Liu et al.—Exceptionally preserved Ordovician conodont apparatuses
because different specimens reveal different sides of the elements. More or less well-preserved Ordovician conodont appara-
tuses are known from only a handful of localities worldwide (e.g., Aldridge et al., 1995, 2013; Repetski, 1997; Tolmacheva and Purnell, 2002; Stewart and Nicoll, 2003; Dhanda, 2004). Establishing the proper names for the apparatuses from the Winneshiek Lagerstätte based on priority rules is not easy because the form species names used in the literature for some morphologically similar conodont elements are based on fragmentary and/or poorly diagnostic type specimens. The present study focuses on two common types of apparatus in the Winneshiek collections, which we identify as Archeognathus primus Cullison, 1938 and Iowagnathus grandis new genus new species. The complete apparatuses and unusual architectures of both taxa are described herein for the first time. Both Archeognathus primus and Iowagnathus grandis
n. gen. n. sp. from the Winneshiek comprise very large elements with robust basal bodies. The largest element reported herein is longer than 16mm, thus they are among the largest known (Liu et al., 2011, 2015). Although most apparatuses from the Winneshiek are less than 20mm in length, an incomplete apparatus (specimen WS13-1 with 8 elements) of I. grandis n. gen. n. sp. is >31mm, and the complete A. primus apparatus shown in Figure 2.1 has a length >13mm. Based on the sizes of apparatuses and individual elements, the I. grandis n. gen. n. sp. apparatuses from the Winneshiek Lagerstätte have an estimated average length of 2–3 cm, which is larger than the Upper Ordovician apparatuses described from the Soom Shale in South Africa (e.g., Theron et al., 1990; Aldridge et al., 2013). Although individual elements from both locations may be of comparable size (up to 15–20 mm), the Winneshiek Shale elements are notably more robust. Based on the conodont anatomy illustrated by the soft-bodied Carboniferous specimens from Scotland, several studies tried to estimate conodont body size from their apparatuses and elements (e.g., Purnell, 1994, 1995). The length of the body of the conodont animals from the Carboniferous of Scotland (Aldridge et al., 1986, 1993) is about 20 times that of the apparatus. Extrapolating this relationship to the Winneshiek specimens suggests that the Iowa conodont animals may have reached lengths of >0.5 m, especially for Iowagnathus grandis.
Geological setting of the Winneshiek Shale
The Winneshiek Shale, which hosts the Winneshiek Konservat- Lagerstätte, is a greenish-brown to dark-gray, laminated sandy shale unit (Fig. 1.4), with some layers containing abundant pyrite and a high content of carbonaceous material. It is dis- conformably overlain by the St. Peter Sandstone, which is widely distributed in the Upper Mississippi Valley, and overlies a local un-named, thick and massive impact breccia unit (Fig. 1.2). In the study area, most of the Winneshiek Shale is buried in its ~25km2 distribution area. The only known expo- sure of the unit is a small section at Decorah, Iowa that is mostly submerged in the Upper Iowa River. This exposure was exca- vated during the summer of 2010 by temporarily damming the river and using earth-moving equipment to dig into the riverbed, exposing a total thickness of ~4m of the upper
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Winneshiek Shale. This 4m section was sampled in detail and more than 2.5 tons of shale were systematically collected from it. This material was stored in the IGS facility building on the Oakdale Research Park Campus of the University of Iowa and kept in water until the shale slabs were split in the laboratory and carefully searched for fossils under binocular microscopes. With the assistance of trained students, this search was completed in three years, and resulted in a collection of more than 5,000 fossil specimens. The full thickness of the Winneshiek Shale was revealed in
two cores, which were drilled for the investigation of the Winneshiek Konservat-Lagerstätte. One of these is located at the single outcrop and the other is in the downtown area of the city of Decorah, ~3.5km distant. The complete Ordovician sedimentary succession in the Decorah region was obtained from local geological data and from investigations of wells, some of which penetrated the Cambrian. Based on rock chips, the Winneshiek Shale has been recognized in at least 24 wells, indicating that the unit is 17–27mthick and is distributed across the impact basin (Liu et al., 2009; McKay et al., 2011; Wolter et al., 2011). Conodonts and other fossils occur through the Winneshiek Shale succession, but most of the fossils dealt with here were collected from the upper four meters of the excavated section.
Age of the Winneshiek Shale
primus were first described from the Dutchtown Formation of Missouri (Cullison, 1938). Branson (1944, pls. 8, 9) published additional illustrations of Dutchtown conodont species, but provided no descriptions. Youngquist and Cullison (1946) gave a more extensive account of this conodont fauna, but did not include any information about A. primus, presumably because they believed that it was not a conodont. Based mainly on new Dutchtown specimens, Klapper and Bergström (1984) descri- bed the “jaw-like” specimens of A. primus in considerable detail and recognized their conodont nature, but their study specimens did not permit any reconstruction of the apparatus. They recognized the similarity between the Dutchtown arche- ognathiform (P) and coleodiform (S) elements and “considered the possibility that they were elements of the same apparatus”, noting that “confirmation of this idea, however, requires additional evidence” (Klapper and Bergström, 1984, p. 974). The present paper provides this evidence. The Dutchtown Formation crops out to a limited extent in
southern and southeastern Missouri, where it rests unconform- ably on the St. Peter Sandstone and is overlain by the Joachim Dolomite (see Branson, 1944, fig. 12). Hence, it differs from the Winneshiek Shale in lying above rather than below the St. Peter Sandstone, although many authors have suggested that the
Among the fossils collected from the Winneshiek Shale, only the conodonts currently furnish useful biostratigraphic evi- dence. However, pending assessment of the entire conodont fauna, our present focus is on the age indications provided by the two multielement species described herein along with one other species, Multioistodus subdentatus. None of these is a zone index, but their distribution elsewhere gives some indication of the age of the Winneshiek Shale. The P (archeognathiform) elements of Archeognathus
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