630
Journal of Paleontology 92(4):611–633 Megaorder Solenata Dall, 1889
Order Hiatellida Carter in Carter et al., 2011 Superfamily Hiatelloidea Gray, 1824 Family Hiatellidae Gray, 1824 Subfamily Panopeinae Bronn, 1862
Genus Panopea Ménard de la Groye, 1807 Type species.—Mya glycimeris Born, 1778 (ICZN, 1986).
Description.—Medium to large, equivalved, quadrate to trape- zoidal, elongate, gaping valves; ligamental nymphs tall; one or two small cardinal teeth; wide pallial sinus. The generic name, Panopea, has been conserved (ICZN, 1986).
Panopea henselli (Hill, 1893) Figure 11.9, 11.10
1893 Pleuromya (?) henselli Hill, p. 31, pl. 4, figs. 1, 2. 1967 Pleuromya (?) henselli; Rodgers in Lokke, p. 127. 1928 Panope (sic) henselli; Adkins, p. 170. 1924 Panopea henselli; Gillet, p. 233. 2002 Panopea henselli; Akers and Akers, p. 473. 1901 Pholadomya henselli; Hill, p. 161. 1937 Panopea hilli; Whitney p. 172, pl. 15, figs. 2, 3.
Lectotype.—Panopea henselli two cotypes NMNH 145646; here the smaller is designated the lectotype.
Diagnosis.—An elongate shell, subcentral beaks, anterior and posterior margins evenly rounded, slight posterior gape; ornament of low, rounded commarginal growth rugae.
Occurrence.—Lower Albian, Somervell County, Texas, about 12m below top of Glen Rose.
Description.—Equivalved, elliptical, elongate valve, umbo subcentral, not prominent; anterior margin rounded, posterior margin subrounded, gaping slightly; angle around beak between dorsal anterior and posterior margins 135°–140°. Ornamented by simple rounded commarginal growth rugae. The average length of the two specimens is 76.1mm; height is 46.95mm; thickness is 33.1mm.
Remarks.—Hill (1893) questioned the generic assignment of this species but created this species because it is characteristic of the Glen Rose Formation. Hill noted the similarities between P. henselli and Thracia myaeformis White (1880). Similar species in Texas Comanchean strata are Panopea texana Shumard 1854 at ‘camp 4 Cross Timbers,’ and Panopea newberryi Shumard 1860 from the Edwards Limestone in north Texas. Both species are illustrated by hand drawings; both are relatively higher than P. henselli. Roemer (1852, p. 45) noted an indeterminate Panopaea (sic) sp. together with Mesorbitolina texana (Roe- mer) near Pedernales that he compared to Panopea regularis d’Orbigny (1843). Panopea sellardsi Whitney 1952 from the middle part of the Glen Rose Formation in central Texas is much smaller than P. henselli, narrower posteriorly, and relatively more inflated.
Panopea is a common genus in pre-Albian strata in
Switzerland and in Argentina (Weaver, 1931). Panopea dupiana d’Orbigny 1843 is about twice as long as high and the umbo is closer to the anterior margin than in P. henselli.
Panopea neocomiensis (Leymerie, 1842) is ornamented by very fine radial striae. Modern Panopea bivalves have aragonite shells that dissolve quickly following death, leaving external molds, which are filled with fine crystalline carbonate. Because of this taphonomy, the resulting casts show mainly external characters.
Conclusions
Species of Pholadomyidea are common and relatively diverse in Albian–lower Cenomanian Comanchean strata in Texas and northern Mexico. Some 22 species have been classed with Pholadomya, Homomya, Pachymya, Myopholas, Panopea, and Pleuromya. Two genera most commonly reported are Homo- mya, order Pholadomyoida, superfamily Pholadomyoidea and Pleuromya, order Pholadida, superfamily Pleuromyoidea. Since 1852, more than a dozen species have been identified as either Homomya or Pleuromya. Because valve morphologies of the two genera are similar in many ways, casts are difficult to separate. Statistical analysis of height and width objectively differentiates species. Eight species are retained in Homomya, four are synonymized with these; two species are provisionally retained in “Homomya” although they differ significantly. Homomyid species in Texas and Mexico range from lower
Albian to lower Cenomanian. They are found in each of three Gulf Coast Comanchean Series groups: upper Aptian to lower Albian Trinity Group, middle to basal upper Albian Freder- icksburg Group, and upper Albian to lower Cenomanian Washita Group. They evolved across this interval, and their ranges vary from one to two million years to up to eight million years. Two sets of species form two clades, the smaller-sized H. knowltoni, H. tarrantensis, H. tlahualiloensis, H. kellumi (Fig. 4) and the larger-sized set of H. cymbiformis,H. austinensis, H. vulgaris,H.budaensis,and H.
auroraensis.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. These infaunal suspension feeders occupied offshore cal- careous mud and carbonate shelf substrates.
Acknowledgments
G. Heumann, Sammlungsverwaltung Paläontologie & Goldfuß-Museum, Steinmann-Institut, Paläontologie, Bonn, kindly provided new photographs of Roemer’s type specimen of Homomya alta. M.S. Florence, Museum Specialist, Smithso- nian Institution, arranged loan of Hill’s types of Pholadomya knowltoni and Pleuromya (?) henselli. A. Molineux and A. Thompson, Non-vertebrate Paleontological Laboratory, Uni- versity of Texas, hosted our visit to the museum and facilitated loan of M.I. Whitney’s specimens and access to the complete Cretaceous collections. D. Miller, University of Michigan Museum of Paleontology, hosted our visit to the museum and arranged loan of Perkins’s types. New photographs of the
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
