Journal of the Geological Society The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater by Derek E.G. Briggs, Huaibao P. Liu, Robert M. McKay, and Brian J. Witzke Journal of the Geological Society Volume 175(6):865-874 November 12, 2018 © 2018 The Author(s). Published by The Geological Society of London

(a) Location of Decorah in the NE of the State of Iowa, USA (a) Location of Decorah in the NE of the State of Iowa, USA. (b) Geological map of Winneshiek and Allamakee counties in the northeastern corner of Iowa State, bounded to the east by the Mississippi River and the State of Wisconsin. (a) Location of Decorah in the NE of the State of Iowa, USA. (b) Geological map of Winneshiek and Allamakee counties in the northeastern corner of Iowa State, bounded to the east by the Mississippi River and the State of Wisconsin. From the map Bedrock Geology of Northeast Iowa; Witzke et al. (1998). (c) Geological cross-section through the western part of the Decorah impact structure. The gently dipping Cambrian and Ordovician formations are disrupted by the impact basin, which is filled with breccia, conglomerate and sandstone (unnamed), overlain by the Winneshiek Shale. Much of the impact basin is overlain by the St Peter Sandstone, which varies in thickness due to the erosional contact at its base. Based on Wolter et al. (2011) and French et al. (2018). Derek E.G. Briggs et al. Journal of the Geological Society 2018;175:865-874 © 2018 The Author(s). Published by The Geological Society of London

The Winneshiek Shale. The Winneshiek Shale. (a–c) Damming and extraction of the Winneshiek Shale from the bed of the Upper Iowa River. (d) Field exposure showing the laminated nature of the Winneshiek Shale. (e) Fractured and deformed Shakopee Formation dolomite just outside the SE rim of the Decorah structure with H. Paul Liu for scale. Photo credits: (a) H.P. Liu; (b, c) Tom Marshall (South Dakota Geological Survey); (d) R.M. McKay; and (e) from French et al. (2018). Derek E.G. Briggs et al. Journal of the Geological Society 2018;175:865-874 © 2018 The Author(s). Published by The Geological Society of London

Eurypterid and other arthropods from the Winneshiek biota. Eurypterid and other arthropods from the Winneshiek biota. (a–g) The megalograptid eurypterid Pentecopterus decorahensis. (a) The second prosomal appendage rotated and showing the array of spines projecting posteriorly (SUI 139941). (b) Dorsal view of the adult; reconstruction by James Lamsdell (West Virginia University). (c) Complete appendage. (SUI 102857). (d) Body segments 4 and 5 showing median rows of scales (SUI 139931, images of part and counterpart superimposed). (e) Appendage 5 isolated from the shale (SUI 139998). (f) Cuticle fragment of underside of prosoma showing setae (SUI 140011). (g) Prosomal appendages 2–4 of a juvenile (SUI 139965). (h, i) Small carbonaceous fossils. (h) Crustacean-type filter plate (SUI 143632-1, WS-9(1), O41/4). (i) Possible euchelicerate coxa (SUI 143607-1, WS-14(1), G49). (a–g) From Lamsdell et al. (2015b); (h, i) from Nowak et al. (2018). Derek E.G. Briggs et al. Journal of the Geological Society 2018;175:865-874 © 2018 The Author(s). Published by The Geological Society of London

Bivalved arthropods from the Winneshiek biota. Bivalved arthropods from the Winneshiek biota. (a) The phyllocarid Ceratiocaris winneshiekensis in right lateral view, showing the trunk and telson extending beyond the valves (SUI 138435). (b) A right valve of Iosuperstes collisionis, which cannot be assigned to a group in the absence of evidence other than the valves (SUI 138464). (c) A right valve of Decoracaris hildebrandi showing a pronounced notch, which may indicate an affinity with Thylacocephala; the rest of the arthropod is unknown (SUI 138453). (d) An undetermined probable leperditiid with the two valves flattened in the butterflied position (SUI 138486). All figures from Briggs et al. (2016), reproduced with permission. Derek E.G. Briggs et al. Journal of the Geological Society 2018;175:865-874 © 2018 The Author(s). Published by The Geological Society of London

Other fossils from the Winneshiek biota. Other fossils from the Winneshiek biota. (a, b) Bedding plane apparatuses of the conodont Archeognathus primus. (a) Dorso-ventral view (SUI 102853). (b) Oblique view (SUI 139882). (c) Complete, but slightly disturbed, apparatus of the conodont Iowagnathus grandis (SUI 139888). (d) Fragment, probably anterior left, of undescribed bilaterally symmetrical vertebrate plate with anterior to right in the figure (SUI 102856). (e–h) Bromalites. (e) Corrugated rod-like form (SUI 145155). (f) Smoother rod-like form (SUI 145164). (g) Variable form (SUI 145169). (h) Form with concentrated quartz sand (SUI 145166). (a–c) From Liu et al. (2017), reproduced with permission; (d) from Liu et al. (2006); (e–h) after Hawkins et al. (2018, fig. 4). Derek E.G. Briggs et al. Journal of the Geological Society 2018;175:865-874 © 2018 The Author(s). Published by The Geological Society of London