Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
Diagnosis.—Shell semielliptical to subrectangular, moderately to strongly convexoconcave (mean length/width ratio 0.63) with subrectangular cardinal extremities; ventral median fold low, almost indistinct near commissure; dorsal sulcus shallow pos- teriorly, impersistent at anterior margin. Radial ribbing sub- equally to unequally parvicostellate. Ventral muscle field bilobed with suboval diductor scars extending 40–45% of valve length. Valve margin strongly thickened with peripheral rim separating disc and trail. Dorsal median ridge initially low and highest at approximately two-thirds valve length.
Occurrence.—Type specimens and other materials considered in this study come from the Las Chacritas Formation exposed at Quebrada La Tuna, Cordón de La Trampa, San Juan Province, Argentina; middle Darriwilian.
Description.—Shell to 16.5mm in width, semielliptical to sub- rectangular in outline, with mean length/width ratio 0.63 (N=15); lateral profile moderately convexoconcave, slightly resupinate, typically 38% as deep as long. Cardinal extremities subrectangular, slightly acute in juvenile specimens. Ventral valve broadly concave with low carinate median fold originat- ing at umbonal area, becoming almost indistinct near commis- sure that is slightly sulcate or rectimarginate. Ventral interarea planar, catacline to gently procline, twice as high as dorsal. Arched imperforated pseudodeltidium covering one-half to two- thirds of delthyrium. Dorsal valve moderately to strongly con- vex, uniformly curved in lateral profile, with broad rounded median sulcus on its posterior third, becoming shallow to impersistent at anterior margin. Dorsal interarea planar, steeply anacline to orthocline. Notothyrium entirely covered by strongly convex chilidium. Ornamentation varying from sub- equally parvicostellate to unequally parvicostellate; accentuated costellae better defined on anterior half of largest individuals, with 6–8 finer parvicostellae between them; costellae 11 or 12 per 2mm (counted at 5mm growth stage). Fine, evenly spaced concentric fila present on entire shell surface. Posterolateral rugae poorly defined or absent. Ventral interior with small, transversely triangular teeth
supported by short strongly diverging dental plates partially masked by secondary deposits. Muscle field bilobed extending anteriorly near 40–45% of valve length; diductor scars suboval, bounded by ridges arising from anterior ends of dental plates; adductor field shorter than diductors, not raised, unbounded anteriorly. Valve margin strongly thickened with rounded subperipheral rim separating disc and trail. Vascula media straight, initially subparallel, subdivided toward valve margin. Gonadal areas poorly defined, striated. Areas between vascular trunks finely pustulose. External ribbing not reflected on valve interior.
787 Dorsal interior with high, bladelike cardinal process erected
on moderately elevated subtriangular notothyrial platform. Socket ridges thickened distally, diverging anteriorly from each other at ~100°, bounding semiconical sockets. Median ridge initially low, becoming highest at approximately two-thirds valve length, fading near anterior margin. Muscle field indistinct in single available internal mold. Dorsal mantle canal system faintly impressed with straight inner branchs of vascula media almost subparallel to median ridge. Internal surface with coarse papillae near margin.
Etymology.—Named after the original locality, Quebrada La Tuna.
Materials.—CEGH-UNC 27172–27183, 12 conjoined speci- mens; CEGH-UNC 27168–27170, three fragmentary ventral valves.
Remarks.—This new species is readily distinguished from Ahtiella argentina by the less transverse subrectangular shell outline, nonalate cardinal extremities, catacline to procline ventral interarea, and larger bilobed ventral muscle field. In addition, the mantle vascular system in A. tunaensis n. sp. is barely impressed, although this could be due to the lack of well-preserved internal molds. Its shell outline is reminiscent of A. quadrata,fromwhich it differsmainly in its much finer radial ribbing. The large bilobed ventralmuscle field of A. tunaensis n. sp. is comparable to that of theBaltic species A. lirata, but differs fromthe latter species in its less convex and smoothly geniculated profile, better defined ventral fold and dorsal sinus, and indistinct rugae. The elongate oval ventral diductors of the Precordilleran species, although rather shorter, resemble those of A.
paucirugosa.However, the Precordilleran species can be distinguished by its subrectangular shell, broad dorsal median ridge, and more deeply impressed mantle canals. The Peruvian species A. zarelae differs in having uniformly costellate radial ribbing, a more prominent ventral fold and deeper dorsal sulcus, a proportionally shorter and wider subpentagonal ventral muscle field, and stronglymarked internal radial ornamentation at all growth stages.
Ahtiella famatiniana new species
urn:lsid:
zoobank.org:act:9AB173B1-73FE-4142-A2AF- A2DF1FD3ED9C Figure 10.9–10.24
2003a Ahtiella sp.; Benedetto, p. 210, pl. 18, figs. 18–20.
Type specimens.—Holotype, CEGH-UNC 27137, internal mold of ventral valve. Paratypes: CEGH-UNC 27135, internal mold of dorsal valve; CEGH-UNC 27149, conjoined specimen.
Figure 11. (1−15) Ahtiella coloradoensis (Benedetto, 1998b); ‘Green Member of the Sepulturas Formation,’ Los Colorados, northwestern Argentina: (1−4) holotype, external mold (1), latex cast (2), internal mold (3), and latex cast (4) of ventral valve, CEGH-UNC 13780; (5, 6) latex cast of exterior (5) and internal mold (6) of dorsal valve, CEGH-UNC 13817; (7) latex cast of ventral exterior, CEGH-UNC 13797; (8) ventral internal mold, CEGH-UNC 13802; (9) latex cast of ventral valve, CEGH-UNC 27187, showing incipient delthyrial cover; (10) latex cast of dorsal interior, CEGH-UNC 13808; (11) internal mold of juvenile dorsal valve, CEGH-UNC 13786; (12) internal mold of juvenile ventral valve, CEGH-UNC 13795; (13) dorsal internal mold, CEGH-UNC 13826; (14, 15) dorsal internal mold (14) and latex cast (15) of dorsal valve, CEGH-UNC 13823, showing detail of cardinalium. (16−18) Ahtiella zarelae Villas in Gutiérrez- Marco and Villas, 2007; San José Formation, Inambari River, Peru (illustrations from Gutiérrez-Marco and Villas, 2007, p. 552, figs. 4D, F, and J, reproduced under the Creative Commons Attribution License CC BY 4.0): (16) latex cast of dorsal valve exterior, MGM 5944X; (17) dorsal internal mold, MGM 5945X; (18) latex cast of ventral interior, MGM 5926X. All specimens dusted with ammonium chloride. Scale bars=5mm.
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
