Freeman et al.—Upper Cambrian linguliform Brachiopods from Texas
condont Zone, in the Threadgill Member of the Tanyard Formation.
Description.—Ventribiconvex, subcircular in outline. Shell thin and fragile. Postlarval ornament of strong parvicostellae crossed by equally strong, rounded lamellae that follow growth lines (Fig. 5.11). Rounded nodes formed by intersection of parvi- costellae, not forming spines. Parvicostellae spaced 20–25 μm apart and lamellae spaced 30–35 μmapart in mid-stages of adult growth, resulting in laterally elongated grid pattern. Lamellae more closely and more irregularly spaced in later growth. Interior of both valves with irregularly spaced, deeply impressed rounded pits, variable in diameter 5–15 μm. Dorsal valve gently convex, with anterior central part of
761
Another difference is a more robust and well-developed plate posterior to the foramen, but because the ventral valves are so fragile, this description is based on a single specimen.
Schizambon sp. Figure 5.15–5.18
Occurrence.—In the Cordylodus lindstromi s. l., Iapetog- nathus, and Cordylodus angulatus conodont zones and the Symphysurina trilobite Zone, in the upper part of the San Saba Member of the Wilberns Formation and the lowest part of the Threadgill Member of the Tanyard Formation.
shell flattened to very faintly and broadly sulcate (Fig. 5.9, 5.10). Dorsal valve transversely suboval in outline, nearly equidimensional with length 95% or more of width. Maximum width may be 3mm or more on mature specimens. Larval shell of dorsal valve ~325 μmin width, bearing two poorly developed nodes separated by median depression. Short, narrow, ortho- cline dorsal pseudointerarea occupying less than half the maximum width of the valve and <5% of maximum length (Fig. 5.12, 5.14). Wide, triangular median groove divides propareas that have a concave surface and upturned anterior margin. Anterior margin of propareas slightly curved in outline. Median buttress extends into low median septum, which extends anteriorally a short distance and tapers into more gently defined median ridge extending to about two-thirds valve length (Fig. 5.12–5.13). Transversely suboval central muscle scars located lateral to median ridge at about one-third length of shell. Ventral valve strongly inflated, with greatest height at
approximately one-third of length from posterior to anterior, width 90–95% of valve length (Fig. 5.1, 5.2). Ventral pseudointerarea low, apsacline, not divided, convex in lateral profile (Fig. 5.3). Posterior margin of pseudointerarea thickened and rounded. Pedicle foramen elongate oval in shape, ~270μm long and 190μm wide (Fig. 5.4). Foramen covered with a thickened plate posteriorally, ~220μm long and 130μmwide. Plate exhibits prominent growth lines and is elevated along its length to a height equal to or greater than the valve laterally, and is bounded by deeply impressed grooves. Intact ventral valves rare in this collection and invariably exfoliated around the internal pedicle opening; details of internal pedicle tube not known. Visceral fields lightly impressed and not easily observable.
Etymology.—After Dennis and Shirley Lange and their daugh- ter Peggy Lange Crenwelge, owners of the Lange Ranch, where this species was found.
Materials.—Two hundred thirty two dorsal valves and 44 mostly fragmented ventral valves, including the holotype and other illustrated material, dorsal valve NPL 45506 and ventral valves NPL 85577–85579.
Remarks.—This material is similar to Schizambon obtusus Holmer et al., 2005 but differs in having a strongly developed dorsal median buttress extending toward the anterior as a short median ridge and in having low nodes, rather than spines, developed at the intersections of the parvicostellae and lamellae.
Description.—Thick shelled and biconvex, with mature ornament of parvicostellae crossed by prominent rounded con- centric lamellae. Lamellae spaced 50–70 μm apart on mature growth, and parvicostellae spaced more irregularly, 25–40 μm apart. Intersections of lamellae and parvicostellae form rounded nodes. Ventral valve with large foramen enlarged by resorption
and filled in to posterior with thick rounded, elevated plate (Fig. 5.17, 5.18). Details of ventral interior not known. Dorsal valve with reduced orthocline pseudointerarea
divided by broad median groove with poorly defined lateral edges (Fig. 5.15). Other details of interior not known.
Materials.—Ten dorsal and seven ventral valves, all fragmental, (Supplemental Data Table 2) including illustrated dorsal valve NPL 45509 and ventral valve NPL 45508.
Remarks.—This material is fragmentary, but distinguishable from the much larger, thin-shelled Schizambon langei n. sp., which co-occurs with this species in one sample. In features that can be observed, it is similar to Schizambon typicalis Walcott, 1884, which occurs in coeval strata in Utah and Nevada (Popov et al., 2002). The short spines characteristic of that species are not present on this material, but the specimens are poorly preserved and clearly abraded.
Genus Siphonobolus Havlíček, 1982 Siphonobolus? sp. Figure 6.1–6.14
Occurrence.—In the Clavohamulus elongatus Subzone of the Cordylodus proavus conodont Zone, the Hirstutodontus simplex Subzone of the Cordylodus intermedius conodont Zone, the Iapetognathus conodont Zone, and the Cordylodus angulatus conodont Zone, and in the Symphysurina trilobite Zone, in the uppermost San Saba Member of the Wilberns Formation and the lowermost Threadgill Member of the Tanyard Formation, Texas.
Description.—Postlarval growth ornamented with short node-like spines and longer hollow spines. Hollow spines along lateral posterior margins of both valves; long hollow spines of two sizes present along line of commissure (Fig. 6.13, 6.14). Hollow spines and tubercles arranged in irregular rows on mature growth (Fig. 6.11, 6.12). Tubercles arranged in poorly expressed radial pattern. Tubercles vary in size from <10 μmin
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
