 59839, 155975; three specimens from Bohemia: NM L 42237, L 42239, L 46556.
Remarks.—Specimens from Gotland and Bohemia are very similar in the shape of the shell, hyponomic sinus, course of growth lines, position and thickness of siphuncle, sutures, and length of phragmocone chambers to D. graftonense from Ohio and Illinois. Two type specimens figured by Foerste (1925, pl. 12, figs. 2, 3) fall within the variability of WER and WWI described herein (Fig. 10). Four endemic species diverged from widely distributed D. graftonense (shell morphology of first whorl in all these species and principal conch parameter [WER] are very similar with D. graftonense): D. ortoni (Meek, 1873) from Ohio differs in having a shallow hyponomic sinus (HW/HH=2.7–3, which is a higher value than those in D. graftonense from Gotland); D. amissus (Barrande, 1865); D. stridsbergi n. sp., which has distinct ribs; and D. lindstroemi n. sp., which has undulated frills, a less-depressed cross section, and a heteromorphic shell.
Discoceras stridsbergi new species Figure 3, Table 2
Type specimens.—Holotype, an almost complete shell RM 59777 (Fig. 3.8), from Stora Vede (Gotland), Slite Beds, early Homerian,Wenlock, Silurian. Paratypes: RM 59452 (Fig. 3.7), 59463 (Fig. 3.6), 59457 (Fig. 3.5), 59719 (Fig. 3.1), 59786 (Fig. 3.4), 59836 (Fig. 3.3), and 59948 (Fig. 3.2).
Diagnosis.—Discoceras with moderately expanding shell and recurrent sharp ribs around the shell, running obliquely to shell
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