Journal of Paleontology, 91(3), 2017, p. 493–511 Copyright © 2017, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.155
Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA
Huaibao P. Liu,1 Stig M. Bergström,2 Brian J. Witzke,3 Derek E. G. Briggs,4 Robert M. McKay,1 and Annalisa Ferretti5
1Iowa Geological Survey, IIHR-Hydroscience & Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, IA 52242, USA
⟨
huaibao-liu@uiowa.edu⟩; ⟨
robert-mckay@uiowa.edu⟩ 2School of Earth Sciences, Division of Earth History, The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA
⟨
bergstrom.1@osu.edu⟩ 3Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USA ⟨
brian-witzke@uiowa.edu⟩ 4Department of Geology and Geophysics, and Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA
⟨
derek.briggs@
yale.edu⟩ 5Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia, via Campi 103, I-41125 Modena, Italy ⟨
ferretti@unimore.it⟩
Abstract.—Considerable numbers of exceptionally preserved conodont apparatuses with hyaline elements are present in the middle-upper Darriwilian (Middle Ordovician, Whiterockian) Winneshiek Konservat-Lagerstätte in
northeastern Iowa. These fossils, which are associated with a restricted biota including other conodonts, occur in fine-grained clastic sediments deposited in a meteorite impact crater. Among these conodont apparatuses, the com- mon ones are identified as Archeognathus primus Cullison, 1938 and Iowagnathus grandis new genus new species. The 6-element apparatus of A. primus comprises two pairs of archeognathiform (P) and one pair of coleodiform (S) elements. The 15-element apparatus of I. grandis n. gen. n. sp. is somewhat reminiscent of the prioniodinid type and contains ramiform elements of alate (one element) and digyrate, bipennate, or tertiopedate types (7 pairs). Both conodont taxa are characterized by giant elements and the preservation of both crowns and basal bodies, the latter not previously reported in Ordovician conodont apparatuses. Comparison of the apparatus size in the Winneshiek specimens with that of the Scottish Carboniferous soft-part-preserved conodont animals suggests that the Iowa animals were significantly larger than the latter. The apparatus of A. primus differs conspicuously from the apparatuses of the prioniodontid Promissum from the Upper Ordovician Soom Shale of South Africa although the apparatus architecture of I. grandis n. gen. n. sp. shows some similarity to it. Based on the Winneshiek collections, a new family Iowagnathidae in Conodonta is proposed.
Introduction
The Winneshiek Konservat-Lagerstätte was discovered in 2005 when geologists of the Iowa Geological Survey (IGS) were
mapping the geology of the Upper Iowa River basin. During the field investigations, a local stratigraphic unit was recognized in the Decorah area in northeastern Iowa (Fig. 1.1, 1.2). From the name of the county where the unit was found, it was named the Winneshiek Shale (McKay et al., 2011; Wolter et al., 2011). Abundant well-preserved fossils, including some with soft-part preservation, occur in the shale, indicating that it is a new Konservat-Lagerstätte (Liu et al., 2006, 2007, 2009). The fauna, particularly the conodonts, indicates that the age of the Winneshiek Lagerstätte is Middle Ordovician (Darriwilian in terms of global classification, and mid to late Whiterockian in regional US Series terminology). TheWinneshiek fossil fauna includes a variety of new forms,
and the invertebrate fossils are dominated by arthropods (Lamsdell et al., 2015a; Nowak et al., 2015; Briggs et al., 2015 [2016]),
including the oldest eurypterid Pentacopterus (Lamsdell et al., 2015b). Possible algae and jawless fish also occur in the fauna. However, the conodonts are the most common fossils (including isolated elements and natural assemblages), and account for 51% of the total number of collected specimens (N = 5,354). Most of the conodont elements from the Winneshiek Shale are hyaline. Their crowns are amber-colored (CAI 1.5–2), indicating minimal thermal maturity. Some elements are found in coprolites or gut contents from carnivores. Of even more interest, the Winneshiek Konservat-Lagerstätte preserves abundant conodont bedding- plane assemblages. Although some contain elements of different taxa, repeated occurrences of bedding-plane assemblages with consistent element composition and non-random relative element positions indicate that these are natural assemblages (i.e., appara- tuses). Several types of conodont apparatuses have been recognized from the Winneshiek Shale, and here we describe two that exhibit large size, unusual architecture and element composi- tion, and which have not been reported previously. These unique conodont materials from the Winneshiek Shale are important for
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