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Review Focus Journal of the Geological Society Published online March 29, 2019 https://doi.org/10.1144/jgs2018-195 | Vol. 176 | 2019 | pp. 609–619

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 1 Biodiversity Institute, University of Kansas, Lawrence, KS 66045, USA 2 Department of Ecology & Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA 3 Pacific Northwest National Laboratory, Richland, WA 99354, USA 4 Department of Geosciences, Colorado State University, Fort Collins, CO 80523, USA JK, 0000-0001-8032-4272; SOE, 0000-0002-3072-286X; BSL, 0000-0002-4353-7874 * 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 with these younger deposits, yet it also contains a remarkable number of unique species. Because of its relatively broad geographical distribution, and the variety of palaeoenvironments and taphonomy, the fossil composition and likelihood of recovering weakly skeletonized (or soft-bodied) taxa varies across localities. The Spence Shale is widely acknowledged not only for its soft-bodied taxa, but also for its abundant trilobites and hyoliths. Recent discoveries from the Spence Shale 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: https://doi.org/10.6084/m9.figshare.c.4423145 Received 31 October 2018; revised 21 February 2019; accepted 28 February 2019

The Early Paleozoic has yielded a remarkable number of fossil- 2015; Foster & Gaines 2016); thus, the depositional environments, bearing sediments preserving weakly skeletonized (or soft-bodied) ichnology and taxonomy are known to an exceptional degree of fossil taxa (Gaines 2014; Van Roy et al. 2015; Muscente et al. 2017; detail. The slightly younger Marjum Formation has also received a Pates & Daley in press). The Great Basin of the western USA significant amount of attention (Elrick & Snider 2002; Brett et al. preserves a significant number of Cambrian Burgess Shale-type 2009; Robison et al. 2015). The Weeks Formation Lagerstätte is the deposits including the Pioche Formation of Nevada (Lieberman youngest of the Burgess Shale-type deposits of Utah (Proagnostus 2003), the Wheeler, Marjum and Weeks formations of western Utah bulbus biozone) and has received relatively little study (Robison & (Robison 1991; Robison et al. 2015; Foster & Gaines 2016; Lerosey- Babcock 2011; Lerosey-Aubril et al. 2012, 2018; Robison et al. Aubril et al. 2018), and the Spence Shale of northeastern Utah and 2015), although it contains some soft-bodied animals (Lerosey- southeastern Idaho (Robison 1991; Liddell et al. 1997; Robison Aubril et al. 2013, 2014, 2018; Lerosey-Aubril 2015; Ortega- et al. 2015). These deposits contain an exceptional number of soft- Hernández et al. 2015). The Spence Shale occupies an intermediate bodied fossils, preserved as 2D mineral films, and thus greatly position between these three formations. Several comprehensive extend our knowledge of Cambrian evolution and palaeoecology. studies of Spence palaeontology exist such that there is a good The Spence Shale is one of five Cambrian Konservat- knowledge of the biota contained within (for a recent review see Lagerstätten that occurs in Utah; the others comprise the (‘deep’) Robison et al. 2015). However, new taxonomic discoveries Wheeler, Marjum and Weeks formations in the House Range and continue to be made from the Spence Shale (e.g. Kimmig et al. the (‘shallow’) Wheeler Formation in the Drum Mountains 2017). In a similar vein, Spence sedimentology and geochemistry (Robison 1991; Briggs et al. 2008; Robison et al. 2015; Foster & have been studied (e.g. Liddell et al. 1997; Garson et al. 2012; Kloss Gaines 2016; Lerosey-Aubril et al. 2018). The Spence Shale et al. 2015), but recent fieldwork conducted by Kimmig and Strotz preserves a diverse fauna of soft-bodied and skeletonized taxa, and and associated taphonomic and sedimentological analyses (Kimmig each of these are dominated by arthropods (Robison et al. 2015); it et al. 2018) have revealed new and distinctive patterns of is also the oldest of the Cambrian Lagerstätten of Utah, dating back palaeoenvironmental and taphonomic variation across the geo- to the early Wuliuan Stage (Robison & Babcock 2011). The graphical and temporal breadth of the Spence Shale. Lagerstätten in the Wheeler, Marjum and Weeks formations of The Spence Shale occupies a distinctive position among the western Utah are younger (Bolaspidella–Cedaria trilobite biozones) Lagerstätten of Utah, as it preserves a range of environments from but have several taxa in common with the Spence Shale Member shallow water carbonates to deep shelf dark shales. Although this by (Liddell et al. 1997; Robison & Babcock 2011; Robison et al. 2015; itself is not unique, the fact that soft-bodied organisms are found in Lerosey-Aubril et al. 2018; Pates et al. 2018). Thus far, the two the mudstones of the Wellsville Mountains and the deeper water Lagerstätten of the Wheeler Formation (House Range and Drum sediments of Idaho allows for a unique opportunity to understand Mountains) have been the most intensively studied Cambrian units the taphonomic pathways of soft-bodied preservation in different containing soft-bodied taxa in Utah (Gaines & Droser 2005; Gaines environments within one member. In addition, the presence of et al. 2005; Brett et al. 2009; Halgedahl et al. 2009; Kloss et al. several laminae and beds preserving soft-bodied fossils in different

© 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 Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

610 J. Kimmig et al. carbonate cycles within each outcrop of the Wellsville Mountains analysed. Samples for thin sections were taken at c. 1 m intervals offers the chance to study changes in taphonomic pathways and along the Spence Shale exposure at Miners Hollow (Fig. 1). This is diagenetic effects on soft-tissue preservation within one locality. perhaps the best-known Spence Shale locality and has also yielded the most diverse soft-bodied biota. Data for the generic presence–absence matrix in the major Spence Material and methods Shale locations were collected from literature (Robison et al. 2015, Skeletonized fossils were photographed dry, and all soft-bodied and references therein; Conway Morris et al. 2015a, b; Kimmig et al. fossils were photographed submerged in ethanol, using a Canon 2017; Pates & Daley 2017; Hammersburg et al. 2018; Pates et al. EOS 5D or 7D Mark II digital SLR camera equipped with Canon 2018) and museum databases (KUMIP; Yale University Peabody 50 mm macro lens, or a Leica DMS 300 digital microscope. The Museum (YPM); Harvard University Museum of Comparative contrast, colour and brightness of images were adjusted using Zoology (MZC); and United States National Museum of Natural Adobe Photoshop. All figured fossils are part of the University of History (USNM)) and iDigBio (www.idigbio.org). Kansas, Biodiversity Institute, Division of Invertebrate Paleontology collections (KUMIP). Locality, geological setting and depositional environment Sedimentological analyses are based on macroscopic and microscopic observations. Thirty ultrathin (<20 µm) polished thin The Spence Shale Member is the middle member of the Langston sections of the shale and limestone stratigraphic intervals were Formation (Fig. 1c; sometimes referred to as the Twin Knobs

Fig. 1. Locations and stratigraphy of the Spence Shale Lagerstätte. (a) Map of the western USA showing the location of the Spence Shale. (b) Geological map (based on the USGS state maps for 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. Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

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Box 1. Sedimentology of the Spence Shale in the Wellsville Mountains The Spence Shale consists of carbonate mudstones to carbonate-rich siliciclastic mudstones that are sub-millimetre- to several centimetre-scale laminated and bedded, and contain abundant millimetre- to decimetre-scale carbonate beds and laminae. Wackestones occur as millimetre-thick lenses. The mudstones are also irregular in thickness, and in some places, lenticular. In several portions throughout the succession, millimetre-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 subparallel to bedding. Organic matter in this unit consists of sub-millimetre 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 decimetre-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, probably a rimmed carbonate platform based on lagoonal components such as the aggregate grains and peloids. The depositional environment shows a transition from carbonate-rich shales in proximal areas to siliciclastic shales distally. High-energy events, probably storms, must have been abundant in all environments as reflected in sharp irregular bases, here 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 dykes probably reflecting synsedimentary earthquakes in the Spence Shale.

Formation or Lead Bell Shale). The Langston Formation is an early sedimentation on a shelf, although no details are currently known middle Cambrian (c. 507.5–506 Ma; Miaolingian: Wuliuan) unit about depositional depth relative to wave base (Liddell et al. 1997). (Albertella to Glossopleura biozones) that crops out in northeastern Exposures vary from c. 9 m at Blacksmith Fork (Walcott 1908; Utah and southeastern Idaho (Fig. 1; Walcott 1908; Maxey 1958; Deiss 1938)toc. 120 m at Oneida Narrows (Liddell et al. 1997), Oriel & Armstrong 1971; Liddell et al. 1997; Robison & Babcock and the most important localities to date are Miners Hollow, 2011). It conformably overlies the lower Cambrian Geertsen Antimony Canyon and Cataract Canyon (all in the Wellsville Canyon Quartzite of the Brigham Group, and is divided into three Mountains) and High Creek, Spence Gulch and Oneida Narrows in members: the Naomi Peak Limestone, Spence Shale and High the Bear River Range (Fig. 1b). However, other localities have also Creek Limestone (Maxey 1958; Liddell et al. 1997; Hintze & yielded a variety of skeletonized and soft-bodied fossils. Kowallis 2009; Garson et al. 2012). The type location of the The palaeontological significance of the Spence Shale has been Langston Formation is in Blacksmith Fork (Fig. 1b), and the recognized for over 100 years (Box 2 ; Walcott 1908; Robison 1965, formation is named after the nearby Langston Creek (Walcott 1908). 1969, 1991; Gunther & Gunther 1981; Conway Morris & Robison The type locality of the Spence Shale is at ‘Spence Gulch’ in 1982, 1986, 1988; Briggs et al. 2008; Robison & Babcock 2011; southeastern Idaho (Fig. 1b). Robison et al. 2015), and important efforts have also focused on The Spence Shale preserves carbonate mudstones (these characterizing the depositional environments within this member predominate) to carbonate-rich siliciclastic mudstones (Box 1; (Liddell et al. 1997; Garson et al. 2012; Kloss et al. 2015). The Fig. 2) and has been interpreted to have been deposited in the middle Spence Shale contains up to eight parasequences, or carbonate carbonate to outer detrital belt of a now west-facing carbonate cycles (Maxey 1958; Liddell et al. 1997; Garson et al. 2012). The platform (Palmer 1971 ; Robison 1991; Liddell et al. 1997). The Wellsville Mountain localities are considered the most proximal Spence Shale shows excellent geographical and stratigraphic Spence deposits, and contain extensive soft-bodied fossils (Liddell exposure over broad areas in northeastern Utah and southeastern et al. 1997; Garson et al. 2012); the depositional setting becomes Idaho (see supplementary material). It is assigned to the Albertella– more distal towards the NE. This is indicated by the reduced Glossopleura trilobite biozones, and is interpreted to represent presence of dolomites and limestones, and the number of soft-

Fig. 2. Sedimentology of the Spence Shale. (a) Lag deposit near base of ‘cycle 3’, consisting of biogenic debris, probably of echinoderm, brachiopod and trilobite remains. It should be noted how bedding bends around the millimetre-size echinoderm bioclasts. The matrix is carbonate-rich siliciclastic mudstone with varying amounts of sub-millimetre carbonate debris. (b) Carbonate-rich siliciclastic mudstone near top of ‘cycle 3’, with several millimetre-long black organic-rich flakes oriented parallel to bedding. Abundant silt-size carbonate particles in the matrix should be noted. Scale bar is 1 mm. Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

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Box 2. Collections history of the Spence Shale 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 Shale 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 for the Spence Shale becoming a deposit so well known for soft-bodied preservation was the extensive and diligent efforts by several private collectors, which tremendously aided and facilitated the work by Robison and colleagues, as well as of subsequent researchers (e.g. Briggs et al. 2005, 2008; Robison & Babcock 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 and Glade; Fig. 3a and b) of Brigham City, UT, winners of the prestigious Strimple Award from the Paleontological Society (USA), 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 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 probably 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. bodied fossils also declines in this direction (Liddell et al. 1997; Taphonomy Garson et al. 2012). Recent investigations of the Langston Formation type locality at Blacksmith Fork, Utah (Bear River There have been numerous hypotheses offered to account for the Range) suggest that it might represent an even more proximal type of soft-bodied preservation seen in the Spence Shale. Some environment than the Wellsville Mountains, as it preserves large have suggested it is the result of an oxygen-depleted environment in amounts of dolomites, indicating shallow water conditions (Maxey conjunction with rapid burial (e.g. Gaines et al. 2012; Garson et al. 1958; J. Kimmig, pers. obs.). The Blacksmith Fork locality has 2012), although oxygen depletion in and of itself cannot account for yielded few soft-bodied fossils to date, but is valuable for inferring soft-tissue preservation (Allison 1988). Intriguingly, the Spence how ecological communities varied along the Cambrian Spence Shale does not record evidence of constant anoxia (Garson et al. Shale shelf from shallow to deep water. Although fossils are in 2012; Kloss et al. 2015; Hammersburg et al. 2018). In fact, in the places well preserved in the Spence Shale, burrows are also Spence Shale, soft-bodied fossils are also found in association with ubiquitous, both in proximal and distal shelf sediments. The bioturbated sediments (Garson et al. 2012; Kimmig & Strotz 2017), presence of trace fossils indicates that dynamic redox conditions and geochemical analysis of some intervals indicates oxygenated may have prevailed during the deposition of the shale-bearing distal bottom waters (Kloss et al. 2015). One of the notable aspects of the Spence environment (Garson et al. 2012; Hammersburg et al. Spence Shale is that there appears to be significant variation in the 2018). However, the fine-scale distribution of burrows and fecal degree of soft-bodied preservation and also the range of taxa strings can be deduced only via thin sections, and although these preserved within any given exposure and across localities were recently collected (Kimmig et al. 2018) they have not yet been (supplementary material; Liddell et al. 1997; Garson et al. 2012; studied in sufficient quantity to ascertain the precise prevalence and Robison et al. 2015). For instance, Broce & Schiffbauer (2017) distribution of dynamic redox states (Egenhoff & Fishman 2013). analysed 10 vermiform fossils from the Spence Shale (eight from

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. Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

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Miners Hollow, one from Antimony Canyon and one from an to hundreds of broken specimens is dominant. Some of the hash is indeterminate locality in the Wellsville Mountains) and found a deposited in ribbon-like and circular forms, resembling coprolites. variety of preservational modes associated with these fossils. The most common form of preservation in these fossils was pyritization, Soft-bodied arthropods but several fossils showed kerogenization and aluminosilicification; Soft-bodied arthropods are the most diverse clade, other than further, phosphatization, as well as barite, monazite and calcite trilobites, from the Spence Shale, with currently 14 species associations were found (Broce & Schiffbauer 2017). identified. Fossils of this type are mostly limited to the Wellsville The different preservational styles in the Spence Shale have Mountains (Box 3, Fig. 4a–l). unfortunately not yet been fully explored at the millimetre-scale but are probably due to changes in ocean water chemistry and sedimentology. This will be important to more precisely ascertain Lobopodians depositional environments as has been done for other well-known Acinocricus stichus Conway Morris & Robison 1988 is the only soft-bodied deposits (e.g. Gabbott et al. 2008). The role that lobopodian species known from the Spence Shale (Robison et al. diagenesis plays in mediating soft-bodied preservation (e.g. 2015). Specimens can be easily identified by their prominent spines Butterfield et al. 2007) has also yet to be unravelled. Thus, (Fig. 5g) and are fairly common, with at least 50 specimens known additional work is required to tease apart the various taphonomic from Miners Hollow, Antimony Canyon and Donation Canyon in pathways involved in soft-bodied preservation in the Spence Shale. the Wellsville Mountains. Complete (or largely complete) speci- This would have the added benefit of helping elucidate the factors mens are rare, however, and most of the time only isolated segments generally responsible for soft-bodied preservation in shales. Further are preserved. Previous studies have concluded that Acinocricus is a studies currently under way (e.g. Kimmig et al. 2018) will be a luolishaniid, a group of ecologically specialized lobopodians with a valuable step forward. worldwide distribution (Spence Shale, Burgess Shale, Chengjiang, Emu Bay and Xiaoshiba) (Yang et al. 2015). Overview of the Spence Shale biota Scalidophorans To date, 87 species in 71 genera that belong to at least 10 phyla have been described from the Spence Shale (Figs 4a–l and 5a–l; Vermiform fossils are abundant in the Spence Shale, but they supplementary material). Two-thirds of the species in the Spence usually do not preserve diagnostic characteristics. However, the few Shale are well skeletonized and such taxa are not only more diverse specimens that are well enough preserved retain extensive detail but also significantly more abundant than the co-occurring soft- (Fig. 5c and d). For instance, a palaeoscolecid is known from Miners bodied taxa. The greatest diversity of soft-bodied taxa is found at Hollow, Wronascolex? ratcliffei (Robison 1969; Conway Morris & Miners Hollow, followed by Antimony Canyon: 20 and 14 soft- Robison 1986; García-Bellido et al. 2013) and is represented by two bodied species, respectively. More information on the fauna, algae, specimens, the holotype (KUMIP 204390, UU1020) and a recently cyanobacteria and trace fossils is provided in the subsequent collected specimen with an everted proboscis (KUMIP 490902, sections, and the soft-bodied arthropods are discussed in Box 3. 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 been reported only Arthropods from Miners Hollow and Antimony Canyon (supplementary Trilobites and agnostoids material), whereas Selkirkia has also been found in the Langston Formation type section at Blacksmith Fork (Resser 1939). The Trilobites are the most diverse group in the Spence Shale and are specimen from Blacksmith Fork comprises solely the external tube. 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 and P. brighamensis. Lophophorates Such a low diversity of agnostoids is among Cambrian soft-bodied Brachiopods and hyoliths are some of the most common fossils in the deposits from Utah unique to the Spence Shale, and might suggest a Spence Shale, and can be preserved in concentrations containing more restricted and/or proximal environment compared with the dozens of specimens. Despite the high abundance, the brachiopods other Utah Lagerstätten. Trilobite diversity is the highest at Spence have received little attention, and the six species known from the Gulch (16 genera) in Idaho, and the Wellsville Mountain localities Spence Shale (Resser 1939; Robison et al.2015; supplementary in Utah (17 genera). Because of the lack of available collections material) probably represent only a fraction of overall diversity. A few from High Creek and Blacksmith Fork, it is unclear how many of the specimens from High Creek preserve chaetae (Fig. 5h). Some of genera are present in these deposits. the hyoliths in the Wellsville Mountains, referable to Haplophrentis Ptychopariid and corynexochid trilobites are the most common reesei, preserve soft tissues, and these have been interpreted as types of trilobites in the Spence Shale and can be found throughout evidence of a lophophore and pharynx (Moysiuk et al.2017). The most of the exposures. Recent comprehensive discussions on these other two genera of hyoliths, Hyolithellus and Hyolithes,areless have been provided by Robison & Babcock (2011) and Robison common; no specimens with soft-bodied preservation are known. et al. (2015). Distinctive biostratigraphic patterns among Spence trilobites were described by Campbell (1974), who argued that there Molluscs may be some turnovers preserved in the trilobite fauna at Antimony Canyon. Further study is needed to confirm whether these turnovers Molluscs are extremely rare in the Spence Shale and are found only appear at other Spence Shale locations. in the Wellsville Mountains. Only two species have been described, In the Wellsville Mountains the trilobites usually appear as Latouchella arguata and Scenella radians (Babcock & Robison isolated specimens, with only one to a few, often complete 1988), and little material has been added since the initial exoskeletons preserved per slab. Preserved soft parts have not descriptions. The soft-bodied Wiwaxia herka (Fig. 5b) is the most been described to this point, but a few unpublished specimens from common mollusc in the Wellsville Mountains. In addition, an Miners Hollow actually display gut structures. In Spence Gulch, undescribed halkieriid has recently been discovered in Miners isolated trilobites are present, but ‘trilobite-hash’ containing dozens Hollow by Paul Jamison. 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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 Antimony Canyon, collected by Ben Datillo. Scale bars represent 10 mm for (a), (e) and (l) and 5 mm for (b)(c), (d) and (f)–(k).

Sponges Echinoderms Sponges are a rare element of the Spence Shale fauna and only three Echinoderms are fairly common in the Spence Shale and can be species have been described, two of which, Vauxia gracilenta and found at most localities. They are represented by at least six species, Vauxia magna (Fig. 5a), are known from only four specimens total the most common being Ctenocystis utahensis, which often appears from the Wellsville Mountains (Rigby 1980; Robison et al. 2015). in mass assemblages, and three species of Gogia: G. granulosa, This is different from other Utah Lagerstätten, where sponges are G. guntheri and G. palmeri. Robison et al. (2015) also mentioned typically the most diverse phylum after arthropods. There are other new Gogia and totiglobid species, but they have not yet been sponges from the Bear River Range: Protospongia hicksi has been described. Lyracystis reesei and Ponticulocarpus robisoni are reported from the Oneida Narrows locality (Fig. 1b), where relatively rare and have thus far been reported from only the hundreds of specimens have been recovered from a 2 m interval Wellsville Mountains (Sumrall & Sprinkle 1999; Sprinkle & (Church et al. 1999). Collins 2006). Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

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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 represent 5 mm. Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

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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 (Conway Morris et al. 2015a; Robison et al. 2015; 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, and other trilobite genera co-occur. At Oneida Narrows Oryctocephalus, Oryctocara and Pentagnostus represent over 90% 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 (supplementary material). Many of the Spence Shale taxa are otherwise known only from the Burgess Shale (e.g. Waptia, Yohoia)or are endemic to the Spence Shale, like the probable stem-chelicerate Utahcaris orion (Conway Morris & Robison 1988; Legg & Pates 2017). Fully articulated, well- preserved specimens are rare when compared with deposits such as the Burgess Shale, but when they are present, they can 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 Shale, 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 and h; Pates et al. 2018). Anomalocaris appears to have been the largest radiodont, whereas 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.

Hemichordates Trace fossils Hemichordates are represented by two species, the proposed Trace fossils are common in the Wellsville Mountains and more enteropneust tube Margaretia dorus (see Nanglu et al. 2016) and than 35 ichnospecies have been described. These range from the pterobranch Sphenoecium wheelerensis (Maletz & Steiner burrows to moving and resting traces to a variety of coprolites 2015). Both species are common in the Wellsville Mountains but (Fig. 4e; Kimmig & Strotz 2017; Hammersburg et al. 2018). have not yet been found in Idaho, or in the more eastern exposures in Ichnofossils have the highest diversity in the Wellsville Mountains, Utah (supplementary material). but Planolites and Diplichnites can be found in Oneida Narrows, and Diplichnites, Rusophycus and Treptichnus have been reported from High Creek (supplementary material; Hammersburg et al. Problematica 2018). Three species of problematic taxa have been described from the Spence Shale, Banffia episoma, Eldonia ludwigi and Siphusauctum Palaeoecology lloydguntheri (Fig. 5e and i)(Conway Morris & Robison 1988; Conway Morris et al. 2015a, b; Kimmig et al. 2017); two of the The Spence biota is similar to other Cambrian Burgess Shale-type three were species originally described from the Burgess Shale. biotas in that the fauna is dominated by arthropods (e.g. Caron & Siphusauctum lloydguntheri (Fig. 5l) is known from a single Jackson 2008; Kimmig & Pratt 2015; Foster & Gaines 2016; specimen from near the top of Antimony Canyon (Kimmig et al. Paterson et al. 2016; Hou et al. 2017; Lerosey-Aubril et al. 2018). 2017); it is a congener of the species described from the Burgess When considering well-skeletonized taxa, trilobites outnumber all Shale. The other two species are all known from multiple specimens the other groups in terms of specimens in museums by a factor of c. and Eldonia can be found in several localities within the Spence 9:1; echinoderms and hyoliths are the next most abundant groups in Shale (supplementary material). 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 Algae and cyanobacteria & Jackson 2008; Robison et al. 2015). The Spence Shale may represent a distinct environment, perhaps more oxygenated based Marpolia spissa is the only alga currently recognized from the on the presence of these as well as the abundant trace fossils. Spence Shale; it has been reported from Antimony Canyon Trilobites also seem to dominate in field samples (J. Kimmig, pers. (Conway Morris & Robison 1988). Its precise affinities among obs.). Notably, there are some well-skeletonized groups that are algae have been debated, and it has even been interpreted as a quite rare in museum collections from the Spence Shale, such as prokaryote (see LoDuca et al. 2017). The possible cyanobacterium molluscs, and this rarity probably represents true rarity in the field, Morania fragmenta has been reported from the Wellsville but the relative paucity of brachiopods in museum collections seems Mountains, although its biological affinities are also questionable to be a matter of sampling (J. Kimmig, pers. obs.). This is something (Handle & Powell 2012), and it might actually represent fecal pellets that has to be considered for future palaeoecological analyses (e.g. (Robison et al. 2015). Lieberman & Kimmig 2018).

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 the Great Basin? Downloaded from http://jgs.lyellcollection.org/ by guest on September 23, 2021

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In terms of soft-bodied fossils, the Spence Shale again is similar University of Kansas, and other institutions: we gratefully acknowledge their to other lower and middle Cambrian Lagerstätten (e.g. Caron & efforts and generosity. We thank J. Ortega-Hernández and R. Lerosey-Aubril for helpful reviews, and R. Lerosey-Aubril for assistance with the figures. Thanks go Jackson 2008; Kimmig & Pratt 2015; Foster & Gaines 2016; to R. LaVine and M. Witte (University of Chicago) and J. Skabelund for Paterson et al. 2016; Hou et al. 2017; Lerosey-Aubril et al. 2018), as discussions and assistance in the field. it is dominated by arthropods (Box 3; supplementary material), which make up about half of the soft-bodied genera. In terms of abundance, only vermiform fossils exceed arthropods. Eldoniids are Funding This research was supported by a Paleontological Society Arthur James Boucot Research Grant and an Association of Earth Science Clubs of locally abundant in the Spence Shale and can occur on slabs with Greater Kansas City Research Grant to J.K. dozens of specimens at Miners Hollow and Cataract Canyon. Many soft-bodied taxa comprise autochthonous benthic species, such as Scientific editing by Philip Donoghue hemichordates, scalidophorans, lobopodians, Wiwaxia, sponges, Correction Notice: Error bars have been added to Fig. 5g and h. The Editorial rare stalked filter feeders, some arthropods and various trace makers, Office apologies for this error. supporting the notion of tolerably well-oxygenated bottom waters (Church et al. 1999; Robison et al. 2015; Kimmig & Strotz 2017; References Kimmig et al. 2017; Kimmig & Pratt 2018; Pratt & Kimmig 2019). Allison, P.A. 1988. The role of anoxia in the decay and mineralization of There were, however, also many putatively nektonic (or even proteinaceous macrofossils. Paleobiology, 14, 139–154. pelagic) taxa such as Banffia, radiodonts and Tuzoia (Conway Babcock, L.E. & Robison, R.A. 1988. Taxonomy and paleobiology of some Morris et al. 2015a, b; Robison et al. 2015; Pates et al. 2018), and Middle Cambrian Scenella (Cnidaria) and hyolithids (Mollusca) from western Marpolia spissa is a possible denizen of the plankton (Kimmig et al. North America. University of Kansas, Paleontological Contributions, 121. Brett, C.E., Allison, P.A., DeSantis, M.K., Liddell, W.D. & Kramer, A. 2009. 2017). Sequence stratigraphy, cyclic facies, and lagerstätten in the Middle Cambrian Based on the generic presence–absence list of Lerosey-Aubril Wheeler and Marjum Formations, Great Basin, Utah. Palaeogeography, et al. (2018) and the list generated for this paper (supplementary Palaeoclimatology, Palaeoecology, 277,9–33, https://doi.org/10.1016/j. palaeo.2009.02.010 material) there are at least 26 genera found in the Spence Shale that Briggs, D.E.G. & Robison, R.A. 1984. Exceptionally preserved nontrilobite have not been reported from other Utah Lagerstätten. Although part arthropods and Anomalocaris from the middle Cambrian of Utah. University of this might be due to the older age of the deposit, several of the of Kansas Paleontological Contributions, 111. taxa have been reported from younger Burgess Shale, suggesting Briggs, D.E.G., Lieberman, B.S., Halgedahl, S.L. & Jarrard, R.D. 2005. A new vetulicolian from the Middle Cambrian of Utah and the phylogenetic position that at least part of it might be due to different environmental of a problematic group. Palaeontology, 48, 681–686, https://doi.org/10.1111/ conditions when compared with the other Cambrian Utah j.1475-4983.2005.00489.x Lagerstätten; that is, better oxygenation, shallower water and Briggs, D.E.G., Lieberman, B.S., Hendricks, J.R., Halgedahl, S.L. & Jarrard, R.D. 2008. Middle Cambrian arthropods from Utah. Journal of Paleontology, possibly higher productivity. 82, 238–254, https://doi.org/10.1666/06-086.1 Broce, J.S. & Schiffbauer, J.D. 2017. Taphonomic analysis of Cambrian Summary vermiform fossils of Utah and Nevada, and implications for the chemistry of Burgess Shale-type preservation. PALAIOS, 32, 600–619, https://doi.org/10. The Spence Shale of northeastern Utah and southeastern Idaho 2110/palo.2017.011 Butterfield, N.J., Balthasar, U.W.E. & Wilson, L.A. 2007. Fossil diagenesis in the preserves a diverse, well-skeletonized and soft-bodied biota of early Burgess Shale. Palaeontology, 50, 537–543, https://doi.org/10.1111/j.1475- middle Cambrian (Miaolingian; Wuliuan) age. It provides insight 4983.2007.00656.x into marine life in Laurentia just before the time of the Walcott Campbell, D.P. 1974. Biostratigraphy of the Albert ella and Glossopleura zones Quarry of the Burgess Shale. Notably, although older than the (lower Middle Cambrian) of northern Utah and southern Idaho. Ms thesis, University of Utah. Burgess Shale and the Wheeler, Marjum and Weeks formations, the Caron, J.-B. & Jackson, D.A. 2008. Paleoecology of the Greater Phyllopod Bed Spence Shale shares several taxa with these deposits, as well as with community, Burgess Shale. Palaeogeography, Palaeoclimatology, the older Pioche Formation in Nevada (supplementary material). It Palaeoecology, 258, 222–256, https://doi.org/10.1016/j.palaeo.2007.05.023 Church, S.B., Rigby, J.K., Gunther, L.F. & Gunther, V.G. 1999. A large seems that during this interval, soft-bodied arthropods (Hendricks Protospongia hicksi Hinde, 1887, from the Middle Cambrian Spence Shale of et al. 2008) and soft-bodied taxa in general (Hendricks 2013) Southeastern Idaho. Brigham Young University Geology Studies, 44,17–25. showed less evolutionary volatility (sensu Lieberman & Melott Conway Morris, S. & Robison, R.A. 1982. The enigmatic medusoid Peytoia and 2013) than trilobites. Let us consider the trilobites, which show a a comparison of some Cambrian biotas. Journal of Paleontology, 56, 116–122, https://doi.org/www.jstor.org/stable/1304497 very high degree of turnover: of 128 species that occur in soft- Conway Morris, S. & Robison, R.A. 1986. Middle Cambrian priapulids and bodied deposits globally, not a single species persists for more than other soft-bodied fossils from Utah and Spain. University of Kansas, one stage (Hendricks et al. 2008). By contrast, among 156 species Paleontological Contributions, 117. Conway Morris, S. & Robison, R.A. 1988. More soft-bodied animals and algae of soft-bodied arthropods, 16 species persist for more than one from the Middle Cambrian of Utah and British Columbia. University of stage, and some of these persisted for several stages (Hendricks Kansas, Paleontological Contributions, 122. et al. 2008). Ultimately, unravelling macroevolutionary patterns in Conway Morris, S., Halgedahl, S.L., Selden, P. & Jarrard, R.D. 2015a. Rare taxa occurring in soft-bodied deposits such as the Spence Shale will primitive deuterostomes from the Cambrian (Series 3) of Utah. Journal of Paleontology, 89, 631–636, https://doi.org/10.1017/jpa.2015.40 probably prove useful for evaluating various hypotheses about the Conway Morris, S., Selden, P.A., Gunther, G., Jamison, P.G. & Robison, R.A. nature and timing of the Cambrian radiation (for discussion of some 2015b. New records of Burgess Shale-type taxa from the middle Cambrian of of these hypotheses, see Lieberman & Cartwright 2011; Daley et al. Utah. Journal of Paleontology, 89, 411–423, https://doi.org/10.1017/jpa. 2015.26 2018). In addition, progress recently has been made in under- Daley, A.C., Antcliffe, J.B., Drage, H.B. & Pates, S. 2018. Early fossil record of standing the geographical distribution of various fossils in the Euarthropoda and the Cambrian Explosion. Proceedings of the National Spence Shale, but much more information is needed about the Academy of Sciences of the USA, 115, 5323–5331, https://doi.org/10.1073/ stratigraphic and sedimentological context of fossils within and pnas.1719962115 Deiss, C.H. 1938. Cambrian formations and sections in part of Cordilleran across localities (Box 4). Only then will it be possible to work out Trough. Geological Society of America Bulletin, 49, 1067–1168, https://doi. the various taphonomic pathways that allowed soft-bodied preser- org/10.1130/GSAB-49-1067 vation in this key window of Cambrian life. Egenhoff, S.O. & Fishman, N.S. 2013. Traces in the dark: sedimentary processes and facies gradients in the upper shale member of the Upper Devonian–Lower Mississippian Bakken Formation, Williston Basin, North Dakota, U.S.A. Acknowledgements We thank P. Donoghue (University of Bristol) for Journal of Sedimentary Research, 83, 803–824, https://doi.org/10.2110/jsr. inviting us to write this paper. This contribution would not have been possible 2013.60 without the dedication and generosity of the Gunther family, as well as P. Jamison Elrick, M. & Snider, A.C. 2002. Deep-water stratigraphic cyclicity and carbonate and P. 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