418
Journal of Paleontology 92(3):412–431
Table 2. Dimensions (in millimeters) in D. stridsbergi n. sp. For abbreviations see Table 1.
N
59447 59448 59452 59453 59456 59457 59458 59459 59699 59719 59764 59768 59771 59772 59773 59777 59780 59781 59786 59806 59836 59840 59905 59906 59948
157058
61.8 51.4 —
66.5 49.6 65.4 53.7 62.9 32 37
67.2 61.5 58
50.6 33.5 67.5 —
46.2 57
42.5 54
37.3 44.5 48
41.6 42.3
SD WW WH WWI WER RI —
24.7 21.3 20.6 26.8 21.3 22.2 21.8 22.5 15.3 24
19.2 15.9 22.1 21.8 17.7 22
23.8 22.9 19
22.5 24
15.5 15.2 16
16.6 16.5
16.8 16.3 17.8 19.2 15.6 18.2 15.2 14.6 9.8
16.3 16.9 13.2 10.7 18.8 17.8 13.9 18 15
14.3 18
16.3 10.6 14.2 15.6 13.4 12.1
1.47 1.31 1.16 1.4
1.37 1.22 1.43 1.54 1.56 1.68 1.32 1.41 1.12 1.46 1.49 1.18 1.22 1.27 1.22 1.5
1.47 1.46 1.07 1.03 1.24 1.36
1.89 2.14 —
1.98 2.13 1.92 1.95 1.70 2.08 2.66 1.87 1.86 1.99 1.83 2.16 1.92 —
2.05 2.14 2.39 2.05 1.95 2.16 2.20 2.18 1.96
—
0.73 — —
0.6 — — —
0.44 0.6 —
0.47 0.44 0.52 0.44 0.46 0.58 0.48 0.46 — — —
0.5
0.43 —
Etymology.—Specific name stridsbergi after Sven Stridsberg (Lund University, Sweden), who has made significant con- tributions to our knowledge of Silurian cephalopods.
Materials.—28 specimens, including types (Table 2).
Remarks.—Discoceras stridsbergi n. sp. most probably diverged from D. graftonense because both species have a similar early shell morphology. Shell shape in both species is similar, but D. stridsbergi n. sp. differs in having a shorter body chamber, sharp ribs around the shell, and a wider hyponomic sinus. Ribs evolved from growth ridges in early whorls, ridges here are much lower and symmetrical in cross section.
Discoceras lindstroemi new species Figures 4–7, 12, 13.3, 13.5, Table 3
Type specimens.—Holotype, a complete shell RM Mo 59511 (Fig. 4.7) from Othem, Kvarnberget 1, Gotland, Slite Beds, late Sheinwoodian, Wenlock, Silurian. Paratypes: RM Mo 40265 (Figs. 4.4, 7.4), 59420 (Figs. 4.1, 12.3), 59432 (Fig. 4.3), 59427 (Fig. 4.6, 4.14), 59442 (Fig. 4.2), 59505 (Fig. 6.1), 157592 (Fig. 4.8), 59506 (Fig. 4.10), 59887 (Fig. 4.13), 59416 (Fig. 4.15), 59419 (Fig. 5.2), 59884 (Fig. 13.3), 59422 (Fig. 5.1), 162186 (Fig. 6.3), and 181926 (Fig. 2.11) from Slite Beds, late Sheinwoodian,Wenlock.
Diagnosis.—Discoceras with heteromorph shell, pseudoumbi- licus and undulate growth frills.
Description.—Evolute, exogastric, slightly depressed shell with a maximum of three whorls (WER: min.=1.83, med.=1.98, max.=2.46, N=19, σ=0.20; Fig. 10). RI: min.=0.33, med.=0.47, max.=0.6, N=16, σ=0.07. Shell heteromorphic: early shell tightly coiled (0.5–1.5 whorl), following part of the shell (about one-half of whorl, Table 3) coiled in loose spire, followed by shell tightly coiled; adapertural part of shell in adult specimens straight. Indication of an additional enrolling, observed in a single specimen (Fig. 13.3), appeared at end of second whorl. The shell before the end of the second whorl is here tightly coiled, in the following one-quarter whorl, a very narrow window between the external shell walls of two neigh- boring whorls appears. Only distal ends of growth ridges oriented adapically are here in contact with the last whorl (body chamber). Umbilical perforation very small. Cross section slightly depressed, impressed zone shallow (WWI: min.=0.73, med.=0.96, max.=1.18, N=17, σ=0.10; Fig. 10). Length of body chamber equal to two-thirds of whorl. Aperture open, in fully grown shells with protruding adapertural part of last whorl, aperture sometimes slightly contracted. Hyponomic sinus V-shaped (HW/HH: min.=1.6, med.=
2.14, max.=3.17, N=11, σ=0.5; Fig. 10). Ocular sinus well developed, situated close to dorsal side (Fig. 4.15). Phragmo- cone chambers relatively long (WH/CL: min.=1.7, med.= 2.71, max.=4.43, N=36, σ=0.69; Fig. 10). In early shell (Fig. 12.2), first five chambers are longer than succeeding chambers. Initial chamber cup-like, with maximum length 1mm, dorsoventral distance in first septum is 2.9mm, length of following chambers measured on ventral side ~2.9mm (second to fourth), following chambers are shorter, length ~1.5mm (Fig. 13.5). Depth of septa during shell growth increases from about one-third of chamber length (second to fourth chamber) to about one-half of phragmocone chamber length in following chambers. Suture forms oblique angle with shallow lateral lobe, but is straight ventrally. Caecum subcentral, slightly expanded within first chamber
(diameter 0.5mm). In second and third chamber, siphuncle shifted toward dorsal side and diameter markedly increased. In the third chamber, siphuncle reaches dorsal side and diameter is about one-sixth of whorl height. Septal necks hemichoanitic; connecting rings relatively thick, tubular or slightly vaulted. Episeptal cameral deposits slightly developed close to siphuncle. First half of whorl smooth or with fine irregular growth lines; growth ridges, accompanied by small constriction of shell, are developed in some specimens at beginning of pseudoumbilicus (WH=5.2–7.2mm; Fig. 12.3); irregularly situated growth ridges appeared between WH 4–7.4mm, they
Figure 4. Discoceras lindstroemi n. sp. from Slite beds, late Sheinwoodian, Gotland; ps: pseudoumbilicus. (1) RM Mo 59420, lateral view, Othem, Spillings: Slite beds; (2) RM Mo 59442, lateral view, counterpart, Färö; (3, 5), RM Mo 59432, lateral and ventral view, Lännaberget; (4) RM Mo 590265, cross section, Othem, Samsuguns 1; (6, 14) RMMo 59427, ventral and lateral view, ps: pseudoumbilicus, Othem, Samsuguns 1; (7, 9, 12) RM Mo 59511, (7) lateral view, (9)detailofearly shell showing lateral furrow (lf) and a healed injury (hi), (12) detail of sculpture, Othem, Samsugun; (8) RMMo 157592, lateral view, polished specimen with undulated frills (uf), Martebo; (10) RMMo 509506, cross section, Othem, Spillings; (11) RM181926 ventral view to adapertural region, Farö; (13) RMMo 59887, lateral view to adapertural region, ocular sinus slightly indicated, Othem, Samsuguns 1; (15) RM Mo 59416, lateral view, protruding last whorl with contracted aperture, Othem, Samsuguns 1, os: ocular sinus, hs: hyponomic sinus. All specimens except Figure 4.8 were coated with ammonium chloride before photographing. Scale bars 10mm.
Occurrence.—Silurian, Wenlock, latest Sheinwoodian–early Homerian; Slite beds; Gotland; Färö, Lannaberget 1; Färö, Lansa 1; Follingbo, Stora Vede 1; Othem, Spillings; Othem, Samsuguns 1; Slite, Kvarnberget 1.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220
