The Winneshiek biota: exceptionally well-preserved fossilsin a Middle Ordovician impact crater

Derek E.G. Briggs1,2*, Huaibao P. Liu3, Robert M. McKay3 & Brian J. Witzke41 Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA2 Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA3 Iowa Geological Survey, IIHR – Hydroscience & Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, IA52242, USA

4 Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USAD.E.G.B., 0000-0003-0649-6417

*Correspondence: derek.briggs@yale.edu

Abstract: The Winneshiek Shale (Middle Ordovician, Darriwilian) was deposited in a meteorite crater, the Decorah impactstructure, in NE Iowa. This crater is 5.6 km in diameter and penetrates Cambrian and Ordovician cratonic strata. It was probablysituated close to land in an embayment connected to the epicontinental sea; typical shelly marine taxa are absent. TheKonservat-Lagerstätte within the Winneshiek Shale is important because it represents an interval when exceptionalpreservation is rare. The biota includes the earliest eurypterid, a giant form, as well as a new basal chelicerate and the earliestceratiocarid phyllocarid. Conodonts, some of giant size, occur as bedding plane assemblages. Bromalites and rarer elements,including a linguloid brachiopod and a probable jawless fish, are also present. Similar fossils occur in the coeval Ames impactstructure in Oklahoma, demonstrating that meteorite craters represent a novel and under-recognized setting for Konservat-Lagerstätten.

Received 18 May 2018; revised 7 August 2018; accepted 7 August 2018

The Winneshiek Konservat-Lagerstätte was discovered in 2005 inthe Winneshiek Shale, which directly underlies the Ordovician StPeter Sandstone, during mapping by the Iowa Geological Survey(Liu et al. 2005) (Fig. 1). The only exposure, which represents thetop of theWinneshiek Shale, is in the valley of the Upper Iowa Rivernear Decorah in Iowa. Collections from this locality yieldedconodonts, linguloid brachiopods, large fragments of eurypteridcuticles and phyllocarid crustaceans (Liu et al. 2005). A coal-likelayer had been reported in local newspapers in the 1920s, whichcould only have been a concentration of eurypterid cuticles, perhapsto encourage speculators. H. Paul Liu recognized the exceptionalnature of these fossils and he and colleagues published a preliminaryaccount of the discovery in 2006 (Liu et al. 2006). A collaborativegrant to the authors funded the construction of a temporary dam in2010, which isolated the exposure of the Winneshiek Shale on thenorth bank of the Upper Iowa River near Decorah (Fig. 2a–c). Thisallowed access to the shale and a sequence of large samples,representing c. 4 m of the section, was extracted by hand with theaid of earth-moving equipment (Fig. 2b, c). The shale was split atthe Iowa Geological Survey sample repository over the next threeyears and yielded >5000 macrofossil specimens.

The biota of theWinneshiek Shale is characterized by exceptionalpreservation combined with a restricted diversity, both of whichreflect the unusual depositional setting. The dominant macro-fossils are arthropods: the giant Pentecopterus decorahensis(Fig. 3) is the earliest described eurypterid (Lamsdell et al. 2015b)and a diversity of bivalved arthropods includes a phyllocarid,Ceratiocaris winneshiekensis, and ostracods (Fig. 4). Conodontelements are the most abundant fossils, many occurring as beddingplane assemblages (Fig. 5), and extrapolation indicates that some arefrom individual animals of very large size (Liu et al. 2017). Rarespecimens interpreted as the head shield of an early armouredvertebrate (Liu et al. 2006) are also present (Fig. 5d). Dissolution of theorganic-rich shale has yielded a variety of microscopic carbonaceous

fossils, including arthropod appendages (Fig. 3h, i) (Nowak et al.2018) and filamentous algae (Nowak et al. 2017).

The meteorite crater and the age of theWinneshiek Shale

The Winneshiek Shale, an unusual greenish brown to dark greyorganic-rich shale up to 26 m thick, was noted in cores and wellcuttings some years prior to the discovery of the exposure nearDecorah (Young et al. 2005). It overlies an unnamed unit dominatedby breccia (Fig. 1c), the two reaching a combined thickness ofc. 200 m. Occurrences of the shale in drill core and in outcropdelimit a circle with a diameter of c. 5.6 km (Liu et al. 2009)(Fig. 1b). This outline was also detected in electromagnetic andgravity data from airborne geophysical surveys. The circularstructure and its sedimentary fill are concealed by the overlying StPeter Sandstone and Quaternary alluvium, except at one smalllocality in the Upper Iowa River Valley near Decorah, where the topof the shale is at the land surface. The circular depositional basin ischaracterized by evidence of local deformation near the perimeter(Fig. 2e) and its unusual stratigraphy. The Winneshiek Shale andunderlying breccia are found only within the circular structure,where the units are completely different from the regionalstratigraphic sequence (Liu et al. 2009; French et al. 2018). Allthese attributes are consistent with deposition within a meteoritecrater. This inference was elegantly confirmed by the demonstrationthat quartz grains observed in drill samples from the breccia showshock-induced phenomena (Liu et al. 2009; McKay et al. 2011),particularly the fractures and deformation features characteristic ofan impact (French et al. 2018).

The stratigraphic context and fossils indicate that the WinneshiekShale is Darriwilian (Middle Ordovician) in age, i.e. from 458.4 to467.3 Ma (Gradstein et al. 2012; Cohen et al. 2013). None of theconodonts associated with the exceptionally preserved fossils arezonal index fossils, but they include Multioistodus subdentatus,

© 2018 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

Review focus Journal of the Geological Society

Published online September 24, 2018 https://doi.org/10.1144/jgs2018-101 | Vol. 175 | 2018 | pp. 865–874

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which is known from Darriwilian strata around Laurentia fromAlberta to NewYork State (Witzke et al. 2011; Liu et al. 2017). Thepresence of the unusual conodont Archeognathus primus (Fig. 5a, b)is consistent with this age. The St Peter Sandstone overlies theWinneshiek Shale (Fig. 1c) and drill cores through this formationin Iowa,Minnesota and Indiana have yielded a diversity of conodontsindicating a late Darriwilian age. Characteristic early Darriwilianconodonts (the Histiodella–Paraprioniodus–Pteracontiodus–Fahraeusodus fauna) are absent in the Winneshiek Shale. Thus theconodont data constrain the age of the exceptionally preserved fossils,which were collected near the top of the Winneshiek Shale, tomiddle–late Darriwilian (Liu et al. 2017). Chemostratigraphic data,based on δ13Corg data from 36 drill core samples, narrow the age ofthe Winneshiek Shale to c. 464–467 Ma (Bergström et al. 2018), i.e.early–middle Darriwilian. Thus the exceptionally preserved fossilsare likely to be middle Darriwilian.

The meteorite impact clearly predates deposition of theWinneshiek Shale and the underlying breccia. The WinneshiekShale provides a minimum age for the impact of c. 464–467 Ma(Bergström et al. 2018). A maximum age is provided by the

Shakopee Formation, the youngest unit penetrated by the crater,which is separated from the overlying St Peter Sandstone by a majorunconformity (Fig. 1c). The Shakopee Formation yields conodontsof Middle Tremadocian age, indicating that the impact occurred noearlier than c. 482 Ma. The impact penetrated a marine embaymentor estuary in the southern part of the palaeocontinent Laurentia(Witzke 1990; Liu et al. 2009, 2017; Witzke et al. 2011). Thecomplexity of the crater-fill and the lithological variability of thebreccia suggest that the meteorite entered a significant depth ofwater (French et al. 2018). Cratering models indicate that asubstantial thickness of younger sediments was present at the timeof the impact, possibly 300–500 m, which was subsequentlyremoved by erosion. This period of erosion may have lasted for10–20 myr, after which the St Peters Sandstone was deposited,burying the crater and the surrounding area (French et al. 2018).

The age of the Decorah impact structure is not well constrained,but it coincides with the break-up of an L-chondrite meteorite parentbody, which resulted in a series of Middle Ordovician impacts(Bergström et al. 2018) at c. 470 Ma (Korochantseva et al. 2007) or468 Ma (Lindskog et al. 2017). A possible link between the asteroid

Fig. 1. (a) Location of Decorah in the NE of the State of Iowa, USA. (b) Geological map of Winneshiek and Allamakee counties in the northeastern cornerof 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 aredisrupted by the impact basin, which is filled with breccia, conglomerate and sandstone (unnamed), overlain by the Winneshiek Shale. Much of the impactbasin 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).

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break-up and the Great Ordovician Biodiversification Event(Schmitz et al. 2008) is uncertain because major diversificationbegan before the meteorite bombardment (Lindskog et al. 2017).The discovery of the Winneshiek Shale biota is notable inidentifying a novel context for exceptional preservation and itprompts a consideration of whether other impact craters could beimportant in revealing new information about soft-bodied taxa andtheir evolutionary and stratigraphic importance (Box 1).

Faunal composition

The most striking member of theWinneshiek fauna is the eurypteridPentecopterus decorahensis (Fig. 3a–g); the size of some of thetergites indicates that it reached lengths of 1.7 m. P. decorahensis isrepresented by >150 specimens, including some juveniles,preserved as carbonaceous cuticular remains. Although eurypteridremains make up <7% of the Winneshiek specimens, they accountfor the bulk of the fossil material. The fossils are mostly partiallydisarticulated elements of the exoskeleton (Fig. 3a, c–e). The way inwhich different parts are preserved in association suggests that thespecimens are a product of moulting (Tetlie et al. 2008); numerousindividuals may have congregated to moult together (Braddy 2001).The specimens found at this site allowed a full reconstruction of theadult eurypterid (Fig. 3b). The distribution of spines on theappendages and the characteristic morphology of the scales,including those forming a narrow median band down the axis of

the trunk tergites (Fig. 3d), indicate that Pentecopterus is related toMegalograptus, which is known from the Upper Ordovician ofOhio. Rare specimens of juvenile P. decorahensis (Fig. 3g) showthat the spiny appendages became more differentiated duringtransition to the adult, revealing a change during ontogeny, which isunusual among chelicerates (Lamsdell et al. 2015b). P. decora-hensis is c. 9 myr older than the previously oldest known eurypterid,Brachyopterus stubblefieldi from the Sandbian of Wales (Størmer1951). Ordovician eurypterids are very rare; <5% of all eurypteridspecies are Ordovician in age (Tetlie 2007). Phylogenetic analysisplaced P. decorahensis at the base of the Megalograptidae, whichoccupy a relatively derived position among the eurypterids. Thisindicates that most eurypterid clades had originated by the MiddleOrdovician (Lamsdell et al. 2015b). Eurypterids must have radiatedrapidly soon after they evolved, or significantly older examples mayremain to be discovered, perhaps in rocks of Cambrian age.

Pentecopterus is not the only chelicerate in the Winneshiekfauna. A much rarer and less well preserved form reveals a semi-circular carapace and 13 tergites, but provides no information on theappendages. This basal euchelicerate, Winneshiekia youngae,shares features with both xiphosurans and with eurypterids andchasmataspids (Lamsdell et al. 2015a). Winneshiekia helps toresolve the relationships of early euchelicerates and confirms thatxiphosurans (‘horse-shoe crabs’) are a paraphyletic group. Smallcarbonaceous fossils extracted from acid-treated samples ofWinneshiek Shale include gnathobasic structures (Fig. 3i), which

Fig. 2. The Winneshiek Shale. (a–c) Damming and extraction of the Winneshiek Shale from the bed of the Upper Iowa River. (d) Field exposure showing thelaminated nature of the Winneshiek Shale. (e) Fractured and deformed Shakopee Formation dolomite just outside the SE rim of the Decorah structure withH. 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).

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may represent the coxae of Winneshiekia or some similarxiphosuran (Nowak et al. 2018).

There are at least seven different taxa of bivalved arthropods inthe Winneshiek Shale, which represent nearly 10% of the fossils byspecimen (Briggs et al. 2016). The small crustacean C. winne-shiekensis (Fig. 4a) is the most abundant and preserves evidence offeatures other than the valves; the trunk segments are often evident,the appendages very rarely. C. winneshiekensis is the oldestrepresentative of one of the most species-rich phyllocarid families,the Ceratiocarididae; the next oldest ceratiocaridid is from theSandbian of Virginia (Collette & Hagadorn 2010). Another

crustacean group, the notostracan branchiopods, may be representedby a single incomplete abdomen with a telson bearing furcal rami.The largest bivalved taxon is Decoracaris hildebrandi with acarapace reaching lengths of 9 cm (Fig. 4b). The valves extendanteriorly into a bulbous projection overlying a shallow notchreminiscent of the outline in thylacocephalans, an extinct group ofprobable crustaceans. If future discoveries confirm that D.hildebrandi is a thylacocephalan, it is the oldest known represen-tative of the group (Haug et al. 2014), although a thylacocephalanidentity for even older bivalved forms has been suggested (Vannieret al. 2006; see Haug et al. 2014 for discussion). A smaller form,

Fig. 3. Eurypterid and other arthropods from the Winneshiek biota. (a–g) The megalograptid eurypterid Pentecopterus decorahensis. (a) The second prosomalappendage rotated and showing the array of spines projecting posteriorly (SUI 139941). (b) Dorsal view of the adult; reconstruction by James Lamsdell (WestVirginia University). (c) Complete appendage. (SUI 102857). (d) Body segments 4 and 5 showing median rows of scales (SUI 139931, images of part andcounterpart 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).

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Iosuperstes collisionis (Fig. 4c), is represented by sub-oval valvesup to 25 mm long, which are sometimes articulated; the absence ofpreserved limbs prevents its affinities from being resolved. Asmaller bivalved arthropod, commonly preserved in a butterfliedconfiguration (Fig. 4d), has a rim on the valves characteristic ofleperditicopes, a group previously considered to be giant ostracods.Leperditicopes are probably crustaceans, but their place within thegroup is uncertain. TheWinneshiek fauna also includes at least threedifferent ostracods (Briggs et al. 2016). Filtering appendagespreserved as small carbonaceous fossils (Fig. 3h) may belong tosome of these taxa (Nowak et al. 2018).

Conodonts are the most abundant fossils in theWinneshiek fauna(Liu et al. 2017), which is dominated by these vertebrates (Aldridgeet al. 1986; Briggs 1992). Isolated elements and bedding planeassemblages account for just over 50% of the fossil specimensrecovered (Liu et al. 2017) and isolated elements are also knownfrom bromalites (Hawkins et al. 2018), indicating that theconodonts fell victim to some unknown predator, perhapseurypterids or larger conodonts. More than a dozen taxa arepresent, most of them awaiting description, including coniform andprioniodinid apparatuses. Two of the more common apparatusesrepresent giant forms. The unusual apparatus of Archeognathusprimus (Fig. 5a, b), previously described from the Ordovician ofMissouri, consists of just six elements: two pairs of archeognathi-form (P) and one pair of coleodiform (S) elements. The elements inthis highly modified apparatus have robust basal bodies and appearto be well equipped for grasping and cutting prey. The apparatusarchitecture of Iowagnathus grandis (Fig. 5c) is more familiar,being reminiscent of the prioniodinid type, and consists of two pairsof P elements, four pairs and one unpaired S element, and a pair ofM elements, all of which also preserve robust basal bodies. The

preservation of basal bodies in the Winneshiek Shale is uniqueamong Ordovician conodont faunas. Although these largerconodont elements tend to be intact, the preservation of otherWinneshiek conodont taxa is variable. The individual elements ofA. primus and I. grandis in these two Winneshiek conodonts aresome of the largest conodont elements known; the unpaired Selement in I. grandis reaches dimensions >16 mm. Inferences oftotal body size based on element or apparatus dimensions areproblematic, but comparison of the apparatus size in theWinneshiek specimens with the Carboniferous conodont animalwith preserved soft parts from Scotland (Briggs et al. 1983;Aldridge et al. 1993) suggests that the Iowa animals may havereached lengths of 0.5–1.0 m. Conodonts are not the onlyvertebrates in the fauna. Rare bilaterally symmetrical, possibledermal, plates ornamented with tubercles have also been found(Fig. 5d). These isolated plates are unlike any such structuresreported from elsewhere and may represent a new early armouredjawless fish (Liu et al. 2006, 2009, 2011).

Bromalites account for just over 25% of the Winneshiek Shalefossils (Hawkins et al. 2018). The term includes both coprolitesand cololites, the latter representing gut contents which havebecome mineralized within a carcase that has decayed. Themorphology of the Winneshiek bromalites is variable. Rod-likeforms (Fig. 5e, f ), near-circular in cross-section, commonly show awrinkled surface (Fig. 5e), which suggests moulding by the gutwall. Some rod-like forms are partly flattened, suggesting a partialgut-fill or more fluid content, or collapse prior to completemineralization. The density of wrinkles (from closely spaced toabsent) and the degree of flattening vary, sometimes even alongthe length of an individual bromalite (Fig. 5g), resulting in amorphological spectrum that presumably reflects the nature of the

Fig. 4. Bivalved arthropods from the Winneshiek biota. (a) The phyllocarid Ceratiocaris winneshiekensis in right lateral view, showing the trunk and telsonextending beyond the valves (SUI 138435). (b) A right valve of Iosuperstes collisionis, which cannot be assigned to a group in the absence of evidenceother than the valves (SUI 138464). (c) A right valve of Decoracaris hildebrandi showing a pronounced notch, which may indicate an affinity withThylacocephala; the rest of the arthropod is unknown (SUI 138453). (d) An undetermined probable leperditiid with the two valves flattened in thebutterflied position (SUI 138486). All figures from Briggs et al. (2016), reproduced with permission.

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faecal material and its taphonomic history. The bromalites arephosphatized, although areas of carbonaceous material occur inassociation with some of them, and they contain phosphatemicrospherules, which may have been features of the gut of theliving animal. In the absence of associated soft tissues, it is oftendifficult to determine whether specimens originated as coprolitesor cololites, and both may be represented in the Winneshiek Shale.Rare examples of a very different type of bromalite, more variablein shape and consisting of aggregates of quartz sand, are alsopresent (Fig. 5h). These structures are likely to be coprolites, butthe source of the sand, and why it was ingested, are uncertain.

Taphonomy

The source of the laminations in the Winneshiek Shale is unknown.The possibility of a tidal influence in the vicinity of the crater hasbeen suggested (Liu et al. 2009), but there has been no analysis totest for tidal rhythmicity. The light colour of the organic-walledalgal fossils indicates a low degree of thermal maturation (Nowak

et al. 2017), consistent with the conodont alteration index values,which range from 1.5 to 2.0 (Liu et al. 2017). Some fossils,including conodont elements (Liu et al. 2017), linguloid brachio-pods and the valves of Decoracaris, Iosuperstes and the probableleperditiid (Briggs et al. 2016), were biomineralized and thereforemore readily preserved. A number of taxa, however, are preserved asorganic remains, including organic-walled algae (Nowak et al.2017), small carbonaceous arthropod remains (Nowak et al. 2018)and the eurypterid Pentecopterus (Lamsdell et al. 2015b). Thephyllocarid C. winneshiekensis is likewise commonly representedby organic cuticles, but some specimens also preserve phosphatizedgut contents and even traces of musculature (Briggs et al. 2016).The bromalites, which are a product of larger animals, are alsopreserved as three-dimensional structures mineralized in apatite.Such a rapid formation of authigenic apatite (Martill 1988, asdiscussed in Briggs 2003) requires high concentrations of phosphateand locally reduced pH, which can occur within a decaying carcase(Briggs & Wilby 1996). It emphasizes the potential for thepreservation of other soft tissues in the Winneshiek Shale,

Fig. 5. 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 quartzsand (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).

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perhaps even of conodonts. In the case of Ceratiocaris, the bodymay have been the phosphate source, while the cuticle provided abarrier to diffusion. The phosphate in the bromalites may have beensourced from the gut contents. Phosphatized soft tissues are alsoassociated with cuticular remains in other Lagerstätten, includingCerin and Solnhofen (Briggs & Wilby 1996).

Palaeoecology

TheWinneshiek Shale is unusual amongMiddle Ordovician marinedeposits in the absence of familiar shelly taxa and graptolites. Thepalaeogeographical setting, the undisturbed organic-rich laminatedsediment (Fig. 2d), the presence of pyrite and the nature of the faunasuggest a restricted basin associated with a large-scale embaymentof the epicontinental sea (Witzke et al. 2011) where bottomconditions, or at least the sediment itself, were anoxic and largelyinimical to life. In spite of the remarkable preservation, it is not clearhow representative the assemblage is of the original communityecology. Soft-bodied organisms without biomineralized skeletalelements or decay-resistant cuticles are unrepresented, exceptperhaps by bromalites. Primary productivity in the plankton isevidenced by a diversity of acritarchs and coenobial green algae(Zippi 2011; Nowak et al. 2017). More than half the Winneshiekspecimens are conodonts, which were probably a major componentof life in the water column (Liu et al. 2017). The complexapparatuses, robust elements and large size of Archeognathus andIowagnathus (Liu et al. 2017) suggest that they may have been high-level predators. The presence of small elements in coprolites showsthat conodonts were also prey, perhaps for larger conodonts or otheranimals. Only a small proportion of the conodont specimens arecomplete apparatuses, so it is difficult to estimate the number ofindividuals represented.

The appendage morphology of the giant eurypterid P. decora-hensis indicates that it was also a predator (Lamsdell et al. 2015b).The apparent dominance of predators in the assemblage isanomalous, but the eurypterid is represented by the remains ofexuviae so, as in the case of conodonts, it is difficult to determinethe number of individuals. Eurypterids congregated in shallowwater to mate and moult where salinity or other factors inhibitedpredators (Braddy 2001; Vrazo & Braddy 2011), in which case theymay have been seasonal visitors to the depositional basin. Only oneexample of a genital appendage of Pentecopterus is known and it isincomplete (Lamsdell et al. 2015b), so it is not possible todetermine whether the skewed sex ratio characteristic of massmoulting is present (Tetlie et al. 2008; Vrazo & Braddy 2011). The

large size of the conodonts and eurypterids suggest a nutrient-richenvironment, but it is not clear how much time they spent in thissetting and evidence for food web interactions is incomplete.

The large, rare bivalved arthropod D. hildebrandi may also havebeen a predator if its affinities lie with thylacocephalans (Briggset al. 2016), which have an anterior raptorial appendage. Thephyllocarid C. winneshiekensis may have been nektobenthic,adapted to low oxygen conditions, allowing it to feed on organicmatter in the sediment (Briggs et al. 2016). One example of multipleCeratiocaris individuals in a coprolite shows that it was preyedupon. Other bivalved arthropods, including ostracods, are rarer andare only represented by carapaces; presumably the thinner cuticlethat made up the rest of the exoskeleton decayed. Benthic animalsare rare, confined to a small linguloid brachiopod (Liu et al. 2006)and a single gastropod specimen (Briggs et al. 2016). Filamentousalgae similar to chlorophytes were probably also benthic, perhapsliving in adjacent shallow water (Nowak et al. 2017).

The place of Winneshiek among Ordovician Konservat-Lagerstätten

The Ordovician, in contrast with the Cambrian, provides relativelyfew windows on the soft-bodied taxa that make up c. 60% of marinediversity (Allison & Briggs 1991, 1993; Muscente et al. 2017). Thelatest compilation of exceptionally preserved fossil assemblages(Muscente et al. 2017) lists just four of Darriwilian age in additionto the Winneshiek biota: the Walcott Rust Quarry of New YorkState, the Valango Formation of northern Portugal and theLlandegley Rocks Lagerstätte and Llanfawr Holothurian Bed ofWales. The Walcott Rust Quarry, which yielded the first examplesof trilobites with preserved appendages, lies within theAmorphognathus superbus zone (Brett et al. 1999) and is Katianand not Darriwilian in age. The Valongo Formation preserves aremarkable fauna of complete trilobites, but no soft parts survive(Gutiérrez-Marco et al. 2009). The Llandegley Rocks Lagerstätte(Botting 2005) is dominated by sponges, some preserving silicifiedsoft tissue, and includes familiar shelly taxa, such as brachiopods,orthoconic nautiloids, trilobites, echinoderms and bryozoans. TheHolothurian Bed of the LlanfawrMudstones (Botting &Muir 2012)preserves possible algae, palaeoscolecidans, cystoids, a uniquearticulated holothurian, and sponges, graptolites, trilobites, bra-chiopods and other shelly taxa. An additional DarriwilianKonservat-Lagerstätte, the Llanfallteg Formation in south Wales(Hearing et al. 2016), yields trilobites, some with phosphatizedguts, and other shelly fossils, non-mineralized arthropods

Box 1. Fossils in impact structures

The discovery of exceptionally preserved fossils in the Decorah impact structure highlights the potential importance of such settings as a source of Konservat-Lagerstätten. Impacts create deep craters, which may favour low-energy sedimentation in a stratified water columnwith anoxic conditions at depth. However, they areoften small in area with a relatively thin infill overlying a crater-fill breccia, and they may not be exposed at the surface. It is not surprising therefore that records offossils preserved in impact craters are rare. The occurrence of significant fossiliferous deposits reflects the circumstances associated with each crater – its size anddepth, marine or non-marine setting, and the nature of infill lithologies – rather than generic features associated with impact cratering.Perhaps the best-known fossil sequence in an impact crater was discovered in the c. 3.5 km diameter Miocene Steinheim Basin in SW Germany, which includes

30–40 m of lacustrine sediments preserving multiple speciation events over time in the gastropod genus Gyraulus (Planorbidae) (Gorthner 1992). The Steinheimsnails were investigated by Franz Hilgendorf in the 1860s, who documented a now classic example of endemic speciation and generated the first phylogenetic tree(Hilgendorf 1867; Rasser 2014). Other Neogene lake sequences in central and southeastern Europe also preserve abundant shelly fossils, but only the Steinheimcrater and the simultaneously formed larger c. 25 km Nördlinger Ries structure originated as the result of an impact (Harzhauser & Mandic 2008).Non-shelly and soft-bodied taxa such as those in the Decorah impact structure are rarely found in impact crater-fill sediments. The Boltysh crater (c. 65 Ma) on the

Ukrainian Shield preserves Paleogene molluscs, ostracods and fish (Gurov et al. 2006). The Darwin Crater in Tasmania (Howard & Haines 2007) provides one of thelongest continuous pollen records in Australia in c. 60 m of late Pleistocene and Holocene lacustrine sediments (Colhoun & van de Geer 1998; Cook et al. 2012).Fossils may also occur in impact breccia and associated sediments, as in the concentration of lignite fragments in the nearshore marine Late Cretaceous WetumpkaCrater in Alabama (King et al. 2006). Volcanic craters show some similarity to impact craters in their circular outline and relatively steep sides. Some of the bestknown examples of exceptional preservation in volcanic crater lakes are those in the Tertiary Central European volcanic belt in Germany, which include importantLagerstätten of Eocene and Oligocene age at Messel, Eckfeld and Enspel (Pirrung et al. 2001). Most lake sediments that yield Lagerstätten, however, occur inorogenic belts or fault-bounded basins (Allison & Briggs 1991).

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(including a xenopod and marrellomorph), a phyllocarid, vermi-form taxa, palaeoscolecids and graptolites. All these Konservat-Lagerstätten, however, like the more remarkable older Ordovician(Tremadocian-Floian) Fezouta Formation of Morocco (Van Royet al. 2015, Lefebvre et al. 2018), preserve a diversity of shelly taxa,indicating an environmental setting representative of more normalmarine conditions than the Winneshiek Shale.

Closer comparisons with the Winneshiek Shale are offered byLate Ordovician Lagerstätten situated around the margins ofLaurentian land masses. The Katian William Lake and AirportCove biotas of Manitoba, Canada include linguloids, conodonts,eurypterids and xiphosurids, although the proportions differbetween them (Young et al. 2007, Fig. 2). The Big Hill biota ofMichigan yields a similar suite of taxa (Lamsdell et al. 2017). Theselithologies are predominantly carbonate, however, in contrast withWinneshiek, but the shared faunal components reflect a shallowmarine marginal setting (Young et al. 2007). There are alsosimilarities between theWinneshiek Shale and the Hirnantian SoomShale of South Africa, even though it was deposited in a cold watershallow marine setting on Gondwana (Gabbott et al. 2016). TheSoom Shale biota is preserved in a laminated mudstone and isdominated by conodonts, some giant, and includes a eurypterid,phyllocarid and ostracods, as well as linguloid brachiopods andbromalites (Aldridge et al. 2006), but a diversity of shelly taxa isalso present. Thus the similarities between the Winneshiek Shaleand these other Ordovician Konservat-Lagerstätten reflect excep-tional preservation combined with atypical marine settings, but donot imply equivalent environments.

The Winneshiek Konservat-Lagerstätte occurs in a very unusualsetting, but it is unlikely to be unique. Other examples ofexceptional preservation originally thought to be one of a kindhave been shown to occur in multiples following furtherexploration: examples include the Cambrian Burgess Shale(Collins et al. 1983) and Ordovician Beecher’s Trilobite Bed(Farrell et al. 2009). The Decorah impact structure is one of anumber of meteorite craters of similar age in North America (Liuet al. 2009; Schmieder et al. 2015): could others also hostexceptionally preserved fossils? The Middle Ordovician Rock Elmstructure in Wisconsin is about the same size as the Decorahstructure. The Rock Elm Shale, which is confined to the crater, hasyielded brachiopods, gastropods, trilobites, crustaceans and poly-chaetes (as scolecodonts) as well as conodonts (Peters et al. 2002;French et al. 2004). This fauna, which has received little attention, isapparently more diverse and includes more normal marine taxa(French et al. 2004) than the Winneshiek Shale; no evidence ofexceptional preservation has been reported.

The Ames structure in NW Oklahoma (Johnson & Campbell1997; Koeberl et al. 2001) is larger than the Decorah impactstructure, up to 15 km in diameter, and is buried 2.75 km below the

surface. Conodont elements found in core chips of black shalewithin the Ames crater included Erismodus and Phragmodus,indicating a mid-Middle Ordovician age (Repetski 1997). Most ofthe elements occur as clusters or bedding plane assemblages, likethose from the Winneshiek Shale, and they include examplesassociated with carbonaceous films, indicating that they representthe remains of carcases (Repetski 1997). The black shale within theAmes structure has yielded two phyllocarids (Hannibal & Feldmann1997), one similar to C. winneshiekensis, the other probablyCaryocaris (Briggs et al. 2016). Rare brachiopods and ‘smallphosphatic masses’ (Hannibal & Feldmann 1997), which mayrepresent bromalites, are also present. Thus the biota, althoughincompletely known, shows some similarities with the Winneshiekassemblage. We are unaware of detailed reports of fossils associatedwith other coeval Ordovician impact sites in North America(Schmieder et al. 2015), at East Clearwater Lake in Québec,Glasford in Illinois and Glover Bluff in Wisconsin. However, it isclear that impact structures represent a potential source of otherexceptionally preserved fossils (Box 1).

Conclusions

The large number of Burgess Shale-type Konservat-Lagerstätten inCambrian rocks provides unparalleled evidence of the history of lifeduring that period, which coincides with the Cambrian radiation.The discovery of the Fezouata formations of Morocco, with theirsimilar preservation of soft-bodied organisms (Van Roy et al.2015), extended this record into the Early Ordovician (Lefebvreet al. 2018). The nature of the animals represented in the Fezouatabiota suggests that the Cambrian radiation and Great OrdovicianBiodiversification Event are more of a continuum than previouslythought. Thereafter the fossil record of soft-bodied taxa is much lessrich, perhaps as a result of the closure of the taphonomic windowthat favoured Burgess Shale-type preservation (Gaines et al. 2012).It has been suggested that the Mid Ordovician asteroid break-up andmeteor shower, which resulted in the Decorah impact structure,might have promoted the Great Ordovician Biodiversification Eventby disrupting communities (Schmitz et al. 2008). There is noobvious mechanism to explain such a relationship on a global scale,however, and biodiversification during the Darriwilian is only onecomponent of a longer and more complex Great OrdovicianBiodiversification Event (Servais & Harper 2018). The WinneshiekKonservat-Lagerstätte is important because it is Middle Ordovicianin age, an interval when non-shelly fossils are poorly represented inthe fossil record. Some of the Winneshiek Shale arthropods are offundamental stratigraphic and phylogenetic importance, includingthe earliest eurypterid, a basal chelicerate and the earliest ceratiocaridphyllocarid. The conodonts provide new evidence of apparatusarchitecture and giant size. The Winneshiek Lagerstätte also reveals

Box 2. Outstanding questions

(1) Some of the Winneshiek Shale fossils await further investigation. What is the nature of other algal material? What is the total conodont fauna?

(2) What new taxa would further excavation reveal?

(3) What is the detailed history of the depositional basin? This would require further drill core data.

(4) What are the details of the crater-fill sequence, including lithologies and thickness?

(5) What were the roles of impact and post-impact processes in generating an environment for exceptional preservation?

(6) How does the Decorah impact structure relate to any proposed ‘cluster’ of Mid Ordovician impact events?

(7) What is the potential of impact structures in general as hosts of Konservat-Lagerstätten and how could modelling determine which kinds of impact structuresshould be targeted?

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the potential importance of impact craters as a novel context forexceptional preservation. Borehole samples from the coeval Amesstructure in Oklahoma contain similar fossils, and sedimentarysequences in other depositional basins created by impacts awaitexploration with the insights provided by the Decorah discovery.Box 2 lists some of the questions yet to be answered about theWinneshiek biota and the nature, history and significance of theDecorah impact structure.

Acknowledgements Steve and Jane Hildebrand, landowners, providedaccess to the outcrop and drill site. S.M. Bergström, D.H. Campbell, A. Ferretti,B.M. French, T.H.P. Harvey, A.D. Hawkins, J. Lamsdell, X. Liu, K. McVey,A.D. Muscente, H. Nowak, B. Schmitz, T. Servais, F. Terfelt, Y. Wang, S. Xiaoand P.A. Zippi collaborated on different aspects of the Winneshiekresearch. R. Rowden, T. Marshall, C. Davis, D. Campbell and C. Monsonassisted in the excavation of the Winneshiek Shale outcrop. M. Spencer,J. McHugh, E. Wilberg, H. Zou, M. Tibbits, W. Cao, J. Matzke, K. McVey,D. McKay, M. Behling, B. Dye, J. Hansen, B. Neale, C. Kuempel, E. Greaves,K. Parson and T. Maher helped in the laboratory with sample preparation and thesearch for fossils. T. Adrain of the University of Iowa Paleontology Repositorycurated specimens. C. Kohrt, Geospatial Administrator, Iowa Department ofNatural Resources, provided the map of the Bedrock Geology of NortheasternIowa and L. Bonneau, Yale Center for Earth Observation, assisted with utilizingthe data. M. Mohanta and E. Martin (Yale University) prepared the figures. Themanuscript benefited from constructive comments from Joe Botting and ananonymous reviewer.

Funding This work was funded by National Science Foundation grants0922054 and 0921245.

Scientific editing by Philip Donoghue

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