sign in sign up
<<
Home , Boltysh crater, Conodont, Rock Elm Disturbance

Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Review focus Journal of the Geological Society Published Online First https://doi.org/10.1144/jgs2018-101

The Winneshiek biota: exceptionally well-preserved fossils in a Middle

Derek E.G. Briggs1,2*, Huaibao P. Liu3, Robert M. McKay3 & Brian J. Witzke4 1 Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA 2 Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06520, USA 3 Iowa Geological Survey, IIHR – Hydroscience & Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, IA 52242, USA 4 Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USA D.E.G.B., 0000-0003-0649-6417 * Correspondence: [email protected]

Abstract: The Winneshiek Shale (Middle Ordovician, Darriwilian) was deposited in a crater, the Decorah , in NE Iowa. This crater is 5.6 km in diameter and penetrates and Ordovician cratonic strata. It was probably situated close to land in an embayment connected to the epicontinental sea; typical shelly marine taxa are absent. The Konservat-Lagerstätte within the Winneshiek Shale is important because it represents an interval when exceptional preservation is rare. The biota includes the earliest eurypterid, a giant form, as well as a new basal chelicerate and the earliest ceratiocarid phyllocarid. , some of giant size, occur as bedding plane assemblages. Bromalites and rarer elements, including a linguloid brachiopod and a probable jawless , are also present. Similar fossils occur in the coeval Ames impact structure 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 in fossils, including arthropod appendages (Fig. 3h, i)(Nowak et al. the Winneshiek Shale, which directly underlies the Ordovician St 2018) and filamentous algae (Nowak et al. 2017). Peter Sandstone, during mapping by the Iowa Geological Survey (Liu et al. 2005)(Fig. 1). The only exposure, which represents the The meteorite crater and the age of the Winneshiek Shale top of the Winneshiek Shale, is in the valley of the Upper Iowa River near Decorah in Iowa. Collections from this locality yielded The Winneshiek Shale, an unusual greenish brown to dark grey conodonts, linguloid brachiopods, large fragments of eurypterid organic-rich shale up to 26 m thick, was noted in cores and well cuticles and phyllocarid crustaceans (Liu et al. 2005). A coal-like cuttings some years prior to the discovery of the exposure near layer had been reported in local newspapers in the 1920s, which Decorah (Young et al. 2005). It overlies an unnamed unit dominated could only have been a concentration of eurypterid cuticles, perhaps by (Fig. 1c), the two reaching a combined thickness of to encourage speculators. H. Paul Liu recognized the exceptional c. 200 m. Occurrences of the shale in drill core and in outcrop nature of these fossils and he and colleagues published a preliminary delimit a circle with a diameter of c. 5.6 km (Liu et al. 2009) account of the discovery in 2006 (Liu et al. 2006). A collaborative (Fig. 1b). This outline was also detected in electromagnetic and grant to the authors funded the construction of a temporary dam in gravity data from airborne geophysical surveys. The circular 2010, which isolated the exposure of the Winneshiek Shale on the structure and its sedimentary fill are concealed by the overlying St north bank of the Upper Iowa River near Decorah (Fig. 2a–c). This Peter Sandstone and Quaternary alluvium, except at one small allowed access to the shale and a sequence of large samples, locality in the Upper Iowa River Valley near Decorah, where the top representing c. 4 m of the section, was extracted by hand with the of the shale is at the land surface. The circular depositional basin is aid of earth-moving equipment (Fig. 2b, c). The shale was split at characterized by evidence of local deformation near the perimeter the Iowa Geological Survey sample repository over the next three (Fig. 2e) and its unusual stratigraphy. The Winneshiek Shale and years and yielded >5000 macrofossil specimens. underlying breccia are found only within the circular structure, The biota of the Winneshiek Shale is characterized by exceptional where the units are completely different from the regional preservation combined with a restricted diversity, both of which stratigraphic sequence (Liu et al. 2009; French et al. 2018). All reflect the unusual depositional setting. The dominant macro- these attributes are consistent with deposition within a meteorite fossils are arthropods: the giant Pentecopterus decorahensis crater. This inference was elegantly confirmed by the demonstration (Fig. 3) is the earliest described eurypterid (Lamsdell et al. 2015b) that quartz grains observed in drill samples from the breccia show and a diversity of bivalved arthropods includes a phyllocarid, shock-induced phenomena (Liu et al. 2009; McKay et al. 2011), Ceratiocaris winneshiekensis, and ostracods (Fig. 4). particularly the fractures and deformation features characteristic of elements are the most abundant fossils, many occurring as bedding an impact (French et al. 2018). plane assemblages (Fig. 5), and extrapolation indicates that some are The stratigraphic context and fossils indicate that the Winneshiek from individual of very large size (Liu et al. 2017). Rare Shale is Darriwilian (Middle Ordovician) in age, i.e. from 458.4 to specimens interpreted as the head shield of an early armoured 467.3 Ma (Gradstein et al. 2012; Cohen et al. 2013). None of the (Liu et al. 2006) are also present (Fig. 5d). Dissolution of the conodonts associated with the exceptionally preserved fossils are organic-rich shale has yielded a variety of microscopic carbonaceous zonal 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 Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

D.E.G. Briggs et al.

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 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). which is known from Darriwilian strata around Laurentia from Shakopee Formation, the youngest unit penetrated by the crater, Alberta to New York State (Witzke et al. 2011; Liu et al. 2017). The which is separated from the overlying St Peter Sandstone by a major presence of the unusual conodont Archeognathus primus (Fig. 5a, b) unconformity (Fig. 1c). The Shakopee Formation yields conodonts is consistent with this age. The St Peter Sandstone overlies the of Middle age, indicating that the impact occurred no Winneshiek Shale (Fig. 1c) and drill cores through this formation earlier than c. 482 Ma. The impact penetrated a marine embayment in Iowa, Minnesota and Indiana have yielded a diversity of conodonts or estuary in the southern part of the palaeocontinent Laurentia indicating a late Darriwilian age. Characteristic early Darriwilian (Witzke 1990; Liu et al. 2009, 2017; Witzke et al. 2011). The conodonts (the –Paraprioniodus–Pteracontiodus– complexity of the crater-fill and the lithological variability of the Fahraeusodus fauna) are absent in the Winneshiek Shale. Thus the breccia suggest that the meteorite entered a significant depth of conodont data constrain the age of the exceptionally preserved fossils, water (French et al. 2018). Cratering models indicate that a which were collected near the top of the Winneshiek Shale, to substantial thickness of younger sediments was present at the time middle–late Darriwilian (Liu et al.2017). Chemostratigraphic data, of the impact, possibly 300–500 m, which was subsequently 13 based on δ Corg data from 36 drill core samples, narrow the age of removed by erosion. This period of erosion may have lasted for the Winneshiek Shale to c.464–467 Ma (Bergström et al. 2018), i.e. 10–20 myr, after which the St Peters Sandstone was deposited, early–middle Darriwilian. Thus the exceptionally preserved fossils burying the crater and the surrounding area (French et al. 2018). are likely to be middle Darriwilian. The age of the Decorah impact structure is not well constrained, The meteorite impact clearly predates deposition of the but it coincides with the break-up of an L-chondrite meteorite parent Winneshiek Shale and the underlying breccia. The Winneshiek body, which resulted in a series of Middle Ordovician impacts Shale provides a minimum age for the impact of c. 464–467 Ma (Bergström et al. 2018)atc. 470 Ma (Korochantseva et al. 2007)or (Bergström et al. 2018). A maximum age is provided by the 468 Ma (Lindskog et al. 2017). A possible link between the asteroid Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Middle Ordovician Winneshiek biota

Fig. 2. The Winneshiek Shale. (a–c) Damming and extraction of the Winneshiek Shale from the bed of the Upper Iowa River. (d)Fieldexposureshowingthe 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). break-up and the Great Ordovician Biodiversification Event the trunk tergites (Fig. 3d), indicate that Pentecopterus is related to (Schmitz et al. 2008) is uncertain because major diversification Megalograptus, which is known from the Upper Ordovician of began before the meteorite bombardment (Lindskog et al. 2017). Ohio. Rare specimens of juvenile P. decorahensis (Fig. 3g) show The discovery of the Winneshiek Shale biota is notable in that the spiny appendages became more differentiated during identifying a novel context for exceptional preservation and it transition to the adult, revealing a change during ontogeny, which is prompts a consideration of whether other impact craters could be unusual among chelicerates (Lamsdell et al. 2015b). P. decora- important in revealing new information about soft-bodied taxa and hensis is c. 9 myr older than the previously oldest known eurypterid, their evolutionary and stratigraphic importance (Box 1). Brachyopterus stubblefieldi from the Sandbian of Wales (Størmer 1951). Ordovician eurypterids are very rare; <5% of all eurypterid Faunal composition species are Ordovician in age (Tetlie 2007). Phylogenetic analysis placed P. decorahensis at the base of the Megalograptidae, which The most striking member of the Winneshiek fauna is the eurypterid occupy a relatively derived position among the eurypterids. This Pentecopterus decorahensis (Fig. 3a–g); the size of some of the indicates that most eurypterid had originated by the Middle tergites indicates that it reached lengths of 1.7 m. P. decorahensis is Ordovician (Lamsdell et al. 2015b). Eurypterids must have radiated represented by >150 specimens, including some juveniles, rapidly soon after they evolved, or significantly older examples may preserved as carbonaceous cuticular remains. Although eurypterid remain to be discovered, perhaps in rocks of Cambrian age. remains make up <7% of the Winneshiek specimens, they account Pentecopterus is not the only chelicerate in the Winneshiek for the bulk of the fossil material. The fossils are mostly partially fauna. A much rarer and less well preserved form reveals a semi- disarticulated elements of the exoskeleton (Fig. 3a, c–e). The way in circular carapace and 13 tergites, but provides no information on the which different parts are preserved in association suggests that the appendages. This basal euchelicerate, Winneshiekia youngae, specimens are a product of moulting (Tetlie et al. 2008); numerous shares features with both xiphosurans and with eurypterids and individuals may have congregated to moult together (Braddy 2001). chasmataspids (Lamsdell et al. 2015a). Winneshiekia helps to The specimens found at this site allowed a full reconstruction of the resolve the relationships of early euchelicerates and confirms that adult eurypterid (Fig. 3b). The distribution of spines on the xiphosurans (‘horse-shoe crabs’) are a paraphyletic group. Small appendages and the characteristic morphology of the scales, carbonaceous fossils extracted from acid-treated samples of including those forming a narrow median band down the axis of Winneshiek Shale include gnathobasic structures (Fig. 3i), which Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

D.E.G. Briggs et al.

Fig. 3. 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). may represent the coxae of Winneshiekia or some similar crustacean group, the notostracan branchiopods, may be represented xiphosuran (Nowak et al. 2018). by a single incomplete abdomen with a telson bearing furcal rami. There are at least seven different taxa of bivalved arthropods in The largest bivalved taxon is Decoracaris hildebrandi with a the Winneshiek Shale, which represent nearly 10% of the fossils by carapace reaching lengths of 9 cm (Fig. 4b). The valves extend specimen (Briggs et al. 2016). The small crustacean C. winne- anteriorly into a bulbous projection overlying a shallow notch shiekensis (Fig. 4a) is the most abundant and preserves evidence of reminiscent of the outline in thylacocephalans, an extinct group of features other than the valves; the trunk segments are often evident, probable crustaceans. If future discoveries confirm that D. the appendages very rarely. C. winneshiekensis is the oldest hildebrandi is a thylacocephalan, it is the oldest known represen- representative of one of the most species-rich phyllocarid families, tative of the group (Haug et al. 2014), although a thylacocephalan the Ceratiocarididae; the next oldest ceratiocaridid is from the identity for even older bivalved forms has been suggested (Vannier Sandbian of Virginia (Collette & Hagadorn 2010). Another et al. 2006; see Haug et al. 2014 for discussion). A smaller form, Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Middle Ordovician Winneshiek biota

Fig. 4. 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.

Iosuperstes collisionis (Fig. 4c), is represented by sub-oval valves preservation of basal bodies in the Winneshiek Shale is unique up to 25 mm long, which are sometimes articulated; the absence of among Ordovician conodont faunas. Although these larger preserved limbs prevents its affinities from being resolved. A conodont elements tend to be intact, the preservation of other smaller bivalved arthropod, commonly preserved in a butterflied Winneshiek conodont taxa is variable. The individual elements of configuration (Fig. 4d), has a rim on the valves characteristic of A. primus and I. grandis in these two Winneshiek conodonts are leperditicopes, a group previously considered to be giant ostracods. some of the largest conodont elements known; the unpaired S Leperditicopes are probably crustaceans, but their place within the element in I. grandis reaches dimensions >16 mm. Inferences of group is uncertain. The Winneshiek fauna also includes at least three total body size based on element or apparatus dimensions are different ostracods (Briggs et al. 2016). Filtering appendages problematic, but comparison of the apparatus size in the preserved as small carbonaceous fossils (Fig. 3h) may belong to Winneshiek specimens with the conodont some of these taxa (Nowak et al. 2018). with preserved soft parts from Scotland (Briggs et al. 1983; Conodonts are the most abundant fossils in the Winneshiek fauna Aldridge et al. 1993) suggests that the Iowa animals may have (Liu et al. 2017), which is dominated by these (Aldridge reached lengths of 0.5–1.0 m. Conodonts are not the only et al. 1986; Briggs 1992). Isolated elements and bedding plane vertebrates in the fauna. Rare bilaterally symmetrical, possible assemblages account for just over 50% of the fossil specimens dermal, plates ornamented with tubercles have also been found recovered (Liu et al. 2017) and isolated elements are also known (Fig. 5d). These isolated plates are unlike any such structures from bromalites (Hawkins et al. 2018), indicating that the reported from elsewhere and may represent a new early armoured conodonts fell victim to some unknown predator, perhaps jawless fish (Liu et al. 2006, 2009, 2011). eurypterids or larger conodonts. More than a dozen taxa are Bromalites account for just over 25% of the Winneshiek Shale present, most of them awaiting description, including coniform and fossils (Hawkins et al. 2018). The term includes both coprolites prioniodinid apparatuses. Two of the more common apparatuses and cololites, the latter representing gut contents which have represent giant forms. The unusual apparatus of Archeognathus become mineralized within a carcase that has decayed. The primus (Fig. 5a, b), previously described from the Ordovician of morphology of the Winneshiek bromalites is variable. Rod-like Missouri, consists of just six elements: two pairs of archeognathi- forms (Fig. 5e, f), near-circular in cross-section, commonly show a form (P) and one pair of coleodiform (S) elements. The elements in wrinkled surface (Fig. 5e), which suggests moulding by the gut this highly modified apparatus have robust basal bodies and appear wall. Some rod-like forms are partly flattened, suggesting a partial to be well equipped for grasping and cutting prey. The apparatus gut-fill or more fluid content, or collapse prior to complete architecture of Iowagnathus grandis (Fig. 5c) is more familiar, mineralization. The density of wrinkles (from closely spaced to being reminiscent of the prioniodinid type, and consists of two pairs absent) and the degree of flattening vary, sometimes even along of P elements, four pairs and one unpaired S element, and a pair of the length of an individual bromalite (Fig. 5g), resulting in a M elements, all of which also preserve robust basal bodies. The morphological spectrum that presumably reflects the nature of the Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

D.E.G. Briggs et al.

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 quartz sand (SUI 145166). (a–c)FromLiu et al.(2017), reproduced with permission; (d) from Liu et al.(2006);(e–h) after Hawkins et al.(2018,fig.4). faecal material and its taphonomic history. The bromalites are et al. 2017), consistent with the conodont alteration index values, phosphatized, although areas of carbonaceous material occur in which range from 1.5 to 2.0 (Liu et al. 2017). Some fossils, association with some of them, and they contain phosphate including conodont elements (Liu et al. 2017), linguloid brachio- microspherules, which may have been features of the gut of the pods and the valves of Decoracaris, Iosuperstes and the probable living animal. In the absence of associated soft tissues, it is often leperditiid (Briggs et al. 2016), were biomineralized and therefore difficult to determine whether specimens originated as coprolites more readily preserved. A number of taxa, however, are preserved as or cololites, and both may be represented in the Winneshiek Shale. organic remains, including organic-walled algae (Nowak et al. Rare examples of a very different type of bromalite, more variable 2017), small carbonaceous arthropod remains (Nowak et al. 2018) in shape and consisting of aggregates of quartz sand, are also and the eurypterid Pentecopterus (Lamsdell et al. 2015b). The present (Fig. 5h). These structures are likely to be coprolites, but phyllocarid C. winneshiekensis is likewise commonly represented the source of the sand, and why it was ingested, are uncertain. by organic cuticles, but some specimens also preserve phosphatized gut contents and even traces of musculature (Briggs et al. 2016). Taphonomy The bromalites, which are a product of larger animals, are also preserved as three-dimensional structures mineralized in apatite. The source of the laminations in the Winneshiek Shale is unknown. Such a rapid formation of authigenic apatite (Martill 1988,as The possibility of a tidal influence in the vicinity of the crater has discussed in Briggs 2003) requires high concentrations of phosphate been suggested (Liu et al. 2009), but there has been no analysis to and locally reduced pH, which can occur within a decaying carcase test for tidal rhythmicity. The light colour of the organic-walled (Briggs & Wilby 1996). It emphasizes the potential for the algal fossils indicates a low degree of thermal maturation (Nowak preservation of other soft tissues in the Winneshiek Shale, Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Middle Ordovician Winneshiek biota

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 column with anoxic conditions at depth. However, they are often 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 of fossils preserved in impact craters are rare. The occurrence of significant fossiliferous deposits reflects the circumstances associated with each crater – its size and depth, 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 Steinheim snails 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 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 (c. 65 Ma) on the Ukrainian Shield preserves Paleogene molluscs, ostracods and fish (Gurov et al. 2006). The in Tasmania (Howard & Haines 2007) provides one of the longest 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 Wetumpka Crater 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 best known examples of exceptional preservation in volcanic crater lakes are those in the Tertiary Central European volcanic belt in Germany, which include important Lagerstätten of Eocene and Oligocene age at Messel, Eckfeld and Enspel (Pirrung et al. 2001). Most lake sediments that yield Lagerstätten, however, occur in orogenic belts or fault-bounded basins (Allison & Briggs 1991).

perhaps even of conodonts. In the case of Ceratiocaris, the body large size of the conodonts and eurypterids suggest a nutrient-rich may have been the phosphate source, while the cuticle provided a environment, but it is not clear how much time they spent in this barrier to diffusion. The phosphate in the bromalites may have been setting and evidence for food web interactions is incomplete. sourced from the gut contents. Phosphatized soft tissues are also The large, rare bivalved arthropod D. hildebrandi may also have associated with cuticular remains in other Lagerstätten, including been a predator if its affinities lie with thylacocephalans (Briggs Cerin and Solnhofen (Briggs & Wilby 1996). et al. 2016), which have an anterior raptorial appendage. The phyllocarid C. winneshiekensis may have been nektobenthic, adapted to low oxygen conditions, allowing it to feed on organic Palaeoecology matter in the sediment (Briggs et al. 2016). One example of multiple Ceratiocaris individuals in a coprolite shows that it was preyed The Winneshiek Shale is unusual among Middle Ordovician marine upon. Other bivalved arthropods, including ostracods, are rarer and deposits in the absence of familiar shelly taxa and graptolites. The are only represented by carapaces; presumably the thinner cuticle palaeogeographical setting, the undisturbed organic-rich laminated that made up the rest of the exoskeleton decayed. Benthic animals sediment (Fig. 2d), the presence of pyrite and the nature of the fauna are rare, confined to a small linguloid brachiopod (Liu et al. 2006) suggest a restricted basin associated with a large-scale embayment and a single gastropod specimen (Briggs et al. 2016). Filamentous of the epicontinental sea (Witzke et al. 2011) where bottom algae similar to chlorophytes were probably also benthic, perhaps conditions, or at least the sediment itself, were anoxic and largely living in adjacent shallow water (Nowak et al. 2017). inimical to life. In spite of the remarkable preservation, it is not clear how representative the assemblage is of the original community ecology. Soft-bodied organisms without biomineralized skeletal The place of Winneshiek among Ordovician Konservat- elements or decay-resistant cuticles are unrepresented, except Lagerstätten perhaps by bromalites. Primary productivity in the plankton is evidenced by a diversity of acritarchs and coenobial green algae The Ordovician, in contrast with the Cambrian, provides relatively (Zippi 2011; Nowak et al. 2017). More than half the Winneshiek few windows on the soft-bodied taxa that make up c. 60% of marine specimens are conodonts, which were probably a major component diversity (Allison & Briggs 1991, 1993; Muscente et al. 2017). The of life in the water column (Liu et al. 2017). The complex latest compilation of exceptionally preserved fossil assemblages apparatuses, robust elements and large size of Archeognathus and (Muscente et al. 2017) lists just four of Darriwilian age in addition Iowagnathus (Liu et al. 2017) suggest that they may have been high- to the Winneshiek biota: the Walcott Rust Quarry of New York level predators. The presence of small elements in coprolites shows State, the Valango Formation of northern Portugal and the that conodonts were also prey, perhaps for larger conodonts or other Llandegley Rocks Lagerstätte and Llanfawr Holothurian Bed of animals. Only a small proportion of the conodont specimens are Wales. The Walcott Rust Quarry, which yielded the first examples complete apparatuses, so it is difficult to estimate the number of of trilobites with preserved appendages, lies within the individuals represented. superbus zone (Brett et al. 1999) and is Katian The appendage morphology of the giant eurypterid P. decora- and not Darriwilian in age. The Valongo Formation preserves a hensis indicates that it was also a predator (Lamsdell et al. 2015b). remarkable fauna of complete trilobites, but no soft parts survive The apparent dominance of predators in the assemblage is (Gutiérrez-Marco et al. 2009). The Llandegley Rocks Lagerstätte anomalous, but the eurypterid is represented by the remains of (Botting 2005) is dominated by sponges, some preserving silicified exuviae so, as in the case of conodonts, it is difficult to determine soft tissue, and includes familiar shelly taxa, such as brachiopods, the number of individuals. Eurypterids congregated in shallow orthoconic nautiloids, trilobites, echinoderms and bryozoans. The water to mate and moult where salinity or other factors inhibited Holothurian Bed of the Llanfawr Mudstones (Botting & Muir 2012) predators (Braddy 2001; Vrazo & Braddy 2011), in which case they preserves possible algae, palaeoscolecidans, cystoids, a unique may have been seasonal visitors to the depositional basin. Only one articulated holothurian, and sponges, graptolites, trilobites, bra- example of a genital appendage of Pentecopterus is known and it is chiopods and other shelly taxa. An additional Darriwilian incomplete (Lamsdell et al. 2015b), so it is not possible to Konservat-Lagerstätte, the Llanfallteg Formation in south Wales determine whether the skewed sex ratio characteristic of mass (Hearing et al. 2016), yields trilobites, some with phosphatized moulting is present (Tetlie et al. 2008; Vrazo & Braddy 2011). The guts, and other shelly fossils, non-mineralized arthropods Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

D.E.G. Briggs et al.

(including a xenopod and marrellomorph), a phyllocarid, vermi- surface. Conodont elements found in core chips of black shale form taxa, palaeoscolecids and graptolites. All these Konservat- within the Ames crater included Erismodus and Phragmodus, Lagerstätten, however, like the more remarkable older Ordovician indicating a mid-Middle Ordovician age (Repetski 1997). Most of (Tremadocian-) Fezouta Formation of Morocco (Van Roy the elements occur as clusters or bedding plane assemblages, like et al. 2015, Lefebvre et al. 2018), preserve a diversity of shelly taxa, those from the Winneshiek Shale, and they include examples indicating an environmental setting representative of more normal associated with carbonaceous films, indicating that they represent marine conditions than the Winneshiek Shale. the remains of carcases (Repetski 1997). The black shale within the Closer comparisons with the Winneshiek Shale are offered by Ames structure has yielded two phyllocarids (Hannibal & Feldmann Late Ordovician Lagerstätten situated around the margins of 1997), one similar to C. winneshiekensis, the other probably Laurentian land masses. The Katian William Lake and Airport Caryocaris (Briggs et al. 2016). Rare brachiopods and ‘small Cove biotas of Manitoba, Canada include linguloids, conodonts, phosphatic masses’ (Hannibal & Feldmann 1997), which may eurypterids and xiphosurids, although the proportions differ represent bromalites, are also present. Thus the biota, although between them (Young et al. 2007, Fig. 2). The Big Hill biota of incompletely known, shows some similarities with the Winneshiek Michigan yields a similar suite of taxa (Lamsdell et al. 2017). These assemblage. We are unaware of detailed reports of fossils associated lithologies are predominantly carbonate, however, in contrast with with other coeval Ordovician impact sites in North America Winneshiek, but the shared faunal components reflect a shallow (Schmieder et al. 2015), at East Clearwater Lake in Québec, marine marginal setting (Young et al. 2007). There are also Glasford in Illinois and Glover Bluff in Wisconsin. However, it is similarities between the Winneshiek Shale and the Hirnantian Soom clear that impact structures represent a potential source of other Shale of South Africa, even though it was deposited in a cold water exceptionally preserved fossils (Box 1). shallow marine setting on Gondwana (Gabbott et al. 2016). The Soom Shale biota is preserved in a laminated mudstone and is Conclusions dominated by conodonts, some giant, and includes a eurypterid, phyllocarid and ostracods, as well as linguloid brachiopods and The large number of Burgess Shale-type Konservat-Lagerstätten in bromalites (Aldridge et al. 2006), but a diversity of shelly taxa is Cambrian rocks provides unparalleled evidence of the also present. Thus the similarities between the Winneshiek Shale during that period, which coincides with the Cambrian radiation. and these other Ordovician Konservat-Lagerstätten reflect excep- The discovery of the Fezouata formations of Morocco, with their tional preservation combined with atypical marine settings, but do similar preservation of soft-bodied organisms (Van Roy et al. not imply equivalent environments. 2015), extended this record into the Early Ordovician (Lefebvre The Winneshiek Konservat-Lagerstätte occurs in a very unusual et al. 2018). The nature of the animals represented in the Fezouata setting, but it is unlikely to be unique. Other examples of biota suggests that the Cambrian radiation and Great Ordovician exceptional preservation originally thought to be one of a kind Biodiversification Event are more of a continuum than previously have been shown to occur in multiples following further thought. Thereafter the fossil record of soft-bodied taxa is much less exploration: examples include the Cambrian Burgess Shale rich, perhaps as a result of the closure of the taphonomic window (Collins et al. 1983) and Ordovician Beecher’s Trilobite Bed that favoured Burgess Shale-type preservation (Gaines et al. 2012). (Farrell et al. 2009). The Decorah impact structure is one of a It has been suggested that the Mid Ordovician asteroid break-up and number of meteorite craters of similar age in North America (Liu meteor shower, which resulted in the Decorah impact structure, et al. 2009; Schmieder et al. 2015): could others also host might have promoted the Great Ordovician Biodiversification Event exceptionally preserved fossils? The Middle Ordovician Rock Elm by disrupting communities (Schmitz et al. 2008). There is no structure in Wisconsin is about the same size as the Decorah obvious mechanism to explain such a relationship on a global scale, structure. The Rock Elm Shale, which is confined to the crater, has however, and biodiversification during the Darriwilian is only one yielded brachiopods, gastropods, trilobites, crustaceans and poly- component of a longer and more complex Great Ordovician chaetes (as scolecodonts) as well as conodonts (Peters et al. 2002; Biodiversification Event (Servais & Harper 2018). The Winneshiek French et al. 2004). This fauna, which has received little attention, is Konservat-Lagerstätte is important because it is Middle Ordovician apparently more diverse and includes more normal marine taxa in age, an interval when non-shelly fossils are poorly represented in (French et al. 2004) than the Winneshiek Shale; no evidence of the fossil record. Some of the Winneshiek Shale arthropods are of exceptional preservation has been reported. fundamental stratigraphic and phylogenetic importance, including The Ames structure in NW Oklahoma (Johnson & Campbell the earliest eurypterid, a basal chelicerate and the earliest ceratiocarid 1997; Koeberl et al. 2001) is larger than the Decorah impact phyllocarid. The conodonts provide new evidence of apparatus structure, up to 15 km in diameter, and is buried 2.75 km below the architecture 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 structures should be targeted? Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

Middle Ordovician Winneshiek biota the potential importance of impact craters as a novel context for Collette, J.H. & Hagadorn, J.W. 2010. The early evolution of phyllocarid – exceptional preservation. Borehole samples from the coeval Ames arthropods: phylogeny and systematics of Cambrian archaeostra- cans. Journal of Paleontology, 84, 795–820. structure in Oklahoma contain similar fossils, and sedimentary Colhoun, E.A. & van de Geer, G. 1998. Pollen analysis of 0–20m at Darwin sequences in other depositional basins created by impacts await Crater, western Tasmania, Australia. International Project on Paleolimnology exploration with the insights provided by the Decorah discovery. and Late Cenozoic Climate, Newsletter, 11,68–89. Collins, D., Briggs, D. & Conway Morris, S. 1983. New Burgess Shale fossil sites Box 2 lists some of the questions yet to be answered about the reveal Cambrian faunal complex. Science, 222, 163–167. Winneshiek biota and the nature, history and significance of the Cook, A., Magee, J., Roberts, K., Douglas, K., O’Callaghan, K. & Sanderson, Decorah impact structure. R. 2012. Australia’s Fossil Heritage. A Catalogue of Important Australian Fossil Sites. Australian Heritage Council. CSIRO Publishing, Clayton, Victoria. Acknowledgements Steve and Jane Hildebrand, landowners, provided Farrell, Ú.C, Martin, M.J., Hagadorn, J.W., Whiteley, T. & Briggs, D.E.G. 2009. access to the outcrop and drill site. S.M. Bergström, D.H. Campbell, A. Ferretti, Beyond Beecher’s Trilobite Bed: widespread pyritization of soft-tissues in the B.M. French, T.H.P. Harvey, A.D. Hawkins, J. Lamsdell, X. Liu, K. McVey, Late Ordovician Taconic Foreland Basin. Geology, 37, 907–910. A.D. Muscente, H. Nowak, B. Schmitz, T. Servais, F. Terfelt, Y. Wang, S. Xiao French, B.M., Cordua, W.S. & Plescia, J.B. 2004. The Rock Elm meteorite and P.A. Zippi collaborated on different aspects of the Winneshiek impact structure, Wisconsin: geology and shock metamorphic effects in research. R. Rowden, T. Marshall, C. Davis, D. Campbell and C. Monson quartz. Geological Society of America Bulletin, 116, 200–218. assisted in the excavation of the Winneshiek Shale outcrop. M. Spencer, French, B.M., McKay, R.M., Liu, H.P., Briggs, D.E.G. & Witzke, B.J. 2018. The J. McHugh, E. Wilberg, H. Zou, M. Tibbits, W. Cao, J. Matzke, K. McVey, Decorah structure, northeastern Iowa: geology and evidence for formation by D. McKay, M. Behling, B. Dye, J. Hansen, B. Neale, C. Kuempel, E. Greaves, meteorite impact. GSA Bulletin, first published online August 8, 2018, https:// K. Parson and T. Maher helped in the laboratory with sample preparation and the doi.org/10.1130/B31925.1 search for fossils. T. Adrain of the University of Iowa Paleontology Repository Gabbott, S.E., Browning, C., Theron, J.N. & Whittle, R.J. 2016. The late curated specimens. C. Kohrt, Geospatial Administrator, Iowa Department of Ordovician Soom Shale Lagerstätte: an extraordinary post-glacial fossil and Natural Resources, provided the map of the Bedrock Geology of Northeastern sedimentary record. Journal of the Geological Society, London, 174,1–9, Iowa and L. Bonneau, Yale Center for Earth Observation, assisted with utilizing https://doi.org/10.1144/jgs2016-076 the data. M. Mohanta and E. Martin (Yale University) prepared the figures. The Gaines, R.R., Hammarlund, E.U. et al. 2012. Mechanism for Burgess Shale type manuscript benefited from constructive comments from Joe Botting and an preservation. Proceedings of the National Academy of Sciences of the USA, anonymous reviewer. 109, 5180–5184. Gorthner, A. 1992. Bau, Funktion und Evolution komplexer Gastropodenschalen in Langzeit Seen. Stuttgarter Beiträge zur Naturkunde B, 190,1–173. Funding This work was funded by National Science Foundation grants Gradstein, F.M., Ogg, J.G., Schmitz, M.D. & Ogg, G.M. 2012. The Geologic 0922054 and 0921245. Time Scale 2012. Elsevier, Amsterdam. Gurov, E.P., Kelley, S.P., Koeberl, C. & Dykan, N.I. 2006. Sediments and impact Scientific editing by Philip Donoghue rocks filling the Boltysh impact crater. In: Cockell, C., Koeberl, C. & Gilmour, I. (eds) Biological Processes Associated with Impact Events. Springer, Heidelberg, 335–358. Gutiérrez-Marco, J.C., Sá, A.A., García-Bellido, D.C., Rábano, I. & Valério, M. References 2009. Giant trilobites and trilobite clusters from the Ordovician of Portugal. Aldridge, R.J., Briggs, D.E.G., Clarkson, E.N.K. & Smith, M.P. 1986. The Geology, 37, 443–446. affinities of conodonts – new evidence from the Carboniferous of Edinburgh, Hannibal, J.T. & Feldmann, R.M. 1997. Phyllocarid crustaceans from a Middle Scotland. Lethaia, 19, 279–291. Ordovician black shale within the Ames Structure, northwest Oklahoma. In: Aldridge, R.J., Briggs, D.E.G., Smith, M.P., Clarkson, E.N.K. & Clark, N.D.L. Johnson, K.S. & Campbell, J.A. (eds) Ames Structure in Northwest Oklahoma 1993. The anatomy of conodonts. Philosophical Transactions of the Royal and Similar Features: Origin and Petroleum Production (1995 Symposium). Society, London, B340, 405–421. Oklahoma Geological Survey Circular, 100, 370–373. Aldridge, R.J., Gabbott, S.E., Siveter, L.J. & Theron, J.N. 2006. Bromalites from Harzhauser, M. & Mandic, O. 2008. Neogene lake systems of central and south- the Soom Shale Lagerstätte (Upper Ordovician) of South Africa: palaeoeco- eastern Europe: faunal diversity, gradients and interrelations. Palaeogeography, logical and palaeobiological implications. Palaeontology, 49, 857–871. Palaeoclimatology, Palaeoecology, 260,417–434. Allison, P.A. & Briggs, D.E.G. 1991. Taphonomy of non-. In: Haug, C., Briggs, D.E.G., Mikulic, D.G., Kluessendorf, J. & Haug, J.T. 2014. Allison, P.A. & Briggs, D.E.G. (eds) Taphonomy: Releasing the Data Locked The implications of a and other thylacocephalan crustaceans for the in the Fossil Record. Plenum Press, New York, 25–70. functional morphology and systematic affinities of the group. BMC Allison, P.A. & Briggs, D.E.G. 1993. Exceptional fossil record: distribution of Evolutionary Biology, 14, 159. soft-tissue preservation through the Phanerozoic. Geology, 21, 527–530. Hawkins, A.D., Liu, H.P., Briggs, D.E.G., Muscente, A.D., McKay, R.M., Bergström, S.M., Schmitz, B., Liu, H.P., Terfelt, F. & McKay, R.M. 2018. High- Witzke, B.J. & Xiao, S. 2018. Taphonomy and biological affinity of three- 13 resolution d Corg chemostratigraphy links the Decorah impact structure and dimensionally phosphatized bromalites from the Middle Ordovician Winneshiek Konservat-Lagerstätte to the Darriwilian (Middle Ordovician) Winneshiek Lagerstätte, northeastern Iowa, USA. Palaios, 33,1–15. global peak influx of . Lethaia, first published online April 17, 2018, Hearing, T.W., Legg, D.A. et al. 2016. Survival of Burgess Shale-type animals in https://doi.org/10.1111/let.12269 a Middle Ordovician deep-water setting. Journal of the Geological Society, Botting, J.P. 2005. Exceptionally well-preserved Middle Ordovician sponges London, 173, 628–633, https://doi.org/10.1144/jgs2015-131 from the Llandegley Rocks Lagerstätte, Wales. Palaeontology, 48, 577–617. Hilgendorf, F. 1867. Über Planorbis multiformis im Steinheimer Süßwasserkalk. Botting, J.P. & Muir, L.A. 2012. Fauna and ecology of the holothurian bed, Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Llandrindod, Wales, UK (Darriwilian, Middle Ordovician) and the oldest Berlin, 1866, 474–504. articulated holothurian. Palaeontologia Electronica, 15.1.9a, 28. Howard, K.T. & Haines, P.W. 2007. The geology of Darwin Crater, western Braddy, S.J. 2001. Eurypterid palaeoecology: palaeobiological, ichnological and Tasmania, Australia. Earth and Planetary Science Letters, 260, 328–339. comparative evidence for a ‘mass–moult–mate’ hypothesis. Palaeogeography, Johnson, K.S. & Campbell, J.A. (eds) 1997. Ames Structure in Northwest Palaeoclimatology, Palaeoecology, 172,115–132. Oklahoma and Similar Features: Origin and Petroleum Production (1995 Brett, C.E., Whiteley, T.E., Allison, P.A. & Yochelson, E.L. 1999. The Walcott- Symposium). Oklahoma Geological Survey Circular, 100,1–396. Rust Quarry: Middle Ordovician trilobite Konservat-Lagerstätten. Journal of King, D.T., Petruny, L.W. & Neathery, T.L. 2006. Paleobiologic effects of the Paleontology, 73, 288–305. Late Cretaceous Wetumpka marine impact, a 7.6-km-diameter impact Briggs, D.E.G. 1992. Conodonts – a major extinct group added to the vertebrates. structure, Gulf Coastal Plain, USA. In: Cockell, C., Koeberl, C. & Gilmour, Science, 256, 1285–1286. I. (eds) Biological Processes Associated with Impact Events. Springer, Briggs, D.E.G. 2003. The role of decay and mineralization in the preservation of Heidelberg, 121–142. soft-bodied fossils. Annual Review of Earth and Planetary Sciences, 31, Koeberl, C., Reimold, W.U. & Kelley, S.P. 2001. Petrography, geochemistry, and 275–301. argon-40/argon-39 ages of impact-melt rocks and from the Ames Briggs, D.E.G., Clarkson, E.N.K. & Aldridge, R.J. 1983. The conodont animal. impact structure, Oklahoma: The Nicor Chestnut 18–4 drill core. Meteoritics Lethaia, 16,1–14. and Planetary Science, 36, 651–669. Briggs, D.E.G. & Wilby, P.R. 1996. The role of the calcium carbonate-calcium Korochantseva, E.K., Trieloff, M. et al. 2007. L-chondrite asteroid breakup tied phosphate switch in the mineralization of soft-bodied fossils. Journal of the to Ordovician meteorite shower by multiple isochron 40Ar–39Ar dating. Geological Society, London, 153, 665–668. Meteoritics and Planetary Science, 42, 113–130. Briggs, D.E.G., Liu, H., McKay, R.M. & Witzke, B.J. 2016. Bivalved arthropods Lamsdell, J.C., Briggs, D.E.G., Liu, H.P., Witzke, B.J. & McKay, R.M. 2015a. A from the Middle Ordovician Winneshiek Lagerstätte, Iowa, USA. Journal of new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) Paleontology, 89, 991–1006. reveals the ground plan of eurypterids and chasmataspidids. The Science of Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.X. 2013. The International Nature, 102,1–8. Commission on Stratigraphy International Chronostratigraphic Chart. Lamsdell, J.C., Briggs, D.E.G., Liu, H.P., Witzke, B.J. & McKay, R.M. 2015b. Episodes, 36, 199–204. The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021

D.E.G. Briggs et al.

megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evolutionary Repetski, J.E. 1997. Conodont age constraints on the Middle Ordovician black Biology, 15,1–31. shale within the Ames Structure, Major County, Oklahoma. In: Johnson, K.S. Lamsdell, J.C., LoDuca, S.T., Gunderson, G.O., Meyer, R.C. & Briggs, D.E.G. & Campbell, J.A. (eds) Ames Structure in Northwest Oklahoma and Similar 2017. A new Lagerstätte from the Late Ordovician Big Hill Formation, Upper Features: Origin and Petroleum Production (1995 Symposium). Oklahoma Peninsula, Michigan. Journal of the Geological Society, London, 174,18–22, Geological Survey Circular, 100, 363–369. https://doi.org/10.1144/jgs2016-059 Schmieder, M., Schwarz, W.H., Trieloff, M., Tohver, E., Buchner, E., Hopp, J. & Lefebvre, B., Gutiérrez-Marco, J.C. et al. 2018. Age calibration of the Lower Osinski, G.R. 2015. New 40Ar/39Ar dating of the Clearwater Lake impact Ordovician Fezouata Lagerstätte, Morocco. Lethaia, 51, 296–311. structures (Québec, Canada) – not the binary asteroid impact it seems? Lindskog, A., Costa, M.M., Rasmussen, C.M.Ø., Connelly, J.N. & Eriksson, Geochimica et Cosmochimica Acta, 148, 304–324. M.E. 2017. Refined Ordovician timescale reveals no link between asteroid Schmitz, B.S., Harper, D.A.T. et al. 2008. Asteroid breakup linked to the Great breakup and biodiversification. Nature Communications, 8, 14066. Ordovician Biodiversification Event. Nature Geoscience, 1,49–52. Liu, H.P., McKay, R.M., Young, J.N., Witzke, B.J., McVey, K.J. & Liu, X. Servais, T. & Harper, D.A.T. 2018. The Great Ordovician Biodiversification 2005. A new soft-bodied Middle Ordovician fauna from the St. Peter Event (GOBE): definition, concept and duration. Lethaia, 51, 151–164. Sandstone in northeast Iowa. Geological Society of America, Abstracts Størmer, L. 1951. A new eurypterid from the Ordovician of Montgomeryshire, with Programs, 37, 116. Wales. Geological Magazine, 88, 409–422. Liu, H.P., McKay, R.M., Young, J.N., Witzke, B.J., McVey, K.J. & Liu, X. 2006. Tetlie, O.E. 2007. Distribution and dispersal history of Eurypterida (Chelicerata). A new Lagerstätte from the Middle Ordovician St. Peter Formation in Palaeogeography, Palaeoclimatology, Palaeoecology, 252, 557–574. northeast Iowa, USA. Geology, 34, 969–972. Tetlie, O.E., Brandt, D.S. & Briggs, D.E.G. 2008. Ecdysis in sea scorpions Liu, H.P., McKay, R.M., Witzke, B.J. & Briggs, D.E.G. 2009. The (Chelicerata: Eurypterida). Palaeogeography, Palaeoclimatology, Winneshiek Lagerstätte, Iowa, USA and its depositional environments. Palaeoecology, 265, 182–194. Geological Journal of China Universities, 15, 285–295 [in Chinese with Vannier, J., Chen, J.-Y., Huang, D.-Y., Charbonnier, S. & Wang, X.-Q. 2006. English summary]. The Early Cambrian origin of thylacocephalan arthropods. Acta Liu, H., Witzke, B.J., Briggs, D.E.G. & McKay, R.M. 2011. Primitive fish from Palaeontologica Polonica, 51, 201–214. the Middle Ordovician Winneshiek Lagerstätte of northeast Iowa. Geological Van Roy, P., Briggs, D.E.G. & Gaines, R.R. 2015. The Fezouata fossils of Morocco; Society of America, Abstracts with Programs, 43, 545. an extraordinary record of marine life in the Early Ordovician. Journal of the Liu, H.P., Bergström, S.M., Witzke, B.J., Briggs, D.E.G., McKay, R.M. & Geological Society, London, 172, 541–549, https://doi.org/10.1144/jgs2015-017 Ferretti, A. 2017. Exceptionally preserved conodont apparatuses with giant Vrazo, M.B. & Braddy, S.J. 2011. Testing the ‘mass-moult-mate’ hypothesis of jaw-like elements from the Middle Ordovician Winneshiek Konservat- eurypterid palaeoecology. Palaeogeography, Palaeoclimatology, Lagerstätte, Iowa, USA. Journal of Paleontology, 91, 493–511. Palaeoecology, 311,63–73. Martill, D.M. 1988. Preservation of fish in the Cretaceous Santana Formation of Witzke, B.J. 1990. Paleoclimatic constraints for Palaeozoic paleolatitudes of Brazil. Palaeontology, 31,1–18. Laurentia and Euramerica. In: McKerrow, W.S. & Scotese, C.R. (eds) McKay, R.M., Liu, H., Witzke, B.J., French, B.M. & Briggs, D.E.G. 2011. Palaeozoic Palaeogeography and Biogeography. Geological Society, Preservation of the Middle Ordovician Winneshiek Shale in a probable impact London, Memoirs, 12,57–73. crater. Geological Society of America, Abstracts with Programs, 43, 189. Witzke, B.J., Ludvigson, G.A. et al. 1998. Bedrock Geology of Northeast Iowa, Muscente, A., Schiffbauer, J.D., Broce, J., Laflamme, M. et al. 2017. Digital Geologic Map of Iowa, Phase 2: Northeast Iowa. Iowa Geological Exceptionally preserved fossil assemblages through geologic time and Survey, Open File Maps, 1998-7. space. Gondwana Research, 48, 164–188. Witzke, B.J., McKay, R.M., Liu, H.P. & Briggs, D.E.G. 2011. The Middle Nowak, H., Harvey, T.H.P., Liu, H.P., McKay, R.M., Zippi, P.A., Campbell, D.H. Ordovician Winneshiek Shale of northeast Iowa – correlation and paleogeo- & Servais, T. 2017. Filamentous eukaryotic algae with a possible graphic implications. Geological Society of America, Abstracts with cladophoralean affinity from the Middle Ordovician Winneshiek Lagerstätte Programs, 43, 315. in Iowa, USA. Geobios, 50, 303–309. Wolter, C.F., McKay, R.M., Liu, H., Bounk, M.J. & Libra, R.D. 2011. Geologic Nowak, H., Harvey, T.H.P., Liu, H.P., McKay, R.M. & Servais, T. 2018. Mapping for Water Quality Projects in the Upper Iowa River Watershed. Iowa Exceptionally preserved arthropodan from the Middle Geological Survey, Technical Information Series, 54, 34. Ordovician Winneshiek Lagerstätte, Iowa, USA. Lethaia, 51, 267–276. Young, J.N., McKay, R.M. & Liu, H.P. 2005. Unusual sections of the Readstown Peters, C.W., Middleton, M.D. & Cordua, W.S. 2002. The paleontology of the Member, St. Peter Formation, at Decorah, northeast Iowa. Geological Society : Pierce County, Wisconsin. Geological Society of of America, Abstracts with Programs, 37, 78. America, Abstracts with Programs, 34, A95. Young, G.A., Rudkin, D.M., Dobrzanski, E.P., Robson, S.P. & Nowlan, G.S. Pirrung, M., Büchel, G. & Jakoby, W. 2001. The Tertiary volcanic basins of 2007. Exceptionally preserved Late Ordovician biotas from Manitoba, Eckfeld, Enspel and Messel (Germany). Zeitschrift der Deutschen Canada. Geology, 35, 883–886. Geologischen Gesellschaft, 152,27–59. Zippi, P.A. 2011. Palynological analysis of Middle Ordovician Winneshiek Shale Rasser, M.W. 2014. Evolution in isolation: the Gyraulus species flock from samples from Iowa. Unpublished report prepared for the Iowa Department of Miocene Lake Steinheim revisited. Hydrobiologia, 739,7–24. Natural Resources, Iowa Geological and Water Survey, Iowa City.