Lucas, S. G., Hunt, A. P. & Lichtig, A. J., 2021, Fossil Record 7. New Mexico Museum of Natural History and Science Bulletin 82. 75 COMPARATIVE TRILOBITE TAPHONOMY FROM THE CRATON TO OUTER SHELF, LATEST EARLY CAMBRIAN (STAGE 4; LATE DYERAN) OF THE SOUTHERN GREAT BASIN, USA

JOHN R. FOSTER

Utah Field House of Natural History State Park Museum, 496 East Main St., Vernal, Utah 84078; [email protected]

Abstract— Analysis of a sample of more than 2600 fossil specimens from fine-grained siliciclastic deposits of the Bolbolenellus euryparia-Nephrolenellus multinodus trilobite zones of the latest Dyeran (Stage 4), across a craton-to-outer shelf transect of the Cambrian passive margin wedge of southwestern Laurentia, demonstrates several patterns from shallow, proximal settings to those of deeper, offshore environments. These patterns include: 1) a decrease in overall fossil abundance from cratonic/nearshore to offshore settings; 2) higher taxonomic diversity on the inner and middle shelves compared to craton/ nearshore, and lowest diversity on the outer shelf; 3) low degree of articulation of trilobites across all environments (~1% of identifiable trilobite specimens), with fragmentation of sclerites increasing offshore and highest degrees of fragmentation and disarticulation on the outer shelf; and 4) trilobite cephalon size and compass orientation show no discernible pattern across the shelf; trilobite cephala up-down orientation varies by site and shows no clear paleogeographic pattern. These results suggest that sclerite burial rates and possibly population densities were higher in middle shelf to cratonic/ nearshore settings, that possibly fluctuating abiotic conditions in nearshore settings limited taxonomic diversity relative to the open shelf, and that long bottom-exposure times and possibly scavenging near or below storm wave base were more responsible for the disarticulation and breakage of trilobite remains than mechanical transport (current action) in more distal settings.

INTRODUCTION trilobite remains in a given formation (Brett and Baird, 1986; The Cambrian passive margin wedge of the southern Great Speyer, 1987; Speyer and Brett, 1988; Speyer, 1991). These Basin of the western United States consists of up to several taphonomic properties reflect many environmental conditions thousand feet of sedimentary rock ranging through much of and may provide refinement of our interpretations of the the Cambrian period in what was northwestern Laurentia paleoenvionmental settings. Several studies have looked at (ancient coordinates). Cambrian deposits range from cratonic/ taphonomic characteristics of Paleozoic deposits, and some have nearshore settings relatively close to the paleo-shoreline out to surveyed a range of formations and Lagerstätten, but few have outer shelf, deeper-water deposits, in a generally southeast to investigated the characteristics from the same time interval and northwest orientation (Stewart, 1970; Palmer, 1971; Pope et al., paleoenvironment across the open shelf (Westrop and Rudkin, 2012), by modern coordinates. Geologic characteristics of the 1999; Karim and Westrop, 2002; Terfelt, 2003; Hunda et al., exposures of Proterozoic and Cambrian sedimentary rocks in 2006; Paterson et al., 2007; Brett et al., 2012). the southern Great Basin led to a three-facies lateral subdivision Previous studies of early Cambrian trilobite taphonomy in (proximal to distal: Craton, Death Valley, and White-Inyo) by the Great Basin region have focused on single sites or formations Nelson (1978). Bahde et al. (1997) used the terms “craton” and and have found a range of taphonomic and biostratinomic “miogeocline” for the above inner two lateral facies, with a characteristics (Webster et al., 2008; Foster, 2011a). The present “craton margin” between them. More recently, lateral positions investigation studies the properties of trilobite taphonomy along this Cambrian margin transect have been divided into from several sites in the latest early Cambrian, from a the craton and inner, middle, and outer shelf settings (Webster craton/nearshore to outer shelf transect, in order to compare 2011a). However labeled or divided, within the Cambrian, the taphonomic conditions. The aim is to test the null hypotheses rocks record the early to late period (Terreneuvian–Furongian) that there are no variations in taxonomic composition, relative accumulation of siliciclastic and carbonate sediments along abundance, diversity, preservation, articulation, breakage, size, several hundred miles of the northern shoreline of Laurentia and orientation of trilobite remains along the craton/nearshore to (ancient coordinates), and preserve environments from the outer shelf transect. shallow near-shore to distal deeper water. These Cambrian Abbreviations – MWC, Museums of Western Colorado, carbonate and siliciclastic sediments can record a complex mix Fruita, Colorado. of environments and deposition affected by multiple factors LOCALITIES (Myrow et al., 2012). The formations and sites investigated for this study Fossiliferous units in this region and along the Cambrian include: the Bright Angel Formation at Frenchman Mountain, margin are numerous, taxonomically diverse, and rich in Clark County, Nevada (cratonic/nearshore setting); the Pioche material and have been studied since the time of C. D. Walcott’s Formation at Oak Spring Summit, Lincoln County, Nevada work in the area in the nineteenth century (Walcott 1884, 1910). (inner shelf setting); the Carrara Formation at Emigrant Pass, Olenellid trilobites, which are particularly abundant in the early Inyo County, California (middle shelf setting); and the Emigrant Cambrian deposits of the Great Basin, often exhibit a range Formation at Split Mountain, Esmeralda County, Nevada (outer of taphonomic characteristics that provide useful information shelf setting) (Fig. 1). about the paleoenvironmental conditions in which they were Cambrian sites on flooded continental craton are generally deposited. Studies of the taphonomy of trilobite deposits in shallow water and are subject to bottom currents caused by suggest that various factors of the environmental settings and tidal ebb and flood (Rose, 2006, 2011), as well as by the wave of trilobite behavior may result in characteristic properties of action caused by wind stress and storms; these sites may also 76 be influenced by variability in temperature and salinity due to freshwater input from rivers. The inner and middle shelves are similarly influenced, through to a lesser degree, by wind and NEVADA US 6 CALIFORNIA Split Mountain storm waves but are more stable in terms of salinity. The outer shelf is generally deeper and well below storm wave base but Oak Spring may be distinguished by its still being affected in some cases by TONOPAH Summit major ocean currents that can exceed 100 cm/sec (Boggs, 1987; Harris et al., 1996). Frenchman Mountain (Craton/Nearshore) US 95 BISHOP US 93 The most proximal site to the Cambrian shoreline, representing the craton/nearshore setting, is Frenchman Death Valley National Park BEATTY Mountain, outside Las Vegas, Nevada (Figs. 1 and 2). This Frenchman section consists of an exposure of lower to upper Cambrian Mountain sedimentary rocks, the base of which includes the Tapeats Sandstone and Bright Angel Formation. The Bright Angel was N LAS VEGAS originally named as “Bright Angel shale” for exposures in the Grand Canyon by Noble (1914, 1922), but it has been renamed Bright Angel Formation due to its diverse lithology by Rose US 395 (2011), a possibly that Noble himself left open. Geologic maps covering the exposures of Frenchman I-15 Emigrant US 95 Mountain contain a mix of Colorado Plateau and Great Basin 100 km Pass stratigraphic names (Matti et al., 1993; Castor et al., 2000) and BAKER map the Bright Angel as Pioche Shale despite the unit being 50 mi under- and overlain by the Tapeats and Muav, respectively. The siliciclastic units between the Tapeats and Muav at Frenchman FIGURE 1. Locality map of the four sites studied for this project Mountain: 1) lithologically resemble the Grand Canyon’s (stars). Inset shows location of main map (gray box) within Bright Angel Formation more than they do the Pioche, and 2) North America, together with scale bars for main map. were translated more than 60 km, and up to as many as 80 km, westward to their present position along the Las Vegas Valley shear zone (Longwell, 1960; Fryxell and Duebendorfer, 2005; Rowland, 2011), from near the Grand Wash Cliffs where the unit is classified as Bright Angel Formation. The shales mapped as Pioche Shale on local geologic maps (Matti et al., 1993; Castor et al., 2000) should be designated as Bright Angel Formation (see also Webster, 2011b). The Bright Angel Formation of the Grand Canyon and

SPLIT OAK SPRING MOUNTAIN Tonopah SUMMIT OAK SPRING Montezuma Delamar Mtns. SUMMIT Bishop Range Grapvine-Funeral Mountains

SPLIT FRENCHMAN MOUNTAIN Sierra Nevada MOUNTAIN

Death Valley

Sheep Range INNER SHELF

Spring Mountains Nopah Range

MIDDLE SHELF

Las Vegas OUTER SHELF Colorado Plateau Colorado Plateau FRENCHMAN MOUNTAIN EMIGRANT Clark Mountains N PASS EMIGRANT PASS

(Craton) Baker NEARSHORE A 100 km B 100 km ~PaleoN FIGURE 2. Modern and paleogeography of the four sites studied here. A, Locations of the four sites relative to mountain ranges of the Basin and Range province, and to the Colorado Plateau and Sierra Nevada Mountains. B, Relative position of the sites after palinspastic restoration of the southern Basin and Range to pre-extension width (based on Snow and Wernicke, 2000), showing reduction of cratonic/nearshore to outer shelf settings of Frenchman Mountain and Split Mountain to ~120 km, and with approximate positions of cratonic/nearshore, inner, and middle shelf transitions restored. 77 Frenchman Mountain represents a cratonic, relatively nearshore levels were combined as representative of the Nephrolenellus setting and, as part of the Tonto Group, has been described multinodus Biozone in the area. paleoenvironmentally as representing an “expansive epicratonic Emigrant Pass (Middle Shelf) estuary”, with a mix of shallow marine, esturine, tidal, and some terrestrial sediments covering a large area of flooded continental The Emigrant Pass locality is in the Carrara Formation crust (Rose, 2006). As such, the Frenchman Mountain section of the southern Nopah Range (Figs. 1 and 2) in Inyo County, in this study represents the most proximal, and probably the eastern California, southeast of Shoshone (Mount 1980; shallowest, setting in the transect; the fossils found at the site Fowler 1999). This section represents the middle shelf setting. (trilobites and hyoliths) clearly indicate that the sampled level Samples were collected from two 15 cm- thick greenish-gray was deposited in a marine environment. The Bright Angel to to pink-gray shale levels stratigraphically about 5.25 m apart, the southeast of Grand Canyon likely represents an upper to near the top of the Dyeran section; these were from the 124.0 lower proximal shore setting (e.g., Łabaj and Pratt, 2016), so m and 129.25 m levels in the lower Pyramid Shale Member the formation at Frenchman Mountain is likely fully marine of the Carrara Formation on Fowler’s (1999) stratigraphic and some distance from the shoreline but shallow. It is only section (Nephrolenellus multinodus Biozone). Each sample was geographically more proximal than the other localities in this similar in trilobite species content, diversity indices, and other study; it is not so proximal as to be a sandy shoreface setting characteristics, and the two samples were combined for this represented by the Tapeats Sandstone to the east. area. The total sample collected from the Carrara Formation at The sample collected from the Bright Angel Formation at Emigrant Pass included more than 650 specimens. Frenchman Mountain for this study came from a thick green Split Mountain (Outer Shelf) shale outcrop interval approximately 45.3–45.6 m above the base of the section in Webster (2011b) (possibly equivalent to The Emigrant Formation section is exposed southwest the Tapeats Sandstone–Bright Angel contact), and the sample of Tonopah in Esmeralda County, Nevada, just west of the consisted of more than 950 individual elements representing Montezuma Range (Figs. 1 and 2), in the hills of Clayton Ridge mostly trilobites and hyoliths. The sample proved to be from near Split Mountain. This section within the Emigrant represents an interval spanning the boundary between the Bolbolenellus the outer shelf setting (Webster, 2011a; Kepper, 1981). The euryparia and Nephrolenellus multinodus trilobite zones. Data section studied here was the Split Mountain East Trench section from a preliminary sampling of the Bright Angel Formation at of Webster (2011d) and Sundberg (2018); this section consists this site (Foster, 2011b) are included within the dataset for this of approximately the lower 1.5 m of the Emigrant Formation, study. just above top surface of the Mule Spring Limestone. Above the 1.5 m level are middle Cambrian trilobites of the Lincolnian Oak Spring Summit (Inner Shelf) (uppermost Stage 4). The sample from the lower Emigrant Representing the inner shelf setting, the Cambrian rocks at Formation consists of 68 specimens collected by Museum of Oak Spring Summit, in the Delamar Mountains west of Caliente, Western Colorado crews in 2013, plus a sample of 18 collected Nevada (Figs. 1 and 2), include a section from the lower Cambrian by Mark Webster and his field assistant (data from Webster, Zabriskie Quartzite up through the Pioche Formation and into pers. comm., 2013). Most specimens were collected between the Lyndon Limestone, Chisholm Formation, and overlying 0.6 m and 0.8 m above the base of the Emigrant Formation, in units. Fossils are known from many Pioche Formation outcrops soft grayish-green shale between the top limestone of the Mule in the Delamar Mountains (including Oak Spring Summit Spring (Mule Spring–Emigrant contact) and a thin tan-white and Hidden Valley) as well as the Chief and Highland ranges limestone just over a meter above the base. This interval appears to the north near the towns of Pioche and Panaca (Merriam, to be within the Nephrolenellus multinodus Biozone (Webster, 1964; Webster, 2011c). The Pioche Formation consists of 2011a). several members, including, in ascending order, the Delamar, Sundberg and McCollum (2003) and Sundberg (2018) Combined Metals, Comet Shale, Susan Duster Limestone, Log identified Olenellus, Bolbolenellus, Crassifimbra, and Cabin, and Grassy Springs members. The Ruin Wash site in the indeterminate ptychopariids from the lower Emigrant Formation, Chief Range is at the top of the Combined Metals Member and in a limestone at the 1.2 m level. These data were not included has produced many hundreds of articulated olenellids and even here, as they were sampled from a different lithology than the some soft-bodied arthropods such as Canadaspis, Tuzoia, and shale being compared between sites in this study. Webster Anomalocaris (Webster et al., 2008; Lieberman, 2003) and is (2011d) reported an indeterminate corynexochid pygidium and equivalent in age to the Oak Spring Summit level sampled here. indeterminate trilobite debris from shale in the lower meter of The samples collected from the Pioche Formation at Oak the Emigrant Formation at the Spilt Mountain East trench site; he Spring Summit for this study came from tan to pinkish shale also reported a single specimen of Olenellus gilberti from same in the upper Combined Metals Member in the Nephrolenellus interval at Split Mountain West; these data were incorporated multinodus Biozone (Webster, 2011a, 2011c). The samples into the sample collected for this study. were from three layers and ranged over a 4.1 m interval from Skovsted (2006) identified a number of mollusks and other approximately 1.2–5.3 m below the basal limestone layer of the small shelly fossils in the same lower Cambrian levels of the Comet Shale Member. This shale-limestone contact also marks Emigrant Formation at Split Mountain studied here. At least two the lower-middle Cambrian boundary in the region. The three types of brachiopods were identified in stratigraphically higher sample levels were approximately 24.7 m, 26.5 m, and 28.8 m limestones of the Emigrant Formation by Lin (2001), and during above the base of the Combined Metals Member (see section in associated exploration of the Split Mountain sites. MWC crews Webster 2011c) and consisted of nearly 900 individual elements also found numerous brachiopods and trilobite fragments in the representing mostly trilobites and hyoliths. Each sampled layer lower Cambrian limestones at the East Trench Section. was approximately 15 cm thick, and the numerically largest METHODS sample proved to be that at the 28.8 m level. All species of trilobites found in the two lower sample levels were also found Trilobite and other specimens were excavated from pits in the 28.8 m level, and there were few significant differences within a limited stratigraphic interval of the Bright Angel, between the three samples, other than in total number of Pioche, Carrrara, and Emigrant formations. Within each pit, specimens. Calculated diversity indices for the levels were specimens were collected from intervals 15–30 cm thick, and also rather similar. For this site, therefore, all three sampled these beds were measured in to the nearest formation or member contact (up from Tapeats–Bright Angel contact at Frenchman 78 Mountain; up from Delamar–Combined Metals contact at Oak cephalon up-down direction were also recorded. Most sites had Spring Summit [also down from the Combined Metals–Comet at least moderate dip to the beds. Sample tests on stereographic Shale]; up from the base of the Carrara Formation at Emigrant restoration of azimuth orientations from strike and dip showed Pass; and up from the Mule Spring–Emigrant contact at Split minimal change, and within any one bedding plane the readings Mountain). were collected by a consistent method relative to each other, so Most trilobite species from formations studied here are all orientation data remain unrestored. Although true compass members of the monophyletic Olenelloidea (Lieberman, 1998, orientation distributions of some sites relative to others might 1999), although many of the “families” within this group may change slightly with restorations, single site rose diagram be paraphyletic (Webster, 2003). Trilobite specimens were distributions should be internally consistent, which is more identified using Palmer (1998), Palmer and Halley (1979), Palmer important in this case. Cephala were measured with digital and Repina (1993), Lieberman (1999), Whittington (1989), calipers for width and length of the cephalon. Each trilobite and Webster (2003), along with museum collections from the specimen was assessed for completeness, and any pre-burial University of California Riverside and Smithsonian Institution. fragmentation of elements was noted; each element identity This paper follows Lieberman’s (1999) use of Mesonacis for was noted. Only fragmentation clearly not related to collection M. fremonti over the previous subgenus designation Olenellus or post-diagenetic weathering was recorded; most incomplete (Mesonacis) of Palmer and Repina (1993). Most genera cephala were on the edge of shale piece breaks and were not (Biceratops, Nephrolenollus, and Bolbolenellus) belong to the counted as being broken. Cephala covered by sediment along “Biceratopsidae”, whereas Olenellus and Mesonacis are within an incomplete edge were not recorded as broken either. Only the possible monophyletic Olenellidae (Webster, 2003). incomplete cephala where a broken sclerite edge was bordered Trilobite specimens were collected marking for the by an expanse of the same bedding plane were counted, original-up and true-north orientation of the shale piece. and the same was true of fragmented thoracic segments. All Azimuth orientations of the anterior direction of the long axis identifications and data were entered into and sorted in Excel, of the glabella, relative to true north, were recorded from with which statistical data were also calculated. Rock samples these markings in the lab with a protractor and angle-finder; from each sampled quarry level were analyzed using hand-

FIGURE 3. Thin section micrographs of the four main shale layers from which trilobites and other fossils were collected for this study. A, Bright Angel Formation at Frenchman Mountain showing angular to subangular silt grains infilling a possible burrow and indistinct silt laminations. B, Pioche Formation at Oak Spring Summit showing relatively low amount of detrital silt grains. C, Carrara Formation at Emigrant Pass showing some larger, more rounded silt to fine sand grains.D , Emigrant Formation at Split Mountain showing clay and silt-sized grains. Images ~2000 µm across. 79 samples and thin sections. the Sequence III–IV maximum flooding surface and within the The goal of the study was to compare taphonomic and interval of the Redlichiid-Olenellid Extinction Carbon isotope biostratinomic characteristics of late Dyeran trilobite skeletal Excursion (ROECE) (Faggetter et al., 2017; Lin et al., 2019). material across a craton-to-outer shelf transect to test how Control for depositional setting was achieved by only conditions may have changed in moving from a relatively sampling shale units, and carbonate and sandstone beds were nearshore to off-shore setting. In order to ensure that relative not sampled. Control of the intensity of sampling across all setting along the transect was the main (or only) variable, it was four sites was accomplished by excavating each locality for necessary to attempt to control all other variables. In order to approximately 45–50 worker-hours (number in crew X number control for geographic setting, all four sites were selected from of hours worked as a group each day X number of days). the southern Great Basin. At each locality, one to three 15–30 Standardizing the sampling effort was important to ensure that cm intervals were sampled in order to find sufficient sample total resulting data between the sites were comparable, if not size (~100+ specimens); if it was determined that species necessarily equal (e.g., Dunhill et al., 2012). preservation, relative abundance, and overall taphonomic and Diversities represented by the collected samples from each preservational characteristics were similar between the sampled site were compared across the transect. As a measure of diversity intervals, they were combined into a single representative for each site sample, Shannon’s Entropy (H) was converted sample of that site. In order to limit the temporal range sampled, to effective richness,eff S (Olszewski, 2010). These measures collections were taken only from a limited interval of the latest take into account both total species diversity and relative two of the Dyeran trilobite zones, the upper Bolbolenellus abundance. Both high straight diversity counts and relatively euryparia and Nephrolenellus multinodus zones (Fig. 4; even abundance distributions within the sample will increase Webster, 2011a). The Oak Spring Summit and Emigrant Pass the effective richness; conversely, low diversity counts and/or sites were sampled purely from the N. multinodus zone near the presence of very abundant and very rare species will lower the Sequence III to Sequence IV boundary of Webster (2011a); at effective richness. The formulae used were Frenchman Mountain the sample interval, though thin (~30 cm), Shannon’s Entropy H = -Σpilog(pi) ranged across the B. euryparia-N. multinodus zone boundary, and with most of the sample dominated by Biceratops nevadensis effective richness S =eH and Olenellus terminatus near the base of the interval and a eff single specimen of N. multinodus at the top (this sample was where p is the proportion of a species in the site sample from near the base of Sequence III; Webster, 2011a); and at and e is the mathematical constant base of the natural logarithm. Split Mountain the sample came from a thin condensed section Shannon’s Entropy is affected by sample size, but the effect is less comprising the N. multinodus Biozone (and/or possibly the B. significant above sample sizes of approximately 100 specimens euryparia Biozone) and both Sequences III and IV (McCollum (Olszewski, 2010). Diversity indices and rarefaction analyses and Sundberg, 2000; Sundberg et al., 2011; Webster, 2011a; were calculated using PAST 4.01. Up-down orientations of Sundberg, 2018). Most sites were also sampled at a level near cephala were compared with binomial analyses run in Stat Trek.

Upper Dyeran Trilobite Zones Outer Middle Inner Craton/ Laurentia Laurentia Global Depositional Shelf Shelf Shelf Series Stage Stage Sequences (Webster, 2011a) Nearshore (Webster, 2011a) Split Emigrant Oak Spring Frenchman Mtn. Pass Summit Mtn. Glossopleura walcotti Lyndon Ls Muav/ Mbr

Hills Sh Lyndon Mexicella mexicana Pahrump Mbr Red Grassy Grassy Spring Pass Pass Ls Mbr Wuliuan Mbr Poliella denticulata Log Cabin Lincolnian Delamaran Amecephalus Susan Duster arrojoensis Ls Mbr Mbr

Eokochaspis nodosa Shale Comet Comet Pyramid IV Shale Mbr Nephrolenellus Formation Emigrant multinodus III

Gold Ace Combined Combined Ls. Mbr Metals Mbr Pioche Formation Pioche Carrara Formation Carrara II Bolbolenellus euryparia Formation Bright Angel Mbr Stage 4 Stage Echo Shale Echo Dyeran

Peachella iddingsi Limestone Waucoban Mule Spring Ls. Mbr Ls. I Bristolia insolens Thimble Delamar Mbr Tapeats Sandstone Bristolia mohavensis Harkless Sh. Mbr

Fm Eagle Mtn. FIGURE 4. Positions of the four sampled layers studied here (stars / interval bars) relative to local sections, cratonic/nearshore- to-outer shelf transect setting, Laurentian trilobite zones, early Cambrian depositional sequences of the southern Great Basin, and stages and series. Gray band correlates Nephrolenellus multinodus zone relative to formations, sequences, formations, and series and stages. Modified from Webster (2011a). 80 GEOLOGIC SETTING by horizontal transport of some distance (Plint, 2010). At each Each of the four sites studied here lies in mountain range site, meters-thick units of shale are separated by occasional thin horsts bounded by normal faults created by Cenozoic extension bioclastic limestones; the only site devoid of limestone units is in the Basin and Range province of the southern Great Basin. that in the Bright Angel Formation at Frenchman Mountain. The lower Cambrian sedimentary rocks studied were part of the Petrographic Analyses lower Paleozoic passive margin wedge of Laurentia, laid down Frenchman Mountain (Bright Angel Formation).–The main along the northern (now western) margin of the continent (Bahde sampled layer in the Bright Angel Formation at Frenchman et al., 1997; Pope et al., 2012). The shallow shelf bordering the Mountain is a pale olive shale (10Y 6/2). In thin section, the continent extended proximally into a massive intertidal expanse layer consists of more than 90% clay, with angular to subrounded of flooded craton, the “expansive epicratonic estuary” of Rose quartz silt grains up to 63 µm diameter scattered throughout and (2006). The present southeast-northwest distance between the concentrated in a few laminations up to 1 mm thick. There are cratonic/nearshore Frenchman Mountain site and the outer also some detrital micas and a few rare, tiny fossil fragments, shelf Split Mountain locality is approximately 300 km, but and an unidentified opaque mineral is scattered throughout (Fig. this distance has been exaggerated by Cenozoic extension of 3A). the Great Basin of at least 65% (Wernicke et al., 1982), which Oak Spring Summit (Pioche Formation).–The main sampled would suggest a distance between the sites during the Cambrian layer in the Pioche Formation at Oak Spring Summit (1.2 m of approximately 181 km. A more recent palinspastic restoration below the Combined Metals-Comet Shale contact) is pale green of the Great Basin ranges (Snow and Wernicke, 2000, their fig. shale (5G 7/2). In thin section, the layer consists of more than 2) indicates that the pre-Cenozoic Frenchman to Split Mountain 95% clay, with a few subrounded to rounded quartz silt grains up distance may well have been closer to 120 km (Fig. 2). This to 100 µm diameter scattered throughout but not concentrated width for the Cambrian shelf, though by no means its maximum in laminations. There are fewer detrital micas than in the Bright even during the early Cambrian in this region, is well within Angel, and a few rare, more intact fossil fragments occur. An the known width range of modern shelves, which may be as unidentified opaque mineral is also scattered throughout (Fig. little as <5 km (Sobarzo and Djurfeldt, 2004) to as much as 3B). >800 km (Romanovskii et al., 2004), with an average width Emigrant Pass (Carrara Formation).–The main sampled of approximately 85 km (Elrod et al., 2004). And it is still not layer in the Carrara Formation at Emigrant Pass is light greenish exactly clear how close to land the Frenchman Mountain site gray shale (5G 8/1). In thin section, the layers consists of may have been. approximately 85–90% clay, with a relatively high percentage The passive margin wedge thickens considerably westward of subrounded and rounded to well-rounded quartz silt and fine in the study area. At Frenchman Mountain, the Bright Angel sand grains up to 127 µm diameter scattered throughout and Formation overlies ~40 m of the Tapeats Sandstone, which in concentrated in several thin laminations mostly less than 1 mm, turn rests on the Great Unconformity (Matti et al., 1993; Castor but up to 2 mm, thick. There are very few detrital micas and et al., 2000; Webster, 2011b). Thus, the sampled level studied few rare, tiny fossil fragments; an unidentified opaque mineral here lies approximately 85 m above Proterozoic metamorphic is scattered throughout (Fig. 3C). and igneous rocks. At Oak Spring Summit, the total Pioche Split Mountain (Emigrant Formation).–The main sampled Formation section below the level of the top of the Combined layer in the Emigrant Formation at Split Mountain is light Metals Member is approximately 130 m (Webster, 2011c); below greenish gray shale (5G 8/1). In thin section, the layer consists this is the Zabriskie Quartzite, which is approximately 730 m of more than 90–95% clay, with no definite detrital micas but thick and is the stratigraphically lowest known sedimentary unit much of an unidentified opaque mineral. In some spots there in the area (Merriam, 1964). At Emigrant Pass, the sampled is abundant fragmented fossil material (possible coprolites?). pits are similarly approximately 125–130 m above the top of There is very little obvious quartz silt, but that which is present is the Zabriskie Quartzite (Fowler, 1999); the Zabriskie overlies subrounded to rounded and approximately 32–62 µm diameter; the Wood Canyon Formation, Stirling Quartzite, Johnnie it is scattered throughout but not concentrated in laminations. Formation, and Noonday Dolomite, which collectively have a Most interesting is a number of small silt- and very fine-sand thickness of approximately 1900 m (Bahde et al., 1997). The sized (<86 µm), square rhombs of possible anhydrite mixed in extremely thin, condensed lower Cambrian section of the lower with the clay (Fig. 3D). This mineral identification is not certain, Emigrant Formation at Split Mountain is nevertheless part of but these silt-sized, rhomb-shaped clasts are clearly not quartz a tremendously thick outer shelf section; below the Emigrant, or feldspar. in descending order, are the Mule Spring Limestone, Harkless Formation, Poleta Formation, and Campito Formation in the RESULTS AND DISCUSSION Cambrian. These formations overlie the Deep Spring Formation Taxa and Reed Dolomite, sedimentary units of the Proterozoic (Nelson, 1978; Hollingsworth, 2011). The total thickness of the The samples yielded most trilobite species reported section below the Emigrant Formation in Esmeralda County, previously from each site, but several rare taxa were not Nevada, from the Mule Spring Limestone down through the encountered at Oak Spring Summit and Emigrant Pass. The sites Reed Dolomite is approximately 3800 m (Nelson, 1978; Ahn at Frenchman Mountain, Oak Spring Summit, and Emigrant et al., 2011). Thus, the inner to outer shelf localities (at 860 m, Pass each produced 4–5 species of trilobites, plus hyoliths; the 2025 m, and 3800 m of sedimentary units below the sampled outer shelf site, Split Mountain, produced brachiopods and at level, respectively) are part of progressively much thicker least two species of trilobites. Webster et al. (2008) found six overall sections of the passive margin wedge, compared with the species of trilobites at the inner shelf Ruin Wash locally in the cratonic/nearshore section at Frenchman Mountain (at 85 m). Pioche Formation northeast of Oak Spring Summit. Ichnofossils Each of the sites (specifically selected for their shale were extremely rare in the sampled shale intervals, with only a lithologies), suggest a relatively low-energy paleoenvironment handful of shallow horizontal traces observed during the entire during deposition. The shale from each sampled locality appears study. to be largely clay mudstone, with a few grains coarser silt Frenchman Mountain.–The sample from the cratonic/ forming laminations or isolated within the clay. Such shale units nearshore Bright Angel Formation at Frenchman Mountain are typical of mud-dominated shelves, where much of the mud consists of 972 specimens and is dominated by the olenellid may be sourced from rivers and distributed throughout the shelf Olenellus terminatus (49.6% of the sample; Fig. 5A–C; see 81

FIGURE 5. Trilobite specimens from the early Cambrian part of the Bright Angel Formation, Frenchman Mountain, Nevada. Cratonic/nearshore setting. A, Cephala of Olenellus terminatus. B, Cephalon of Olenellus terminatus, MWC 7155. C, Cephala of Olenellus (mostly O. terminatus) typical of the densest parts of the deposit, MWC 7154. D, Articulated thorax and fragment of cephalon of Biceratops nevadensis (MWC 7172). E, Three cephala of Biceratops nevadensis. F, Cephalon of Biceratops nevadensis, MWC 6926. G, Shale piece with several olenellid cephala, including Olenellus gilberti, MWC 7160. H, Cephalon of Olenellus gilberti, MWC 7160. I, Cephalon of Nephrolenellus multinodus, MWC 8097. All scale bars = 1 cm. All Museums of Western Colorado specimens. 82 also Fig. 10) with the biceratopsid Biceratops nevadensis (Fig. may be environmental differences of bottom sediment or depth 5D–F; Pack and Gayle, 1971) less common at 21.8%, and the between the cratonic/nearshore to middle shelf sites, but they olenellid O. gilberti (Fig. 5G–H) rare (3.8%; see also Fig. 10). are not obvious from the hand and thin-section samples studied Nephrolenellus multinodus (Fig. 5I) is represented by a single from each locality. specimen from the upper part of the sampled interval (0.2%). The lower diversity in the outer shelf setting may be an The rest of the sample consists of olenelloid fragments and actual biological pattern, or it might be a result of this level hyoliths (see Fig. 11). Isolated cephala and thoracic segments of the Emigrant Formation representing a condensed section are abundant (Fig. 5C; see Fig. 11) and articulated thoraces (Fig. (McCollum and Sundberg, 2000; Sundberg et al., 2011; 5D) are very rare (see Fig. 11). Sundberg, 2018); low sedimentation rate and long bottom Oak Spring Summit.–The sample from the inner shelf exposure times may result in high degrees of disarticulation Pioche Formation at Oak Spring Summit consists of 904 (Speyer, 1987), reduce the number of preserved sclerites, and specimens and is dominated by the olenellid Olenellus gilberti thus lower the number of taxa preserved. (49.7 %; Fig. 6C, Fig. 6F, Fig. 10); Nephrolenellus geniculatus The LAD of Biceratops appears to be at the top of the (Fig. 6D), O. terminatus (Fig. 6A–B), and O. chiefensis (Fig. Bolbolenellus euryparia zone (Webster, 2011a), and even 6F) are less common at 14.8%, 9.7%, and 8.7%, respectively though that trilobite zone was only sampled at Frenchman (Fig. 10). Olenellus fowleri (Fig. 6E) is rare at 1.7% (Fig. 10). Mountain during this study, Biceratops has not been reported This is approximately the same pattern of abundance found at a from lower levels of the Pioche, Carrara, or Emigrant formations similar stratigraphic level of the Combined Metals Member of either. Indeed, Biceratops appears to be restricted to the Bright the Pioche Formation at Ruin Wash by Webster et al. (2008, table Angel Formation, not just in this study, but also considering an 2). The remainder of the Oak Spring Summit sample consists area encompassing the southern Great Basin and the Colorado of indeterminate olenelloids and hyoliths (Fig. 10). Complete Plateau. The genus has not been reported from any Cambrian articulated specimens (cephalon and thorax; Fig. 6A) are very section other than Frenchman Mountain, except for a specimen rare, and most of the sample consists of isolated cephala and illustrated by McKee and Resser (1945, plate 19, fig. 25) thoracic segment fragments (Fig. 6F and 11). and listed as “Olenellus sp.” from the Tapeats–Bright Angel Emigrant Pass.–The sample from the middle shelf Carrara transition zone in Quartermaster Canyon of the western Grand Formation at Emigrant Pass consists of 647 specimens, many Canyon. Biceratops thus may have been endemic to cratonic/ relatively distorted tectonically, and the sample is dominated nearshore settings; most other trilobite species encountered by Olenellus terminatus (Fig.7A–B) at 40.0% (Fig. 10). during this study (both common and rare species) appear to be Nephrolenellus multinodus (Fig. 7E–F) and O. gilberti (Fig. relatively widespread within the southern Great Basin. 7C) are less common at 10.4% each, and Mesonacis fremonti Ptychopariid trilobites can be abundant, and co-occur (Fig. 7D) is least abundant at 7.3% (Fig. 10). Indeterminate with olenelloids, in some facies within lower Cambrian units olenelloids and hyoliths account for almost a third of the total in the Great Basin. Webster and Zelditch (2011) reported sample (Fig. 10). Fragmented cephala (Figs. 7G and 7H) and well preserved Crassifimbra walcotti from carbonates in the thoracic segments (Figs. 7I and 7J) also occur. Isolated cephala Combined Metals Member of the Pioche Formation, though and thoracic segments dominate the sample, and complete well below the level studied here. However, not a single element specimens and articulated series of thoracic segments (Fig. 8) that could be attributed to Ptychopariida was identified in the are extremely rare (Fig. 11). samples collected from shale for this study. This suggests Split Mountain.–The sample from the outer shelf Emigrant an environmental preference of ptychopariids for carbonate Formation at Split Mountain consists of more than 80 specimens environments (or possibly for shallow water deposits) versus and is dominated by brachiopods (52.2%; Fig. 10). Trilobites siliciclastic settings during the early Cambrian; by early in the are comparatively rare and most are unidentifiable olenelloid middle Cambrian ptychopariids were commonly occurring in cephalon and thoracic segment fragments (66.6% of trilobite shale deposits, but they are usually very rare in those of early specimens; Figs. 9–11). The only non-olenelloid in the entire Cambrian age. In the slightly older early Cambrian Harkless four-site study sample is a single corynexochid pygidium from Formation at Split Mountain, ptychopariids are relatively more this site, collected by Mark Webster (pers. comm., 2012). common (F. Sundberg, pers. comm., 2019). These results (Fig. 10) suggest several possible patterns, Diversity and Abundance including: 1) the domination of the outer shelf setting by brachiopods rather than olenelloids; 2) the domination of the The overall sample from the four sites demonstrates two cratonic/nearshore to middle shelf settings by Olenellus, either trends: decreasing overall fossil abundance from the cratonic/ O. terminatus or O. gilberti; 3) distinctly lower diversity in the nearshore to the outer shelf (lower sample size at each more outer shelf setting (measured by effective richness) and possibly distal site, for comparatively the same sampling effort as the next slightly lower diversity in cratonic/nearshore settings; 4) that proximal; see Table 1 and N values in Figures 10–13) and higher Biceratops was possibly endemic to cratonic/nearshore settings; diversity in the inner and middle shelf settings relative to the and 5) that early Cambrian ptychopariids of western Laurentia craton/nearshore and especially the outer shelf, as indicated by may have had an environmental preference for carbonate shoals each respective effective richness value (Fig. 10). The effective rather than siliciclastic settings (see below). richness value of the upper Combined Metals Member of the It is unclear whether the abundance of brachiopods at Split Pioche Formation at Oak Spring Summit (Fig. 10) is comparable Mountain (outer shelf) is a result of these animals truly being to that of the same level at the Ruin Wash site in the nearby abundant relative to olenolloids during the early Cambrian Chief Range. The upper level, most productive layer at Ruin or whether this is a preservational bias. The reasons for the Wash (Surface 3) has six species, a Shannon index of 1.075, and dominance of Olenellus terminatus at Frenchman Mountain and an effective richness of 2.929; based on data in Webster et al., Emigrant Pass, and the dominance of O. gilberti at Oak Spring 2008. Low species richness on the outer shelf in this study may Summit (and Ruin Wash), are not immediately apparent. Given be affected by the small numerical sample size we were able to the overall lithologic similarities of the sites, yet their having find in the Emigrant Formation (N<100). Interestingly, however, some differences apparent in thin section (see Geologic Setting), Skovsted’s (2006; Wotte and Sundberg, 2017) small shelly these different taxonomic dominances may be the expression of fauna from the early Cambrian part of the Emigrant Formation, subtle paleoenvironmental preferences of the respective species, which included mollusks such as Anabarella, Parkula, and different biogeographic ranges, or preservational bias. There Costipelaiella, was of relatively low diversity as well. 83

FIGURE 6. Trilobite specimens from the early Cambrian upper Combined Metals Member of the Pioche Formation, Oak Spring Summit, Delamar Range, Nevada. Inner shelf setting. A, Nearly complete exoskeleton of Olenellus terminatus. B, Cephalon of Olenellus terminatus. C, Cephalon of Olenellus gilberti. D, Cephalon of Nephrolenellus geniculatus. E, Cephalon of Olenellus fowleri. F, Shale piece showing typical preservation and density of material, with broken thoracic segments, two cephala of Olenellus gilberti, and one cephalon of Olenellus chiefensis. All scale bars = 1 cm. All Museums of Western Colorado specimens. 84

FIGURE 7. Trilobite specimens from the early Cambrian part of the Pyramid Shale Member of the Carrara Formation, Emigrant Pass, Nopah Range, California. Middle shelf setting. A, Cephalon of Olenellus terminatus, MWC 7177. B, Slightly tectonically distorted cephalon of Olenellus terminatus, MWC 7180. C, Olenellid cephalon, MWC 7175. D, Cephalon of Mesonacis fremonti. E and F, Cephala of Nephrolenellus multinodus. G, Partial cephalon of Olenellus gilberti? H, Partial cephalon of Olenellus gilberti? I, Isolated partial T3 thoracic segment with intact pleural spine. J, Partial, isolated thoracic segment. All scale bars = 1 cm. All Museums of Western Colorado specimens. 85 If diversities truly were lower in cratonic/nearshore settings (e.g., Frenchman Mountain), it is possible that fluctuating temperature, turbidity, and salinity close to estuarine settings may have limited the number of species adapted to tolerate such conditions. The pattern demonstrated along this early Cambrian craton/ nearshore-outer shelf transect, with highest diversity in the inner and middle shelf, lower diversity nearshore, and lowest diversity in the outer shelf setting, is quite the opposite of that measured for some modern shelf transects, which demonstrate highest diversity on the outer shelf while parts of estuaries have moderate diversities that were in turn higher than the shallow open shelf (Boesch, 1972). Still, there is some evidence that variable conditions limit benthic invertebrate diversity. Some studies have found an apparent increase in diversity with decreasing salinity range within modern estuaries so that areas with the most variable abioitic conditions have lower diversities FIGURE 8. Trilobite specimens from the early Cambrian part of than more stable sample areas (Attrill, 2002). And there may be the Pyramid Shale Member of the Carrara Formation, Emigrant different trends in per-sample diversity both across a range of Pass, Nopah Range, California. Middle shelf setting. A, Shale depths and between samples of similar depth on moderate- to piece with olenellid cephala (including Olenellus terminatus) deep-water parts of modern continental shelves (Gray, 2000), and one partial exoskeleton along edge. B, Series of larger so additional sampling may well indicate a similarly complex articulated thoracic segments of an olenellid with small, nearly picture for the Cambrian shelf. At this early stage of taphonomic complete exoskeleton of Olenellus terminatus, MWC 7197. All study of early Cambrian trilobite deposits we cannot yet assume scale bars = 1 cm. Museums of Western Colorado specimens. that diversity trends along the shelf were similar to or different from those of modern shelves. Either possibility seems to be equally plausible. Rarefaction analysis of the four settings demonstrates similar diversity levels for the Bright Angel, Pioche, and Carrara formations at each inner locality (all within 95% confidence intervals), but equivocal results for the Emigrant Formation at Split Mountain due to low sample size. Preserved Elements Element representation is similar across the shelf and only differs in the outer shelf setting, almost certainly due to small sample size. At the cratonic/nearshore to inner and middle shelf localities, cephala and thoracic segment fragments dominate the sample to approximately the same degree (Fig. 11). Hypostomes, complete (unfragmented) thoracic segments, and articulated series of thoracic segments are rare. Cephalon fragments appear to increase in relative abundance slightly in the middle and outer shelf environments (Fig. 11). In the outer shelf environment FIGURE 9. Trilobite specimens from the early Cambrian part of of the lower Emigrant Formation (otherwise dominated by the lower Emigrant Formation, Split Mountain, Clayton Ridge, brachiopods), the trilobite sample consists of nearly three- Nevada. Outer shelf setting. A, Shale piece with cephalon quarters thoracic segment fragments, and cephala are rare (Fig. fragment of olenelloid trilobite. B, Shale piece with trilobite 11). The high abundance of cephala relative to thoracic segments thoracic segment fragment. C, Shale piece with two trilobite and hypostomes at all sites may be due to taphonomic biases thoracic segment fragments. D, Shale piece with large trilobite in preservation and not necessarily mechanical winnowing (see thoracic segment fragment. All scale bars = 1 cm. All Museums also discussion under Size) of Western Colorado specimens. Articulation and Fragmentation The apparent trend of higher fossil abundance in proximal The degree of articulation (articulated specimens and settings and lower abundance overall in offshore settings may articulated series of 3+ thoracic segments/total number of be the result of a biotic abundance pattern in the animals living sclerites) at all four sites is very low, ranging from a complete lack in the marine setting during the early Cambrian, or it may be of any articulated elements in the sample from Split Mountain an artifact of preservation, perhaps the result of cyclic variation (outer shelf) to a high of 0.7% of elements at Frenchman in sedimentation rates in local sections (Brett et al., 2006), Mountain (craton/nearshore); the inner and middle shelf settings possibly related to the sites’ positions in regional sequences at Oak Spring Summit and Emigrant Pass have articulation or parasequences (as in other formations, e.g., Adams and frequency of approximately 0.5% each (Fig.11). Considering Grotzinger, 1996 and Liddell et al., 1997). For example, lower complete specimens and articulated series of thoracic segments sedimentation rates in the condensed section (Sequences III and relative to identifiable specimens only (complete specimens IV; Webster 2011a) of the outer shelf setting at Split Mountain and isolated cephala), the degrees of articulation are still just may have resulted in overall lower fossil abundance (and thus 1.3% at Frenchman Mountain, 1.7% at Oak Spring Summit, and diversity) in this interval; although such a deposit would have 1.2% at Emigrant Pass. These are comparable to the articulation been greatly time averaged and thus might have had high fossil percentages in the slightly older lower Cambrian Latham Shale input to single layers, long bottom exposure times may have led (Foster, 2011a) but are considerably lower than the total Ruin to sclerite destruction and taphonomic bias against preservation. Wash sample (5.0%), although Surface 3 at Ruin Wash had 86 OUTER SHELF MIDDLE SHELF INNER SHELF CRATON/NEARSHORE Emigrant Formation Carrara Formation Pioche Formation Bright Angel Formation Split Mountain Emigrant Pass Oak Spring Summit Frenchman Mountain

Olenellus sp. Hyoliths Olenellids indet. Olenellus gilberti 1.2% 15.1% Hyoliths Hyoliths Olenelloids 3.1% 0.33% 2.7% 1.2% indet. Olenellus 8.1% Olenellus terminatus fowleri 9.7% 1.7% Olenellids Olenellids Olenellus Olenellus indet. Trilobita indet. terminatus chiefensis 22.0% Olenellus Brachiopods indet. 28.8% 8.7% 40.0% Biceratops terminatus 63.9% 25.6% Olenellus 49.6% gilberti nevadensis 49.7% 21.8%

Corynexochida Mesonacis fremonti Olenellus indet. Nephrolenellus Nephrolenellus Olenellus 1.2% 7.3% Nephrolenellus gilberti geniculatus multinodus 10.4% 14.8% multinodus gilberti 10.4% 0.18% 3.76% N = 86 N = 260 N = 298 N = 559

Shannon Index (H): 0.1763 Shannon Index (H): 1.2549 Shannon Index (H): 1.1979 Shannon Index (H): 0.9207 E ective Richness (Se ): 1.19 E ective Richness (Se ): 3.51 E ective Richness (Se ): 3.31 E ective Richness (Se ): 2.51 FIGURE 10. Relative abundances of fossil species preserved at the four sites studied here, along with diversity indices and sample sizes. Arranged left to right from outer shelf to craton/nearshore (approximately northwest to southeast along the transect). OUTER SHELF MIDDLE SHELF INNER SHELF NEARSHORE Emigrant Formation Carrara Formation Pioche Formation Bright Angel Formation Split Mountain Emigrant Pass Oak Spring Summit Frenchman Mountain Complete, Complete, Pygidia articulated articulated 11.1% Pygidia trilobites trilobites 0.33% 50.0% Single, 0.2% complete thoracic Cephala segments Thoracic Thoracic 22.2% Thoracic 0.3% segment Thoracic segment Cephala fragments segment fragments segment 38.9% fragments Cephala 33.6% 42.8% 52.5% Cephala fragments Cephalon 37.7% 55.6% 33.3% fragments Single, 33.3% complete thoracic Articulated segments series of 3-8 Articulated 1.2% thoracic series of 3-8 thoracic segments Hypostomes Articulated Hypostomes 0.22% Hypostomes segments Cephalon 2.6% 1.2% series of 3-8 fragments 1.1% Cephalon 0.5% thoracic 15.5% fragments Cephalon segments 6.5% fragments 0.46% 8.5% N = 9 N = 647 N = 904 N = 972 FIGURE 11. Skeletal element preservation for trilobites at the four sites studied here. Arranged left to right from outer shelf to craton/nearshore (approximately northwest to southeast along the transect). no articulated remains in a sample of 95 specimens (0%; see from all sites are hard to assess because most specimens are Webster et al., 2008). positive or negative impressions of sclerites and do not preserve Apparently mostly postmortem (non-injury, taphonomic) the sclerite itself. breakage and fragmentation of sclerites is very common at all The abundance of disarticulated sclerites, with frequent sites. At Frenchman Mountain 99.1% of thoracic segments breakage of the remains, may indicate long-term, time-averaged are broken, and 13.2% of cephala have confirmed pre-burial accumulations in relatively low-energy environments with breakage. At Oak Spring Summit and Emigrant Pass breakage low sedimentation rates (Brett and Baird, 1986; Speyer, 1991; comprises 99.5% and 97.2% of thoracic segments, respectively, Martin, 1999), as the preservation of articulated whole carcasses and 6.6% and 28.5% of cephala. These thoracic segment and molts is more common in paleoenvironments demonstrating fragmentation rates are comparable to those reported from the rapid burial (Speyer, 1991). The low degree of articulation in the Ruin Wash and Latham Shale sites (Webster et al., 2008; Foster, four samples studied here (~1%) may indicate low sedimentation 2011a). At Split Mountain all thoracic segments and all but one rates and long exposure times on the sea bottom (Speyer, 1987), of the cephala are fragmented. Breakage of thoracic segments is especially at Split Mountain in the outer shelf setting. Most often at the axial ring-inner pleural region junction, due to the trilobite elements preserved at these sites, even uncompacted zone of weakness on either side of the axial ring at that point. olenelloid cephala, were probably flat enough to have been Breakage and fragmentation also sometimes occur elsewhere buried by sedimentation rates of <1cm/100yr, proposed as a on the inner pleural region, and this would seemingly require quantification of “low” by Brett and Baird (1986), although greater force, although in general olenellid sclerites appear to it is possible that higher rates were required in some cases. have been relatively thin. Corrosion and abrasion of specimens The sedimentology of the sites, shale with minor laminations 87 OUTER SHELF MIDDLE SHELF INNER SHELF NEARSHORE Emigrant Formation Carrara Formation Pioche Formation Bright Angel Formation Split Mountain Emigrant Pass Oak Spring Summit Frenchman Mountain

80 120 120

70 100 100 60 80 80 50 40 60 60 No Intact Cephala Yet Found 30 In Shale Layers 40 40 20 20 20 Number of Specimens Number of Specimens Number of Specimens 10 0 0 0 0-5 5-10 10-15 15-20 20-25 25-30 30+ 0-5 5-10 10-15 15-20 20-25 25-30 30+ 0-5 5-10 10-15 15-20 20-25 25-30 30+ Cephalon Length (mm) Cephalon Length (mm) Cephalon Length (mm) Mean: 10.63 Mean: 10.68 Mean: 8.34 Standard Deviation: 5.58 Standard Deviation: 5.49 Standard Deviation: 5.95 Range: 2.25-30.83 Range: 3.15-46.89 Range: 0.87-40.86

N = 0 N = 181 N = 214 N = 330 FIGURE 12. Size distributions (measured on cephalon length) for trilobite specimens from the four sites studied here. Arranged left to right from outer shelf to craton/nearshore (approximately northwest to southeast along the transect).

OUTER SHELF MIDDLE SHELF INNER SHELF NEARSHORE Emigrant Formation Carrara Formation Pioche Formation Bright Angel Formation Split Mountain Emigrant Pass Oak Spring Summit Frenchman Mountain

150 350 350

300 300 120 250 250 90 200 200 No Intact Cephala Yet Found 60 150 150 In Shale Layers 100 100 30 Number of Specimens Number of Specimens Number of Specimens 50 50

0 0 0 Convex-up Convex-down Convex-up Convex-down Convex-up Convex-down

Cumulative Binomial Probability Cumulative Binomial Probability Cumulative Binomial Probability P(X<127): 0.2305 P(X<321): 0.9999 P(X<245): 0.0003 P(X>127): 0.7304 P(X>321): 5x10-15 P(X>245): 0.9996 N = 0 N = 265 N = 476 N = 571 FIGURE 13. Up-down orientations of trilobite cephala for the four sites studied here. Arranged left to right from outer shelf to craton/nearshore (approximately northwest to southeast along the transect).

TABLE 1. Sample size, stratigraphic level, sampling intensity, and geologic setting of the four lower Cambrian sites studied here. #Sampling intensity is measured as the number of workers x number of hours worked each day x number of days worked. ##Includes hours worked by M. Webster and one assistant. *Stratigraphic level measured from base of Combined Metals Member of Pioche Formation. **Stratigraphic level measured from base of respective formation.

Locality Setting Formation Member Sampling Stratigraphic Sample Locality Intensity# Level Size Total

Frenchman Mountain Nearshore Bright Angel – ~44 hrs 45.4 m** 972 972

Oak Spring Summit Inner Shelf Pioche Combined Metals ~45 hrs 28.8* 780 – 26.5* 84 – 24.7* 35 899

Emigrant Pass Middle Shelf Carrara Pyramid Shale ~51 129.5** 410 – 124.3** 245 655

Split Mountain Outer Shelf Emigrant – ~47 hrs## 0–1.0 m** 86 86 88 of silt, indicates low environmental energy (bottom currents two other areas are, like the sites studied here, right(positively)- <10 cm/sec, as defined by Brett and Baird, 1986). Breakage skewed. This size-frequency pattern thus appears to be common and fragmentation of thoracic segments and cephala may be a in lower Cambrian deposits of the Great Basin, but it probably result of scavenging or predation by trilobite and non-trilobite reflects more a preservational effect than actual population arthropods (e.g., Pratt, 1998; Zhu et al., 2004; Haug et al., 2012), structures, which appear to be more normal in most arthropods as mechanical breakage appears to be rare, demonstrated by the (Sheldon, 1988; Hartnoll and Bryant, 1990). fact that genal spines and fragile pleural spines are often intact. Cephala do not appear to be size sorted, a potential Most olenellid non-lethal predation injuries appear to be to the taphonomic bias that may in turn affect taxonomic relative lateral cephalon and lateral parts of thoracic segments (Pates abundance patterns (Westrop, 1986); but relatively low and Bicknell, 2019), and those spine elements seemingly most abundance of thoracic segments at all sites indicates at least vulnerable to mechanical breaks are usually well preserved some taphonomic sorting of elements. Although cephalon length (pers. obs.; F. Sundberg, pers. comm., 2019). distributions from the four sites show little indication of size- The sedimentology of the sites also would seem to sorting, very small, immature specimens are somewhat under- suggest that mechanical agitation of the material in currents represented. The paucity of hypostomes, on the other hand, would be rare and not particularly strong; it may be difficult may relate more to biochemical preservational differences in to significantly fragment even thin olenelloid sclerites in these elements and is not necessarily indicative of mechanical what currents were present. Shell fragmentation and breakage sorting (Speyer, 1987). Still, the apparent lack of size sorting in by high-energy agitation in nearshore settings may be due to the overall samples of cephala, apparent in the cephalon length any one of a complex set of, or to a combination of several, histograms (Fig. 12), suggests little influence of current-driven factors, including scavenging, predation, current agitation, and transport or winnowing (Speyer and Brett, 1988) of these late bioturbation. The cause of this fragmentation and breakage in Dyeran deposits. It is possible that taphonomic sorting of the the nearshore setting may not be distinguishable from that in samples was a result of other, non-transport related factors. outer shelf settings where the damage is often a result of biotic factors, at least for modern bivalves (Zuschin et al., 2003). Biostratinomy Sclerite breakage due to current and wave turbulence would be Orientation data on trilobite cephala could again only be most likely in coarser grained sandstone and grainstone settings collected for the cratonic/nearshore to middle shelf settings. (e.g., Pratt and Bordonaro, 2007). Thus, scavenging bites, as Convex-up versus convex-down comparisons of trilobite opposed to current agitation, might be a better explanation for cephala from the three sites indicate different preservation at at least some of the fragmentation seen in the present material, each. The sample from the Bright Angel Formation at Frenchman especially in the middle to outer shelf settings. Mountain is 57.1% convex down; the sample from the Pioche The high degree of disarticulation and high (in thoracic Formation at Oak Spring Summit is 67.5% convex up; and the segments) to medium (in cephala) degree of fragmentation at sample from the Carrara Formation at Emigrant Pass is 52.1% the craton/nearshore (Frenchman Mountain) to middle shelf convex down (Fig. 13). Binomial results suggest that only (Emigrant Pass) sites in this study are congruent with relatively the Carrara Formation sample has a statistically insignificant low rates of sedimentation and environmental turbulence difference; the Bright Angel and Pioche samples appear to be (Speyer and Brett, 1988). The higher degree of fragmentation significantly dominated by convex-down and convex-up cephala, of cephala at Emigrant Pass (28% vs. 13.2% and 6.6% at respectively. These binomial results are considered tentative, Frenchman Mountain and Oak Spring Summit) might indicate however, as they were calculated with an assumption that the a higher incidence of scavenging on the middle shelf. At Split probability of one orientation versus another (i.e., convex-up vs. Mountain, presumably the lowest degree of sedimentation and convex-down) is 0.50 as a result of passive settling, something turbulence rates agree with the highest degree of disarticulation that has not yet been experimentally established for olenelloid and possible dysoxic conditions (i.e., long bottom exposure cephala. Interestingly, the Pioche Formation sample at Ruin time and possible fewer scavengers); a significant factor in the Wash, stratigraphically equivalent to the Oak Spring Summit characteristics of open shelf setting shell deposits is sediment sample here, is also dominated by convex-up cephala (84.4%; starvation (Norris, 1986). see Webster et al., 2008). Size (Cephalon Length) The cause of these trends is difficult to assess, but a convex- up dominant mode (as at Oak Spring Summit and Ruin Wash) may Cephalon length was measured because it is less susceptible be attributed to reorientation by bottom surface currents, and a to shape (and thus linear measurement) change than is cephalon convex-down dominant mode (as at Frenchman Mountain) may width in compacted olenelloid cephala (Sundberg, 1983; Webster suggest a quiet water setting, a low degree of bioturbation, and/ and Hughes, 1999). Where cephalon length could be compared or settling of sclerites intermittently stirred up by wave action (craton/nearshore to middle shelf), the range, mean and standard (Speyer and Brett, 1986, table 2; Speyer and Brett, 1988, table 3). deviation of the samples was very similar, indicating no distinct Lask (1993) suggested that convex-down dominant modes may pattern to the preservation of size (Fig. 12). In each case the occur in oscillatory wave-swept environments but indicated that modal size class was 5–10 mm, with fewer very small cephala sedimentary structures such as ripples might be expected in the and a right (positively)-skewed distribution. However, the lower same setting with trilobite sclerites; such sedimentary structures limit of the range and the mean were somewhat smaller in the were not observed at Frenchman Mountain. An essentially equal craton/nearshore at Frenchman Mountain (Bright Angel); the up-down orientation (as at Emigrant Pass) may be indicative of smaller mean was the result of an abundance of cephala in the 0–5 significant bioturbation or little current activity. No tool or roll mm size class (Fig. 12). On average, cephala of Nephrolenellus marks were observed in the shale at any of the four sites, further multinodus and N. geniculatus are smaller than those attributable suggesting a lack of strong currents in the deposits. Such marks to various species of Olenellus, and to Mesonacis at Emigrant (especially tool marks) are sometimes observed in fossiliferous Pass. No complete cephala could be measured from the shale layers of Cambrian shales of the region (e.g., the Spence Shale at layers in the outer shelf setting. Miners Hollow in Utah) and indicate that even very fine-grained Cephala preserved at the four localities studied here are deposits with abundant trilobites and other fossils can have had generally similar in mean and modal cephalon length to those at significant bottom currents. Ruin Wash (Webster et al., 2008) and in the Latham Shale of the Field azimuth readings on olenelloid cephala (toward Marble Mountains (Foster, 2011a). The size distributions of these anterior, long axis of glabella) indicate random orientations 89 for the samples from the inner and middle shelf settings at Oak decay, gentle wave action, and possibly scavenging. Silty beds Spring Summit (Pioche Formation) and Emigrant Pass (Carrara of trilobite “hash”, present at most sites but not studied here, Formation); the cratonic/nearshore sample from the Bright were likely deposited during stronger storms. Angel Formation at Frenchman Mountain demonstrates a weak The four sites studied along the craton/nearshore to preferential orientation to the west-northwest and east-southeast outer shelf transect in this study appear to represent biogenic (Fig. 14). Webster et al. (2008) found a preferred east-southeast and mixed sedimentologic-biogenic skeletal accumulations orientation for cephala on Surface 3 at Ruin Wash (Oak Spring (Kidwell et al., 1986). Biogenic accumulations may have been Summit equivalent), although the sample size was relatively caused by behavior of the trilobite species themselves and by small (N = 32), and when all bioclasts were included in the scavenging and other actions of other species. In most cases, analysis no trend was apparent. laminated sedimentologic concentrations at these sites probably These results suggest a relative lack of current sorting on the are related to gradual accumulations during low sedimentation; inner and middle shelf. The statistically insignificant difference in the rarity of articulated material suggests rapid burial events up-down orientation of cephala on the middle shelf is congruent were infrequent. Interestingly, the inner to outer shelf settings with these data; however, the fact that at both Oak Spring Summit just above and below storm wave base most typically preserve and Ruin Wash cephala are strongly preferentially oriented in a the biogenic and sedimentologic accumulations, and the more convex-up position is at odds with the lack of preferred azimuth proximal settings (lagoons and possibly beaches and tidal flats), orientation, if it is assumed that current influence will result in near where we might expect the cratonic/nearshore sample a both a preferred convex-up (i.e., hydrodynamically stable) of the Bright Angel Formation at Frenchman Mountain to be position (Speyer and Brett, 1988; Speyer, 1991) and non-random situated, demonstrate a wide variety of common accumulation azimuth orientation. These results indicate that either currents types (Kidwell et al., 1986, fig. 5). This might in part account were influencing the up-down orientations of cephala but not for some of the anomalous characteristics demonstrated by the their azimuth orientations, or that whatever was orienting the Frenchman Mountain sample in terms of diversity and cephalon up-down position of the cephala, it was not bottom currents. orientations. In the cratonic/nearshore setting, the weak bimodal orientation The four sites also seem to best fit Taphofacies 3, 4, and 6 of the cephala (Fig. 14) is perplexing considering the preferred of Speyer and Brett (1988) for Paleozoic epeiric seas in having convex-down orientation of these sclerites, again assuming that high degrees of disarticulation, moderate reorientation/sorting, preferred azimuth orientation should be correlated with convex- and moderate to high fragmentation. These taphofacies range up (hydrodynamically stable) position. The results from the from oxic to anoxic bottom conditions, which probably fluctuate Bright Angel Formation at Frenchman Mountain imply either a seasonally on the shelf (Tyson and Pearson, 1991). decoupling of current-caused azimuth orientation from up-down Whether these transect trends hold true for other regions orientation of olenelloid cephala or that an unknown, secondary of the shelf or for other stages of the Cambrian will require influence was over-printing the up-down orientations. additional sampling. Apparent trends found as a result of this Taphofacies study may be tested with additional data from the many other sites within each of the craton/nearshore-to-outer shelf zones As with other early Cambrian deposits in fine-grained investigated here. siliciclastic settings, the sites studied here appear to have been deposited near or below storm wave base of about 20–40 m CONCLUSIONS (Coe, 2003). Even the coarsest grains in the sampled layers • The samples from the latest early Cambrian from the craton/ were silt to fine-grained sand, and no sedimentary structures nearshore to outer shelf of southwestern Laurentia are dominated were observed; that this is true even of the relatively nearshore by olenellid and biceratopsid trilobites; only one corynexochid Frenchman Mountain site suggests quiet-water conditions were was found previously (outer shelf condensed section of the prevalent. The rounding of quartz silt and sand grains present Emigrant Formation at Split Mountain). among the clay of the sampled layers increased steadily from the • Ptychopariids were not found in any of the shale layers cratonic/nearshore to middle shelf settings, and the amount of silt studied here, although they are common in some early Cambrian decreased in the outer shelf setting. This relatively rounded sand carbonates in the region. and silt may have been windblown. Disarticulation, winnowing, • Olenellus terminatus or O. gilberti dominate most sites and fragmentation of trilobite remains were probably a result of numerically.

OUTER SHELF MIDDLE SHELF INNER SHELF NEARSHORE Emigrant Formation Carrara Formation Pioche Formation Bright Angel Formation Split Mountain Emigrant Pass Oak Spring Summit Frenchman Mountain

N N N 15 25 15

12 20 12

9 15 9

6 10 6

3 5 No Intact Cephala Yet Found 3 In Shale Layers

N = 0 N = 107 N = 210 N = 110 FIGURE 14. Rose diagram of azimuth orientations of trilobite cephala for the four sites studied here (measured toward anterior along the glabellar axis). Arranged left to right from outer shelf to craton/nearshore (approximately northwest to southeast along the transect). 90 • Biceratops is restricted to cratonic/nearshore outcrops. Perry, Zeb Miracle, ReBecca Hunt-Foster, Ray Bley, Josh • Brachiopods and hyoliths were also encountered in all Bonde, Chris Vulpe, Ethan Bull-Vulpe, Nancy Cummings, Bob samples. and Dave Lame, Bill Mason, and Rachel and Sue Foldy, who • Overall fossil abundance declines from the craton/nearshore assisted in the field by collecting most of the specimens used in to the outer shelf, although the stratigraphic thicknesses of the N. the study. Mark Webster shared data from some sites and helped multinodus Biozone intervals at the cratonic/nearshore through with initial access to some of the localities; thanks also to Fred middle shelf localities are of roughly comparable thickness. Sundberg and Stew Hollingsworth for discussions. Reviews • Diversity of the samples is highest on the inner and were provided by three anonymous reviewers, and their help is middle shelf, lower nearshore, and lowest in the outer shelf greatly appreciated. setting (perhaps due to extremely low sedimentation rates in a REFERENCES condensed section). • Most trilobite samples are dominated by cephala and broken Adams, R. D., and Grotzinger, J. 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