Journal of Paleontology, 92(4), 2018, p. 611–633 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.139
Albian infaunal Pholadomyida (Cretaceous Bivalvia), Comanchean Carbonate Shelf, Texas
Robert W. Scott and Bradley W. Claggett Geosciences Department, University of Tulsa, Tulsa, Oklahoma 74104, USA ⟨
rwscott@cimtel.net⟩, ⟨
bradclaggett@fastmail.net⟩
Abstract.—Species of the megaorder Poromyata, although common and relatively diverse in Albian–lower Cenomanian Comanchean strata in Texas and northern Mexico, have been neglected as biostratigraphic markers and paleoecological indicators. Since 1852, more than a dozen species have been identified as Homomya Agassiz, order Pholadomyida, superfamily Pholadomyoidea or Pleuromya Agassiz, order Pholadida, superfamily Pleuromyoidea. Because valve morphologies of the two genera are similar in many ways, casts are difficult to separate. Statistical analysis of key morphological properties objectively defines species concepts and assesses synonymies. Eight species are retained in Homomya; four are synonymized with these. Two species are provisionally retained in “Homomya” although they differ significantly. One species is reassigned to Liopistha (Sergipemya) alta (Roemer, 1852). Pleuromya henselli (Hill, 1893) is reassigned to Panopea. Homomyid species range from upper Aptian to lower Cenomanian of the Comanchean Series in Texas and Mexico.
Their ranges vary in duration from one million years up to eight million years. These infaunal suspension feeders occupied calcareous mud and carbonate shelf substrates. Two sets of species are distinct morphotypes: a smaller-sized set of H. knowltoni Hill, 1895; H. tarrantensis Perkins, 1961; H. tlahualiloensis Perkins, 1961; and H. kellumi Perkins, 1961 and a larger-sized set of H. cymbiformis Perkins, 1961; H. austinensis Shattuck, 1903; H. vulgaris Shattuck, 1903; H. budaensis Whitney, 1911; and H. auroraensis Perkins, 1961. Two end-member morphotypes are represented by the “streamlined” Homomya knowltoni, which is an elongate, slightly inflated form with a relatively high umbo, and the cylindrical Homomya budaensis, which is a very elongate, tubular, inflated form with a very low umbo.
Introduction
Mesozoic soft, muddy, carbonate marine shelf substrates hosted a variety of infaunal suspension-feeding bivalves similar to the modern Pholadomya candida (Sowerby, 1823), which burrows in shallow muddy shelves. These large, elongate, thin-shelled bivalves feed by extending the foot through the anterior gape in search of detritus, pedal feeding (Morton, 1981). Pholado- myidae survived the end-Permian mass extinction (Stanley, 2015). Beginning in the Triassic and expanding in the Jurassic, this family evolved into several different clades. The more common Cretaceous genera were Pholadomya Sowerby, 1823; Homomya Agassiz, 1843; Pachymya J. de C. Sowerby, 1826 (in Sowerby and Sowerby, 1812–1846); and Pleuromya Agassiz, 1843, whose species generally were long ranging within and even across more than one stage. Two common and relatively diverse Albian genera
reported from Comanchean strata are Homomya, order Pholadomyoida Newell (1965), superfamily Pholadomyoidea King (1844), and Pleuromya, order Pholadida Gray (1854), superfamily Pleuromyoidea Zittel (1895). The valve morpho- logies of the two groups are similar in many ways and casts are difficult to distinguish. Both groups have edentulous hinges although some Pholadomyidae King, 1844 species have small knobs at the hinge line. Pholadomyoidae differ from Pleuromyoidae by the ligament structure, which is rarely
preserved in external casts. Genera of Pholadomyoidae have an external opisthodetic, parivincular ligament and some have ligamental pseudonymphs. Homomya in the family Pholado- myidae has no lunule, but its escutcheon has marginal ridges that fade anteriorly. The external posterior ligament of Pleuro- myoidea is opisthodetic, and the anterior ligament becomes subumbonal supported by weak chondrophores. Pleuromya species have no lunule or escutcheon and the right valve (RV) slightly overlaps the left valve (LV). The posterior shell outline of Homomya tends to be straight and sloping either toward the posteroventral corner or toward the dorsal margin, whereas the posterior margin of Pleuromya is evenly rounded. Consequently, Comanchean species previously assigned to Pleuromya are here reassigned to Homomya. The order Pholadomyoida and the superfamily Pholado-
myoidea contain Pholadomyidae King (1844), Margaritariidae Vokes (1964), Ucumariidae Sánchez (2003), and Arenigomyi- dae Carter (Carter et al., 2011). The superfamily Pholadomyoi- dea ranges from Early Ordovician to Holocene. Its common Cretaceous genus Homomya first appeared in Early Triassic and became extinct in middle Eocene but possibly continued into late Oligocene (personal communication, J.G. Carter, Uni- versity of North Carolina at Chapel Hill, 2016). Carter et al. (2011) placed the family Pleuromyidae in the order Pholadida in the superfamily Pleuromyoidea with the families Ceratomyidae Arkell 1934 and Vacunellidae Astafieva-Urbajtis 1973. This
611
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
