Liu et al.—Exceptionally preserved Ordovician conodont apparatuses
St. Peter Sandstone is not the same age everywhere in the Midcontinent region (e.g., Ross et al., 1982). It should also be noted that regional subsurface studies suggest that the St. Peter Sandstone interfingers with the calcareous Dutchtown Formation (e.g., Rexroad et al., 1982, fig. 2; Witzke and Metzger, 2005). No archeognathiform elements have been reported from the Joachim Dolomite (Branson and Mehl, 1933; Andrews, 1967) or younger strata in Missouri. Among the Dutchtown conodonts, Phragmodus flexuosus
and P. polonicus are biostratigraphically important. The former is a geographically widespread species in upper Darriwilian and lower Sandbian strata in North America. The latter species, which has been collected from the type locality of the formation by one of us (SMB), is characteristic of the middle-upper Darriwilian of North America and occurs in coeval strata in Baltoscandia, Poland, and Siberia (see Ethington and Clark, 1981 and Dzik, 1994 for discussions of the taxonomy and other aspects of this species). The Siberian Platform occurrences (Moskalenko, 1970, 1973, 1983) in the Vihorevian Stage are of special interest because, as recognized by Moskalenko (1970), the conodont fauna of this stage is similar to that of the Dutch- town Formation. The Siberian fauna includes, among other taxa, archeognathiform elements that may be congeneric with Archeognathus and one element (see synonymy in the Sys- tematic section) that resembles the P2 element of Iowagnathus n. gen. of the Winneshiek fauna. Moskalenko described all her material only as form taxa and they have not yet been revised in modern multielement taxonomy. Moskalenko (1983) correlated her Vihorevian fauna with that of the Darriwilian Eoplacognathus suecicus Zone in Baltoscandia. This might be correct although the absence of biostratigraphically important Baltoscandian taxa in the Siberian fauna makes precise correlation difficult. In the Winneshiek conodont fauna, Multioistodus
subdentatus, a species originally described from the Dutchtown Formation (Cullison, 1938), has been identified. This species has been recorded from the lower part of the Simpson Group of the Arbuckle Mountains of Oklahoma (e.g., Bauer, 2010), the Burger and Tyner formations of northeastern Oklahoma (Bauer, 1989), and several formations in Utah and Nevada (Ethington and Clark, 1981). All currently known occurrences of this species are of Darriwilian age. Because of their successional relationship, the biostrati-
graphic age of the overlying St. Peter Sandstone would provide a minimum age for the Winneshiek Shale. Although the St. Peter Sandstone is essentially unfossiliferous, shaly inter- vals, particularly in the lower part of the unit, have yielded some biostratigraphically significant conodonts in drillcores from Iowa and Minnesota (Witzke and Metzger, 2005). The most diagnostic fauna, which is from the Camp Quest core of Lemars, Plymouth County, in northwestern Iowa, includes representa- tives of Phragmodus flexuosus, Leptochirognathus sp., Erraticodon sp., Cahabagnathus sp. cf. C. friendsvillensis, Archeognathus sp., and Eoplacognathus sp., indicating a late
497
Darriwilian age. This age is consistent with the resemblance of this conodont fauna to that of the McLish Formation of Oklahoma (Bauer, 1987). The St. Peter Sandstone in south- western-most Indiana (i.e., even farther afield than northwest Iowa) has also yielded a somewhat similar, but taxonomically less diverse, conodont fauna (Rexroad et al., 1982). A taxonomically unusual, but probably approximately coeval, conodont fauna occurs in strata identified as the Dutchtown Formation in southeastern Indiana (Ethington et al., 1986). All the data indicate that conodont faunas with archeognathiform elements were widely distributed in the Midwest in the late Darriwilian. In view of the fact that the Winneshiek Shale is older than at least part of the St. Peter Sandstone, its fauna is also likely to be older. However, the geographically widespread Histiodella-Paraprioniodus-Pteracontiodus-Fahraeusodus fauna of early Darriwilian age is absent from both the Dutchtown and Winneshiek formations, which suggests that the Winneshiek Shale is of middle-late Darriwilian age. This is consistent with the fact that the early Darriwilian fauna occurs in the upper Everton Formation, which underlies the St. Peter Sandstone, in Missouri and Arkansas.
Paleoecology and taphonomy of the Winneshiek fauna
Paleogeographic studies indicate that northeastern Iowa occupied a marginal to near-shore setting in the tropical zone of Laurentia during the Middle Ordovician (Fig. 1.3) (Witzke, 1990; Niocaill et al., 1997; Jin et al., 2013). Recent analyses of subsurface data indicate that the Winneshiek Shale and the underlying breccia unit are confined to a circular basin with a diameter of 5.6km in the Decorah area. Multiple lines of geological evidence indicate that this circular basin was formed by a meteorite impact (Liu et al., 2009; McKay et al., 2011). The shape and dimension of the crater have recently been established by aerial geophysical surveys conducted by the U.S. Geological Survey (Koontz and McKay, 2013), and the crater has been named the Decorah Impact Structure. The crater resulted in an enclosed basin where anoxic conditions developed in the relatively undisturbed bottom water, which provided appropriate taphonomic conditions for the exceptional preservation that characterizes the Winneshiek Konservat-Lagerstätte. Although the deposit is marine influenced, many typical
Ordovician open-marine fossils, such as trilobites, graptolites, corals, bryozoans, and echinoderms, are absent in the Winneshiek fauna, which includes eurypterids (Lamsdell et al., 2015b), phyllocarids (Briggs et al., 2015 [2016]), ostracodes, jawless fish, and linguloids. The Winneshiek Shale also yields abundant bromalites (mineralized gut contents and coprolites) and algae. The unusual composition of the fauna suggests that it inhabited a restricted environment, likely in brackish water, with a low salinity that was inhospitable to typical marine taxa (Liu et al., 2006, 2007, 2009; Witzke et al., 2011).
Figure 2. Three bedding-plane conodont apparatuses of Archeognathus primus Cullison, 1938 from the Winneshiek Shale. Note that all these 6-element apparatuses, which we interpret as complete, contain only one pair of coleodiform S elements and two pairs of ramiform P elements (P1 and P2), all with large basal bodies. Also note that the elements are consistently arranged, from left to right in Figure 2.1 and 2.2, as paired P1,P2, and S: (1) SUI 102853; (2) SUI 139882 (WL143); (3) SUI 139883 (WS16-1). Scale bar = 2mm.
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