912 Materials and methods
In a recent survey of available collections at the Gotland Museum (Visby), the Swedish Museum of Natural History (Stockholm), and the Natural History Museum (London), a fairly large number of specimens of Pentamerus gothlandicus were examined in order to clarify the phylogeny and evolu- tionary patterns of Pentamerus because this Silurian hallmark taxon can provide important information on the faunal evolution and paleobiogeography of the period. Examination of museum collections, as well as new collections from Tjeldersholm, one of the P. gothlandicus localities on Gotland, through naturally exposed dorsal interiors and transversely cut sections, immediately reveals that the dorsal internal structures of P. gothlandicus are distinctly different from those of the typical Pentamerus. Rather, the ventral sides of the outer hinge plates show various degrees of fusion to form a ventrally projecting plate, resembling a medially crested arch, usually around the mid-length of the hinge plates (Fig. 1.7–1.11). The inner hinge plates and the dorsal portions of the outer hinge plates, however, are clearly discrete, as is typical of Pentamerus. Such a tent-like structure, called a “closed brachiophorium” by Sapelnikov (1972), has been reported in several pentamerid genera of Wenlock–Ludlow age, such as Pentamerifera Khodalivich, 1939; Capelliniella Strand, 1928; and Brooksina Kirk, 1922, which are most common in the Ural Mountains, Alaska, and western North America (e.g., Kirk, 1922, 1925; Sapelnikov, 1972, 1985; Johnson et al., 1976; Boucot and Johnson, 1979; Rong and Zhang, 1988; Sheehan and Harris, 1997). Thus, it appears that this brachial structure was exclusively a post- Llandovery evolutionary trend among some genera of the Pentameroidea, since such a structure is unknown in Llandovery or older members of the superfamily. Sections of other well- preserved dorsal interiors of the Gotland species have also confirmed the presence of a brachiophorium with a fused crest, thus warranting the establishment of a new genus, to be called Karlsorus n. gen. in this study. The main purpose of this study, therefore, is to clarify the morphology of Karlsorus gothlandi- cus (n. comb.) and to discuss the implications of its revised taxonomy for pentamerid evolution and paleobiostratigraphy in the Silurian. A recent summary and update of the geology of Gotland
and the litho- and biostratigraphy of the Slite beds can be found in Calner et al. (2004). Details of the geology of the Pentamerus gothlandicus-bearing localities (Tjeldersholm 1, Stora and Lilla Karlsö, Valle 1, and Valle 2) can be found in Hede (1927), Laufeld (1974), Calner et al. (2004), and Bassett (2005). The Karlsorus-bearing upper Slite beds lie in the upper Wenlock Cyrtograptus lundgreni Biozone, where a minor extinction event has been recognized, but it affected mainly planktonic fossil groups (e.g., graptolites; see Jaeger, 1991; Lenz et al., 2006) rather than the shelly benthos (e.g., brachiopods; Copper, 2004).
Repositories and institutional abbreviations.—Specimens examined and cited in this study, including figured specimens, are from the following institutions: Gotland Museum (LMG), Visby, Gotland, Sweden; Swedish Museum of Natural History
Journal of Paleontology 91(5):911–918
(Riksmuseum, Br), Stockholm, Sweden; Natural History Museum(B), London, UK; Paleontological Institute (VSEGEI), St. Petersburg, Russia.
Evolutionary and biostratigraphic significance
Historically, Pentamerus has been a taxonomic dump bag for a variety of smooth-shelled pentameride genera. This has greatly diminished the biostratigraphic utility of this globally wide- spread taxon and obscured its evolutionary trend during the early Silurian, although true Pentamerus has been thought largely as a middle Aeronian–late Llandovery genus. The first appearance of Pentamerus during the middle
Aeronian has been widely recorded in Laurentia, Baltica, Avalonia, Siberia, and some of their adjacent small terranes (see summaries of Boucot and Johnson, 1979; Sapelnikov, 1985; Jin and Copper, 2000). The early forms of Pentamerus developed a nearly equibiconvex shell, were most abundant from high tropical to subtropical regions, and adapted to a vertical life position on a muddy, level-bottom substrate in tightly packed
clusters (Jin, 2008). By the late Aeronian, Pentamerus diversi- fied into the paleoequatorial regions to give rise to Sulcipenta- merus, a strongly ventribiconvex shell with an arched ventral valve and a flattened and reduced dorsal valve. This has been interpreted as an adaption for a recumbent life position in the generally low-energy paleoequatorial environment devoid of frequent severe storms, with the large and deep ventral valve partly embedded in the sediments, and the reduced dorsal valve behaving like a mobile lid (Rong et al., 2007; Gushulak et al., 2016).
By the middle Telychian, Pentamerus went through a
major speciation event and gave rise to Pentameroides Schu- chert and Cooper, 1931, which became a common pentameroid in Laurentia and Baltica during the late Telychian (Johnson, 1979; Baarli and Johnson, 1988; Jin and Copper, 2000). This cladogenetic event was marked by the formation of a cruralium, through the convergence of inner hinge plates onto a low median septum in the dorsal valve. The Pentamerus-Pentameroides transition has been regarded as a reliable biostratigraphic marker for a middle Telychian age for Laurentia and Baltica (Baarli and Johnson, 1988). The post-Llandovery evolution of Pentamerus has been
confusing because the taxonomic validity of many smooth- shelled pentamerids has been a matter of debate. In North America, some large-shelled forms of late Telychian–Wenlock age, such as Apopentamerus and Pentamerus (Supertrilobus) proposed by Boucot and Johnson (1979), have not been studied in detail regarding their internal structures. Apopentamerus, for example, was regarded by Sapelnikov (1985) as a junior syno- nym of Harpidium. The relationship between the smooth- shelled Harpidium Kirk, 1925 and Pentamerus also remains to be investigated. The strongly ventribiconvex shells of Harpidium, ranging from late Llandovery to Ludlow, may be similar to the evolution of Sulcipentamerus that adapted to low-energy level bottom or sheltered reefal depositional environments (Gushulak et al., 2016). Pentamerus and Karlsorus n. gen., as well as many other genera of the family Pentameridae showed various
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 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238