422
Journal of Paleontology 91(3):417–433
(Kozlowski, 1923) nearly 250m above Keidel’s bed, in the Punta Negra Formation in the Loma de los Piojos section near Jáchal (Rustán and Vaccari, 2010, p. 1084), strongly supports this stratigraphic interpretation. Thus, the top of the Talacasto Formation should be con-
sidered diachronous, with earliest Emsian levels in the southern sections (Quebrada de Talacasto area) and late Emsian or younger levels in the northernmost sections (Río Jáchal area) (Figs. 2 and 3).
Materials and methods
After new collecting efforts and a revision of previous paleon- tological collections, we recognized nearly thirty bactritid specimens coming from different localities in the Talacasto Formation (Fig. 1). Surveyed sections in San Juan Province include (from south to north): Quebrada de la Cortadera, Quebrada de Talacasto, Quebrada de Las Aguaditas, and Loma de los Piojos. The single locality in La Rioja Province is the isolated outcrop at the Sierra de las Minitas (Rustán et al., 2011a).
Bactritids were recovered from levels below, within,
and directly above Keidel’s marker unit. The lowest beds with bactritids correspond to the lower argillaceous interval of the Talacasto Formation (greenish and black mudstones and shales) from the Quebrada de la Cortadera and Sierra de las Minitas localities, which are considered Lochkovian in age (Benedetto et al., 1992; Herrera, 1993; Rustán et al., 2011a). Thus, the stratigraphic range of bactritids in the Talacasto Formation spans from the Lochkovian to the Emsian, presuming the stratigraphic interpretation is correct. The preservation of specimens differs depending on the
lithology and locality. Fossils are preserved either in the clayey mudstone matrix or inside small nodules in the lower argillac- eous interval, in massive grayish sandstones just below Keidel’s bed, within large (up to 50 cm) calcareous nodules within
Keidel’s bed, and in greenish mudstones in the upper part of the unit. Some consist of internal molds of incomplete phragmo- cones or body chambers. In these cases, the usually poor, frag- mentary preservation does not allow further taxonomic determination. However, specimens from Keidel’s level pre- serve phragmocones with part of the body chamber, frequently exhibiting complete remains of the septa, siphuncle, shell wall, which provides information on the ornamentation and mor- phology of the early ontogenetic stages. Specimens were prepared using pneumatic “vibro-tools”
and needles under a binocular microscope. Polished sections were also prepared. In some cases, silicon-rubber casts were made in order to see the ornamentation. Both fossils and rubber casts were sometimes coated with black Chinese ink and then coated with ammonium chloride sublimate before they were photographed. In some cases, the external shell was treated with 30%
chlorhydric acid to uncover the sutures. Uncoated specimens were photographed dry, submersed in water or alcohol, or covered with vaseline (particularly those cut and polished), using a Canon Power Shot S50 digital camera mounted on a Leica MZ75 binocular loupe. Characters measured are set out in the explanation of
Table 2. All measurements were taken using digital calipers with a resolution of 0.1mm.
Repositories and institutional abbreviations.—The material from San Juan Province is housed under the prefix CEGH-UNC in the paleontological collections of the CIPAL (Centro de Investigaciones Paleobiológicas, Córdoba, Argentina), at the building of the CICTERRA (Centro de Investigaciones en Ciencias de la Tierra), CONICET-Universidad Nacional de Córdoba, Córdoba, Argentina. The material from La Rioja Province is housed under the prefix PULR-I in the Museo de Ciencias Naturales of the Universidad Nacional de La Rioja, La Rioja, Argentina.
Table 2. Measurements of Bactrites gracilis (Blumenbach, 1803) and Bactrites sp. from the Lower Devonian Talacasto Formation, Argentine Precordillera. The stratigraphic position of each specimen is detailed in the Systematic Paleontology section. LF = length of the fragment; Diaor = conch diameter at adoralmost preserved end; Diaap = conch diameter at adapicalmost preserved end (usually the diameter measured laterally or dorsoventrally doesn’t differ significantly, therefore we took just one value), units = mm; AA = apical angle = tangents-1 of the expansion rate (Diaor - Diaap)/distance between both diameters), unit = degrees; Si. Dia. = siphuncle diameter = ratio of the siphuncle diameter to the conch diameter at the same point of measure, expressed as a percentage; Sep. Dep. = septal depth = ratio of the distance between the plane of the suture and the culmination of the septum to the diameter of the phragmocone at the same point expressed as a percentage;
L.Li.Cham. = length of the preserved fragment of the body chamber, unit = mm; Cam. Rat. = cameral ratio = ratio of the dia- meter to the length of the chambers (or the number of camerae in a distance equivalent to the conch diameter at the same ontogenetic stage). Sometimes there is a range of measurements; we put them as a–b, indicating the minimum and the maximum values. ** CEGH-UNC 27082 consists of a few disaggregated chambers. *Approximate measures because of poor preservation or deformation.
specimen
CEGH-UNC 27103 CEGH-UNC 27080 CEGH-UNC 27081 CEGH-UNC 27082 CEGH-UNC 27083 CEGH-UNC 27084 CEGH-UNC 27085 CEGH-UNC 27086 CEGH-UNC 27087 CEGH-UNC 27088 CEGH-UNC 27089 CEGH-UNC 27090 CEGH-UNC 27091 PULR-I 1
CEGH-UNC 27092
Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites gracilis Bactrites sp. Bactrites sp. Bactrites sp. Bactrites sp.
taxonomic assignment LF (mm) Diaor (mm) Diaap (mm) AA (º) Si. Dia. (%) Sep. Dep. (%)
L.Li.Cham Cam. Rat. 13–15
112.4 42.0 35.0 **
12.2 5.5
28.7 12.3 6.6 5.2
37.5 23.7 8.0
32.3 9.3
6.6 5.6 5.0 5.8 5.4 4.0 4.7 -
10.0 4.1
3.6 3.9 7.4 7.6 7.9
0.5 3.0 3.1 5.5 4.8 3.6 3.2 5.4 3.1 3.6 5.2 6.0 7.0 6.8
3.4*
2.8 4.0 3.0 -
4.3 3.8 3.4 3.7
6.4* 5.7
4.3*
3.1 3.5 3.1 -
15 16
9–12 9
12 14 16 13 14 15 10 10 10
-
27 - -
32 20 38 28 30 20 10 42 -
25 23 -
44.0 ? - - - -
? - - -
12.5
30? -
23.7? -
1.3–1.8 1.1–1.3 1.6–2.0 1.8–2.0 1.7–1.8 -
?
2.0–2.5 -
1.8 1.8 2.6
0.7 -
2.0
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