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Exceptionally preserved arthropodan microfossils from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA

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posted on 2017-11-22, 17:13 authored by Hendrik Nowak, Thomas H. P. Harvey, Huaibao P. Liu, Robert M. McKay, Thomas Servais
The Middle Ordovician (Darriwilian) Winneshiek Shale from Winneshiek County, Iowa, USA, hosts a Konservat-Lagerstätte 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 fossils’ (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 topological similarities and implied functional equivalence to the coxae of xiphosurid chelicerates, in particular, outweigh the resemblance to certain branchiopodan and hexapodan mandibles. In turn, however, the restricted size range and lack of associated 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.

Funding

This work was funded by the ANR (Agence National de la Récherche, France) as part of the ‘RALI – The Rise of Animal Life’ project (Grant number ANR-11-BS56-0025), and by the NSF of USA (grant numbers EAR 0921245 and EAR 0922054). Acid treatment and slide preparation were conducted by Laurence Debeauvais (CNRS/Université Lille 1). Paul K. Strother (Boston College) found the first possible xiphosuran coxae in our material. Massimo Morpurgo (Museum of Nature South Tyrol) provided exuviae of Limulus polyphemus. We thank James C. Lamsdell (Yale University), Petra Tonarová (Tallinn University of Technology, Czech Geological Survey), Olle Hints (Tallinn University of Technology), Denis Audo (Université de Rennes 1), Carolin Haug (LMU Munich), Joachim T. Haug (LMU Munich) and Alexander Blanke (Hull, UK) for discussing our findings, and two anonymous reviewers for their helpful comments. This work is a contribution to IGCP project 653 – ‘The onset of the Great Ordovician Biodiversification Event’ and to the CPER research project CLIMIBIO. The authors thank the French Ministère de l'Enseignement Supérieur et de la Recherche, the Hauts-de-France Region and the European Funds for Regional Economic Development for their financial support to this project and the Department of Innovation, Research and University of the Autonomous Province of Bozen/Bolzano for covering the Open Access publication costs.

History

Citation

Lethaia, 2017

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/School of Geography, Geology and the Environment

Version

  • VoR (Version of Record)

Published in

Lethaia

Publisher

Wiley, Lethaia Foundation

issn

0024-1164

eissn

1502-3931

Acceptance date

2017-06-01

Copyright date

2017

Available date

2017-11-22

Publisher version

http://onlinelibrary.wiley.com/doi/10.1111/let.12236/full

Language

en

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