Small shelly fossils and carbon isotopes from the early Cambrian (Stages 3–4) Mural Formation of western Laurentia

The extraordinary window of phosphatized and phosphatic small shelly fossils (SSF) during the early and middle Cambrian is an important testament to the radiation of biomineralizing metazoans. While SSF are well known from most Cambrian palaeocontinents during this time interval, western Laurentia has relatively few SSF faunas. Here we describe a diverse SSF fauna from the early Cambrian (Stages 3–4) Mural Formation at three localities in Alberta and British Columbia, Canada, complemented by carbon isotope measurements to aid in a potential future bio‐chemostratigraphic framework. The fauna expands the recorded SSF assemblage diversity in western Laurentia and includes several brachiopods, four bradoriids, three chancelloriids, two hyoliths, a tommotiid and a helcionellid mollusc as well as echinoderm ossicles and specimens of Microdictyon, Volborthella and Hyolithellus. New taxa include the tommotiid genus Canadiella gen. nov., the new bradoriid species Hipponicharion perforata sp. nov. and Pseudobeyrichona taurata sp. nov. Compared with contemporaneous faunas from western Laurentia, the fauna is relatively diverse, particularly in taxa with originally phosphatic shells, which appear to be associated with archaeocyathid build‐ups. This suggests that the generally low faunal diversity in western Laurentia may be at least partly a consequence of poor sampling of suitable archaeocyathan reef environments. In addition, the tommotiid Canadiella filigrana appears to be of biostratigraphical significance in Cambrian Stage 3 strata of western Laurentia, and the unexpected high diversity of bradoriid arthropods in the fauna also suggests that this group may prove useful for biostratigraphical resolution in the region.

L O W E R Cambrian fossils of the Mural Formation have been studied for over 100 years and a rich fauna of trilobites and brachiopods has been documented (Walcott 1913;Fritz & Mountjoy 1975;Fritz 1992;Balthasar 2004Balthasar , 2007Balthasar , 2008Balthasar , 2009). The discovery of exceptionally preserved fossils such as soft-shelled brachiopods (Balthasar & Butterfield 2009), anomalocaridids and other nonmineralized biota (Sperling et al. 2018) has also sparked interest in the formation. However, Burgess Shale-type exceptional preservation in the Mural Formation is at best marginal (a Tier 3 deposit), and there is little geochemical evidence for anoxic conditions that might result in more spectacular preservation (Sperling et al. 2018). The bulk of the known fauna has been derived from mudstones of the middle part of the Mural Formation. Here, we describe skeletal fossils from both the underlying carbonate-dominated lower Mural Formation and from carbonate storm beds within the middle Mural Formation.
In the early Cambrian (Terreneuvian and Cambrian Series 2), small shelly fossils (SSF) constitute a significant portion of the total taxonomic diversity of the metazoan fossil record (Maloof et al. 2010). SSF share nothing beyond a commonly small size range (typically < 2 mm; which may be taphonomically biased for some taxa; Mart ı Mus et al. 2008) and the fact that the fossils are resistant to digestion of the carbonate host rock in weak acids (fossils are typically either phosphatic by original composition or secondarily phosphatized or silicified calcareous shells). The fossils include various shells, sclerites, spicules and other skeletal elements belonging to a multitude of early animal groups representing stem and crown members of various phyla across the metazoan tree of life (Budd & Jensen 2000;Kouchinsky et al. 2012).
Relatively few SSF faunas have been described from the Cambrian of western Laurentia. From the earliest Cambrian (Fortunian), Conway Morris & Fritz (1980) reported a single protoconodont specimen (probably from the Ingta Formation in the Mackenzie Mountains of north-western Canada; see Aitken 1989), and Pyle et al. Hence, the SSF fossil record from western Laurentia is relatively meagre compared with contemporaneous faunas from eastern Laurentia (Skovsted 2006b;Skovsted & Peel 2007, 2011 and other palaeocontinents (Qian & Bengtson 1989;Bengtson et al. 1990;Kouchinsky et al. 2012Kouchinsky et al. , 2015. Biostratigraphic subdivision and correlation of Cambrian strata traditionally rests on trilobites (Shergold & Geyer 2003;Zhu et al. 2019). However, a significant part of the Cambrian Period is pre-trilobitic (Terreneuvian) and in the overlying unnamed Cambrian Series 2, trilobite faunas have proven to be highly endemic with resulting problems for intercontinental correlation. Carbon isotope stratigraphy has also emerged as a robust correlation method in lower Cambrian strata (e.g. Smith et al. 2016) but both patterns of negative/positive excursions and absolute values can be non-unique. Consequently, additional sources of age control are needed. Recently, non-trilobite shelly fossils were used to define a new biostratigraphic subdivision of Cambrian Series 1-2 strata from South Australia (Betts et al. 2016(Betts et al. , 2017(Betts et al. , 2018, and various SSF taxa are currently being considered as index fossils for the boundary between Cambrian Series 1 and 2 (see review in Zhang et al. 2017). However, the biostratigraphical control of SSF assemblages from western Laurentia remains to be tested.
The excellent preservation and relatively high taxonomic diversity presented here makes the fauna of the Mural Formation one of the most diverse and best preserved SSF faunas described from the lower Cambrian of western Laurentia. The results highlight that an increased sampling focus on archaeocyathan reefs and associated sediments might increase the known SSF diversity in western Laurentia. Our results suggest that specific SSF taxa such as tommotiids and bradoriid arthropods have a good potential for biostratigraphical resolution in Cambrian Stage 3 of western Laurentia, although further work is required to realize this potential. With the combination of carbon isotope chemostratigraphic data in conjunction with SSF data, as we present here, it may be possible in the future to build an improved bio-chemostratigraphic framework. Ultimately this will be key to placing western Laurentian SSF diversity in the global picture and determining if the observed low diversity is due to sampling, ecology, or taphonomic effects.

GEOLOGICAL SETTING
The Mural Formation is located in the southern Canadian Cordillera and was deposited on the western Laurentian margin, approximately during the rift-drift transition and initial Palaeozoic flooding of North America (Sauk transgression) (Pope et al. 2012). The Mural was deposited in a relatively shallow-water environment, with the Laurentian craton to the east and deeper water conditions developing to the west. The Mural Formation itself thins substantially across the Peace River Arch, a major eastnorth-east-trending structure in northern Alberta and British Columbia (Fig. 1;McMechan 1990). However, the tripartite stratigraphic motif of the Mural is recognized from Mexico to Yukon during the Lower Cambrian (Series 2, Age 3 and 4; Waucoban Series, Montezuman-Dyeran Stages in a North American timescale; Nevadella-Bonnia-Olenellus trilobite zones in older literature). This motif consists of a lower carbonate (often archaeocyathan limestone mounds/biostromes and ooid grainstone), a medial shale/siltstone, and an upper carbonate, again often containing archaeocyaths (Pope et al. 2012). In most localities, the Nevadella-Bonnia-Olenellus boundary, which is also the Montezuman-Dyeran boundary, is located in the medial shale (Fig. 2). Additional data on the geology, sedimentology, and palaeontology of the Mural Formation can be found in Fritz & Mountjoy (1975), Balthasar (2004) and Sperling et al. (2018). The Waucoban Series is traditionally divided into a series of trilobite zones: the Fallotaspis, Nevadella and Bonnia-Olenellus Zones in ascending order. This biostratigraphical framework has recently been revised and refined with a number of new trilobite zones (Hollingsworth 2011;Webster 2011). However, given that the trilobites of the Mural Formation have yet to be re-studied in this biostratigraphical framework, consequently we refer to the older zone names herein.

MATERIAL AND METHOD
The Mural Formation was sampled at three different locations: the type Mumm Peak section in Jasper National Park in western Alberta (sample prefix MP; see Balthasar 2004 andSperling et al. 2018 for locality information); a new section in a glacial valley to the north-west of Mumm Peak in eastern British Columbia (the informally named Rocky Lake camp, sample prefix RL); and the Dezaiko Range further to the north in British Columbia (sample prefix DR). At Mumm Peak the basal limestone and middle shale units were sampled for SSF while carbon isotope samples were collected from a measured stratigraphic section of the entire formation, including the upper carbonate unit that was heavily dolomitized. At Rocky Lake the basal limestones were sampled for SSF in relative stratigraphic order while the upper carbonate (limestone) was sampled for carbon isotopes from a F I G . 1 . Locality map of investigated Mural Formation localities in the southern Canadian Cordillera. Map is modified from Norford (2012) and shows the generalized present-day distribution of middle Cambrian -Middle Ordovician rocks from 49°N to Peace River. The transition from the shallow-water platform to deep-water conditions was probably not as sharp in the early Cambrian during deposition of the Mural Formation, but the presence of deep-water slope deposits in the underlying late Neoproterozoic stratigraphy immediately to the west of our study sites (Ross & Arnott 2007) suggests that a westward-deepening basin had been established for some time prior to Mural Formation deposition. Our collection sites in the Dezaiko Range, Mumm Peak, and Rocky Lake sit on the southern edge of the Peace-Athabasca Arch (McMechan, 1990 Carbonate samples (600-1700 g; for details see Skovsted et al. 2020, appendix S1) were digested in buffered, 10% acetic acid at the Microfossil Laboratory at Lund University, Sweden, following protocols established for conodont extraction (Jeppsson et al. 1999). The resulting residues were scanned for fossils under a stereo microscope and selected specimens were gold-coated and pictured using the Hitachi scanning electron microscope at the Swedish Museum of Natural History in Stockholm, Sweden.
For carbon and oxygen isotope analyses, only samples of pure carbonates were analysed. Hand samples were cut at Stanford University perpendicular to bedding and individual laminae were drilled for powder, avoiding veins or obvious alteration. Samples were then analysed at Yale University using a Thermo Scientific Kiel IV Carbonate Device connected to a Thermo Finnegan MAT 253 mass spectrometer. Long-term precision on a marble reference material was AE0.05 per mil (&) for d 13 C and AE0.06& for d 18 O. All measured isotope data are reported in Skovsted et al. (2020, appendix S2).

AGE AND CORRELATION OF THE MURAL FORMATION
The recovered SSF fauna of the Mural Formation ( Fig. 2; Table 1) includes some elements that are known from roughly coeval strata in western Laurentia such as Canadiella filigrana. This tommotiid species was originally described from the Cassiar Mountains in northern British Columbia (Conway Morris & Fritz 1984) and is also found in Sonora, Mexico (McMenamin 1984, 2001 and eastern California (Signor & Mount 1986). The brachiopod Kutorgina perugata is also known to occur in Nevada and eastern California (Walcott 1912;Signor & Mount 1986). Microdictyon sp. from the Mural fauna is closely comparable to specimens reported as Microdictyon cf. rhomboidale from the Mackenzie Mountains of Northwest Territories (Bengtson et al. 1986) and from the Great Basin (Wotte & Sundberg 2017). Volborthella tenuis is another widespread taxon in the Great Basin and the Canadian Rocky Mountains (Fritz & Yochelson 1988;Hagadorn & Waggoner 2002). However, in general the non-trilobite fauna from strata of Cambrian Series 2 in western Laurentia is extremely poorly known and many of the fossil taxa reported here from the Mural Formation have not been reported from the region before.
Although the reported SSF fauna from the Mural Formation is of limited value for biostratigraphical correlation within Laurentia due to the present poor state of knowledge of Laurentian SSF faunas, we note that several taxa appear to be both geographically widespread and have a restricted stratigraphic range in western Laurentia. Particularly, this applies to Canadiella filigrana, with a demonstrated range in the Motezuman from northern British Columbia to Mexico. The geographic and stratigraphic distribution of the brachiopods Kutorgina perugata and Mickwitzia muralensis (see McMenamin 1992) and the problematic Volborthella tenuis is similar, although these species have not been reported from T A B L E 1 . List of SSF samples from Mumm Peak, Rocky Lake and Dezaiko Range with sample number, absolute or approximate height above base of formation, and fossil content.   (Shu 1990;Hou et al. 2002;Zhang 2007), Australia (Fleming 1973;Skovsted et al. 2006;Topper et al. 2007Topper et al. , 2011aBetts et al. 2014) and different areas of peri-Gondwana (Hinz-Schallreuter 1993;Gozalo & Hinz-Schallreuter 2002;Gozalo et al. 2004). Furthermore, Betts et al. (2017) showed in a recent analysis of the biostratigraphy of the lower Cambrian sequence of South Australia that bradoriids hold great potential for regional correlation within Australia and for intercontinental correlation between Australia and particularly South China, Antarctica and Siberia.
Four different bradoriids were discovered in the acidresistant residues from the Mural Formation: Hipponicharion perforata sp. nov.; Pseudobeyrichona taurata sp. nov.; Beyrichona sp.; and Liangshanella? sp. In most cases only fragmentary or poorly preserved valves are present, precluding definite species assignment, but two samples yielded more complete material, allowing the characterization of two new species. In light of the demonstrated global high taxonomic diversity and wide palaeogeographic distribution of bradoriids in the early Cambrian, the low diversity of contemporaneous bradoriid faunas from Laurentia is likely to reflect insufficient sampling rather than lower original diversity. The fact that the fauna of the Mural Formation documented here includes four new bradoriid species seems to lend support to this interpretation. Even more strikingly, the new species represents some of the first bradoriids from Cambrian Stages 3-4 strata of western Laurentia known to date (Siveter & Williams 1997). In addition, Devaere et al. (2019) recently reported a bradoriid from the Puerto Blanco Formation, in Sonora, Mexico, which may belong to P. taurata. It is anticipated that future investigations will reveal a much larger bradoriid diversity in this region.

Carbon isotope stratigraphy
To aid in current and future correlations we also generated carbonate carbon isotope data from our sections. whereas the closed circles represent limestones from Rocky Lake, with samples correlated on the base of the upper carbonate. d 18 O averages À12.4& at Mumm Peak and À12.1& at Rocky Lake, below general cut-offs for alteration of a sample's carbon isotope composition (e.g. Knoll et al. 1995). Given that no other diagenetic evaluation was conducted in this study, these carbon isotope results should be considered with this caveat in mind. d 13 C values at Mumm Peak start around 0& and then undergo a series of c. 1& oscillations through the archaeocyathan biohermal limestones. At the transition to interbedded packstones/wackestones and shales, in the Nevadella Zone, values decrease from c. 0& to c. À3&. The only other Laurentian formation in this time interval to have received comprehensive carbon isotope study is the Sekwi Formation, studied in the Mackenzie Mountains, Northwest Territories by Dilliard et al. (2007). There, the Montezuman-Dyeran (Nevadella-Bonnia-Olenellus) transition interval is marked by an unconformity and a period of clastic deposition. The stratigraphically highest Nevadella samples in the Sekwi Formation do show negative carbon isotope trends, although these excursions start from more positive values than in the Mural and do not reach values as low as À3&. Several possibilities exist to explain the discrepant carbon isotope curves from the two formations: (1) the basal limestone in the Mural Formation records carbonate deposition not present in the Sekwi Formation (either time lost in the unconformity or during clastic intervals); (2) the Mural Formation negative carbon isotope excursion represents negative excursion D in the Sekwi Formation (which occurs there in the basal part of the Bonnia-Olenellus zone); implying time-transgressive trilobite zones; or (3) the negative excursion in the Mural Formation is artefactual and represents progressive loss of carbonate buffering capacity in the transition from the basal limestone to the middle shale. d 13 C from the upper carbonate at Rocky Lake is between c. À0.5 and À2.0& and trends slightly more negatively upsection. Dolomitized samples from the upper carbonate at Mumm Peak are similar but slightly more negative than at Rocky Lake. These results are consistent with results from the Sekwi Formation Bonnia-Olenellus zone, but given that there are no carbon isotopic excursions in this interval the correlations are non-unique.

SSF OF WESTERN LAURENTIA
The poorly known SSF assemblages from western Laurentia stand in stark contrast to faunas of much higher taxonomic diversity from eastern Laurentia and from other continental blocks, primarily Siberia (Missarzhevsky 1989;Kouchinsky et al. 2011Kouchinsky et al. , 2015, Australia (Bengtson et al. 1990;Gravestock et al. 2001;Betts et al. 2016Betts et al. , 2017Betts et al. , 2018 In contrast, in western Laurentia knowledge of SSF assemblages is limited to a handful of isolated faunas representing different time intervals (Terreneuvian-Wuliuan) and to localities distributed over an immense distance from Mexico (McMenamin 1984(McMenamin , 1985 2) from the White-Inyo region of eastern California, but the listed fossils were compiled from a host of older papers and unpublished theses and were not illustrated. This assemblage is in dire need of taxonomic revision before its importance and true taxonomic diversity can be assessed. The same applies to the fauna from the Mackenzie Mountains of the Northwest Territories described by Voronova et al. (1987).
In two recent publications, SSF assemblages from the Great Basin and Sonora were described in greater detail than ever before. Wotte & Sundberg (2017) investigated material from nine different stratigraphic sections in the Great Basin, derived from four formations spanning outer to inner shelf environments in the Montezuman-Delamaran time interval (Cambrian Stage 3-Wuliuan). The reported fauna is composed of four molluscs, two hyoliths, three chancelloriids, three nominal species of Microdictyon (probably synonymous, see discussion below) and three problematic taxa in addition to echinoderm ossicles and trilobite debris; no brachiopods were reported. The total diversity of this fauna (15 nominal species, not counting trilobite remains) is of similar richness as the fauna from the two sections of the lower Mural Formation reported herein (15 species, excluding brachiopods, archaeocyaths and trilobites; Fig. 3  The underlying reasons behind low taxonomic diversity of SSF assemblages in western Laurentia are speculative, but Wotte & Sundberg (2017) suggested that the preservation of originally calcareous shells by secondary phosphatization is rare in this region. This hypothesis seems to explain why molluscs, hyoliths, chancelloriids and echinoderms are preserved only in a few of the investigated sections and samples from the Great Basin, given that all of these taxa are known to have calcareous shells that are not readily recovered in SSF assemblages in the absence of diagenetic mineralization. This observation dovetails nicely with recent investigations into the secondary phosphatization of calcareous shells, which is shown to be highly facies dependent and is tied to sediment starvation and the development of hardgrounds (Pruss et al. 2018;Freeman et al. 2019;Jacquet et al. 2019). It is noteworthy that the fauna of the Mural Formation is also poor in taxonomic diversity of these calcareous fossil groups, although some of the investigated samples yielded a large number of specimens of particular taxa (mainly chancelloriid sclerites or hyoliths; Table 1). However, the suggested hypothesis fails to explain the low diversity of originally phosphatic fossils in the Great Basin compared with the Mural Formation, in particular given that organophosphatic brachiopods are known to be common in the same formations in the Great Basin (brachiopods were not reported by Wotte & Sundberg but see Rowell 1966Rowell , 1977Skovsted & Holmer 2006;Butler et al. 2015). Of the taxa reported from the Great Basin by Wotte & Sundberg (2017), four are phosphatic in original composition (25%; Fig. 3A). However, three of these are nominal, co-occurring species of Microdictyon that are likely to be synonymous (see Systematic Palaeontology below and discussion in Devaere et al. 2019) while seven species from the Mural Formation had phosphatic shells by original composition (48%; Fig. 3C). The fauna from Sonora (Devaere et al. 2019) is also to a large extent dominated by secondarily phosphatized calcareous shells while originally phosphatic shells are represented only by three taxa (15%; Fig. 3B). The fauna is largely derived from three stratigraphically narrow intervals, which reinforces the impression that secondary phosphatization of calcareous shells is rare in western Laurentian sections, perhaps coinciding with generally higher sedimentation rates precluding the development of hardgrounds. We note that in our material from the Mural Formation, the highest total taxonomic diversity is in samples from storm beds in the middle shale unit, although bioclastic limestones associated with archaeocyathid reefs in the basal limestone unit collectively yielded a higher number of species (Table 1). If only originally phosphatic species are counted, the taxonomic diversity is highest in samples from the basal limestone unit. This pattern indicates that although secondary phosphatization is important for the diversity of recovered SSF assemblages, the distribution of originally phosphatic shells may be more strongly controlled by the environmental preferences of the organisms that secreted the shells. The direct association of a number of the taxa recovered from the Mural Formation with archaeocyathan build-ups mirrors previously reported patterns of distribution of tommotiids and other SSF in and around archaeocyathan build-ups in South Australia (Holmer et al. 2008;Skovsted et al. 2011Skovsted et al. , 2015Betts et al. 2016Betts et al. , 2017Betts et al. , 2018. A similar pattern was also reported from the early Cambrian of Mexico and California The SSF fauna from the Mural Formation described here is one of the richest faunas ever discovered in the Cambrian successions of western Laurentia, particularly when its limited stratigraphic range is taken into account. The excellent preservation of many taxa in the fauna reveals new taxonomic and palaeobiological details and further increases its importance. In addition, the discovery that originally organophosphatic fossils such as tommotiids and bradoriids were associated with archaeocyathid reefs may be useful for biostratigraphical resolution in western Laurentia. Remarks. As previously reported (Walcott 1913;Balthasar 2004Balthasar , 2007Balthasar , 2008Balthasar , 2009 (Table 1).
Description. Kutorginid brachiopod with ventribiconvex shell; transversely ovate to sub-rectangular in outline with almost straight posterior margin and marginal apex in both valves. Ventral valve convex with weakly developed fold (Fig 4D, H); apex slightly overhanging the posterior margin; greatest height slightly anterior of apex (Fig. 4H). Dorsal valve almost flat or gently convex with a broad, weakly developed sulcus (Fig. 4A, F, G). External ornament in both valves of concentric rugae of variable amplitude ( Fig. 4F-H), often inconsistently developed and sometimes interrupted by prominent nickpoints (Fig. 4D). Micro-ornament of elongate or rhomboidal elevations separated by narrow furrows (Fig. 4C). Ventral larval shell smooth (Fig. 4D). Dorsal larval shell bilobed, c. 280 lm wide (Fig. 4A). Diagnosis. Kennardiid tommotiid with three distinct sclerite types (A, B, C); A sclerites bilaterally symmetrical pyramidal; B sclerites asymmetrical pyramidal with rectangular cross-section; C sclerites laterally compressed cone-shaped with crescentic cross-section; initial shell and first 2-3 growth increments of B and C sclerites narrow, spine shaped with ornament of pustules of two size ranges; adult shell with clear differentiation of co-marginal ribs and inter-rib grooves and radial plicae concentrated to specific sclerite regions; adult ornament of spineshaped pustules with superimposed reticulate network in interrib grooves; larger, radially arranged pustules on co-marginal ribs form pseudo plicae.

Remarks
Remarks. Conway Morris & Fritz (1984)  The new genus differs from Kennardia by the presence of radial plicae and from both Kennardia and Dailyatia by the presence of minute spine-like pustules in inter-rib grooves as well as the development of the apical spine (elongated sclerite tips) in B and C sclerites, formed by the initial shell and the first 2-3 growth increments. In terms of ornamentation Canadiella is most similar to species of Dailyatia with subdued radial plicae and clear pseudoplicae (i.e. D. bacata Skovsted, Betts, Topper &Brock, 2015 andD. odyssei Evans &Rowell, 1990). However, the differences in morphology and shell ornament outlined above clearly distinguish the new genus. Unfortunately, too few complete sclerites are known to be able to determine sclerite variability in general or if specific sclerite subtypes exist. In particular, the A sclerite is poorly represented in the current material, and more complete material will be needed to clearly outline its morphology.
Devaere & Skovsted (2017) recently redescribed Lapworthella schodackensis (Lochman, 1956) based on collections from North-East Greenland and noted the presence of tubercles in inter-rib grooves with a superimposed reticulate network that makes this species more similar to Canadiella than other species of Lapworthella. In addition, the most common sclerite type in L. schodackensis is a pyramidal sclerite with a rectangular crosssection (B sclerites), which is comparable to the B sclerites of Canadiella. However, L. schodackensis lacks sclerites with a crescentic cross-section (C sclerites) as well as radial plicae and pseudoplicae and also exhibits sections of shell with co-marginal striations representing regular intervals of small-scale incremental growth, which lack counterparts in Canadiella or other kennardiids where the external surface was formed by a succession of growth sets (composed of one co-marginal rib and one inter-rib groove formed by a single shell lamina; see description of shell structure and sclerite formation in Dailyatia in Skovsted et al. 2015, p. 67).
Even though Devaere & Skovsted (2017) demonstrated the presence of distinct sclerite types in Lapworthella schodackensis (Lochman, 1956) from Greenland, the genus Lapworthella remains one of the least poorly understood of all camenellan tommotiids, despite its apparently global distribution. Widely differing species concepts have been applied to lapworthellids in the past and combined with a high degree of variability in sclerite shape and ornament, this has led to much confusion (Devaere & Skovsted 2017). We anticipate that renewed study of lapworthellid assemblages in the future will lead to significant taxonomic refinement of this problematic fossil group, as exemplified by the present discovery of the kennardiid affinity of 'Lapworthella' filigrana.
Occurrence. Late early Cambrian (Series 2, Montezuman Stage, Nevadella Trilobite Zone) of western Laurentia; northern and eastern British Columbia and western Alberta (Canada), Sonora (Mexico) and possibly eastern California (USA). Material. 3 A sclerites, 4 B sclerites, 10 C sclerites and 67 juvenile or fragmentary specimens of uncertain sclerite type. All specimens from the lower part of the basal limestone unit of the Mural Formation at Mumm Peak and Rocky Lake (Table 1).
Apical area of all sclerite types with differentiated growth regimen and ornamentation compared with the adult shell. The A sclerite is represented only by one fragmentary sclerite preserving mainly the posterior and left lateral fields (Fig. 5A, B) in addition to two possible small specimens representing early growth stages (Fig. 5C-E). The larger specimen has a rectangular cross-section, elongated along the anterior-posterior axis and appears to be bilaterally symmetrical although the first 2-3 growth increments are slightly displaced compared with later growth along the posterior margin (Fig. 5B). The apex is missing but appears to have been slightly inclined over the posterior field, which is developed into a gently domed deltoid (Fig. 5A). No clearly defined posterolateral plications are present but the lateral field is delimited anteriorly by a well-developed anterolateral plication (Fig. 5B). The co-marginal ribs on the lateral field exhibit a distinct apical bend. The apex of the small specimens is a dome-shaped structure with an oval outline, elongated along the anterior-posterior axis and the first two co-marginal ribs replicate this shape (Fig. 5C).
The B sclerite is represented by several well-preserved specimens representing different growth stages. The sclerites are pyramidal with an elongate rectangular cross-section and a moderate helical twist with the apex inclined over one of the wide lateral fields (Fig. 5G). The anterior and posterior fields are narrow and bounded by weakly expressed radial plicae (Fig. 5G). The subapical lateral field is divided into two regions by a median fold: a concave posterolateral sector with several radial plicae and a convex anterolateral sector with only pseudoplicae (Fig. 5F). The supra-apical lateral field is straight or gently convex and with only pseudoplicae (Fig. 5H).
The asymmetrical C sclerites are more numerous than the A and B sclerites and exhibit a pyramidal shape with a crescentic cross-section (Fig. 6B, E, H, K). The inflated dorsal surface is divided into a central, strongly convex zone and two narrow lateral zones by weakly expressed folds (Fig. 6A, E, H). The central zone is typically ornamented by multiple pseudoplicae, increasing in number with sclerite size (Fig. 6A, H). The proximal zone (over which the apex curves) has weakly expressed pseudoplicae (Fig. 6B). The distal zone is characterized by 2-5, strongly developed and closely set radial plicae (Fig. 6E, J). Co-marginal ribs in the folds between plicae are curved towards the apex. The ventral surface is moderately to strongly concave with co-marginal ribs curved towards the apex but without radial ornament (Fig. 6C, F, J).
Both B and C sclerites exhibit distinct apical spines, elongated spine-shaped structures consisting of a tubular initial shell and the 2-3 first co-marginal growth sets (Figs 5F, J, L, 6A, G, L). After the formation of this apical spine the rate of expansion increases dramatically to initiate the adult morphology (sometimes with a single growth set of intermediate expansion; Fig. 5L).
The shell ornament consists of growth sets of a deeply concave inter-rib groove and a convex, flat-topped rib (Figs 5A, K, 6M). Growth sets are separated by a narrow slit at the base of the ad-apertural slope of the rib (Fig. 6N). The surface of interrib grooves is ornamented by irregularly distributed rounded pustules with a weakly expressed superimposed reticulate pattern (Fig. 6M, N). Co-marginal ribs are smooth or with large pustules, elongated in the direction of growth and aligned across successive growth sets to form pseudoplicae (Fig. 5K). The initial shell and first growth sets of B and C sclerites are ornamented by two orders of pustules: large pustules conforming in size and arrangement with the pustules of inter-ribs of the adult shell, and a second set of smaller pustules that are more or less irregularly arranged on, and between the larger pustules (Fig. 5I, L, M). The ornament of the initial shell of A sclerites has a single order of densely set pustules but adult ornament conforms closely with the adult ornament of B and C sclerites.
Remarks. The sclerite morphology and ornamentation of the tommotiid sclerites from the Mural Formation is essentially identical to that of sclerites from the Cassiar Mountains described as Lapworthella filigrana by Conway Morris & Fritz (1984), and the respective specimens are considered conspecific. The only notable difference is the more regular hexagonal pattern formed by the smaller pustules on the initial shell in the Cassiar Mountain specimens (compare Fig. 5I Consequently, it appears that Canadiella filigrana is a widespread tommotiid taxon in Cambrian Stage 3 strata of western Laurentia (Montezuman) with a distribution from northern Mexico to northern British Columbia.
Conway Morris & Fritz (1984) recognized two sclerite types in Canadiella filigrana from the Cassiar Mountain: one 'A' sclerite with polygonal cross-section and one 'B' sclerite with rapidly expanding aperture and a central 'saddle ', andMcMenamin (1984, 2001) followed this sclerite designation. According to our interpretation, the 'A' sclerite of Conway Morris & Fritz (1984) is equivalent to the B sclerites in kennardiids (Laurie 1986;Skovsted et al. 2015), and sclerites of this morphology are consequently referred to as B sclerites herein. The 'B' sclerite of Conway Morris & Fritz (1984) is equivalent to the kennardiid C sclerites and this designation is followed herein. In addition to these sclerite types we also recognize a bilaterally symmetrical sclerite morph in C. filigrana, equivalent to the A sclerites of kennardiids. The material of the A sclerite is, however, limited to fragmentary specimens and its morphology is uncertain. The smaller possible A-type sclerites in the collection represent a low dome-shaped initial shell and this contrasts with the spiniform initial shell of the associated B and C sclerites. A similar difference in initial shell morphology between A and B + C sclerites was documented in Dailyatia  Holotype. Articulated valve RBCM P1411 (Fig. 7C) from sample MP15, lower part of middle shale unit, Mural Formation, Mumm Peak Section, Alberta, Canada.
Material. Holotype and 29 additional specimens, including valve fragments from sample MP15, lower part of middle shale unit, Mural Formation, Mumm Peak Section (Table 1).
Diagnosis. Species of Hipponicharion with elongate, postplete subtriangular shell with strongly marked angular anterodorsal curve; three strongly developed and clearly separated lobes; anterior and posterior lobes long, high and narrow; well developed, transversely elongate central lobe located close to dorsal margin; ornament of fine, circular perforations or pits separated by low nodular ridges.
Description. Equivalved, rounded triangular shell, postplete in lateral outline with length greater than height. Greatest length coincides with anterodorsal curve and crest of anterior lobe (Fig. 7A, F). Hinge line almost straight. Three distinct and well developed nodes: anterior lobe straight, strongly elevated, reaching from anterodorsal corner to close to the ventral margin at about midvalve, separated from flattened lateral margin by a clearly demarcated furrow (Fig. 7A, C, F); posterior lobe straight, strongly elevated and slightly shorter than anterior lobe, reaching from posterolateral corner to close to the ventral margin but clearly separated from anterior lobe (Fig. 7A, C, F); central lobe transversely elongated oval in outline, located close to dorsal margin, with greatest width roughly parallel to the margin (Fig. 7C, D, F). Anterodorsal curve strongly marked and angular, well separated from anterior lobe (Fig. 7A, C). Shell ornament of fine pits or perforations separated by uneven, nodular ribs, which may be developed as discrete pustules on the anterior and posterior nodes (Fig. 7D, E). A well-defined circular area situated between anterior and central lobes lacking perforations, exhibits much finer, anastomosing ridges forming a fingerprint-like pattern (Fig. 7E). A single articulated specimen (Fig. 7G, H) with the left valve partly broken away, exhibits internal structures in the form of a sheet-like inner lamella, partly covering the internal cavities of the prominent anterior and posterior nodes and an elongate, tapering and posteriorly curving and structure emanating from the anterocentral part of the right-hand shell. These structures are partly covered by an anastomosing network of filamentous structures.
Remarks. Hipponicharion perforata differs from the type species, H. eos from Avalonia and Baltica by the more strongly postplete valve outline, the widely separated anterior and posterior lobes and the transverse elongation of the central node, as well as in the pitted ornament (Siveter & Williams 1997;Dies Alvarez et al. 2008). The new species is similar to H. geyeri Hinz-Schallreuter, 1993 from Morocco in the punctate ornament of the shell. However, the pits of H. geyeri are much larger and more widely dispersed on an otherwise smooth shell surface (Hinz-Schallreuter 1993, pl. 12, fig. 1), which is different from the ornament of fine pits separated by nodular ridges in H. perforata (Fig. 7E). The species also differs from H. geyeri in the marked anterodorsal curve and the much longer posterior lobe. Hipponicharion perforata also differs from H. australis Topper et al., 2007 from South Australia in the higher anterior and posterior lobes, the presence of a well-defined central lobe and the clearly marked anterolateral curve as well as in the pitted surface ornament. The new species differs from H. skovstedi Peel, 2017a from North Greenland in the longer valve profile (length greater than width) and the more equally developed anterior and posterior nodes. The species also differs from three morphologically similar and possibly synonymous species (H. hispanicum, H. taidaltensis and H. elickii) reported from the lower Cambrian of Morocco, Spain and Germany by Gozalo & Hinz-Schallreuter (2002) in the subtriangular valve outline with marked anterodorsal curve and the dorsal position of the strongly developed central node.
The marked change in surface ornament from pits separated by nodular ridges to much finer, anastomosing ridges, in a subcircular zone between the anterior and central nodes (Fig. 7E), mirrors the position of a smooth zone behind the anterior spine in Pseudobeyrichona taurata described below (Fig. 8C). Similar anterodorsal zones of reduced or unusual ornaments are present in some other hipponicharionid taxa, such as Hipponicharion geyeri from Morocco (Hinz-Schallreuter 1993, pl. 12, fig. 1 (1999) interpreted the small, rounded anterior (anterodorsal) lobes of the kunmingellid bradoriid Kunmingella Huo, 1956 from the Chengjiang Lagerst€ atte of South China as specific eye lobes. The extensive anterior lobes of hipponicharionid bradoriids do not conform closely with the eye lobes of Kunmingella, but the interpretation of the anterodorsal region of unusual ornament as possible eye spots, suggests that these bradoriids also had well-developed eyesight. The internal structures exposed in a single bivalved specimen with left valve partly broken away (Fig. 7G, H) may represent strongly degraded phosphatized soft parts, including the internal lamella and a large posteriorly projecting limb as well as other unclear structures in the anterior portion of the shell. However, these features are partly covered by filamentous structures presumably representing a phosphatized bacterial cover, which limits biological interpretations. This specimen, together with all other specimens of Hipponicharion perforata, were recovered from a limestone layer close to the base of the middle shale unit of the Mural Formation.
Occurrence. Lower part of middle shale unit, Mural Formation at Mumm Peak in eastern British Columbia. Genus PSEUDOBEYRICHONA Shu, 1990 Remarks. The hipponicharionid genus Pseudobeyrichona was proposed by Shu (1990) (Table 1).
Diagnosis. Equivalved, amplete or weakly postplete hipponicharionid with moderately inflated subtriangular valves; prominent anterior lobe drawn out into an anterodorsally projecting spine with lenticular cross-section; posterior lobe weakly developed; straight hinge line; wide marginal rim; shell ornamented by a network of shallow, circular pits developed into anastomosing furrows on anterior spine.
Description. Equivalved bradoriid with rounded triangular shell, amplete or slightly postplete in lateral outline with straight hingeline (Fig. 8A). Greatest length coincides with angular anterodorsal curve. Anterior and posterior nodes are confluent with a uniform ventral swelling (Fig. 8B). Anterior node inflated and drawn out in the anterodorsal direction, forming a long, flattened spine (Fig. 8A, D). Posterior node weakly developed and restricted to dorsal half of valve (Fig. 8A). Marginal rim wide, with greatest width posteriorly and slightly narrower on anterior side, separated from valve by a prominent furrow (Fig. 8A, B). Shell ornamented by network of irregularly distributed fine pits developed as anastomosing furrows on anterior spine. A circular region at the base of the anterior lobe lacks pits (Fig. 8C).

Remarks.
A single bradoriid valve from the Puerto Blanco Formation of Mexico (Bradoriid sp.; Devaere et al. 2019) is closely comparable to Pseudobeyrichona taurata in valve shape, spine morphology and the width of the marginal rim and may belong to the same species. However, the described valve ornament of the Mexican specimen differs by the presence of pustules around the anterior spine and, until further material is described, the taxonomic identity is considered questionable. Pseudobeyrichona taurata differs from the type species and from P. the anterior spine. Hipponicharion skovstedi from Kap Troedsson Formation of North Greenland is a spinose hipponicharionid from the lower Cambrian of Laurentia (Peel 2017a, fig. 4) but P. taurata differs from this species by the postplete valve outline, the forwardly directed anterior spine and the weakly developed posterior lobe. The circular zone lacking the characteristic pitted surface sculpture at the dorsal side of the base of the prominent anterior spine in the best preserved specimen of Pseudobeyrichona taurata is reminiscent of the possible eye spot present in Hipponicharion perforata described above and may have had a similar function.
Occurrence. Lower Mural Formation at Mumm Peak in eastern British Columbia, possibly the Puerto Blanco Formation of Cerro Raj on, Sonora, Mexico.

Figure 8E-H
Material. 20 specimens from the base of the basal limestone unit of the Mural Formation, Rocky Lake section (Table 1).
Description. Beyrichonid bradoriid with smooth shell surface. Valve outline uncertain but appear to be elongated subtriangular with evenly inflated valve centre and two relatively small but well-constrained and equally developed anterior and posterior nodes ( Fig. Fig. 8E, G). The nodes appear to be restricted to dorsal half of the valve (Fig. 8G, H). Marginal rim narrow with poorly defined furrow, slightly uneven, indicating a possible gape adjacent to anterior lobe (Fig. 8H).
Remarks. All available specimens of this species are fragmentary, mainly preserving the central part of the valves but sometimes with the anterior and posterior nodes preserved. However, the specimens are clearly different from the other bradoriid species in the Mural Formation. The nodes appear to be restricted to the upper half of the valves, which differs from the situation in Hipponicharion perforata where the nodes extend almost from the dorsal to the ventral edge. The presence of two almost equally developed low, rounded nodes also differs from the spine-like anterior and subdued posterior node of Pseudobeyrichona taurata as well as from Liangshanella sp., which lack distinct lobes. The fragmentary specimens are most similar to the hipponicharionid genus Albrunnicola Martinsson, 1979 and the beyrichonid Beyrichona Matthew, 1886. Albrunnicola is best known from the lower Cambrian of South Australia (Skovsted et al. 2006;Topper et al. 2011a) and South China (Zhang 2007) but Albrunnicola sp. has also been reported from the Bastion Formation of North-East Greenland (Skovsted 2006b; Peel 2017a). However, Albrunnicola have typically very reduced lobes, particularly the posterior lobe, and the specimens from the Mural Formation seem to have more strongly pronounced lobes of more or less equal development, which makes them more closely comparable to Beyrichona. This genus is common in Avalonia (Siveter & Williams 1997; Williams & Siveter 1998) but also occurs in Baltica (Dies Alvarez et al. 2008) as well as Kazakhstan (Melnikova et al. 1997). Recently, Beyrichona avganna Peel, 2017a was described from the lower Cambrian of North Greenland (Peel 2017a) and this species is similar to the specimens from the Mural Formation in lobation and general outline, suggesting that they may be congeneric. However, the fragmentary nature of the Mural specimens precludes detailed comparison and hence they are consequently left in open nomenclature.
Occurrence. Basal part of the basal limestone unit of the Mural Formation at Rocky Lake (sample RL3), eastern British Columbia, Canada.
Description. Two isolated, articulated but partly deformed specimens. Sub-rounded, postplete shell with straight dorsal hinge, a likewise straight posterodorsal margin and obtuse anterodorsal corner with a rounded anterodorsal curve (Fig. 8I). The ventral margin is not well-preserved and the shell is partly compressed with irregular folds in the anterior part. The shell surface preserves a fine reticulate pattern (Fig. 8J).
Remarks. These specimens appear to represent a postplete bradoriid with rounded valves with a distinctive straight posterodorsal margin but without lobes. This morphology resembles the cosmopolitan svealutid genus Liangshanella Huo, 1956, particularly the widely dispersed species Liangshanella sayutinae (Melnikova, 1988  Occurrence. Lower-middle Mural Formation, Cambrian Stage 3, Alberta and British Columbia, Canada.

Phylum UNCERTAIN
Class HYOLITHA Marek, 1963 Remarks. In the Mural Formation, hyoliths are relatively common in acid residues from both the basal limestone and middle shale units but are almost exclusively represented by internal moulds of the conical conchs. At least two species are present. Indeterminable hyolithids, represented by internal moulds with a subtriangular cross-section are common throughout the investigated sections ( Fig. 9G-J). Based on differences in rate of expansion of the conchs, more than one species may be represented but better preserved material will be required to confirm this. Gently tapering and curved orthothecid conchs with a circular cross-section may also represent more than one species. The majority of specimens are terminated by a convex transverse wall separated from the conch wall by a countersunk rim, suggesting the genus Cupitheca as described below. Two internal moulds of a hyolith operculum with circular outline may represent the same taxon.
Description. Elongate internal moulds of gently curved hyolith conchs with circular cross-section. Specimens are up to 2.5 mm long (Fig. 9A) and rate of expansion is 8°. Many specimens terminated apically by a rounded transverse wall with countersunk rim (Fig. 9A-F). The margins of the rim often ornamented by short, densely spaced tubercles or rod-like units (Fig. 9D). In a single small specimen, almost completely embedded in matrix, the phosphatized shell is preserved with the countersunk rim ornamented by fine circular pits and a well preserved star-like pattern on the transverse wall (Fig. 9M, N). Weakly impressed star-like impressions are also preserved in some internal moulds (Fig. 9K, L). Internal moulds of subcircular opercula with deep impressions of spine-like cardinal processes with rounded triangular base ( Fig. 9O-Q). Inclined, rod-like clavicles inserted behind the cardinal processes and forming an angle of c. 80° (Fig. 9P), enclosing the rounded apex and a dome-shaped triangular area. The area of the mould between cardinal processes and clavicles slightly depressed, indicating that this surface was elevated in the original shell (Fig. 9Q). Internal surface ornamented by small pustules, particularly on the rounded apex and the depressed area between cardinal processes and clavicles (Fig. 9Q).
Remarks. The specimens of Cupitheca sp. from the Mural Formation are almost exclusively internal moulds of the conch. Only a single small and fragmentary specimen with the outer shell preserved was found. Two internal moulds of opercula were found in direct association with fragmentary conchs, and the circular cross-section indicates that these specimens belong to the same species. Many specimens probably represent the living chamber of the hyolith terminated by the characteristic mould of a convex septum with countersunk rim (Fig. 9A, E, F), while others represent tube segments that were released during the growth of the organism (compare discussion in Bengtson et al. 1990). However, some specimens (see Fig. 9B) seem to preserve moulds of septa at both terminal ends and these specimens presumably represent intermediate stages in tube development with multiple septa. Such specimens with multiple septa have previously been described in material of Remarks. Chancelloriid sclerites are among the most common fossils in the investigated material but most specimens are fragmentary internal moulds, often representing single, isolated rays, and are consequently difficult to identify. However, at least three different genera are represented by rare, better-preserved specimens. Star-shaped sclerites with a rosette of lateral rays surrounding a central, vertical ray (6 + 1, 7 + 1) are referable to Chancelloria Walcott, 1920 (Fig. 10A). Other sclerites with four rays, one of which is strongly recurved over the rest (4 + 0), represent Archiasterella Sdzuy, 1969 (Fig. 10B), while sclerites with four or five sub-equal rays bent away from the basal surface (4 + 0, 5 + 0) are more closely comparable to Allonnia Dor e & Reid, 1965 (Fig. 10C, D). Although much of chancelloriid taxonomy is based on articulated specimens from Burgess Shale-type Lagerst€ atten, sclerites of all three genera are common components in SSF assemblages of early and middle Cambrian age worldwide and a number of sclerite-based species of each genus are recognized. The Mural Formation has not yet yielded any articulated chancelloriid specimens and, although recent investigations have shown that it may be possible to correctly classify disarticulated sclerites ( (Table 1).
Remarks. Disarticulated echinoderm ossicles are common in samples from the basal limestone and middle shale units of the Mural Formation. The majority of specimens are preserved as secondary phosphatic infill of the cavities in the echinoderm stereome structure (Fig. 10F). The morphology of the ossicles varies considerably but most are subcircular or polygonal in outline with smooth or scalloped surfaces (Fig. 10E, G, H). The generalized nature of the majority of specimens precludes identification to any particular echinoderm type.  Yochelson (1977) proposed that agmatans are so different from other fossil and extant taxa that they should be classified in a separate phylum. However, agmatans are known only from the Cambrian and the validity of a phylum-level grouping with only extinct members is uncertain. Agmatan genera are mainly separated from each other by structural differences. In Salterella the agglutinated deposits are F I G . 1 0 . Small shelly fossils from the Mural Formation. All specimens except I-J from Mumm Peak. A, Chancelloria sp., partly phosphatized RBCM P1447 from sample MP-16, middle shale unit, star-shaped sclerite (6 + 1). B, Archiasterella sp., partly phosphatized sclerite RBCM P1404 from sample MP-15 middle shale unit, with recurved vertical ray (4 + 0). C-D, Allonnia sp., internal mould of star-shaped sclerite RBCM P1408 from sample MP-15 middle shale unit, with all rays recurved ( sandwiched between narrow calcareous layers inside a mineralized calcareous shell (Yochelson 1977;Peel & Yochelson 1982;Skovsted 2003;Peel 2017b), while Volborthella seems to lack calcareous components (Hagadorn & Waggoner 2002;Yochelson & Kisselev 2003). Agglutinated cone-shaped fossils occur in samples from both the basal limestone member and in the middle shale member of the Mural Formation (Fig. 10I, J). The fossils have a uniform angle of divergence (c. 30°), a smooth outer surface (Fig. 10J) and a narrow central canal (Fig. 10I). This morphology indicates that the material belongs to the genus   (Table 1).
Remarks. Phosphatic, net-like specimens represent sclerites of the lobopodian Microdictyon Bengtson et al., 1986. All recovered specimens are fragmentary (Fig. 10K, M) but preserve the node morphology ( Fig. 10L) and the basal structure of the perforations in relatively good detail (Fig. 10N) fig. 8.24-32). In our view all the reported specimens probably belong to a single species and the reported differences probably represent different degrees of abrasion. The specimens are not sufficiently preserved to allow a detailed comparison with the material from western Laurentia illustrated by Bengtson et al. (1986) or herein, and should be referred to Microdictyon sp., although Devaere et al. (2019) suggested that all the different node morphologies present in these specimens can be accommodated in M. multicavus. In addition, strongly corroded perforated fragments were referred by Wotte & Sundberg (2017) (Table 1).
Remarks. Helcionelloid molluscs are represented by a small number of imperfectly preserved silicified shells. The specimens represent a single planispiral (Fig. 11A, D), openly coiled (up to c. 180°) and laterally flattened species (Fig. 11C). The specimens exhibit prominent, widely spaced and acutely pointed comarginal ribs that appear to be continuous across the dorsum (Fig. 11B, C). Areas between the co-marginal ribs are ornamented by fine longitudinal striations (Fig. 11D). In gross morphology the specimens may resemble internal moulds of Davidonia Parkhaev, 2017, which are common in sediments from Cambrian Series 2 in Mexico, Greenland and the Taconic allochthons (Landing & Bartowski 1996;Skovsted 2004;Devaere et al. 2019), but the corrugated sculpture of these moulds reflects internal ribs in this genus, which are not reflected on the external shell surface (Bengtson et al. 1990;Gravestock et al. 2001;Skovsted 2004). Instead, the specimens from the Mural Formation are reminiscent of helcionelloid genera such as Oelandiella Vostokova, 1962, Latuochella, Cobbold, 1921and Capitoconus Skovsted, 2004, which have external ribs on the shell. However, due to the poor preservation, precise determination is not possible based on the current material.   (Table 1).
Remarks. Narrow phosphatic tubes with a circular cross-section and external ornamentation of regular annulations occur in the basal limestone and middle shale units. The average rate of expansion is 3.1°and on average the tubes exhibit 24 annulations per mm with individual annulations variable from c. 25 to 75 lm in width. The tubes are straight (Fig. 11E) or gently curved (Fig. 11F) and may bend up to 90° (Fig. 11H). The circular crosssection and finely annulate ornament facilitates identification of the material as belonging to the genus Hyolithellus Billings, 1872. However, due to the high degree of variability, tubes of Hyolithellus are difficult to identify to species based only on the morphology of individual specimens (Skovsted & Peel 2011;Devaere et al. 2019). Although the specimens from the Mural Formation are numerous and generally well-preserved, the variability is also great. A number of specimens with an extremely slow rate of expansion and occasional sharp bends or contorted morphologies (Fig. 11H)  The narrow end of several specimens exhibits a flaring aperture representing a basal increase in diameter of c. 60% (Fig. 11F-G). This flaring, funnel-shaped base is reminiscent of holdfasts in other fossil and recent mineralized tubes (Vinn 2006), and may suggest that tubes were formed by an epibiont, attaching to hard substrates.
Acknowledgements. We thank Jen Wasylyk at Parks Canada for permissions and logistical help, and the pilots of the Yellowhead helicopters for safe flying. We gratefully acknowledge support by the National Geographic Society's Global Exploration Fund -Northern Europe GEFNE113-14, NSF grant EAR-1324095, which helped finance fieldwork, and NSF grant DEB-1747731 for support to EAS. This expedition was conceived at a workshop at the University of Bristol with funds from the Dean's Black Swan fund to JV. We thank Kevin Taylor and Amber Shipley of Bearpaw Heli-Skiing for logistical help during Dezaiko fieldwork. Laboratory preparation of SSF samples was financed by a grant from the Swedish Museum of Natural History and the Swedish A-C, partially silicified specimen RBCM P1366 from sample RL-6: A, lateral view; B, view from dorsum; C, view from supra-apical side. D, lateral view of partially silicified specimen RBCM P1365 from sample RL-6. E-H, Hyolithellus sp. E, lateral view of long, straight specimen RBCM P1329 from sample RL-3, Rocky Lake, with even rate of expansion. F-G, long, bent specimen RBCM P1330 from sample RL-3, with possible holdfast: F, lateral view; G, oblique view of possible holdfast. H, tube fragment RBCM P1333 from sample RL-3, with 90°bend. Scale bar represents: 500 lm (A-F, H); 100 lm (G).
Research Council (VR2016-04610). We thank Tom Boag, Justin Strauss, and Brad Erkilla for assistance with carbon isotope analyses. Therese Sallstedt is gratefully acknowledged for picking fossils from etched residues. Constructive reviews by John L. Moore and Michael J. Vendrasco as well as by the technical journal editor Sally Thomas greatly improved the manuscript and are gratefully acknowledged.   191-197.