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Accepted Manuscript

Journal of the Geological Society

The Spence Shale Lagerstätte: an Important Window into

Cambrian Biodiversity

Julien Kimmig, Luke C. Strotz, Sara R. Kimmig, Sven O. Egenhoff & Bruce S.

Lieberman

DOI: https://doi.org/10.1144/jgs2018-195

Received 31 October 2018

Revised 21 February 2019

Accepted 28 February 2019

© 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative

Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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The Spence Shale Lagerstätte: an Important Window into Cambrian Biodiversity

Julien Kimmig1*, Luke C. Strotz1,2, Sara R. Kimmig1,3, Sven O. Egenhoff4 & Bruce S. Lieberman1,2

1Biodiversity Institute, University of Kansas, Lawrence, KS 66045, USA

2Department of Ecology & Evolutionary Biology, University of Kansas, Lawrence, KS, USA

3Pacific Northwest National Laboratory, Richland, WA 99354, USA

4Department of Geosciences, Colorado State University, Fort Collins, CO 80523, USA

*Correspondence: [email protected]

Abstract: The Spence Shale Member of the Langston Formation is a Cambrian (Miaolingian:

Wuliuan) Lagerstätte in northeastern Utah and southeastern Idaho. It is older than the more well-

known Wheeler and Marjum Lagerstätten from western Utah, and the Burgess Shale from Canada.

The Spence Shale shares several species in common with these younger deposits, yet it also contains

a remarkable number of unique species. Because of its relatively broad geographic distribution, and

the variety of different palaeoenvironments and taphonomy, the fossil composition and likelihood of

recovering weakly skeletonized (or soft-bodied) taxa varies across localities. The Spence Shale is not

only widely acknowledged for its soft-bodied taxa, but also for its abundant trilobites and hyoliths.

Recent discoveries from the Spence include problematic taxa and provide insights about the nature

of palaeoenvironmental and taphonomic variation between different localities.

Supplementary material: A generic presence/absence matrix of the Spence Shale fauna and a list of

the Spence Shale localities are available at …

Introduction

The Early Paleozoic has yielded a remarkable number of fossil-bearing sediments preserving weakly

skeletonized (or soft-bodied) fossil taxa (Gaines 2014; Van Roy et al. 2015; Muscente et al. 2017;

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mailto:[email protected]


Pates & Daley 2018). The Great Basin of the western USA preserves a significant number of

Cambrian Burgess Shale-type (BST) deposits including the Pioche Formation of Nevada (Lieberman

2003), the Wheeler, Marjum, and Weeks formations of western Utah (Robison 1991; Robison et al.

2015; Foster & Gaines 2016; Lerosey-Aubril et al. 2018), and the Spence Shale of northeastern Utah

and southeastern Idaho (Robison 1991; Liddell et al. 1997; Robison et al. 2015). These deposits

contain an exceptional number of soft-bodied fossils, preserved as two-dimensional mineral films,

and thus greatly extend our knowledge of Cambrian evolution and palaeoecology.

The Spence Shale is one of five Cambrian Konservat-Lagerstätten that occurs in Utah, the

others comprise the (‘deep’) Wheeler, Marjum, and Weeks formations in the House Range and the

(‘shallow’) Wheeler Formation in the Drum Mountains (Robison 1991; Briggs et al. 2008; Robison et

al. 2015; Foster & Gaines 2016; Lerosey-Aubril et al. 2018). The Spence Shale preserves a diverse

fauna of soft-bodied and skeletonized taxa, and each of these are dominated by arthropods (Robison

et al. 2015); it is also the oldest of the Cambrian Lagerstätten of Utah, dating back to the early

Wuliuan Stage (Babcock & Robison 2011). The Lagerstätten in the Wheeler, Marjum, and Weeks

formations of western Utah are younger (Bolaspidella–Cedaria trilobite biozones) but share several

taxa in common with the Spence Shale Member (Liddell et al. 1997; Robison & Babcock 2011;

Robison et al. 2015; Lerosey-Aubril et al. 2018; Pates et al. 2018). Thus far, the two Lagerstätten of

the Wheeler Formation (House Range and Drum Mountains) have been the most intensively studied

Cambrian units containing soft-bodied taxa in Utah (Gaines & Droser 2005; Gaines et al. 2005; Brett

et al. 2009; Halgedahl et al. 2009; Kloss et al. 2015; Foster & Gaines 2016); thus, the depositional

environments, ichnology, and taxonomy are known to an exceptional degree of detail. The slightly

younger Marjum Formation has also received a significant amount of attention (Elrick & Snider 2002;

Brett et al. 2009; Robison et al. 2015). The Weeks Formation Lagerstätte is the youngest of the BST

deposits of Utah (Proagnostus bulbus biozone) and has received relatively little study (Robison &

Babcock 2011; Lerosey-Aubril et al. 2012; Robison et al. 2015; Lerosey-Aubril et al. 2018), though it

contains some soft-bodied animals (Lerosey-Aubril et al. 2013, 2014; Lerosey-Aubril 2015; Ortega-

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Hernández et al. 2015; Lerosey-Aubril et al. 2018). The Spence Shale occupies an intermediate

position between these three formations. Several comprehensive studies of Spence palaeontology

exist such that there is a good knowledge of the biota contained within (see Robison et al. 2015 for a

recent review). However, new taxonomic discoveries continue to be made from the Spence (e.g.

Kimmig et al. 2017). In a similar vein, Spence sedimentology and geochemistry have been studied

(e.g. Liddell et al. 1997; Garson et al. 2012; Kloss et al. 2015), but recent fieldwork conducted by

Kimmig and Strotz and associated taphonomic and sedimentologic analyses (Kimmig et al. 2018)

have revealed new and distinctive patterns of palaeoenvironmental and taphonomic variation across

the geographic and temporal breadth of the Spence Shale.

The Spence Shale occupies a distinctive position among the Lagerstätten of Utah, as it

preserves a range of environments from shallow water carbonates to deep shelf dark shales. While

this by itself is not unique, the fact that soft-bodied organisms are found in the mudstones of the

Wellsville Mountains and the deeper water sediments of Idaho allows for a unique opportunity to

understand the taphonomic pathways of soft-bodied preservation in different environments within

one Member. In addition, the presence of several laminae and beds preserving soft-bodied fossils in

different carbonate cycles within each outcrop of the Wellsville Mountains offers the chance to

study changes in taphonomic pathways and diagenetic effects on soft-tissue preservation within one

locality.

Material and methods

Skeletonized fossils were photographed dry, and all soft-bodied fossils were photographed

submerged in ethanol, using a Canon EOS 5D or 7D Mark II digital SLR camera equipped with Canon

50mm macro lens, or a Leica DMS 300 Digital Microscope. The contrast, colour, and brightness of

images were adjusted using Adobe Photoshop. All figured fossils are part of the University of Kansas,

Biodiversity Institute, Division of Invertebrate Paleontology collections (KUMIP).

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Sedimentological analyses are based on macroscopic and microscopic observations. Thirty

ultrathin (< 20 µm) polished thin sections of the shale and limestone stratigraphic intervals were

analyzed. Samples for thin sections were taken at ~ 1 m intervals along the Spence Shale exposure at

Miners Hollow (Fig. 1). This is perhaps the best-known Spence locality and has also yielded the most

diverse soft-bodied biota.

Data for the generic presence/absence matrix in the major Spence Shale locations were

collected from literature (Robison et al. 2015 and references therein; Conway Morris et al. 2015a,b;

Kimmig et al. 2017; Pates & Daley 2017; Hammersburg et al. 2018; Pates et al. 2018) and museum

databases (KUMIP; Yale University Peabody Museum [YPM]; Harvard University Museum of

Comparative Zoology [MZC]; and United States National Museum of Natural History [USNM]) and

iDigBio (www.idigbio.org).

Locality, geological setting and depositional environment

The Spence Shale Member is the middle member of the Langston Formation (Fig. 1c; sometimes

referred to as the Twin Knobs Formation or Lead Bell Shale). The Langston Formation is an early

middle Cambrian (~507.5–506Ma; Miaolingian: Wuliuan) unit (Albertella to Glossopleura biozones)

that outcrops in northeastern Utah and southeastern Idaho (Fig. 1; Walcott 1908; Maxey 1958; Oriel

& Armstrong 1971; Liddell et al. 1997; Robison & Babcock 2011). It conformably overlies the lower

Cambrian Geertsen Canyon Quartzite of the Brigham Group, and is divided into three members: the

Naomi Peak Limestone, Spence Shale, and High Creek Limestone (Maxey 1958; Liddell et al. 1997;

Hintze & Kowallis 2009; Garson et al. 2012). The type location of the Langston Formation is in

Blacksmith Fork (Fig. 1b), and the formation is named after the nearby Langston Creek (Walcott

1908). The type locality of the Spence Shale is at ‘Spence Gulch’ in southeastern Idaho (Fig. 1b).

The Spence Shale preserves carbonate mudstones (these predominate) to carbonate-rich

siliciclastic mudstones (Box 1; Fig. 2) and has been interpreted to have been deposited in the middle

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http://www.idigbio.org/


carbonate to outer detrital belt of a now west-facing carbonate platform (Palmer 1971; Robison

1991; Liddell et al. 1997). The Spence shows excellent geographic and stratigraphic exposure over

broad areas in northeastern Utah and southeastern Idaho (Suppl. 1). It is assigned to the Albertella-

Glossopleura trilobite biozones, and is interpreted to represent sedimentation on a shelf, although

no details are currently known about depositional depth relative to wave base (Liddell et al. 1997).

Exposures vary from ~ 9 m at Blacksmith Fork (Walcott 1908; Deiss 1938) to ~ 120 m at Oneida

Narrows (Liddell et al. 1997), and the most important localities to date are Miners Hollow, Antimony

Canyon, and Cataract Canyon (all in the Wellsville Mountains) and High Creek, Spence Gulch, and

Oneida Narrows in the Bear River Range (Fig. 1b). However, other localities have also yielded a

variety of skeletonized and soft-bodied fossils.

The palaeontological significance of the Spence Shale has been recognized for over 100

years (Box 2; Walcott 1908; Robison 1965, 1969, 1991; Conway Morris & Robison 1982, 1986, 1988;

Gunther and Gunther 1981; Briggs et al. 2008; Robison & Babcock 2011; Robison et al. 2015), and

important efforts have also focused on characterizing the depositional environments within this

member (Liddell et al. 1997; Garson et al. 2012; Kloss et al. 2015). The Spence contains up to eight

parasequences, or carbonate cycles (Maxey 1958; Liddell et al. 1997; Garson et al. 2012). The

Wellsville Mountain localities are considered the most proximal Spence deposits, and contain

extensive soft-bodied fossils (Liddell et al. 1997; Garson et al. 2012); the depositional setting

becomes more distal towards the northeast. This is indicated by the reduced presence of dolomites

and limestones, and the number of soft-bodied fossils also declines in this direction (Liddell et al.

1997; Garson et al. 2012). Recent investigations of the Langston Formation type locality at

Blacksmith Fork, Utah (Bear River Range) suggest that it might represent an even more proximal

environment than the Wellsville Mountains, as it preserves large amounts of dolomites, indicating

shallow water conditions (Maxey 1958; J. Kimmig pers. obs.). The Blacksmith Fork locality has

yielded few soft-bodied fossils to date, but is valuable for inferring how ecological communities

varied along the Cambrian Spence Shale shelf from shallow to deep water. While fossils are in places

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well preserved in the Spence Shale, burrows are also ubiquitous, both in proximal and distal shelf

sediments. The presence of trace fossils indicates dynamic redox conditions may have prevailed

during deposition of the shale-bearing distal Spence environment (Garson et al. 2012; Hammersburg

et al. 2018). However, the fine-scale distribution of burrows and fecal strings can only be deduced

via thin sections, and although these were recently collected (Kimmig et al. 2018) they have not yet

been studied in sufficient quantity to ascertain the precise prevalence and distribution of dynamic

redox states (Egenhoff & Fishman 2013).

Taphonomy

There have been numerous hypotheses offered to account for the type of soft-bodied preservation

seen in the Spence Shale. Some have suggested it is the result of an oxygen depleted environment in

conjunction with rapid burial (e.g. Gaines et al. 2012; Garson et al. 2012), although oxygen depletion

in and of itself cannot account for soft-tissue preservation (Allison 1988). Intriguingly, the Spence

Shale does not record evidence of constant anoxia (Garson et al. 2012; Kloss et al. 2015;

Hammersburg et al. 2018). In fact, in the Spence, soft-bodied fossils are also found in association

with bioturbated sediments (Garson et al. 2012; Kimmig & Strotz 2017), and geochemical analysis of

some intervals indicates oxygenated bottom waters (Kloss et al. 2015). One of the notable aspects of

the Spence Shale is that there appears to be significant variation in the degree of soft-bodied

preservation and also the range of taxa preserved within any given exposure and across localities

(Suppl. 2; Liddell et al. 1997, Garson et al. 2012; Robison et al. 2015). For instance, Broce &

Schiffbauer (2017) analyzed ten vermiform fossils from the Spence Shale (eight from Miners Hollow,

one from Antimony Canyon, and one from an indeterminate locality in the Wellsville Mountains) and

found a variety of preservational modes associated with these fossils. The most common form of

preservation in these fossils was pyritization, but several fossils showed kerogenization and

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aluminosilicification; further, phosphatization, as well as barite, monazite, and calcite associations

were found (Broce & Schiffbauer 2017).

The different preservational styles in the Spence have unfortunately not yet been fully

explored at the millimeter-scale but are likely due to changes in ocean water chemistry and

sedimentology. This will be important to more precisely ascertain depositional environments as has

been done for other well-known soft-bodied deposits (e.g. Gabbott et al. 2008). The role that

diagenesis plays in mediating soft-bodied preservation (e.g., Butterfield et al. 2007) has also yet to

be unraveled. Thus, additional work is required to tease apart the various taphonomic pathways

involved in soft-bodied preservation in the Spence Shale. This would have the added benefit of

helping elucidate the factors generally responsible for soft-bodied preservation in shales. Further

studies currently underway (e.g. Kimmig et al. 2018) will be a valuable step forward.

Overview of the Spence Shale biota

To date, 87 species in 71 genera that belong to at least 10 phyla have been described from the

Spence Shale (Fig. 4a–l and 5a–l; Suppl.2). Two thirds of the species in the Spence Shale are well

skeletonized and such taxa are not only more diverse but also significantly more abundant than the

co-occurring soft-bodied taxa. The greatest diversity of soft-bodied taxa is found at Miners Hollow,

followed by Antimony Canyon: 20 and 14 soft-bodied species, respectively. More information on the

fauna, algae, cyanobacteria and trace fossils is provided in the subsequent sections, and the soft-

bodied arthropods are discussed in Box 3.

Arthropods

Trilobites and agnostoids

Trilobites are the most diverse group in the Spence Shale and are represented by 41 species in 25

genera. Agnostoid trilobites can be abundant locally, but are generally less common and only two

species are known; Peronopsis bonnerensis, P. brighamensis. Such a low diversity of agnostoids is

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among Cambrian soft-bodied deposits from Utah unique to the Spence, and might suggest a more

restricted and/or proximal environment compared to the other Utah Lagerstätten. Trilobite

diversity is the highest at Spence Gulch (16 genera) in Idaho, and the Wellsville Mountain localities in

Utah (17 genera). Due to the lack of available collections from High Creek and Blacksmith Fork, it is

unclear how many genera are present in these deposits.

Ptychopariid and corynexochid trilobites are the most common types of trilobites in the

Spence Shale and can be found throughout most of the exposures. Recent comprehensive

discussions on these have been provided by Robison & Babcock (2011) and Robison et al. (2015).

Distinctive biostratigraphic patterns among Spence trilobites were described by Campbell (1974),

who argued that there may be some turnovers preserved in the trilobite fauna at Antimony Canyon.

Further study is needed to confirm whether these turnovers appear at other Spence Shale locations.

In the Wellsville Mountains the trilobites usually appear as isolated specimens, with only one

to a few, often complete exoskeletons preserved per slab. Preserved soft-parts have not been

described to this point, but a few unpublished specimens from Miners Hollow actually display gut

structures. In Spence Gulch, isolated trilobites are present, but ‘trilobite-hash’ containing dozens to

hundreds of broken specimens is dominant. Some of the hash is deposited in ribbon-like and circular

forms, resembling coprolites.

Soft-bodied arthropods

Soft-bodied arthropods are the most diverse clade, other than trilobites, from the Spence Shale with

currently 14 species identified. Fossils of this type are mostly limited to the Wellsville Mountains

(Box. 3, Fig. 4a–l).

Lobopodians

Acinocricus stichus Conway Morris & Robison 1988 is the only lobopodian species known from the

Spence Shale (Robison et al. 2015). Specimens can be easily identified by their prominent spines (Fig.

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5g) and are fairly common, with at least 50 specimens known from Miners Hollow, Antimony

Canyon, and Donation Canyon in the Wellsville Mountains. Complete (or largely complete)

specimens are rare though, and most of the time only isolated segments are preserved. Previous

studies have concluded that Acinocricus is a luolishaniid, a group of ecologically specialized

lobopodians with a worldwide distribution (Spence Shale, Burgess Shale, Chengjiang, Emu Bay, and

Xiaoshiba) (Yang et al. 2015).

Scalidophorans

Vermiform fossils are abundant in the Spence Shale, but they usually do not preserve diagnostic

characteristics. However, the few specimens that are well enough preserved retain extensive detail

(Fig. 5c, d). For instance, a palaeoscolecid is known from Miners Hollow, Wronascolex? ratcliffei

(Robison 1969; Conway Morris & Robison 1986; García-Bellido et al. 2013) and is represented by two

specimens, the holotype (KUMIP 204390, UU1020) and a recently collected specimen with an

everted proboscis (KUMIP 490902, Fig. 5c). Ottoia prolifica and two species of Selkirkia, S. spencei

and S. cf. columbia, represent the only other scalidophorans in the Spence Shale (Robison et al.

2015). Ottoia has only been reported from Miners Hollow and Antimony Canyon (Suppl. 2), while

Selkirkia has also been found in the Langston Formation type section at Blacksmith Fork (Resser

1939). The specimen from Blacksmith Fork comprises solely the external tube.

Lophophorates

Brachiopods and hyoliths are some of the most common fossils in the Spence Shale, and can be

preserved in concentrations containing dozens of specimens. Despite the high abundance, the

brachiopods have received little attention, and the six species known from the Spence Shale (Resser

1939; Robison et al. 2015; Suppl. 2) likely represent only a fraction of overall diversity. A few of the

specimens from High Creek preserve chaetae (Fig. 5h). Some of the hyoliths in the Wellsville

Mountains, referable to Haplophrentis reesei, preserve soft-tissues, and these have been interpreted

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as evidence of a lophophore and pharynx (Moysiuk et al. 2017). The other two genera of hyoliths,

Hyolithellus and Hyolithes, are less common; no specimens with soft-bodied preservation are known.

Molluscs

Molluscs are extremely rare in the Spence Shale and are only found in the Wellsville Mountains. Only

two species have been described, Latouchella arguata and Scenella radians (Babcock & Robison

1988), and little material has been added since the initial descriptions. The soft-bodied Wiwaxia

herka (Fig. 5b) is the most common mollusc in the Wellsville Mountains. In addition, an undescribed

halkieriid has recently been discovered in Miners Hollow by Paul Jamison.

Sponges

Sponges are a rare element of the Spence Shale fauna and only three species have been described,

two of which, Vauxia gracilenta and Vauxia magna (Fig. 5a), are known from only four specimens

total from the Wellsville Mountains (Rigby 1980; Robison et al. 2015). This is different from other

Utah Lagerstätten, where sponges are typically the most diverse phylum after arthropods. There are

other sponges from the Bear River Range: Protospongia hicksi has been reported from the Oneida

Narrows locality (Fig. 1b), where hundreds of specimens have been recovered from a 2 m interval

(Church et al. 1999).

Echinoderms

Echinoderms are fairly common in the Spence Shale and can be found at most localities. They are

represented by at least six species, the most common being Ctenocystis utahensis, which often

appears in mass assemblages, and three species of Gogia: G. granulosa, G. guntheri, and G. palmeri.

Robison et al. (2015) also mentioned new Gogia and totiglobid species, but they have not yet been

described. Lyracystis reesei and Ponticulocarpus robisoni are relatively rare and have thus far only

been reported from the Wellsville Mountains (Sumrall & Sprinkle 1999; Sprinkle & Collins 2006).

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Hemichordates

Hemichordates are represented by three species, the proposed enteropneust tube Margaretia dorus

(see Nanglu et al. 2016), and the pterobranch Sphenoecium wheelerensis (Maletz & Steiner 2015). All

three species are common in the Wellsville Mountains but have not yet been found in Idaho, or in

the more eastern exposures in Utah (Suppl. 2).

Problematica

Three species of problematic taxa have been described from the Spence Shale, Banffia episoma,

Eldonia ludwigi, and Siphusauctum lloydguntheri (Fig. 5e, i) (Conway Morris & Robison 1988; Conway

Morris et al. 2015a, b; Kimmig et al. 2017); two of the three were species originally described from

the Burgess Shale. Siphusauctum lloydguntheri (Fig. 5l) is known from a single specimen from near

the top of Antimony Canyon (Kimmig et al. 2017); it is a congener of the species described from the

Burgess Shale. The other two species are all known from multiple specimens and Eldonia can be

found in several localities within the Spence Shale (Suppl. 2).

Algae and cyanobacteria

Marpolia spissa is the only alga currently recognized from the Spence Shale; it has been reported

from Antimony Canyon (Conway Morris & Robison 1988). Its precise affinities among algae have

been debated, and it has even been interpreted as a prokaryote (see LoDuca et al. 2017). The

possible cyanobacterium Morania fragmenta has been reported from the Wellsville Mountains,

although its biologic affinities are also questionable (Handle & Powell 2012), and it might actually

represent fecal pellets (Robison et al. 2015).

Trace fossils

Trace fossils are common in the Wellsville Mountains and more than 35 ichnospecies have been

described. These range from burrows to moving and resting traces to a variety of coprolites (Fig. 4e;

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Kimmig & Strotz 2017; Hammersburg et al. 2018). Ichnofossils have the highest diversity in the

Wellsville Mountains, but Planolites and Diplichnites can be found in Oneida Narrows, and

Diplichnites, Rusophycus and Treptichnus have been reported from High Creek (Suppl. 2;

Hammersburg et al. 2018).

Palaeoecology

The Spence biota is similar to other Cambrian BST biotas in that the fauna is dominated by

arthropods (e.g., Caron & Jackson 2008; Kimmig & Pratt 2015; Foster & Gaines 2016; Paterson et al.

2016; Hou et al. 2017; Lerosey-Aubril et al. 2018). When considering well skeletonized taxa,

trilobites outnumber all the other groups in terms of specimens in museums by a factor of ~ 9:1;

echinoderms and hyoliths are the next most abundant groups in Spence Shale museum collections.

The diverse echinoderm fauna is unique relative to other Cambrian Lagerstätten of Laurentia, as

usually sponges are the second most dominant phylum (e.g., Caron & Jackson 2008; Robison et al.

2015). This Spence Shale may represent a distinct environment, perhaps more oxygenated based on

the presence of these as well as the abundant trace fossils. Trilobites also seem to dominate in field

samples (J. Kimmig pers. obs.). Notably, there are some well skeletonized groups that are quite rare

in museum collections from the Spence, such as molluscs, and this rarity likely represents true rarity

in the field, but the relative paucity of brachiopods in museum collections seems to be a matter of

sampling (J. Kimmig pers. obs.). This is something that has to be considered for future

palaeoecological analyses (e.g., Lieberman and Kimmig 2018).

In terms of soft-bodied fossils, the Spence again is similar to other lower and middle

Cambrian Lagerstätten (e.g., Caron & Jackson 2008; Kimmig & Pratt 2015; Foster & Gaines 2016;

Paterson et al. 2016; Hou et al. 2017; Lerosey-Aubril et al. 2018), as it is dominated by arthropods

(Box 3; Suppl. 2), which make up about half of the soft-bodied genera. In terms of abundance, only

vermiform fossils exceed arthropods. Eldoniids are locally abundant in the Spence Shale and can

occur on slabs with dozens of specimens at Miners Hollow and Cataract Canyon. Many soft-bodied

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taxa comprise autochthonous benthic species, such as hemichordates, scalidophorans, lobopodians,

Wiwaxia, sponges, rare stalked filter feeders, some arthropods, and various trace makers,

supporting the notion of tolerably well oxygenated bottom waters (Church et al. 1999; Robison et al.

2015; Kimmig et al. 2017; Kimmig & Strotz 2017; Kimmig & Pratt 2018; Pratt & Kimmig 2019). There

were, however, also many putatively nektonic (or even pelagic) taxa such as Banffia, radiodonts, and

Tuzoia (Conway Morris et al. 2015a, b; Robison et al. 2015; Pates et al. 2018), and Marpolia spissa

comprises a possible denizen of the plankton (Kimmig et al. 2017).

Based on the generic presence/absence list of Lerosey-Aubril et al. (2018) and the list

generated for this publication (Suppl. 2) there are at least 26 genera found in the Spence that have

not been reported from other Utah Lagerstätten. While part of this might be due to the older age of

the deposit, several of the taxa have been reported from younger Burgess Shale, suggesting that at

least part of it might be due to different environmental conditions when compared to the other

Cambrian Utah Lagerstätten, i.e. better oxygenation, shallower, and possibly higher productivity.

Summary

The Spence Shale of northeastern Utah and southeastern Idaho preserves a diverse, well

skeletonized and soft-bodied biota of early middle Cambrian (Miaolingian; Wuliuan) age. It provides

insight into marine life in Laurentia just before the time of the Walcott Quarry of the Burgess Shale.

Notably, although older than the Burgess Shale and the Wheeler, Marjum, and Weeks formations,

the Spence shares several taxa in common with these deposits, as well as with the older Pioche

Formation in Nevada (Suppl. 2). It seems that during this interval, soft-bodied arthropods (Hendricks

et al. 2008) and soft-bodied taxa in general (Hendricks 2013) showed less evolutionary volatility

(sensu Lieberman & Melott 2013) than trilobites. Consider the trilobites, which show a very high

degree of turnover: of 128 species that occur in soft-bodied deposits globally, not a single species

persists for more than one stage (Hendricks et al. 2008). By contrast, among 156 species of soft-

bodied arthropods, 16 species persist for more than one stage, and some of these persisted for

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several stages (Hendricks et al. 2008). Ultimately, unraveling macroevolutionary patterns in taxa

occurring in soft-bodied deposits such as the Spence will likely prove useful for evaluating various

hypotheses about the nature and timing of the Cambrian radiation (see Lieberman & Cartwright

2011 and Daley et al. 2018 for discussion of some of these hypotheses). In addition, progress

recently has been made in understanding the geographic distribution of various fossils in the Spence,

but much more information is needed about the stratigraphic and sedimentologic context of fossils

within and across localities. Only then will it be possible to work out the various taphonomic

pathways that enabled soft-bodied preservation in this key window of Cambrian life.

Acknowledgements We thank Phillip Donoghue (University of Bristol) for inviting us to write this

paper. This contribution would not be possible without the dedication and generosity of the Gunther

Family, as well as Paul Jamison and Phil Reese, who have donated a large number of excellent

specimens to the University of Kansas, and other institutions: we gratefully acknowledge their

efforts and generosity. We thank Javier Ortega-Hernández and Rudy Lerosey-Aubril for helpful

reviews, and R. Lerosey-Aubril for assistance with the figures. Thanks to Rhiannon LaVine and

Matthew Witte (University of Chicago) and Jacob Skabelund for discussions and assistance in the

field.

Funding This research was supported by a Paleontological Society Arthur James Boucot Research

Grant and an Association of Earth Science Clubs of Greater Kansas City Research Grant to J.K.

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Boxes

Box 1. Sedimentology of the Spence Shale in the Wellsville Mountains

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The Spence Shale consists of carbonate mudstones to carbonate-rich siliciclastic mudstones that are

sub-millimeter- to several centimeter-scale laminated and bedded, and contain abundant

millimeter- to decimeter-scale carbonate beds and laminae. Wackestones occur as millimeter-thick

lenses. The mudstones are also irregular in thickness, and in some places, lenticular. In several

portions throughout the succession, millimeter-thick accumulations of biogenic carbonate debris are

intercalated into the succession that have sharp, irregular bases in places and pinch out laterally.

Isolated biogenic carbonate debris is common throughout the succession and is generally oriented

parallel to sub-parallel to bedding. Organic matter in this unit consists of sub-millimeter wide

flattened flakes, in many places associated with pyrite, which is generally concentrated in distinct

laminae. Locally, it forms lens-shaped slightly inclined accumulations directly adjacent to biogenic

debris. All siliciclastic mudstones contain abundant silt-size carbonate grains that are generally

rounded to some degree, as well as platy clay minerals. In places, macroscopically visible burrows

are common in these rocks.

The carbonate beds are of irregular thickness laterally, and can be nodular. They consist of

either carbonate mudstones with lenses of silty packstones composed of peloids, or up to

decimeter-thick carbonate wacke- to packstones made up of biogenic debris and aggregate grains.

Grains in this facies are poorly sorted, generally recrystallized, and contain a micritic outer rim.

Burrows with varying orientations are common in these rocks. Fractures showing a clear zigzag

pattern occur in both the carbonates and the siliciclastic mudstones but are more common in the

mudstones, and are filled with a clear carbonate cement that is structureless.

The abundance of carbonate mud and grains suggests that the Spence Shale was the distal

equivalent of a carbonate system, likely a rimmed carbonate platform based on lagoonal

components such as the aggregate grains and peloids. The depositional environment transitioned

from carbonate-rich shales in proximal areas to siliciclastic shales distally. High-energy events, likely

storms, must have been abundant in all environments as reflected in sharp irregular bases, here

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interpreted as scours at the base of carbonate and siliciclastic mudstone beds, lens-shaped bedding

indicating bed-load transport, and carbonate debris forming lags. The great quantity of burrows in

the carbonates indicates abundant benthic life during deposition of this unit in carbonate facies; the

same appears partly true with the siliciclastic mudstones. The cement-filled fractures are here

interpreted as recrystallized, originally mud-filled clastic dikes likely reflecting synsedimentary

earthquakes in the Spence Shale.

Box 2. Collections history of the Spence

The Spence Shale was first described by Charles Walcott (1908) from Spence Gulch in Idaho (Fig. 1b).

Since then, palaeontologists have unearthed a treasure trove of fossils from the unit, especially in

the Wellsville Mountains north of Brigham City, Utah. Its potential scientific value was already

recognized by Walcott (1908, p. 8), who called it “an extremely abundant and varied lower Middle

Cambrian fauna”. The scientific merit of the Spence began to come to fruition through the work of

Deiss (1938), Resser (1939), and Maxey (1958), and especially in publications by Dick Robison and

colleagues that highlighted the soft-bodied biota (Robison 1969, 1991; Robison & Richards 1981;

Briggs & Robison 1984; Conway Morris & Robison 1986, 1988; Babcock & Robison 1988). One of the

chief reasons that the Spence became a deposit so well known for soft-bodied preservation was the

extensive and diligent efforts by several private collectors which tremendously enabled and

facilitated the work by Robison and colleagues, as well as of subsequent authors (e.g. Briggs et al.

2005, 2008; Babcock & Robison 2011; Conway Morris et al. 2015a, b; Kimmig et al. 2017; Pates et al.

2018).

In particular, the contributions of the famous Gunther family (Lloyd, Val, Glade; Fig. 3a, b) of

Brigham City, UT, winners of the prestigious Strimple Award from the Paleontological Society (U.S.),

stand out, including their scientific publications (e.g., Gunther & Gunther 1981). They started

collecting in the Spence Shale, as well as in the Wheeler, Marjum and Weeks formations, around

1965, and their efforts have contributed over 75% of all known Spence Shale specimens to museum

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collections: a remarkable legacy. The Gunther family was later joined by Phil Reese and Paul Jamison

(Fig. 3c), and they extended the tradition of giving scientifically important specimens to museums.

Indeed, it is no exaggeration to state that these collectors are the major reason the diversity of the

Spence Shale is so well understood; without their contributions the many taxonomic studies of

Spence Shale fossils would not have been possible. In many respects this legacy continues, as these

enthusiastic private collectors are still one of the driving forces behind the exploration of the

Cambrian deposits of Utah. The majority of soft-bodied fossils would likely never have been found

were it not for the passion of these people. Given that new species are still being described from this

unit, and that there are several specimens in museum collections that are currently unidentifiable,

palaeontologists can only hope that such collection efforts continue, further contributing to our

knowledge of Cambrian biodiversity.

Box 3. Soft-bodied arthropods of the Spence Shale

Arthropods are the dominant component throughout the Spence Shale and are currently

represented by 57 species in 40 genera (Robison et al. 2015; Conway Morris et al. 2015a; Pates &

Daley 2017; Pates et al. 2018). The majority of the species are trilobites and agnostoids, comprising

43 species. The most abundant trilobites in the Wellsville Mountains are Amecephalus, Athabaskia

and Ogygopsis, while other trilobite genera co-occur. At Oneida Narrows Oryctocephalus, Oryctocara

and Pentagnostus represent over 90 percent of the diversity and several dozen specimens can

appear on one slab, possibly indicating a restricted environment. The 14 species of soft-bodied

arthropods, with the exception of some carapaces, are restricted to localities in the Wellsville

Mountains north of Brigham City, Utah, in particular Miners Hollow and Antimony Canyon (Suppl. 2).

Many of the Spence Shale taxa are otherwise only known from the Burgess Shale (e.g., Waptia,

Yohoia) or are endemic to the Spence Shale, like the probable stem-chelicerate Utahcaris orion

(Conway Morris and Robison 1988; Legg & Pates 2017). Fully articulated, well preserved specimens

are rare when compared to deposits like the Burgess Shale, but when they are present, they can

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preserve fine details of the appendages, limbs, and other parts of the body (Fig. 4a–l). Four bivalved

arthropods have been described from the Spence, Canadaspis cf. C. perfecta, Dioxycaris argenta,

Isoxys sp., and Tuzoia retifera. They rarely have body parts associated, and often are isolated

carapaces which is indicative of decomposition before burial or possible predation (Kimmig & Pratt

2016, 2018; Kimmig & Strotz 2017). Tuzoia represents the largest bivalved arthropod from the

Spence Shale, with some valves reaching 12 cm long by 8 cm wide. Radiodonts are also fairly

common in the Spence Shale and at least three species are known, an indeterminate Anomalocaris

species (Briggs et al. 2008), Caryosyntrips camurus (Pates & Daley 2017) and at least one species of

Hurdia, H. victoria (Pates et al. 2018). It is likely that there are more species present, as some

specimens have not yet been assigned to species or genus (Fig. 4g, h; Pates et al. 2018).

Anomalocaris appears to have been the largest radiodont, while most hurdiids were fairly small

(Briggs et al. 2008; Pates et al. 2018). The radiodonts of the Spence Shale have a variety of

interpreted feeding habits, including grasping, grasping-slicing, and sediment sifting.

Box 4. Outstanding Questions

(1) What factors make the Wellsville Mountains localities more likely to preserve soft-bodied fossils

than other Spence Shale localities?

(2) What are the patterns of ecological association in the Spence Shale?

(3) What are the stratigraphic relationships among the various Spence Shale localities?

(4) How does the Spence Shale correlate with other deposits within and outside of the Great Basin?

Figure captions:

Fig. 1. Locations and stratigraphy of the Spence Shale Lagerstätte. (a) Map of the western USA

showing the location of the Spence Shale. (b) Geologic map (based on the USGS state maps for

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Google Earth Pro) of northern Utah and southern Idaho showing the principal localities within the

Spence Shale. AC = Antimony Canyon, BF = Blacksmith Fork, CC = Cataract Canyon, CFC = Calls Fort

Canyon, DC = Donation Canyon, EC = Emigration Canyon, HC = Hansen Canyon, HCR = High Creek,

MH = Miners Hollow, ON = Oneida Narrows, PP = Promontory Point, SG = Spence Gulch, TMC = Two

Mile Canyon. (c) Simplified stratigraphy of the Langston Formation.

Fig. 2. Sedimentology of the Spence Shale. (a) Lag deposit near base of ‘cycle 3’, consisting of

biogenic debris, likely of echinoderms, brachiopod, and trilobite remains. Note how bedding bends

around the millimeter-size echinoderm bioclasts. The matrix is carbonate-rich siliciclastic mudstone

with varying amounts of sub-millimeter carbonate debris. (b) Carbonate-rich siliciclastic mudstone

near top of ‘cycle 3’, with several millimeter-long black organic-rich flakes oriented parallel to

bedding. Note abundant silt-size carbonate particles in the matrix.

Fig. 3. Important private collectors. (a) Lloyd Gunther of Brigham City, UT. (b) Val (right) and Glade

(left) Gunther of Brigham City, UT. (c) Paul Jamison of Logan, UT.

Fig. 4. Selected soft-bodied arthropods from the Spence Shale. (a) KUMIP 204511 holotype of

Meristosoma paradoxum from Miners Hollow, collected by the Gunther family. (b) KUMIP 314041,

Mollisonia symmetrica from Miners Hollow, collected by the Gunther family. (c) KUMIP 314038,

Waptia cf. W. fieldensis from Cataract Canyon, collected by Val and Glade Gunther. (d) KUMIP

312404, Isoxys sp. from Miners Hollow, collected by Arvid Aase. (e) KUMIP 314036, Tuzoia sp. with

burrows under the carapace from Miners Hollow, collected by Phil Reese. (f) KUMIP 204783,

Leanchoilia superlata? from Miners Hollow, collected by Val and Glade Gunther. (g) KUMIP 314027,

Hurdiid H-element from Miners Hollow, collected by the Gunther family. (h) KUMIP 491056, Hurdia

sp. appendage from Miners Hollow, collected by Paul Jamison. (i) KUMIP 204777, Arthropod

appendage from Antimony Canyon, collected by Val Gunther. (j) KUMIP 491904, Dioxycaris argenta

from Miners Hollow, collected by the Gunther family (k) KUMIP 357406, holotype of Yohoia utahana

from Miners Hollow, collected by Paul Jamison. (l) KUMIP 204784, holotype of Utahcaris orion from

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Antimony Canyon, collected by Ben Datillo. Scale bars are 10 mm for (a, e, l) and 5 mm for (b, c, d, f–

k).

Fig. 5. Selected fossils from the Spence Shale. (a) KUMIP 491902 and KUMIP 491903, Vauxia magna

from Miners Hollow, collected by Rhiannon LaVine. (b) KUMIP 287449, holotype of Wiwaxia herka

from Miners Hollow, collected by Phil Reese and the Gunther family. (c) KUMIP 490902,

Wronascolex? ratcliffei from Miners Hollow, collected by Riley Smith. (d) KUMIP 314115, Selkirkia

spencei from the Wellsville Mountains, collected by the Gunther family. (e) KUMIP 204370, Eldonia

ludwigi from Antimony Canyon, collected by Lloyd and Val Gunther. (f) KUMIP 339907, Sphenoecium

wheelerensis from Miners Hollow, collected by the Gunther family. (g) KUMIP 491080, Acinocricus

stichus from Miners Hollow, collected by Paul Jamison. (h) KUMIP 490932, Micromitra? sp. from

High Creek, with chaetae preserved, collected by Paul Jamison. (i) KUMIP 491805,’enrolled’

Amecephalus laticaudum from Miners Hollow, collected by Paul Jamison. (j) KUMIP 491808,

Zacanthoides liddelli from High Creek, collected by Paul Jamison (k) KUMIP 491853, Oryctocephalus

walcotti from Oneida Narrows, collected by the Gunther family. (l) KUMIP 135150, holotype of

Siphusauctum lloydguntheri from Antimony Canyon, collected by Lloyd Gunther. Scale bars are 5

mm.

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