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Exceptionally Preserved Arthropodan Microfossils from the Middle Ordovician Winneshiek Lagerstätte, Iowa

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Exceptionally Preserved Arthropodan Microfossils from the Middle Ordovician Winneshiek Lagerstätte, Iowa Exceptionally preserved arthropodan microfossils from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA Hendrik Nowak, Thomas Harvey, Huaibao Liu, Robert Mckay, Thomas Servais To cite this version: Hendrik Nowak, Thomas Harvey, Huaibao Liu, Robert Mckay, Thomas Servais. Exceptionally pre- served arthropodan microfossils from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA. Lethaia, Wiley, 2018, 51 (2), pp.267-276. 10.1111/let.12236. hal-02408755 HAL Id: hal-02408755 https://hal.archives-ouvertes.fr/hal-02408755 Submitted on 3 Sep 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License Exceptionally preserved arthropodan microfossils from the Middle Ordovician Winneshiek Lagerst€atte, Iowa, USA HENDRIK NOWAK , THOMAS H. P. HARVEY, HUAIBAO P. LIU, ROBERT M. MCKAY AND THOMAS SERVAIS Nowak, H., Harvey, T.H.P., Liu, H.P., McKay, R.M. & Servais, T. 2018: Exceptionally preserved arthropodan microfossils from the Middle Ordovician Winneshiek Lagerst€atte, Iowa, USA. Lethaia, Vol. 51, pp. 267–276. The Middle Ordovician (Darriwilian) Winneshiek Shale from Winneshiek County, Iowa, USA, hosts a Konservat-Lagerst€atte that has yielded a diverse fauna including soft-bodied fossils. The shale is rich in organic content; in particular, algal material and fragmentary cuticular remains. Palynological acid treatment alongside modified, low-manipulation processing enables the extraction of these ‘small carbonaceous fos- sils’ (SCFs) from the matrix, allowing a more detailed view of their morphology. Together these methods have yielded exceptionally well-preserved crustacean-type setae and a population of distinctive microfossils which we identify as the feeding appendages of a small-bodied arthropod. We present two hypotheses for their identity: as either pancrustacean mandibles, or euchelicerate coxae. Overall, the detailed topo- logical similarities and implied functional equivalence to the coxae of xiphosurid che- licerates, in particular, outweigh the resemblance to certain branchiopodan and hexapodan mandibles. In turn, however, the restricted size range and lack of associ- ated limb or carapace fragments pose a taphonomic conundrum, suggesting an extreme biostratinomic bias. By comparison with previously described arthropodan SCFs from the Cambrian of Canada, the Winneshiek fossils extend the geographic, palaeoenvironmental and temporal range of this taphonomic window and provide a complementary tool for reconstructing the diversity and ecology of the Winneshiek biota. □ Arthropods, Chelicerata, Crustacea, Konservat-Lagerst€atte, Middle Ordovician, small carbonaceous fossils. Hendrik Nowak ✉ [[email protected]], Museum of Nature South Tyrol, Binder- gasse/Via Bottai 1 39100 Bolzano, Italy; Hendrik Nowak [[email protected]] and Thomas Servais [[email protected]], Univ. Lille, CNRS, UMR 8198 - Evo- Eco-Paleo, F-59000 Lille, France; Thomas H. P. Harvey [[email protected]], Depart- ment of Geology, University of Leicester, University Road Leicester LE1 7RH, UK; Huai- bao P. Liu [[email protected]] and Robert M. McKay [[email protected]], Iowa Geological Survey, IIHR-Hydroscience & Engineering, The University of Iowa, 340 Trowbridge Hall Iowa City, IA 52242, USA; manuscript received on 20/02/2017; manu- script accepted on 1/06/2017. The Winneshiek Shale in Iowa, USA, is a Darriwilian Wales (Whittington 1993; Legg & Hearing 2015). The (Middle Ordovician) Konservat-Lagerst€atte (site with documented occurrence of diverse arthropods in the exceptionally well-preserved fossils; see Seilacher Winneshiek biota, combined with the exceptional 1970) that is notable not least for its arthropod fossils. preservation of eurypterid cuticle (Lamsdell et al. It has yielded the currently oldest described eurypter- 2015a) and various algal palynomorphs (P.A. Zippi ids (Lamsdell et al. 2015a), phyllocarids with soft- 2011, unpublished report), has prompted a search for tissue preservation (Liu et al. 2006; Briggs et al. ‘small carbonaceous fossils’ (SCFs) (sensu Butterfield 2016), ostracods and other bivalved arthropods & Harvey 2012). Previous SCFs reported from the (Briggs et al. 2016), and a dekatriatan-like euchelicer- Cambrian include crustacean remains including bran- ate (Lamsdell et al. 2015b). Apart from this, unusually chiopodan filter plates (Harvey & Butterfield 2008; large conodonts (elements up to 16 mm in length) Harvey et al. 2012b) and branchiopod, copepod and with complete apparatuses (Liu et al. 2017), possible ostracod-type mandibles (Harvey & Butterfield 2008; jawless fishes, linguloid brachiopods, molluscs, algae Harvey et al. 2012a,b; Harvey & Pedder 2013), pro- and acritarchs have been found (Liu et al. 2006; P.A. viding valuable insight into the evolution and ecology Zippi 2011, unpublished report). This site is one of of early crustaceans. The attempt to search for organic few Konservat-Lagerst€atten in the Ordovician (Van microfossils in the Winneshiek Shale has now yielded Roy et al. 2015), the only other Konservat-Lagerst€atte numerous objects that we tentatively identify as crus- known so far from the Middle Ordovician (also Darri- tacean filter plates and coxae of a diminutive euche- wilian) being the Llanfallteg Formation in south-west licerate, which are described herein. DOI 10.1111/let.12236 © 2017 The Authors. Lethaia Published by John Wiley & Sons Ltd on behalf of Lethaia Foundation. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 268 Nowak et al. LETHAIA 51 (2018) Geological context depth below samples St Peter The Winneshiek Shale (Darriwilian, Middle Ordovi- (m) 0 WS-Top cian) is found around the city of Decorah, Win- WS-1 neshiek County, Iowa, USA. It is a well-laminated, WS-2 greenish brown to medium or dark grey, sandy shale WS-3 with a thickness of 18–27 m (Liu et al. 2006). Its dis- tribution is limited to a circular area with a diameter 1 WS-4 x 1 of about 5.6 km (25 km2). Multiple geological evi- dences have indicated that this structure is an impact WS-5 crater, the ‘Decorah impact structure’. The fossil WS-6 x 7 WS-7 x 7 component, as well as the results from palaeoenvi- WS-8 x 5 ronmental studies, indicates that the Winneshiek WS-9 x 10 2 Shale was deposited in a restricted, brackish, shallow WS-10 x 6 WS-11 x 5 marine environment (Liu et al. 2006, 2009). WS-12 x 3 Only one small outcrop of the Winneshiek Shale is known, which is mostly submerged under the Upper WS-13 x 6 Iowa River. At this site, the Iowa Geological Survey WS-14 x 2 (IGS) excavated the upper four metres of the Win- 3 WS-15 x 2 WS-16 x 3 neshiek Shale in 2010 by damming the river temporar- ily. Otherwise, the shale succession is accessed by wells WS-17 x 2 (Liu et al. 2009). The Winneshiek Shale overlies an WS-18 x 7 unnamed impact depositional unit and is uncon- formably overlain by the widely distributed sandstones 4 of the St Peter Formation (Liu et al. 2006). Setal arrays Feeding appendages Fig. 1. Stratigraphical position relative to the St Peter Formation and yield of samples from the Winneshiek Shale. Materials and methods Several tons of the Winneshiek Shale were collected from the drained section of the Upper Iowa River Results during the excavation in 2010. This section was sub- divided into 18 sampling intervals, which represent All samples, except for one from near the boundary the uppermost about 4 m of the Winneshiek Shale with the overlying St Peter Formation, yielded a rich and its discordant upper boundary with the St Peter organic fossil content (Fig. 1). Aside from fragmen- Formation (Fig. 1). A borehole throughout the Win- tary eurypterid cuticle (Fig. 6A shows an example; neshiek Shale was drilled and sampled in close prox- compare Lamsdell et al. 2015a), and cyanobacterial imity to the excavated section. sheaths, algae and acritarchs similar to those recov- A few tens of grammes of rock from each sampling ered during a separate earlier study (P.A. Zippi 2011, interval were treated with a standard palynological unpublished report), we recovered organic-walled maceration technique, employing hydrofluoric and microfossils with possible chelicerate and crustacean hydrochloric acid. The residue was filtered with mesh affinity that presently cannot be linked to macrofos- sizes of 51 and 15 lm. Extracts of the size fraction sil groups known from this section. between 15 and 51 lm were then mounted in resin on permanent palynological slides. The same material Setal arrays and samples from the borehole were also treated with a low-manipulation approach for the extraction of Description. – Among the Winneshiek SCFs are rare SCFs (modified from Butterfield & Harvey 2012). specimens of arthropodan setae, occurring either as Specimens were studied and photographed under a isolated units or in semi-articulated arrays (Fig. 2). ZEISS Axio Imager.A2 transmitted light microscope A variety of setal morphologies are present in the with a mounted AxioCam ERc5s camera. assemblage, including ‘simple’ forms, that is tubular All samples are reposited at the University of Iowa shafts lacking finer-scale elaboration (following the Paleontology Repository, Department of Earth and terminology of Garm & Watling 2013; see Fig. 2A, Environmental Sciences (SUI). B), and those with well-preserved setules, that is LETHAIA 51 (2018) Arthropodan microfossils in the Winneshiek Shale 269 filamentous side branches much less than 1 lmin (e.g. Fig. 2C, D) often have roles in filter feeding, diameter. The setule-bearing setae have plumose whereas fans of simple setae (Fig.
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