Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA

Journal Pre-proof Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA

Andrew J. Wendruff, Loren E. Babcock, Joanne Kluessendorf, Donald G. Mikulic PII:

S0031-0182(20)30075-4

DOI:

https://doi.org/10.1016/j.palaeo.2020.109631

Reference:

PALAEO 109631

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date:

23 July 2019

Revised date:

30 January 2020

Accepted date:

4 February 2020

Please cite this article as: A.J. Wendruff, L.E. Babcock, J. Kluessendorf, et al., Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA, Palaeogeography, Palaeoclimatology, Palaeoecology (2020), https://doi.org/10.1016/j.palaeo.2020.109631

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© 2020 Published by Elsevier.

Journal Pre-proof Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA

Andrew J. Wendruff, Loren E. Babcock, Joanne Kluessendorf, and Donald G. Mikulic

Andrew J. Wendruff, Department of Biology and Earth Science, Otterbein University, Westerville, Ohio 43081, USA [[email protected]]; Loren E. Babcock, Department of

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Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA [[email protected]];

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Donald G. Mikulic, Weis Earth Science Museum, University of Wisconsin-Fox Valley, Menasha,

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Wisconsin 54952, USA [[email protected]]; Joanne Kluessendorf, Weis Earth Science

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Museum, University of Wisconsin-Fox Valley, Menasha, Wisconsin 54952, USA

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[[email protected]]

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Abstract

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The Silurian (Llandovery, Telychian) Waukesha Lagerstätte in the Brandon Bridge

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Formation of Wisconsin, USA, is preserved in a dolomitic plattenkalk representing deposition in a shallow-water sediment trap on a carbonate platform. Renewed study of the deposit demonstrates biodiversity far richer than previously reported and includes both biomineralizing and non-biomineralizing organisms. At present, remains of at least 12 animal phyla are known, in addition to algae and microorganisms. Of these, remains of cnidarians, brachiopods, mollusks and echinoderms were previously unknown from this deposit. The macrobiota is dominated by trilobites, graptolites, conulariids, non-biomineralizing arthropods (some of which were previously unreported), and some worms (palaeoscolecids and annelids). New abundant evidence

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Journal Pre-proof of microbial mats and associated microbial structures indicates a thriving microorganismal community. Evidence, including remnants of preserved microbes on macrofossils, demonstrates that microbial processes played a leading role in both burial and preservation of nonbiomineralizing tissues. This process, referred to as microbial entombment, is a biologically mediated sedimentary process by which organic remains are entombed within a microbial mat.

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Keywords

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Exceptional preservation; Silurian; Lagerstätte; soft-bodied preservation; microbial mat;

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entombment.

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1. Introduction

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The Brandon Bridge Formation of southeastern Wisconsin, USA (Figs. 1, 2) hosts the Waukesha Biota, an important Silurian Konservat-lagerstätte yielding a diverse assemblage

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(Mikulic et al., 1985a, b; Wendruff, 2016; Wendruff et al., 2016) of arthropods (Figs. 3–5) and

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numerous other organisms including non-biomineralized taxa (Figs. 6–8). The biota includes members characteristic of the Silurian Laurentian platform (Weller, 1900; Weller, 1907; Feldman, 1989; Peters and Bork, 1999) but also has organisms that could be considered ‘holdovers’ better known from Cambrian Burgess Shale-type deposits (e.g., lobopodians). Some taxa, including a conodont retaining soft tissues (Smith et al., 1987), a noncalcified alga (LoDuca et al., 2003), a synziphosurine (Moore et al., 2005), an early thylacocephalan crustacean (Haug et al., 2014) and three species of Ceratiocaris (Jones et al., 2016), have been described and descriptions of others are in preparation. Biomineralizing taxa, apart from trilobites, are rare.

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Journal Pre-proof Palaeoscolecid worms, great-appendage arthropods, lobopodians and a chordate (possibly nonconodont) comprise some of the non-biomineralized fauna. Other notable taxa include an early arachnid and a leech. Several ‘worm’ and arthropod species preserve digestive tracts. The Waukesha Biota is among the earliest Silurian Konservat-lagerstätten (Wendruff et al., 2020), and therefore provides an important taphonomic window soon after the end-Ordovician extinction.

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In this paper, we address the overall biotic composition of the Waukesha Lagerstätte and

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explore its taphonomic history. New information highlights the importance of microbial

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processes in promoting entombment and preservation in the deposit. We document here for the

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first time a number of biomineralizing animals common to Silurian platform carbonate deposits but previously thought to have been absent from the Waukesha biota. They are cnidarians,

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brachiopods, cephalopods, and echinoderms. New non-biomineralizing animals from the

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Waukesha biota that are first documented here include lobopodians, new arthropods, polychaete worms and non-conodont chordates. A recently described scorpion (Wendruff et al. 2020) is also

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included here. Preserved gut tracts are illustrated from various Silurian trilobites for the first

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time. Preserved gut tracts are also illustrated from several non-biomineralizing arthropods and polychaetes. Trace fossils, microbial structures and body fossils of microbes are illustrated for the first time from the Waukesha Lagerstätte.

2. Significance of the Waukesha Lagerstätte

Konservat-lagerstätten (or simply ‘Lagerstätten’) are deposits yielding exceptionally preserved fossils (Seilacher, 1970). Exceptional preservation commonly refers to fossils in which

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Journal Pre-proof non-biomineralized parts, such as chitin (arthropod cuticle) and internal organs are preserved, not merely hard parts such as bones, teeth and shells. Hard part preservation dominates much of the fossil record (Kidwell and Flessa, 1995), and exceptional preservation is a relatively uncommon occurrence. A large proportion of Holocene marine ecosystems comprises organisms lacking hard tissues with poor preservational potential (Conway Morris, 1986). For this reason, Konservat-lagerstätten provide glimpses into biotic diversity that is commonly lost from the

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stratigraphic record (Conway Morris, 1986, Conway Morris et al., 1987; Conway Morris, 1989;

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Butterfield, 1995). These deposits (e.g., Burgess Shale, Chengjiang, Hunsrück Slate, Mazon

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Creek and Solnhofen) are celebrated and intensely studied because they provide important

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glimpses into, among other things, evolutionary history, paleoecology, paleobiology, paleoenvironments and the processes of fossilization (taphonomy). Some Lagerstätten have

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attracted an enormous amount of interest because they offer insight into the evolution of animals

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during a critical time in Earth history.

Two intervals of the Paleozoic have a disproportionately large number of non-

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concretionary marine Lagerstätten, the Cambrian and Silurian (e.g., Allison and Briggs, 1993;

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Bottjer et al., 2002b; Muscente et al., 2017). With few exceptions (e.g., eurypterids; Clarke and Ruedemann, 1912; Andrews et al., 1974; Kluessendorf, 1994; Ciurca and Tetlie, 2007; Tetlie et al, 2007; Vrazo et al., 2016), exceptionally preserved fossils from Silurian deposits are not as well-known and are less studied than those of the Cambrian. Strictly from a large-scale perspective, Cambrian and Silurian Lagerstätten share some interesting similarities. A number of Burgess Shale-type taxa are present in the Waukesha Lagerstätte including a number of nonbiomineralizing arthropods, lobopodians and palaeoscolecid ‘worms.’ Similar to the Cambrian deposits, those of the Silurian record biotic events during a critical interval of Earth history: it

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Journal Pre-proof was the time during which, for example, jawed fishes evolved, the Mid-Paleozoic Marine Revolution (Signor and Brett, 1984) was in early stages, coral-stromatoporoid reefs became common and widespread, vascular land plants were evolving, and arthropods first became fully terrestrial. Kluessendorf (1994) provided an analysis of North American Silurian Konservatlagerstätten and concluded that they have a predictable pattern of occurrence and provide

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information about the less commonly preserved components of Silurian epeiric seas. In contrast

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with the Cambrian, many Silurian Lagerstätten are in dolomitic carbonate platform deposits,

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including plattenkalk-type deposits (Wendruff et al., 2020). Some examples of Silurian

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Lagerstätten have obrution (or sediment-smothered) beds (see Brett et al., 1997; Brett and Seilacher, 1991) and relatively few non-biomineralized taxa (e.g., Rochester and Waldron

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shales). Exceptional preservation is commonly attributed to dysoxic or anoxic conditions or

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sediment smothering in the Cambrian (e.g., Whittington, 1971; Conway Morris, 1986; Caron and Jackson, 2006; Gaines, 2014). Predominantly, these Silurian Lagerstätten of the paleocontinent

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Laurentia occur in shallow carbonate platform settings (Lowenstam, 1957; Kluessendorf, 1994),

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including tidal flats, sabkhas, and inter-reef patches. Preservational mechanisms responsible for exceptional preservation in the Silurian may have occurred locally, involved fluctuating environmental conditions, even perhaps time-specific facies (compare Brett et al., 2012a; Babcock et al., 2015). The Waukesha deposit is one of a number of Konservat-lagerstätten preserved in lime mudstone facies of the Great Lakes region (Kluessendorf, 1994; LoDuca and Brett, 1997; von Bitter et al., 2007). They include sites in the Lockport Group (New York; Brett et al., 1995), Bertie Formation or Group (New York and Ontario; Clarke and Ruedemann, 1912; Caron et al.,

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Journal Pre-proof 2004), Eramosa Formation (Ontario; von Bitter et al., 2007), Lecthaylus Shale (Illinois; Lowenstam, 1948), Kokomo Limestone (Indiana; Clarke and Ruedemann, 1912) and Pointe-auxChenes Shale (Michigan; Alling and Briggs, 1961). These Lagerstätten yield diverse organisms, including some that were lightly and non-biomineralized, and add considerably to the overall known biodiversity of the Silurian.

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3. Material and methods

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This study is based mostly on specimens collected from dolostone of the Waukesha

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Lagerstätte in the Brandon Bridge Formation (Silurian), Waukesha Lime and Stone Company quarry, Waukesha, Wisconsin (see Mikulic et al., 1985a, b; Kluessendorf and Mikulic, 1996;

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Wendruff, 2016); 43.04° N, 88.21° W. Specimens were collected mostly in situ. Additional

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specimens were collected from blast piles in the Franklin Aggregate quarry, Franklin, Wisconsin (42.91° N, 87.99° W), which exposes the same unit. Specimens illustrated in this paper that are

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the prefix UWGM.

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deposited in the University of Wisconsin Geology Museum in Madison, Wisconsin, USA bear

Specimens were photographed with a Canon EOS Rebel T3i Digital SLR with a Canon MP-E 65 mm macro lens and full spectrum lighting. Images were stitched using Adobe Photoshop CC. Some samples were studied by Scanning Electron Microscopy (SEM) using a FEI Quanta Field Emission (FEQ) SEM located in the School of Earth Sciences at The Ohio State University.

4. Geological setting and stratigraphy

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4.1 Geological setting

During the Silurian Period, present-day Wisconsin was part of the Laurentian paleocontinent, which lay astride the equator and was largely covered by epeiric seas. The present-day Great Lakes region was characterized by carbonate platforms and shallow

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epicontinental basins. Extensive reef and non-reef subtidal environments were developed (e.g.,

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Lowenstam, 1957; Ingels, 1963; Mikulic, 1987; Mikulic and Kluessendorf, 1998). Subtidal,

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intertidal and supratidal settings hosted microbial communities manifested as crusts, mats,

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microbialites and as interstitial components of the sediment (see Riding, 2000). Exceptionally preserved fossils from the Brandon Bridge Formation near Waukesha,

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Wisconsin, occur in finely laminated carbonate mudstone that was deposited along the western

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edge of the Michigan Basin (Kluessendorf and Mikulic, 1996). In a broader context, the Michigan Basin is an intracratonic sedimentary basin bounded by the Wisconsin Dome along the

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west and the Kankakee Arch along the southwest (Howell and van der Pluijm, 1990: fig. 1).

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Hundreds of fossilized reefs occur along the margin of the basin (Lowenstam, 1957). Inter-reef facies consist of argillaceous lime mudstone and tend to be rich in trilobites, brachiopods and rooting organisms such as benthic graptolites and pelmatozoans (Lowenstam, 1957; see also Weller, 1900; Weller, 1907; Lowenstam, 1948; Lowenstam, 1957; Feldman, 1989; Peters and Bork, 1999).

4.2 Stratigraphy

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Journal Pre-proof The Waukesha Lagerstätte consists predominantly of a dolomitized, rhythmically bedded plattenkalk. It is fine-grained and lacks a sugary texture. That, together with the observation that fossils are commonly preserved in great detail, suggests that the dolomitization was penecontemporaneous with sedimentation. Barthel et al. (1990) used the terms Flinz and Fäule to describe endmember petrographic variation in the Solnhofen (Jurassic) plattenkalk. We follow their terminology here. Flinz (plural, Flinzen) refers to sheets of pure micritic carbonate that

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often show conchoidal fracture. Bedding surfaces of the Flinz layers are not normally smooth,

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but slightly crenulated (probably due to the influence of microbial mats). Flinzen may represent

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relatively short-term event layers that became cemented through microbial processes during

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intervals of low siliciclastic sedimentation. Fäule (plural, Fäulen) refers to fossiliferous, shaley carbonate layers. Bedding surfaces of the Fäule layers are normally smooth. Fäulen may

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represent ‘background’ sedimentation, comprising mud-sized sediments of mixed carbonate-

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siliciclastic origin. Lithologies intermediate between Fäule and Flinz are designated by terms such as Blätterflinz (fissile Flinz) and zähe Fäule (tough or tenacious Fäule).

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In the Waukesha Lagerstätte, fossils are preserved predominantly in thinly laminated (mm

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scale), non-fissile, silt-sized dolomudstone (Flinz) or thinly laminated (< 1 mm thick), fissile, organic-rich, clay-sized argillaceous dolomudstone (Fäule). Overall, Flinz predominates in thickness and Fäule layers form thin intercalations. The two lithologies are readily distinguishable by color and occur in an alternating rhythmic-like pattern. Flinz layers are a light-grey to tan-grey. Fäule layers are commonly medium to dark grey or greenish-grey and rarely pinkish-grey. Both lithologies occur with lighter and darker sedimentary couplets, suggesting tidally influenced sedimentation. Intermediate lithologies, Blätterflinz and zähe Fäule, occasionally occur as well. Scattered thin accumulations of skeletal debris are present. Above the

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Journal Pre-proof most productive layers, the Brandon Bridge Formation commonly occurs as weakly laminated, crystalline dolomudstone (LoDuca et al., 2003). The Brandon Bridge Formation in southeastern Wisconsin (Fig. 1) has a maximum thickness of 8 m and is composed mostly of dolomudstone (Fig. 2). The Brandon Bridge overlies an unconformity (sequence boundary), which is manifested at the Waukesha Lime and Stone Company quarry as a scarp (Kluessendorf and Mikulic, 1996, fig. 3). The Brandon Bridge

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Formation wedges out over the Manistique Formation in southeastern Wisconsin and is overlain

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by the Waukesha Formation (Kluessendorf and Mikulic, 1996).

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Exceptional preservation occurs within an interval approximately 1 m thick. This is the

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Waukesha Lagerstätte. This interval begins approximately 2 m above the disconformity with the Manistique Formation (Kluessendorf and Mikulic, 1996). Most animal fossils described to date

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are from the lower 12 cm of this interval (e.g., Smith et al., 1987; Moore et al., 2005; Haug et al.,

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2014; Jones et al., 2016; Wendruff et al., 2020). In the upper part of the interval, exceptional preservation is limited primarily to non-calcified green algae and graptolites (LoDuca et al.,

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2003). A variety of sedimentary structures including rill marks, wrinkle structures, intraclasts,

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interference ripples, as well as Chondrites trace fossils, have been reported (Kluessendorf and Mikulic, 1996; LoDuca et al., 2003). Chondrites are common in offshore deposits, including in the Silurian, and small Chondrites are characteristically thought of as low-oxygen indicators (Bromley and Ekdale, 1984). Most of this work is based on material from the lower 12 cm of the Waukesha Lagerstätte in the Waukesha Lime and Stone Company west quarry, and unless otherwise specified, statements refer to this material. Supplementary information comes from material collected from Brandon Bridge Formation in the Franklin Aggregate quarry.

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Journal Pre-proof Collecting in the Franklin Aggregate quarry was less extensive than in the Waukesha Lime and Stone quarry (Fig. 1). Exceptionally preserved fossils are in blocks from an interval correlative with the lower part of the Brandon Bridge Formation at the Waukesha quarry. Exceptionally preserved fossils include a variety of non-biomineralized and lightly skeletonized organisms, some of which also occur in the Waukesha Lime and Stone quarry, though at Franklin they occur in more concentrated accumulations (Fig. 5G). Missing members include the

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trilobites. A chordate (non-conodont?) is present in the Franklin quarry (Figs. 8F, G) but is

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unknown from the Waukesha quarry. The most abundant sedimentary structures present include

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fine lamination and hummocky structures.

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The Konservat-lagerstätte layers were deposited as part of a transgressive systems tract (Kluessendorf and Mikulic, 1996). This episode of eustatic rise is attributed to partial melting of

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the Gondwanan Ice Sheet (see Spengler and Read, 2010). The Lagerstätte layers contain

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graptolites indicative of the Oktavites spiralis Zone including O. spiralis (Mikulic et al., 1985a, b; Saunders et al., 2009). Recent biostratigraphic assessment (Kleffner et al., 2018) of the

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Waukesha Lagerstätte shows that conodonts present include Pterospathodus eopennatus, the

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eponymous indicator of the P. eopennatus Superzone (Telychian Stage; c. 437.5-436.5 Ma).

5. Biota

More than 50 genera, some of which are undescribed, representing at least 12 phyla, are present in the Waukesha Biota (Figs. 3–8). The Waukesha Konservat-lagerstätte yields organisms typically ranging from 1 mm to 20 cm in maximum dimension. Whereas many Konservat-lagerstätten display greater biological diversity by preserving soft and hard tissue

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Journal Pre-proof (shelly) organisms, shelly organisms other than trilobites and conulariids are uncommon in the Waukesha Lagerstätte. The Waukesha biota seems to represent an assemblage of organisms, some of which were washed in from nearby areas, that was buried in a shallow-water sediment trap. Taxa present are a combination of forms typical of shallower, or more proximal, environmental settings (e.g., leperditocopid ‘ostracodes,’ crinoids, corals, and scorpions), and ones characteristic of ‘deeper,’ or more offshore settings (e.g., Arctinurus, dalmanitid trilobites,

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dendroid graptolites such as Oktavites, cephalopods, and Chondrites traces).

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Trilobites—At least 11 trilobite species are present in the Waukesha Biota (Figs. 3, 4, 9)

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Common forms are an undescribed dalmanitid, Stenopareia and a harpid (Scotoharpes?). Less

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common forms include Arctinurus, Meroperix, a calymenid, a phacopid, an otarionid, two odontopleurids and at least one questionable cheirurid. The dalmanitid, Stenoparaeia,

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Arctinurus, Meroperix and one odontopleurid are known from articulated specimens. Most

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articulated specimens show the dorsal surface, and few ventral views are known. Some of the trilobites preserve mineralized gut tracts. Digestive tracts are common in the undescribed

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dalmanitid (Figs. 3D, F) but are also known from Arctinurus (Fig. 3C) and Meroperix (Figs. 3A,

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B). Many of the trilobite exoskeletons appear to be decalcified leaving a ‘ghosted’ preservation often lacking significant details (e.g., Figs. 3A, 3C, 4B, 9H, 9I, 11). As suggested in the Discussion, this preservation style results from decalcification of carbonate skeletal material under a microbial biofilm. Articulated, usually outstretched, dalmanitid exoskeletons are common on some bedding planes (Figs. 3A, 9). As indicated in the Discussion, their occurrence in this manner suggests either gregarious behavior (compare Speyer and Brett, 1985; Robison and Babcock, 2011; Brett, 2015) or a patchy distribution resulting from post-mortem transport and concentration.

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Journal Pre-proof Non-trilobite arthropods—At least 14 non-trilobite arthropods are present (Fig. 5). They include synziphosurines (see Moore et al., 2005; Fig. 5L), a scorpion (see Wendruff et al., 2020; Fig. 5A), three species of Ceratiocaris (Jones et al., 2016; Fig. 5N), a thylacocephalan crustacean (Haug et al., 2014) and leperditicopid ‘ostracodes’ (Fig. 5H). Arthropods of uncertain affinity include three great-appendage arthropods, a myriapod-like animal, two enigmatic bivalved arthropods and a number of others, mostly represented by fragmentary material. Some of these

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taxa show preserved gut tracts (e.g., Fig. 5F).

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Lobopodians—At least two lobopodian taxa are present in the Waukesha Biota. They

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include a small ‘armored’ lobopodian with round dorsal ‘plates’ (Fig. 5E) and a large stout-

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limbed ‘xenusiid’ lobopodian (Fig. 5D). One lobopodian specimen shows a preserved gut tract (Fig. 5D).

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Problematica (?Cnidaria)—Two conulariids, Conularia niagarensis Hall, 1852 (Fig. 6B)

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and Metaconularia cf. manni Roy, 1935 (Fig. 6A), plus Sphenothallus are present. Sphenothallus is known from complete tubes (Figs. 6C) and holdfasts, which are attached to specimens of both

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conulariid species (Figs. 6A, B). Conulariids are among the best preserved shelly fossils, likely

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due to their phosphatic composition.

Brachiopods—Phosphatic inarticulate brachiopods are represented by a single known specimen (Fig. 6E). Several poorly preserved, undetermined articulate brachiopods, which originally had calcite shells, are present. They include possible orthids (Fig. 6D) and rhynchonellids (Fig. 6F). Cephalopods—Poorly preserved cephalopods are present in the Waukesha Biota (Figs. 6I, J). One form (Fig. 6I) shows longitudinal ridges, which are characteristic of the family Kionoceratinae (compare Holland, 2000: fig. 1h).

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Journal Pre-proof Cnidaria—A partial tabulate coral, possibly Favosites, is known from a single incomplete specimen (Fig. 6G). Worms—Several ‘worms’ with soft tissue are present, including at least two palaeoscolecids (Figs. 7D, E), an aphroditid polychaete (Fig. 7F), a spiny polychaete worm (Figs. 7G, H), a wide-bodied annelid (Fig. 7B), and a ‘leech’ (Figs. 7A, C). Scolecodonts also have been recovered (Mikulic et al., 1985a, b). Some worm specimens show preserved gut tracts

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(Figs. 7A, B, D, F, G).

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Hemichordates—Graptolites, often carbonized, are common in this biota. Taxa present

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include Oktavites spiralis (Fig. 8D), cf. Desmograptus (Fig. 8A), cf. Dictyonema (Fig. 8B),

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Thallograptus? sp. (Fig. 8E) and an undetermined benthic form with large isolated conical theca (Fig. 8C).

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Echinoderms—Disarticulated pelmatozoan ossicles are scattered (Fig. 10D) through a

crinoid is known (Fig. 6H).

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number of slabs from the Waukesha quarry, but are rare overall. An undetermined articulated

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Chordates—A partial conodont (‘conodont animal’) with an assemblage of phosphatized

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elements and decayed soft tissue was described from the Waukesha Lime and Stone quarry (Smith et al., 1987). The Franklin Aggregate quarry has produced numerous specimens of a wellpreserved chordate, which is unknown from the Waukesha quarry, that preserves a notochord, vshaped myomeres and caudal fin-like marginal structures (Figs. 8F, G). Conodont elements were not preserved in direct association with these specimens. The soft tissue remains of the Franklin Aggregate quarry chordate resembles the soft tissues of the conodont Clydagnathus windsorensis (Carboniferous; compare Sweet and Donoghue, 2001, fig. 3) and the cephalochordate Cathaymyrus diadems (Cambrian; compare Shu et al., 1996; Donoghue and Keating, 2014, fig.

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Journal Pre-proof 2e) in general form and in the presence of myomeres.

6. Microbial and sedimentary structures

Cracked sediment coated with an elephant skin texture is present (Figs. 10A, B) and indicates the buildup of microorganisms in a mat-influenced sedimentary environment (compare

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Bottjer and Hagadorn, 2007, fig. 4(a)-6b). Interpretation of the cracks is somewhat ambiguous;

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they could represent either mudcracks or syneresis cracks. The cracks usually are connected,

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continuous, and form polygonal patterns, which leads us to think that at least some were

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subaerially exposed. Some slabs show what appear to be microbially encrusted mud curls at their edges (Fig. 10A). Exceptional preservation is associated with some of these cracks in the

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Franklin quarry, but not in the Waukesha quarry.

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Additional microbial structures are visible in cross-sections of rock, and they include dark, crinkled laminae, wavy laminae, and domal structures (Figs. 10C, E; compare Pflüger, 1999, fig.

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6D). Torn and ruptured laminae (Fig. 10C) indicate the presence of microbial mats (compare

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Pflüger, 1999, fig. 6d; Schieber, 1999, fig. 5). Decay halos (Fig. 5D, E, M) around some nonbiomineralized and lightly skeletonized organisms indicate the former presence of microbial activity (see Borkow and Babcock, 2003). Possible microbial-induced gas escape structures (Fig. 10K) are also present (compare Dornbos et al., 2007, fig. 4(d)-2c, d; 4(d)-3b). Trace fossils are rare and observed only in the Fäule layers. The lower interval of the Waukesha Lagerstätte has an ichnofabric index (Droser and Bottjer, 1986) of 1. Vertical burrows are not evident. Traces observed are horizontal or bedding-plane-parallel. They include a few examples of Diplichnites (Fig. 10G). In addition, Gordia-like pyrite-infilled burrows (compare

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Journal Pre-proof Babcock and Peel, 2007) on the cuticle of a phyllocarid has been reported (Jones et al., 2016, figs. 2.2, 2.5).

7. Discussion

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7.1 Taphonomy and biotic composition

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The Waukesha Lagerstätte provides a somewhat skewed view of Silurian biodiversity.

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Animals characteristic of normal marine communities such as corals, echinoderms, bryozoans,

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brachiopods, and mollusks are rare or absent, and prior to this report, tabulate corals, echinoderms, brachiopods, and cephalopods were not known. Even at the time of this writing,

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rugose corals, bryozoans, gastropods, and bivalves are not known to be present in the Waukesha

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biota. The Waukesha fossils likely represent a death/molt assemblage (taphocoenosis), including some biotic elements washed in from nearby areas including deeper water. The Waukesha

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Lagerstätte favors preservation of organisms poorly known from other Silurian marine deposits

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and helps fill in our perception of biodiversity during the Silurian.

7.2 Taphonomic overprint

Fossils of the Waukesha Lagerstätte display a range of taphonomic styles that are related in part to the organisms’ original compositions and lithologies in which they are preserved. Typically, Fäule layers yield finer preservation of non-biomineralized and lightly skeletonized anatomy, whereas Flinz layers primarily yield biomineralizing organisms with occasional examples of poorly preserved non-biomineralized and lightly skeletonized arthropods. 15

Journal Pre-proof Preservation involving calcium phosphate is common in the Waukesha Lagerstätte, and organisms originally incorporating phosphate often preserve differently from those that were secondarily phosphatized. Organisms having phosphatic hard parts such as conulariids (Figs. 6A, B), an inarticulate brachiopod (Fig. 6E), annelid worms (Fig. 7) and ceratiocaridid phyllocarids (Fig. 5N) tend to be well-preserved and without apparent secondary mineral overgrowth. Commonly non-biomineralized organisms are compressed and secondarily phosphatized with an

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organic carbon film beneath (Fig. 5F). Secondary phosphatic overgrowths tend to be thicker and

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crinkled on non-biomineralized and lightly skeletonized organisms. Calcium phosphate also

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appears to have infilled and coated anatomical structures preserving but often distorting or

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obscuring morphological details (see Moore et al., 2005).

Organisms originally having calcium carbonate skeletons are commonly preserved as

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‘ghosted’ fossils, or they are preserved as molds. Trilobites are most numerous in the Fäule

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layers (Figs. 3D, 9), and exoskeletons tend to be preserved in various stages of decalcification (Mikulic et al., 1985b; Wendruff, 2016; herein Figs. 1A–C, 10H, I). Most examined specimens

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retain a thin carbonate overgrowth or film that is likely the result of carbonate remobilization

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(dissolution and reprecipitation) within or under a microbial mat (see Babcock and Peel, 2007; herein Figs. 10H, I). Trilobite exoskeletons also show a range of states of compression. Most are flattened and without marginal fractures related to sedimentary compaction. Trilobites possessed an exoskeleton composed of calcium carbonate with an external chitinous layer (Towe, 1973; Mutvei, 1981; Dalingwater and Mutvei, 1990; Wilmot, 1990). The absence of fractures on nearly flat trilobites suggests that carbonate was removed early in their fossilization history, leaving the more pliable chitin. Phosphatized gut tracts are present in some trilobites (Figs. 3A–D, F; Babcock et al., 2016), but preserved appendages are rare. Echinoderms are represented mostly by

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Journal Pre-proof disarticulated ossicles, preserved as molds (Fig. 10D). Mollusks (Fig. 6J) and articulate brachiopods (Figs. 6D, F) are mostly moldic. Organisms with carbonized preservation include graptolites, non-calcifying algae, ‘worms,’ and some arthropods. Especially at the Franklin Aggregate quarry, chordates, ‘worms’ and non-biomineralized arthropods are commonly preserved with carbon, and little or no

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phosphate is evident (Figs. 5G, I).

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7.3 Disarticulation and transportation

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Many of the examined Waukesha trilobites are articulated and fully outstretched. Rare specimens of the dalmanitid are in a loosely folded posture (Fig. 9B). Tightly enrolled trilobites

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from this locality have not been observed. It is possible that the dalmanitid lacked the capability

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for tight enrollment; however, another species of dalmanitid (from the Devonian of Illinois) has been documented showing tight enrollment and articulation of the vincular structures (Roy,

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1933, figs. 1, 4B; Levi-Setti, 1975, pls. 33, 34; Levi-Setti, 1993, pl. 49). Assemblages of

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outstretched, articulated dalmanitids, mostly of the same size class (holaspides), are common on some bedding planes (Figs. 3D, 9A). Most specimens have lost their limbs, antennae, and hypostomes, indicating that they were molts or corpses. The occurrence of dalmanitids in this manner suggests either gregarious behavior (compare Speyer and Brett, 1985; Robison and Babcock, 2011; Brett, 2015), or patchy distribution of transported and concentrated remains. Speyer and Brett (1985) described monospecific aggregations of trilobites from the Hamilton Group (Devonian) of New York as associations of molted exoskeletons shed in the context of mating behavior. Similarly, Robison and Babcock (2011) attributed monospecific aggregations

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Journal Pre-proof of trilobites from the Cambrian of Utah to the same process. Following molting, exoskeletons were rapidly buried under distal tempestite beds. The occurrence of dalmanitids in clusters in the Waukesha Lagerstätte also may be related to similar processes. The orientation of many Waukesha dalmanitid trilobites preserved in clusters indicates that remains were current-transported or reoriented. It is likely that these fossils were parautochthonous. Alignment of trilobite exoskeletons on slabs, commonly bidirectional,

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suggests current transport and alignment (Fig. 9A). In addition, relative consistency in up/down

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orientation of exoskeletons (Fig. 3D, 9A) suggests current reorientation of exoskeletons. The

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slabs were not collected in place, making it uncertain as to which side of each slab was originally

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stratigraphically up. Taphonomic experiments have shown that fresh arthropod carcasses can remain at the sediment surface for weeks before complete disarticulation occurs (Babcock and

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Chang, 1997; Babcock et al., 2000). Limbs and gills are among the first structures to be lost, and

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they begin disarticulating in about one week (Babcock and Chang, 1997; Babcock et al., 2000; McCoy and Brandt, 2009). Transportation has been observed to have little, if any, effect on

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disarticulation (Babcock and Chang, 1997; Babcock et al., 2000).

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Evidence that macroorganisms were alive at or close to the time of burial in the lower Brandon Bridge Formation is scant. The presence of Diplichnites on several slabs from various intervals in the Brandon Bridge Formation denotes the activity of some live arthropods, but it is uncertain which arthropods made the tracks and whether they routinely lived at the burial site or were infrequent or accidental visitors to it (washed in). Tightly enrolled trilobites are an indicator of inhospitable environmental conditions, sometimes including rapid burial events (Henry and Clarkson, 1974; Babcock and Speyer, 1987; summarized in Brett et al., 2012b). Lacking enrolled trilobites suggests that quickly changing conditions did not contribute to their deaths. Dendroid

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Journal Pre-proof graptolites characteristically lack their rooted holdfasts in the lower 12 cm of the Waukesha Lagerstätte, indicating that they were not preserved in place (Fig. 8A). Stemmed echinoderms, mostly preserved as molds, are nearly exclusively known from fragmentary remains having disarticulated prior to burial (compare Ausich and Sevastopolo, 1994; Ausich et al., 1999). Evidence indicates that most macroorganisms in the Waukesha Lagerstätte were transported,

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7.4 Microbial environments and entombment

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Kluessendorf, 1998; LoDuca et al., 2003; Wendruff, 2016).

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from nearby areas, often as molts and carcasses (Mikulic et al., 1985a, b; Mikulic and

Here, we report evidence that microbial processes were of primary importance in both

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entombment and early diagenetic alteration of bodily remains in the Waukesha Lagerstätte.

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In earlier work, rapid burial coupled with anoxia was offered as a mechanism for exceptional preservation in this deposit (Mikulic et al., 1985b; LoDuca et al., 2003; Moore et al., 2005).

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Haug et al. (2014) pointed to an anoxic, possibly brackish, environment with restricted

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circulation as a factor in exceptional preservation. In addition, it is also a possibility that hypersalinity, at least at times, was a factor governing exceptional preservation in the Waukesha Lagerstätte. Evidence supporting the hypothesis of a hypersaline environment include the presence of teepee structures, microbial mats, and the absence of mat grazers such as gastropods (discussed below). Some microbial structures were previously noted in the Brandon Bridge Formation (see Kluessendorf and Mikulic, 1996) but a direct association with exceptional preservation was not reported. Phosphatized biofilms or thin microbial mats are commonly preserved on body fossils

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Journal Pre-proof of macroscopic organisms from the Waukesha Lagerstätte. Figure 11 illustrates a thin coating of phosphatized biofilm on a decalcified or ‘ghosted’ dalmanitid trilobite. In the enlarged view (Fig. 11B), individual coccoid microbial bodies are evident within the biofilm. Other evidence of microbial biofilm or mat-formers present in the Waukesha Lagerstätte (lower Brandon Bridge Formation) includes wrinkled/crinkled or elephant skin layers (Figs. 10A, B, F), domal layers (Fig. 10E), ‘teepee’ structures (Fig. 10J), torn mat layers (Fig. 10C), decay halos (e.g., Fig. 5D,

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E, M), and shrinkage cracks (Fig. 10A, B). Gas escape structures are also known from the

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Waukesha Lagerstätte (Fig. 10K). Mat grazers, such as gastropods, are completely unknown

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from the Waukesha Lagerstätte. Some tiny burrows are present and interpreted to have been

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formed by small organisms ‘mining’ the mat (Jones et al., 2016, figs. 2.2, 2.5). Finely laminated layers (Figs. 10C, E) and a paucity of large trace fossils indicates that

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burrowing and vagile macroorganisms were mostly excluded from the Brandon Bridge

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depositional site. This, together with the presence of widespread microbial mat development, indicates an environment hostile to many macroscopic life forms. The large number of body

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fossils compared to the low number of trace fossils suggests that most organisms were washed in

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as carcasses or molts. These organisms were likely transported from neighboring areas and then accumulated in sedimentary traps. Below the Brandon Bridge Formation, the Schoolcraft and Burnt Bluff carbonates were subaerially exposed prior to the time of Brandon Bridge sedimentation. This created an 8-m-high scarp with a gentle slope, and epikarstic and intertidalsupratidal features, in which the remains of organisms collected. Epikarstic features present (Moore et al., 2005) include kamenitzas (solution pans), kluftkarren (vertical dissolution structures), broad scalloping, notching, and runnel troughs. Any of these features could have served as repositories for organismic remains. Once there, bodily remains adhered to sticky

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Journal Pre-proof microbial surfaces, and were then covered by growing mat-formers. Covering of organic remains or traces at the sediment surface by a microbial mat and sediment trapped within it is a form of ‘burial,’ and we use the term microbial entombment (following Varejão et al., 2019 with modification; herein, see Fig. 12) to describe this microorganism-mediated sedimentary and early diagenetic process. Microbial entombment incorporates elements of microbial sealing (a biological process), microbially mediated mineral

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precipitation (which may be accompanied by dissolution), and physical sediment accumulation

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within a microbial mat. This process also has been discussed as sealing by microbial mats or

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formation of a microbial mat sarcophagus (Iniesto et al., 2016, 2017). Seilacher et al. (1985)

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mentioned a process, probably the same one, referring to it alternatively as bacterial sealing, cyanobacterial sealing, and algal sealing. We prefer using the term microbial in this context

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because of the likelihood that the microbial aggregations consisted of varied bacterial and fungal

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taxa, at least, and perhaps others such as archeans. Microbial entombment takes place on short time scales, normally within weeks of the time

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an organism’s remains arrive at the mat surface. Iniesto et al. (2016, 2017) observed that the

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process of entombment by a microbial mat may take place over a time span of a few days up to 30 days. Decay, predation, scavenging, and other forms of biogenic disruption would have been limited by low oxygen conditions, hypersalinity, and organisms that were quickly enveloped by a microbial mat (compare Allison and Briggs, 1991a, b; Gehling, 1999; Briggs, 2003b). Microbial mats trap and stabilize sediment (Decho, 2000; Paerl et al., 2000; Riding, 2000) through the secretion of a sticky film known as extracellular polymeric substance (EPS), which binds and lithifies sediment through the precipitation of calcium carbonate (Decho, 2000). Microbial-stabilized marine sedimentary environments served an important role in the

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Journal Pre-proof fossilization of non-biomineralized taxa during the Ediacaran Period (Gehling, 1999; Hagadorn and Bottjer, 1999; Seilacher, 1999) but declined in abundance during the Phanerozoic due to the appearance of burrowers and grazers that disrupted and devoured the mats (see Gehling, 1999; Pflüger, 1999; Seilacher, 1999; Bottjer et al., 2002a; Gehling and Droser, 2009; Stal, 2012). In the Phanerozoic, microbial mats have tended to proliferate in environments from which burrowers, grazers, and other disrupters were excluded. Commonly these include environments

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of high salinity, temperature fluctuation, and restricted circulation that could lead to low

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oxygenation (e.g., Riding, 2000). Physical, chemical, and biological characteristics of the

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Brandon Bridge depositional setting were conducive to the development of microbial mats. Little

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evidence of grazers and burrowers is known from the lower Brandon Bridge Formation. Paleogeographic location and early dolomitization of the carbonate layers suggests high salinity

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(compare Wilson, 1975). The diversity of trilobites suggests more normal marine conditions;

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however, it is likely that many remains were washed into their final location. The Burnt Bluff

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7.5 Early diagenesis

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scarp may have restricted local circulation of water (Kluessendorf and Mikulic, 1996: fig. 3).

Microbial mats are complex stratified ecosystems in which diverse consortia of microorganisms occupy different layers (Paerl et al., 2000; Riding, 2000; Puckett et al., 2011). Upper mat levels typically are aerobic and dominated by photoautotrophs, whereas lower levels tend to be anaerobic and dominated by chemoautotrophs and heterotrophs (Puckett et al., 2011). Upper layers, commonly dominated by cyanobacteria, provided a sticky surface (EPS) for organisms to adhere (Gehling, 1999). Organisms trapped in the mat would have quickly become

22

Journal Pre-proof enveloped through microbial entombment (Fig. 12B). Dissolution of calcium carbonate (from shelly biota) and precipitation of early diagenetic minerals including pyrite (e.g., Briggs et al., 1996; Borkow and Babcock, 2003) and calcium phosphate (e.g., Babcock et al., 2005; Briggs et al., 2005) are some of the biochemical processes facilitated within microbial mats (Briggs, 2003a; Fig. 11). Once exposed to lower mat levels, organismal remains were subjected to localized microbially mediated chemical

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microenvironments (semi-closed systems) under which dissolution and precipitation could occur,

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depending on pH conditions, oxygen levels, and ion concentrations (Fig. 12C). Dysoxic or

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anoxic microenvironments can yield differential preservation of biomineralized parts and nonbiomineralized tissues (e.g., Dick and Brett, 1986; Babcock and Speyer, 1987; Briggs et al.,

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1993; Wilby et al., 1996; Briggs et al., 2005; Moore et al., 2005; Moore et al., 2011; Zatoń et al.,

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2014). Below the mat surface, decaying organisms produce free phosphorus ions that can

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accumulate (Briggs et al., 1993; Briggs, 2003a, b) and become the source for calcium phosphate precipitation; pH is the primary driver of this process (Allison, 1988; Wilby et al., 1996). A

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microbial mat will have layers, and oxygen levels in those layers will range from oxic through

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anoxic. Changes in pH associated with those layers can promote dissolution of calcium carbonate and precipitation of calcium phosphate in some, and precipitation of carbonate minerals in other layers. These microbially mediated processes likely facilitated mineralization leading to the preservation of non-biomineralized and lightly skeletonized organisms in the Waukesha Lagerstätte and possibly other Silurian Lagerstätten in Laurentia (e.g., Bertie and Eramosa formations). The rarity of organisms with original calcium carbonate hard parts, other than trilobites, is likely the result of a combination of original restriction or low abundance of certain shelly biota (e.g., bivalves, brachiopods, cephalopods, gastropods, echinoderms, corals,

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Journal Pre-proof bryozoans, stromatoporoids) as well as post-depositional dissolution. Molds of disarticulated pelmatozoans are present in cross sections of Fäule layers but have not been observed in the Flinz layers. Much like modern carbonate platform environments (e.g., San Salvador; Puckett et al., 2011), Fäule layers would have cemented quickly, via microbial mediation, into a stabilized matground.

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7.6 Waukesha, a carbonate Konservat-lagerstätte

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Previous authors have commonly drawn comparisons between the Waukesha Lagerstätte

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(e.g., Mikulic et al., 1985b; Meyer and Gunderson, 1986), which involves preservation in

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carbonate mud, and other types of lagerstätten. Comparisons with carbonate-hosted deposits of exceptional preservation such as the Bertie Group (Silurian of New York and Ontario; Caron et

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al., 2004) and the Bear Gulch Limestone (Carboniferous of Montana; McRoberts and Stanley,

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1989; Briggs and Bartels, 2010) are more suitable than comparisons with classic, fine-grained, siliciclastic-hosted lagerstätten such as the Burgess Shale (Cambrian of British Columbia) and

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the Hunsrückshiefer (Devonian of Germany). From a taxonomic standpoint, such comparisons are inevitable, but from a sedimentary process standpoint, such comparisons tend to mask the dominant preservational mechanisms. The Waukesha Lagerstätte is notable for the fact that exceptional preservation has taken place in a non-concretionary carbonate setting. Exceptional preservation is well documented from carbonate concretions such as ‘Orsten’-type concretions (Cambrian of Scandinavia and China; Maas et al., 2006), Mazon Creek-type concretions (Carboniferous of North America, the United Kingdom, and continental Europe; Nitecki, 1979; Baird et al., 1985; Shabica and Hay, 1997), and concretions from the Santana Formation

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Journal Pre-proof (Cretaceous of Brazil; Maisey, 1991), among many other occurrences. However, as noted earlier (Babcock, 2011; Babcock et al., 2011) a number of Konservat-lagerstätten are hosted in finegrained, non-concretionary carbonate rock, not unlike the celebrated Solnhofen Limestone (Seilacher et al., 1985; Bartels et al., 1990). Some other examples of non-concretionary carbonate lagerstätten include the Huaqiao Formation (Cambrian of South China; Babcock et al., 2011), Trenton Limestone (Ordovician of New York; Babcock, 2011); Eramosa Formation

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(Silurian of Ontario; von Bitter et al., 2007), Bertie Group (Silurian of New York and Ontario;

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Clarke and Ruedemann, 1912), Bear Gulch Formation (Carboniferous of Montana; Grogan and

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Lund, 2002; Hagadorn, 2002), Hamilton Lagerstätte (Carboniferous of Kansas; Mapes and

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Mapes 1988; Cunningham et al., 1993), Crato Formation (Cretaceous of Brazil; Martill et al., 2007; Varejão et al. 2019), and the Green River Formation of the western United States (Grande,

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1984, 2013). One important distinction between classic siliciclastic-hosted Konservat-

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lagerstätten and those hosted in non-concretionary carbonates is the inferred influence of microbial entombment in some of the carbonate deposits. Microbial entombment has been

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documented from the Crato Formation (Varejão et al. 2019), but also can be inferred from

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photographs of specimens from other deposits published previously (e.g., Bertie Group; Vrazo et al., 2016). Microbial entombment has not been demonstrated to date from classic Burgess Shaletype deposits, or from Hunsrück-type deposits.

8. Conclusions

Renewed study of the Waukesha Lagerstätte reveals a far richer biodiversity than previously recorded. Representatives of at least 12 animal phyla are now known, and many show

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Journal Pre-proof preserved soft and lightly mineralized tissues. Exceptional preservation has been mediated by microbial processes. We infer that microbial entombment has played a leading role in the preservation of non-biomineralized organisms and the non-preservation (or rarity) of shelly organisms. Conditions at the Waukesha locality were evidently favorable for the development of a microbially-rich environment. Organisms were initially coated by a microbial mat, and this was followed by mat-stabilized sedimentation. Finally, remains were preserved (often through

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mineral precipitation) or partly destroyed (through dissolution) in microenvironments below the

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mat surface. The ‘ghosted’ preservation of carbonate skeletons in the Waukesha Lagerstätte

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likely reflects remobilization (dissolution and reprecipitation) of carbonate under the influence of

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a microbial biofilm or mat. This study invites the possibility that preservation of nonbiomineralized tissues in other carbonate Konservat-lagerstätten likewise may have been

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Acknowledgments

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dominated by similar microbial processes.

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We wish to thank Rodney M. Feldmann, William I. Ausich and John Hunter for valuable advice and comments on earlier versions of this work. This manuscript benefited from the reviews of C.E. Brett, S.T. LoDuca and an anonymous reviewer. Carrie Eaton graciously provided access to specimens at University of Wisconsin Geology Museum. Gerald O. Gunderson and Ronald C. Meyer collected some of the illustrated material. Mats E. Eriksson provided valuable insight on the identification of ‘worms’. Stig M. Bergström provided identifications of some chordate specimens. Wade T. Jones provided a photograph of a phyllocarid. Julie Sheets provided valuable time and assistance with the SEM. SEM images were

26

Journal Pre-proof acquired at the Subsurface Energy Materials Characterization and Analysis Laboratory (SEMCAL), Schools of Earth Sciences, The Ohio State University. This work was supported in part by a grant from the Subsurface Energy Research Center of The Ohio State University to L.E. Babcock.

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Journal Pre-proof Fig. 1. Location of Waukesha Lime and Stone Company west quarry and Franklin Aggregate quarry. A. Map of the United States showing the location of Wisconsin. B. Map of Wisconsin showing the locations of the Waukesha Lime and Stone Company west quarry (Waukesha Quarry) and Franklin Aggregate quarry (Franklin Quarry). C. Part of Waukesha 7.5’ topographic quadrangle map, Wisconsin (U.S. Geological Survey, 2013, 1:24,000) showing the location of the Waukesha Lime and Stone Company quarry (WQ). D, Part of Greendale 7.5’ topographic

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the Franklin Aggregate quarry (FQ). Scale bar equals 1 km.

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quadrangle map, Wisconsin (U.S. Geological Survey, 2013, 1:24,000) showing the location of

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Fig. 2. Stratigraphic section of uppermost Ordovician and overlying Silurian strata in southeastern Wisconsin. Arrow indicates position of Waukesha Lagerstätte within the Brandon

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Bridge (BB) Formation.

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Fig. 3. Large trilobites from the Waukesha Lagerstätte in the Brandon Bridge Formation,

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Silurian (Llandovery, Telychian), Wisconsin. A. Meroperix sp. showing exoskeletal dissolution (‘ghosting’) and preserved gut tract (FMNH). B. Close-up of Meroperix gut tract (FMNH). C.

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Arctinurus sp., ‘ghosted’ with preserved gut tract (FMNH). D. Multiple plate of dalmanitid trilobites showing preserved gut tracts (UWGM 2340). E. Broken cephalon of a dalmanitid trilobite, arrow denotes bite mark (UWGM 2338). F. Close-up of dalmanitid from slab in D preserving gut tract (UWGM 2340). G. Pygidium of Arctinurus sp., arrow denotes bite mark (UWGM 2335). Scale bar equals 2 cm for A–C, F–G, 5 cm for D; 1 cm for E.

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Journal Pre-proof Fig. 4. Small trilobites from the Waukesha Lagerstätte in the Brandon Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Leonaspis sp. (UWGM 2337). B. Harpid, Scotoharpes?, ‘ghosted’ through exoskeletal dissolution (UWGM 2332). C. Stenopareia sp. (UWGM 2329). D. Phacopid (UWGM 2334). E. Otarionid (UWGM 2327). F. Pygidium of odontopleurid (UWGM 2328). G. Pygidium of calymenid (UWGM 2336). H. Undetermined cheirurid cephalic fragment, form 1 (UWGM 2331). I. Undetermined cheirurid pygidium, form 2

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(UWGM 2326). J. Undetermined cheirurid pygidium, form 3 (UWGM 2577). Scale bar equals

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10 mm for A–D, G–J; 5 mm on E–F.

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Fig. 5. Non-biomineralized and lightly skeletonized arthropods from the Waukesha Lagerstätte in the Brandon Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Scorpion

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(UWGM 2162). B. Cheloniellid arthropod, ventral surface (UWGM 2436, formerly UW4001/4).

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C. Cheloniellid arthropod, dorsal surface (UWGM 2345). D. Lobopodian with stout limbs (UWGM 2427). E. Lobopodian with rounded plates and narrow limbs (UWGM 2428). F. Large

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arthropod with two pairs of antenniform appendages (UWGM 2450). G. Plate from the Franklin

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Aggregate quarry covered in non-biomineralized organisms (FMNH). H. Leperditicopid ‘ostracode’ with splayed valves (UWGM 2586). I, Enlargement of boxed area in G showing a myriapod-like animal. J. Enigmatic bivalved arthropod (UWGM 2449). K. Arthropod grasping appendage (UWGM 2339). L. Venustulus waukeshaensis, synziphosurine (UW 4001/21). M. Vermiform arthropod with large headshield and small grasping appendage (UWGM 2451). N. Phyllocarid, Ceratiocaris papilio (UWGM 1926), photograph provided by Wade T. Jones. Arrows in D, E, and M indicate the presence of a decay halo. Abbreviations: (cf) carbon film, (phos), phosphate. Scale bar equals 5 mm for A, E, I, J, N; 1 cm for B–D, H, K–M; 2 cm for F–

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Journal Pre-proof G.

Fig. 6. Shelly organisms from the Waukesha Lagerstätte in the Brandon Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Conulariid, Metaconularia cf. manni (UWGM 2448) with Sphenothallus holdfasts. B. Conulariid, Conularia niagarensis (UWGM 2447) with Sphenothallus holdfasts. C. Sphenothallus tube (UWGM 2580). D. Decalcified articulate

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brachiopod, possible orthid (UWGM 2425). E. Orbiculoid (inarticulate) brachiopod (UWGM

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2453). F. Decalcified (or ‘ghosted’) articulate brachiopod, possibly rhynchonellid (UWGM

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2454). G. Partial tabulate coral (UWGM 2435). H. Undetermined decalcified crinoid (UWGM

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2579). I. Kionoceratinae cephalopod (UWGM 2425). J. Internal mold of an undetermined

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cephalopod (UWGM 2426). Scale bar equals 10 mm for A, B, H–J; 5 mm for C–G.

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Fig. 7. ‘Worms’ from the Waukesha Lagerstätte in the Brandon Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Putative leech preserving the mouth (UWGM 2422). B.

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Large, wide bodied annelid worm (UWGM 2430). C. Putative leech with detailed soft tissue

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(UWGM 2584). D. Palaeoscolecid worm preserving gut (UWGM 2431). E. Palaeoscolecid worm (UWGM 2578). F. Aphroditid polychaete (UWGM 2434). G. Spinose polychaete worm (UWGM 2433). H. Polychaete worm (UWGM 2432). Scale bar equals 2 cm for A–C; 1 cm for D–H.

Fig. 8. Hemichordates and chordates from the Waukesha Lagerstätte in the Brandon Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Graptolite, cf. Desmograptus, lacking holdfast (UWGM 2444). B. Graptolite cf. Dictyonema (UWGM 2445). C, Undetermined

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Journal Pre-proof benthic graptolite (UWGM 2443). D. Oktavites spiralis (UWGM 2446). E. Thallograptus? sp. (UWGM 2442). F. Chordate with myomeres and notochord (FMNH). G. chordate preserving tail (left) and trunk with v-shaped myomeres and a notochord (FMNH). Scale bar equals 10 mm for A, B, D–G; 5 mm for C.

Fig. 9. Dalmanitid trilobites preserved on Flinz from the Waukesha Lagerstätte in the Brandon

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Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Plate (UWGM 5581)

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showing bidirectional alignment in rose diagram. Azimuths of complete trilobite exoskeletons

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(n=49) were measured along mean axial direction with cephalon pointed forward and plotted in

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15o classes. B. Loosely folded dalmanitid trilobite (FMNH). C. Another specimen of a loosely

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folded dalmanitid trilobite (FMNH). Scale bar equals 20 cm for A; 2 cm for B, C.

Fig. 10. Sedimentary and microbial structures from the Waukesha Lagerstätte in the Brandon

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Bridge Formation, Silurian (Llandovery, Telychian), Wisconsin. A. Mudcracks coated in

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microbial mat causing an elephant skin texture (UWGM 2457). B. Close-up of microbially induced elephant skin texture (UWGM 2457). C. Sliced cross section of plattenkalk with both

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Fäule (top darker grey layer) and Flinz (predominantly lighter grey layer) with microbial mat structures (UWGM 2585). D. Cross section showing molds of pelmatozoan stem fragments (UWGM 2585). E. Domal microbial buildup (UWGM 2451). F. Bedding plane view of crinkled sedimentary surface due to preserved microbial mat. G. Diplichnites trace fossil (UWGM 2456.) H. ‘Ghosted’ specimen of a dalmanitid trilobite showing loss of calcite from the exoskeleton (UWGM 2455). H. Microbial dissolution of a dalmanitid trilobite (UWGM 2455). I. Cross section of dalmanitid trilobite showing that the exoskeleton has been dissolved (UWGM 2455).

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Journal Pre-proof J. Cross section of microbial ‘teepee’ structures. K. Possible gas escape structure. Scale bar equals 2 cm for A, C–E, F, G, J; 1 cm for B, H, I, K.

Fig. 11. Preserved biofilm on a decalcified or ‘ghosted’ dalmanitid trilobite (FMNH). A. Trilobite in large scale. B. SEM showing coccoid microbe biofilm on the trilobite. Scale bar

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equals 1 cm for A; 50 μm for B.

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Fig. 12. Diagram explaining the process of microbial entombment. A. Generalized diagram of

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microbial mat. B. Washed in organisms (e.g., trilobites and crinoids) adhere to the sticky surface

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of the microbial mat. C. Over time, the mat surface covers the organisms exposing them to the dysoxic/anoxic portions of the microbial mat, causing the formation of decay halos and both the

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dissolution (e.g., calcite) and precipitation of minerals (e.g., pyrite and apatite). D. Eventually the

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organisms are below the living portion of the microbial mat and no longer undergo further

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described.

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dissolution or precipitation of minerals. Yellow highlighting indicates which layer is being

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Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

preservation

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Highlights • Detailed revision of the Waukesha Biota yielded many new members and phyla (+12) • Soft-tissues preserved on many arthropods including gut tracts of 3 trilobite species • Body fossils of microbes noted for first time at the deposit (coccoid bacteria) • Microbially influenced sedimentary structures noted for first time at this deposit • Depositional model (microbial entombment) proposed to explain exceptional

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