Sheffield and Sumrall—Generic revision of North American Holocystitidae
759
Figure 4.
(1) Gomphocystites indianensis Miller, 1889, a nonholocystitid
diploporitan from the Silurian of North America. Note the long, spiraling ambulacra. Brachiole facets are borne from the left side of the ambulacra only. True diplopores situated within shallow, elliptical peripores are clearly seen in this image (FMNH 19708). (2) Middle Silurian Eucystis specimen from the Bainbridge Formation of Missouri. Oral area pictured shows five multibranching ambulacra extending across the orals and proximal thecal plates and ending in various numbers of large brachiole facets (CMCIP 53630). Scale bars = 1 cm.
strongly from holocystitids. It has a typical oral plate–bearing oral area but lacks O7, and plates O2 and O5 are not in contact with the peristomial opening. The recumbent ambulacra are long and spiraling and wrap around the theca, but they bear brachiole facets only on the left side and seem to be borne on floor plates that are restricted to the left side. They also bear true diplopores instead of the humatipores of holocystitids. The only other known Silurian diploporitan from North
America is a recently discovered and undescribed species of Eucystis from Wenlock-age strata of the Bainbridge Group of Missouri (Sheffield and Sumrall, 2015b). These specimens (Fig. 4.2) share strong morphological similarities with Eucystis from the Ordovician Baltican and peri-Gondwanan faunas. Like their Baltican and peri-Gondwanan counterparts, these speci- mens have five multibranching ambulacra extending across the orals and proximal thecal plates without underlying floor plates. These food grooves each end in a brachiole facet. The diplopores
are simple, and the theca bears an unusually large holdfast that flares slightly at the attachment surface (whether this flaring is characteristic of this taxon or was formed around an unusually large bioclast is unclear). However, these specimens share a similar, slightly clockwise rotation of the oral plates with respect to the ambulacra with Holocystites and
Glyptosphaerites.Itis clear, from strong morphological deviations, that neither Gomphocystites nor these recently discovered eucystitid speci- mens are closely related tomembers of Holocystitidae. Details of the relationships between these taxa are pending phylogenetic analyses that are currently in progress.
Previous phylogenetic analysis.—An evolutionary hypothesis of the Holocystitidae was proposed by Frest et al. (2011), based on assumed trends in peristomial morphology. A second ana- lysis based on a stratocladistic model is not discussed here. The resulting phylogeny (Fig. 5) shows a complicated evolutionary history with drastic changes in the oral area of the diploporitans from an inferred hypothetical ancestor to the more derived taxa (Table 1). Holocystites and Trematocystis are depicted as grades of organization at nodes rather than as monophyletic groupings of taxa. Frest et al. (2011) drew the conclusion that more- derived holocystitids trended toward a reduced number of plates
Figure 5. A proposed evolutionary hypothesis proposed by Frest et al. (2011), based on changes within peristomial morphology; note that Holocystitinae, Holocystites, and Trematocystis are all paraphyletic. This analysis interpreted a trend toward a reduction in oral plates in advanced holocystitids, such as Pentacystis, whose species were described as having vestigial or absent oral plates. Figure modified from Frest et al. (2011).
within the oral area. As mentioned in the preceding, the number of plates observed was affected by taphonomy and specimen preparation and is, therefore, not based on the evolutionary history of the taxa involved. Further, Paul (1971) and Frest et al. (2011) misidentified an oral plate (O7) as a facetal plate, which influenced their interpretations. Some of the characters within their analysis were based on counting the numbers of plates present in proposed taxa. However, characters based on the number of plates present in the absence of a clear understanding of which homologous elements are present and absent among taxa are not properly constructed because the alternate states are not derived from a single character transformation.
Materials and methods
Repositories and institutional abbreviations.—All taxa studied for this analysis, alongwith their locality and age information, are listed in Table 2. All specimens are housed in research collections from the following museums or institutions: CincinnatiMuseum Center (CMCIP), Field Museum of Natural History (FMNH; UC), TheUniversity of Iowa (SUI),Miami University (MUMG), and Yale Peabody Museum (YPM). Brightonicystis was not examined for this study because of a lack of available material.
Systematic paleontology
Subphylum Blastozoa Sprinkle, 1973 Class Diploporita Müller, 1854
Superfamily Sphaeronitida Neumayr, 1889 Family Holocystitidae Miller, 1889
Type genus.—Holocystites Hall, 1861
Other genera.—Trematocystis Jaekel, 1899; Pentacystis Paul, 1971; Pustulocystis Paul, 1971; Brightonicystis Paul, 1971; Paulicystis Frest and Strimple, 2011 in Frest et al., 2011.
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 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272 |
Page 273 |
Page 274 |
Page 275 |
Page 276 |
Page 277 |
Page 278 |
Page 279 |
Page 280 |
Page 281 |
Page 282 |
Page 283 |
Page 284 |
Page 285 |
Page 286 |
Page 287 |
Page 288
