Mapes and Doguzhaeva—New Pennsylvanian coleoids from Nebraska and Iowa
Discussion.—The single specimen of Starkites compressus n. gen. n. sp. is unique in the Stark Shale coleoids with its com- bination of the ink sac and lack of a mineralized body chamber. The large apical angle of the phragmocone of ~10º may be a partial product of shell crushing; however, that the shell is a brevicone is supported by the close septal spacing. The position of the siphuncular elements on the presumed ventral side sup- ports a lateral crushing orientation for the specimen. The ink sac is the only trace of the soft body of the animal. Without the presence of the undisputed and undistorted ink sac, this speci- men would be viewed as a rather ordinary, poorly preserved and unidentifiable othoceratid nautiloid or prehaps a parabactritoid. However the discovery and validation of the ink sac filled with ink implies the soft body was present when the animal came to rest of the ocean floor. This specimen represents a rather important stage of
coleoid evolution in the upper Carboniferous in proving that the coleoids had evolved in having an internal mineralized phragmocone without a mineralized body chamber to contain the soft body tissues and without a well-developed rostrum as is seen in the Upper Mississippian coleoid Hematites, which also lacks a body chamber (Doguzhaeva et al., 2002a). This condition was thought to have evolved in the Mesozoic, and so this Pennsylvanian age specimen is important in documenting that this condition began much earlier in geological time in several different lineages.
Genus Pabianiconus new genus
Type species.—Pabianiconus starkensis new genus new species.
Diagnosis.—As for the type species. Types.—Holotype (UNSM 16568A).
Etymology.—After Roger Pabian in recognition of his numer- ous contributions to the understanding of invertebrate fossils from the upper Paleozoic of Nebraska and surrounding regions.
Occurrence.—From the Stark Shale (Pennsylvanian, Missourian = Kasimovian) at the 36th Street Quarry in Bellevue, Nebraska. No other information is available.
Discussion.—Pabianiconus n. gen. is only somewhat compar- able with Donovaniconus in that both have a mineralized body chamber; however, the body chamber of Donovaniconus is long and parallel-sided and the body chamber of Pabianiconus n. gen. is relatively short and flared with a relatively high angle (~20º) from the terminal septum to the aperture. Pabianiconus n. gen. has no ornament on the exterior of the shell wall and no evidence that a rostrum or a pro-ostracum-like structure was present; whereas, Donovaniconus has longitudinal ridges as ornament
151
and curved growth lines on the dorsum, indicating that a short, broad pro-ostracum-like structure was present. Additionally, the body chamber length in Donovaniconus is longer and the length of the ink sac compared to the length of the body chamber length is shorter. Pabianiconus n. gen., with its flared body chamber, differs from Starkites n. gen., which lacks a body chamber.
Pabianiconus starkensis new species Figure 3.1–3.3
Types.—Holotype (UNSM 16568A, B).
Diagnosis.—Rostrum absent, incomplete and crushed phrag- mocone 21.3mm long with an apical angle of ~10º, body chamber 23.3mm long with an apical angle of ~20º; septa - closely spaced, mural part of septa about one-third of camera length; ink sac about three-fourths of body chamber length and located immediately orad of terminal septum; multilayered shell wall of body chamber surrounds entire body mass.
Description.—Around the exterior of the specimen there is an oval stain in the black shale suggesting that the animal tissues during bacterial degradation produced a micro chemical envir- onment that aided the preservation of the specimen. The oval is 52 x 45mm and probably encompasses the entirety of the fossil specimen, including any soft tissues that were present. If this was the case, then the specimen lacked arm hooks because none was observed on the bedding plane that contains the specimen. The conch of Pabianiconus starkensis n. sp. is crushed
laterally, based on the assumption that the siphuncle, which is presumed to have been ventral and marginal, is now positioned in the center of the crushed phragmocone. Thus, after death, the animal came to rest on the anoxic sea floor on its side. The only remnants of the siphuncle are nine circular rings, which are the foramens of the septal necks and are relatively well preserved because they had thin siphuncular deposits around the apical and adapical septal neck surfaces that allowed them to better survive the crushing and diagenetic alteration of the specimen. In the body chamber, which is ~27.5mmlong and 25.2mmwide, there is an ink-filled, cylindrical organ with a flattened narrow duct on the orad end that extends to near the edge of the body chamber. The ink sac is 23mm long and 5mm in maximum width. In places the ink sac is covered by the shell wall of the body chamber on the upper and lower surfaces, indicating the body chamber was mineralized around its complete circumference. The posterior end of the ink sac is immediately adjacent to the terminal septum of the phragmocone. Patches of striated brown material are present in the body chamber and on the shell covering the ink sac; this material is interpreted to be muscular mantle tissue. The phragmocone is represented by the septal necks, which were described previously, and nine fragmentary septa with their mural edges extending approximately one-third
Figure 2. Nebraskaconus whitei n. gen. n. sp. (1, 2) Part and counterpart, respectively, of holotype UNSM 8161 showing a three-dimensional ink sac (is) filled with ink, part of the body chamber (bc), and a crushed breviconic phragmocone (ph); (3), fine lirae between the septa of the phragmocone suggesting the conch had a “wrinkle layer”, which if true, then the test of the conch and the sheath-like rostrum must have been very thin. (4–7) Starkites compressus n. gen. n. sp. (4, 5) Part and counterpart, respectively, of holotype UNSM 16562A showing the three-dimensional ink-filled sac (is), the breviconic phragmocone (ph), and aligned circular septal necks (sn). Note there is no trace of a body chamber; (6) enlargement of the ink-filled sac in position in front of the terminal septum without any trace of a body chamber; (7) enlargement of the cyrtochoanitic septal necks (sn) as donut-shaped structures. Scale bars are 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
