Can. J. Earth Sci. 43: 487–496 (2006) doi:10.1139/E05-119 © 2006 NRC Canada

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Small shelly fossils from the basal EmigrantFormation (Cambrian, uppermost Dyeran Stage) ofSplit Mountain, Nevada

Christian B. Skovsted

Abstract: A latest Early Cambrian fauna of helcionelloid molluscs and small shelly fossils from the basal EmigrantFormation of Nevada is described. The fauna is the first of its kind to be described from trilobite-bearing strata in theGreat Basin and is well preserved, but of limited diversity. At the specific level the assemblage is largely endemic, butit contains several genera with global distribution in the Lower and Middle Cambrian. Costipelagiella nevadense n.sp.in the fauna is the oldest representative of the widespread, but little known genus Costipelagiella Horný, 1964. Twoadditional new taxa are described: Anabarella chelata n.sp. and Parkula esmeraldina n.sp.

Résumé : Une faune de mollusques helcionelloïdes et de petits fossiles à coquilles du Cambrien précoce terminal de labase de la Formation d’Emigrant, au Nevada, est décrite. Cette faune, la première en son genre observée dans des stratesà trilobites du Grand Bassin à faire l’objet d’une description, est bien préservée, mais sa diversité est limitée. Sur leplan des espèces, l’assemblage est principalement endémique, mais il renferme plusieurs genres dont la distribution auCambrien précoce et moyen est globale. Costipelagiella nevadense n. sp., présent dans cette faune, est le plus vieuxreprésentant de Costipelagiella Horný, 1964, un genre répandu, bien que peu connu. Deux autres nouveaux taxons sontégalement décrits : Anabarella chelata n. sp. et Parkula esmeraldina n. sp.

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Introduction

Extensive and accessible outcrops of Lower and MiddleCambrian rocks are found in the Great Basin Region of thewestern United States. Although trilobite and brachiopodfaunas from these rocks are well known (e.g., Rowell 1977,1980; Sundberg and McCollum 2000, 2003), other fossilgroups have received little attention. In other Cambrianregions, notably Siberia, China, Australia, and the Avalonianmicrocontinent, molluscs, hyoliths, and problematic fossilsof unknown biological affinity (collectively, small shelly fossils)have been studied extensively (e.g., Rozanov et al. 1969;Landing et al. 1989; Qian and Bengtson 1989; Bengtson etal. 1990). A few studies on Cambrian small shelly fossilsfrom western Canada have been published (Nowlan et al.1985; Voronova et al. 1987), and diverse assemblages ofsmall shelly fossils have recently been documented from theLower Cambrian of Greenland and the Taconic region ofeastern Laurentia (e.g., Landing and Bartowski 1996; Landinget al. 2002; Malinky and Skovsted 2004; Skovsted 2004, inpress; Skovsted et al. 2004). The potential of small shellyfossils for biostratigraphy and paleogeography has been dis-

cussed by Geyer and Shergold (2000), Brock et al. (2000),and Gubanov (2002).

In this report, an assemblage of helcionelloid molluscsand small shelly fossils is described from the basal EmigrantFormation at Split Mountain in Esmeralda County, Nevada.This is the first such fauna to be described from the trilobite-bearing Cambrian strata of the Great Basin. The assemblageis of limited diversity but is well preserved by secondaryphosphatization of originally calcareous shells.

In recent years, attempts by the Cambrian Subcommissionof the International Commission on Stratigraphy (ICS) toidentify internationally recognizable Cambrian time unitshave led to intensified research in the stratigraphy and pale-ontology of potential stratotype sections around the world(e.g., Fletcher 2003; Babcock et al. 2005; Geyer 2005).Sundberg and McCollum (2003) implied that the SplitMountain section could be considered a candidate strati-graphical type section for a potential Cambrian stage bound-ary defined by the lowest appearance of the polymeridtrilobite Oryctocephalus indicus. The present investigation isconfined to material from a narrow interval at the base of theEmigrant Formation, about 14 m below the O. indicus Zone,and adds considerable detail to our knowledge of the faunaassociated with the last olenellid trilobites in the GreatBasin.

Geological setting

The Emigrant Formation of southern Nevada is a condenseduppermost Lower Cambrian to Lower Ordovician sequenceof pelagic and hemipelagic sedimentary rocks deposited inan outer shelf setting (Sundberg and McCollum 2000; Waggoner2003). A detailed stratigraphical analysis of the basal Emigrant

Received 11 August 2005. Accepted 6 December 2005.Published on the NRC Research Press Web site athttp://cjes.nrc.ca on 25 May 2006.

Paper handled by Associate Editor J. Jin.

C.B. Skovsted. Centre for Ecostratigraphy and Palaeobiology,Department of Earth and Planetary Sciences, MacquarieUniversity, NSW 2109, Sydney, Australia (e-mail:cskovste@els.mq.edu.au).

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Formation and the underlying uppermost Mule Spring Lime-stone at the Split Mountain Locality was presented by Sundbergand McCollum (2003), who recognized four biozones in thebasal 20 m of the formation. The Mule Springs Limestonewas deposited in shallow water, and the abrupt facies changeto the basal Emigrant Formation may be related to eustaticsea-level rise (Ketner 1998). The lowermost Emigrant For-mation at Split Mountain contains Early Cambrian trilobites(Olenellus Zone), and the Lower–Middle Cambrian boundary(as conventionally defined by the disappearance of olenellidtrilobites) occurs �1.5 m above the base of the formation(Sundberg and McCollum 2003). The overlying Eokochaspisnodosa and Amecephalus arrojosensis Zones span �13.5 m.The lowest appearance of O. indicus at Split Mountain is�15 m above the base of the Emigrant Formation (Sundbergand McCollum 2003) (Fig. 1).

Material and methods

Three samples of �1 kg each were collected near the baseof the Emigrant Formation at the Split Mountain section atClayton Ridge in Esmeralda County, Nevada, during theLaurentia 99 field conference to the Great Basin of the Cam-brian Stage Subdivision Working Group of the ICS (McCollumand Sundberg 1999). Two samples of limestone (CS990917-01and CS990917-02) were collected close to the base of theEmigrant Formation, and a third sample (CS990917-03) wascollected �1.4 m above the base of the formation (Fig. 1).The limestone samples were digested in 10% phosphate-buffered acetic acid at the Department of Earth Sciences,

Uppsala University, Uppsala, Sweden. Insoluble residues weresieved (mesh dimensions 1, 0.5, 0.25, and 0.125 mm) andscreened for fossils. Selected fossils were gold coated andphotographed with Phillips and JEOL scanning electronmicroscopes (SEM). All figured specimens are housed at theUS National Museum of Natural History (USNM) at theSmithsonian Institution in Washington, DC.

Split Mountain fauna

Acid-resistant residues from the Emigrant Formation containabundant trilobite spines and phosphatic brachiopods inaddition to a suite of secondarily phosphatized shells ofmolluscs and other small shelly fossils. No major differencesin the fauna were observed between the three samples. Threegroups of organophosphatic brachiopod are present (actrotretids,acrotheloids, and paterinids). Brachiopods from Early–MiddleCambrian transitional beds of the Great Basin were describedby Rowell (1980) but are not dealt with taxonomically in thispaper. The assemblage of secondarily phosphatized fossilsdescribed in this paper is numerically dominated by disarti-culated echinoderm ossicles and chancelloriid sclerites, andphosphatic internal molds of one helcionelloid mollusc andone hyolithid are relatively common. Other components ofthe fauna are rare and are represented by a few specimensonly.

It is noteworthy that almost all secondarily phosphatizedspecimens are small, rarely exceeding 1 mm in length, whileassociated brachiopods are represented by larger specimens(up to 5 mm in diameter) in addition to juveniles. All taxa

Fig. 1. Location and stratigraphy. (A) Generalized map of the United States with location of the studied area indicated. (B) Map ofsouthern Nevada with location of the Split Mountain section indicated. (C) Simplified stratigraphical column for the uppermost MuleSprings Limestone and basal Emigrant Formation of the Split Mountain section (after Sundberg and McCollum 2003) showing trilobitezones and stratigraphical position of samples.

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represented in the fauna are closely related to species knownto attain much larger size at other localities, and the smallsize of the phosphatized specimens from Split Mountain isprobably related to taphonomic or diagenetic rather than bio-logical factors.

The diversity of the Split Mountain fauna is limited, andonly three taxa are adequately preserved to allow detaileddescriptions (e.g., Anabarella chelata n.sp., Costipelagiellanevadense n.sp., and Parkula esmeraldina n.sp.). All threespecies are new, but other species of the same genera arewidespread in the Early or Middle Cambrian. Faunal com-parisons of the Split Mountain assemblage are further ham-pered by a general lack of data on small shelly fossils fromthe latest intervals (Toyonian and equivalents) of the EarlyCambrian (as conventionally defined). Although many smallshelly fossil assemblages from around the world have beendescribed, most are considerably older (Tommotian to Botomanor equivalent in age). This is true for all assemblages describedfrom Laurentia, including those described from the Yukon(Nowlan et al. 1985) and the Mackenzie Mountains (Voronovaet al. 1987) of Canada. Within Laurentia, the only possibleexception is a fauna associated with trilobites of the latestEarly Cambrian Bicella bicensis trilobite assemblage fromthe Taconic allochthon at Ville Guay in Quebec (Landing etal. 2002). The Ville Guay fauna shows no clear ties to theSplit Mountain assemblage and instead shows similarities toolder small shelly faunas from other parts of the Taconicallochthon (Landing and Bartowski 1996), Greenland (Malinkyand Skovsted 2004; Skovsted 2004, in press), and westernNewfoundland (personal observation, 2004).

Thus, the Split Mountain assemblage cannot be readilycompared to any other fauna of small shelly fossils describedin the literature. It is a very small fauna, derived from onlythree samples, and it is the first of its kind to be describedfrom the Great Basin. This study demonstrates that secondaryphosphatization of calcareous shells did occur in the GreatBasin and that investigation of small shelly fossil assemblagesmay contribute to our knowledge of the paleontology andbiostratigraphy of this region, as well as other Cambrianregions of the world.

Systematic paleontology

Phylum Mollusca Cuvier, 1797Class Helcionelloida Peel, 1991Order Helcionellida Geyer, 1994Family Helcionellidae Wenz, 1938Genus Anabarella Vostokova, 1962Type species Anabarella plana Vostokova, 1962

COMPOSITION: Type species, Anabarella australis Runnegarin Bengtson, Conway Morris, Cooper, Jell and Runnegar1990, Anabarella simesi MacKinnon, 1985, and Anabarellachelata n.sp.

DIAGNOSIS: Small, bilaterally symmetrical univalve molluscwith advolute or slightly involute, rapidly expanding and later-ally compressed shell coiled through about one half to onewhorl; the shell is smooth or weakly ornamented by closelyspaced transverse growth lines; protoconch is small and glob-ular.

DISCUSSION: Anabarella differ from most helcionelloid molluscsby its great lateral compression. Among genera with similarlycompressed planispiral shells it differs from Stenotheca Salterin Hicks, 1872 by the more strongly coiled shell and convexapertural margins (in lateral view); from MellopegmaRunnegar and Jell, 1976 by the more strongly coiled shellwith overhanging apex, and from Planutenia Elicki, 1994 bythe small, cap-shaped apex.

Anabarella chelata n.sp.Figs. 2A–2E

HOLOTYPE AND TYPE LOCALITY: USNM 530265 from sampleCS990917-03 of the basal Emigrant Formation, Split Moun-tain, Esmeralda County, Nevada (Fig. 2A).

DIAGNOSIS: Species of Anabarella with low shell and stronglyoverhanging apex reaching well beyond the subapical margin.

ETYMOLOGY: Latin chela, meaning claw. Derived from theclaw-like appearance of the fossils in lateral view.

ADDITIONAL MATERIAL: USNM 530266 – USNM 530268 and48 unfigured specimens from the basal Emigrant Formation,Split Mountain, Esmeralda County, Nevada.

DESCRIPTION: Narrow internal molds of a small and stronglycompressed univalve. The fossils are bilaterally symmetricaland are 0.5–0.8 mm long, 0.3–0.4 mm high, and about0.2 mm wide. The molds are coiled through one quarter toone half of a whorl and rapidly expanding in the plane ofcoiling. The apex is strongly overhanging and reaches wellbeyond the subapical margin of the aperture. In lateral viewthe aperture is gently and evenly convex, except at thesubapical end where it is more strongly flexed towards theapex, forming a deep sinus (Figs. 2A–2C). The mold of thesubapical sinus is slightly expanding laterally. A less pro-nounced sinus is present at the supra-apical end of the aper-ture (Figs. 2B, 2C). The mold of the protoconch is small andglobular, but not clearly delineated from the rest of the conch.Faint furrows radiating from the very apex are present in onespecimen (Figs. 2B, 2E). The surfaces of the molds are other-wise ornamented by faint comarginal lineations.

DISCUSSION: Anabarella chelata n.sp. is represented in acidresidues by internal molds only. The relatively tight coilingand strong lateral compression of the molds are characteristicof Anabarella Vostokova, 1962. The species differs fromother species of Anabarella by the more rapid expansion ofthe conch, resulting in a lower, more elongated shell with amore strongly overhanging apex. Internal molds of A. australisRunnegar in Bengtson et al. (1990) from the Atdabanian toBotoman of South Australia (Parkhaev in Gravestock et al.2001), Germany (Elicki 1996), and Greenland (Gubanov etal. 2004) are highly variable and frequently exhibit an over-hanging apex, but not to the same degree as in A. chelata.Further, the faint comarginal ornamentation of A. australis isnot reflected on the surface of internal molds. Anabarellaaustralis is a very variable species and a variant with a moredistinctly overhanging apex (originally described separatelyas Anabarella argus Runnegar in Bengtson et al. 1990; seediscussion by Parkhaev in Gravestock et al. 2001) differsfrom A. chelata by the higher and more tightly coiled shell.

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Anabarella simesi MacKinnon, 1985 from the Middle Cam-brian of New Zealand and New South Wales (Brock 1998)does exhibit faint comarginal lineations on internal molds,but the shell of this species is much higher than that of

A. chelata and the apex rarely projects beyond the subapicalmargin in lateral view.

The maximum size of A. chelata is small compared withthat of other species of the genus. As discussed previously,

Fig. 2. Molluscs from the basal Emigrant Formation of Split Mountain, Nevada. (A–E) Anabarella chelata n.sp. (A) USNM 530265(holotype) from sample CS990917-03, lateral view. (B, E) USNM 530266 from sample CS990917-01, lateral view and closeup of apicalregion. (C) USNM 530267 from sample CS990917-02, lateral view. (D) USNM 530268 from sample CS990917-01, oblique dorsalview. (F–J) Costipelagiella nevadense n.sp. (F) USNM 530269 from sample CS990917-03, view from umbilical side of internal mold.(G) USNM 530270 from sample CS990917-03, view from umbilical side of secondarily phosphatized specimen. (H–J) USNM 530271(holotype) from sample CS990917-01, secondarily phosphatized specimen from spiral side, from smooth dorsum and in oblique aperturalview. Scale bar represents 100 µm for (A–D), 75 µm for (E), and 150 µm for (F–J).

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this applies to all secondarily phosphatized taxa from thebasal Emigrant Formation and may be a diagenetic feature.

OCCURRENCE: Uppermost Lower Cambrian of Nevada.

Genus Costipelagiella Horný, 1964Type species Costipelagiella zazvorkai Horný, 1964

COMPOSITION: Type species, Costipelagiella kochi Peel, 1988,Costipelagiella sp. A (sensu Bischoff 1990), and Costipelagiellanevadense n.sp.

DIAGNOSIS: Univalve, dextrally coiled shell composed of oneor two rapidly expanding whorls. Spiral surface flat, slightlyconvex, or concave. The last whorl is wide with oval orsubtriangular cross section. External ornamentation of coarse,comarginal ribs or folds.

DISCUSSION: Pelagiellids are the most widely distributed of allhelcionelloid molluscs in the Cambrian Period. Most speciesare referred to Pelagiella Matthew, 1895, but a few formsfrom the Middle Cambrian have been referred to CostipelagiellaHorný, 1964, based on a distinctive external ornament ofcomarginal ribs. The type species of Costipelagiella,Costipelagiella zazvorkai Horný, 1964 from the MiddleCambrian of the Czech Republic (Horný 1964), is onlyknown from a single specimen, and the same applies toCostipelagiella kochi Peel, 1988 from the Middle Cambrianof North Greenland (Peel 1988). Costipelagiella cf. zazvorkaiwas described from the Middle Cambrian of New Zealandby MacKinnon (1985), and closely comparable specimensfrom the Middle Cambrian of New South Wales were reportedas Costipelagiella sp. A by Bischoff and Pendergast (1987).Bischoff (1990) suggested that specimens from New Zealandand Australia are conspecific. In Costipelagiella sp. A, theexternal ornament is sometimes evident on internal molds,but this is not the case in the new species from the GreatBasin. Without the preserved external shell, internal moldsof C. nevadense n.sp. are indistinguishable from similarlypreserved Pelagiella.

The best known species of the related genus Pelagiella,Pelagiella subangulata (Tate, 1892) from the Lower Cam-brian of Australia and Greenland, exhibits a high degree ofvariability in shell shape and ornamentation (Parkhaev inGravestock et al. 2001; Skovsted 2004). The three previouslyrecognized species of Costipelagiella are distinguished bydifferences in shell shape and ornamentation that are so subtlethat they would easily fall within the range of variability andontogenetical change of P. subangulata. As two out of threespecies are known from one specimen only, however, it isdifficult to evaluate their interrelationships and potential syn-onymy.

Flexure of the ribs on the dorsum of Costipelagiella wasinterpreted by MacKinnon (1985), Peel (1988), and Bischoff(1990) to reflect a sinus on the aperture of the shell. Pelagiellasubangulata has an external ornament of fine ridges thatconverge to form an inverted V-shaped pattern on the dorsum(e.g., Skovsted 2004, figs. 8b, 8c) The aperture of P. subangulatais planar and the V-shaped ridges on the dorsum are notassociated with any notch or sinus on the aperture. Thus, theexternal ornament in Pelagiella does not reflect growth incre-ments. The same may apply to the external ornamentation of

Costipelagiella, but this cannot be assessed until specimenswith a well-preserved aperture are found.

Costipelagiella nevadense n.sp.Figs. 2F–2J

HOLOTYPE AND TYPE LOCALITY: USNM 530271 from sampleCS990917-01 from the basal Emigrant Formation, SplitMountain, Esmeralda County, Nevada.

DIAGNOSIS: Species of Costipelagiella with spiral side flat andan oval cross section of the last whorl. External ornament ofgently curved fold-like ribs on both sides of the smooth dor-sum.

ETYMOLOGY: From the location of the type locality in Nevada.

Additional material: USNM 530269, USNM 530270, and sixunfigured specimens from the basal Emigrant Formation,Split Mountain, Esmeralda County, Nevada.

DESCRIPTION: Small, dextral shells, coiled through about onewhorl. The spiral side is nearly flat, and the umbilical side ismore strongly convex. The cross section of the whorl is oval.The suture separating the last whorl from the apex is a deep,narrow furrow. The protoconch is globose, smooth, anddelineated by a distinct furrow. The aperture is not pre-served in any of the available specimens. Both the spiral andumbilical sides are ornamented by closely spaced (averagedensity 43 ribs/mm), fold-like ribs and intervening narrowgrooves that seemingly run parallel to the aperture. Close tothe smooth dorsum the ribs curve slightly towards the apexand become progressively subdued. Co-occurring internal moldshave unornamented surfaces.

All specimens exhibit traces of the activity of endolithicalga, either as hollow, vermiform borings through the shell(Fig. 2I, top of specimen) or as narrow undulating molds ofbacterial strands on internal molds (Fig. 2F).

DISCUSSION: All pelagiellid specimens recovered from the basalEmigrant Formation are assigned to Costipelagiella Horný,1964 because of their distinct external ornament of fold-likeribs on secondarily phosphatized specimens. Costipelagiellanevadense n.sp. differs from all previously known species ofCostipelagiella by the complete absence of the folds on thedorsum. As all specimens of C. nevadense with externalshell preserved are relatively small, the paucity in ornamen-tation could be interpreted as a juvenile character. This is notconsidered likely, however, as the width of the smooth zoneon the dorsum appears to increase towards the aperture.

The latest Early Cambrian age of the basal Emigrant For-mation means that C. nevadense is the oldest known speciesof Costipelagiella. Other reports of the genus are exclusivelyfrom Middle Cambrian strata. In the Middle Cambrian,Costipelagiella occurs in different paleogeographic provinces(East and West Gondwana, Laurentia), indicating that thegenus was very widespread by this time. Its potential forbiostratigraphy is hampered, however, by insufficient knowledgeon the morphology and variability of the described species.

Bischoff (1990) described fossilized remains of possibledigestive glands in two specimens of Costipelagiella sp. Afrom New South Wales. No comparable structures wereobserved in C. nevadense.

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OCCURRENCE: Uppermost Lower Cambrian of Nevada.

Phylum uncertainClass Hyolitha Marek, 1963Order Hyolithida Syssoiev, 1957Family unassignedGenus Parkula Bengtson in Bengtson, Conway Morris,

Cooper, Jell and Runnegar, 1990Type species Parkula bounties Bengtson in Bengtson,

Conway Morris, Cooper, Jell and Runnegar, 1990

COMPOSITION: Type species and Parkula esmeraldina n.sp.

EMENDED DIAGNOSIS: Straight conchs with lenticular crosssection, faint dorsal median ridge, and semielliptical ligula.Surface sculpture of transverse striations and sometimesirregular longitudinal wrinkles. Opercula with strongly con-vex conical shield, short cardinal processes, and one pair ofclavicles ending in blade-like projections.

DISCUSSION: Parkula differ from Microcornus Mambetov, 1972,in the very weak dorsal median ridge and the elliptical crosssection of the conch. The operculum of Microcornus is notwell known, although the holotype of M. parvulus Mambetov,1972 is preserved with the operculum still in the aperture ofthe conch. The operculum has a narrow cardinal shield andwell-developed comarginal ornamentation, but its internalmorphology is not known. The type species of Parkula,P. bounties Bengtson in Bengtson et al. 1990, was originallydescribed from the Lower Cambrian (Atdabanian to Botomanequivalents) of South Australia but has also been reportedfrom Antarctica (Wrona 2003) and northeast Greenland(Malinky and Skovsted 2004).

Parkula esmeraldina n.sp.Figs. 3A–3I

HOLOTYPE AND TYPE LOCALITY: USNM 530272 from sampleCS990917-03 of the basal Emigrant Formation, Split Mountain,Esmeralda County, Nevada.

DIAGNOSIS: Species of Parkula with well-developed ornamenta-tion of transverse striations on conch and operculum. Apertureof conch has a shallow median dorsal sinus. Operculum withshort but well-developed cardinal processes projecting wellbeyond the cardinal shield.

ETYMOLOGY: From the location of the type locality in EsmeraldaCounty, Nevada.

Additional material: USNM 530273 – USNM 530275 and18 unfigured specimens from the basal Emigrant Formation,Split Mountain, Esmeralda County, Nevada.

DESCRIPTION: Straight and slender conchs (angle of divergence20°) up to approximately 1 mm long, but some specimenslack the apical portion. The transverse cross section is lenticular,with the dorsal side more strongly convex than the ventralside. The dorsal and ventral surfaces meet along the smoothlyrounded lateral edges. The dorsal side has a very weak medianridge. The ligula is semielliptical and approximately twotimes wider than long. The aperture has a well-defined dorsalmedian sinus, and the conch is ornamented by well-developedrounded comarginal ribs.

Co-ocurring opercula (Figs. 3D–3I) have a narrow sigmoidalcardinal shield that reflects the dorsal median sinus of theconch and poorly defined tectula. The conical shield is stronglyconvex with well-defined comarginal ribs. The cardinal pro-cesses are short, but in lateral view project beyond the cardinalshield. In effect they are situated in front of the cardinalshield rather than under it. One pair of clavicles that ends inblade-like projections is situated at the junction of the cardinaland conical shields.

One operculum has a series of pits on the internal surfaceof the conical shield between the clavicles (Figs. 3G–3I).One centrally placed pit just below the junction of cardinalprocesses and clavicles reflects the apex of the conical shield.Situated slightly behind the apical pit and immediately belowthe mid-point of the clavicles is a pair of shallow furrows orelongated pits (indicated by arrows in Figs. 3G and 3H). Theshort furrows run approximately parallel to each other andthe anterior–posterior axis of the operculum.

DISCUSSION: Parkula esmeraldina n.sp. differs from P. bountiesin the surface sculpture of rounded comarginal ribs on theconch without longitudinal ribs or wrinkles and in the positionof the cardinal processes in front of rather than under thecardinal shield of the operculum (as seen in lateral view;compare Fig. 3E with Figs. 150d, 150h, and 151c of Bengtsonet al. 1990). Similar cardinal processes are present in twoopercula described as Parkula? by Malinky and Skovsted(2004) from the Bastion Formation of northeast Greenland,where they occur together with P. bounties. Parkula? sp. hasa smooth conical shield with subdued comarginal ornamenta-tion, however.

Macroscopic hyoliths have previously been described fromthe Lower Cambrian of the Great Basin (e.g., Babcock andRobison 1988). No previously described hyolithid matchesthe conchs and opercula of P. esmeraldina, however. Thegeneral problem of comparing macroscopic and microscopichyoliths was discussed by Malinky and Skovsted (2004).

The shallow pits on the internal surface of the conicalshield in Parkula esmeraldina probably represent sites ofmuscle insertion. Muscle scars are known to occur in variouspositions in different post-Cambrian hyolithids, and musclesprobably associated with the movement of the helens arecommonly inserted on the conical shield (Marti-Mus andBergström 2005).

OCCURRENCE: Uppermost Lower Cambrian of Nevada.

Miscellaneous fossilsEchinoderm scleritesFigs. 4A, 4B

MATERIAL: USNM 530276, USNM 530277, and hundreds ofadditional specimens from the basal Emigrant Formation,Split Mountain, Esmeralda County, Nevada.

DISCUSSION: Fossils with a preserved stereome ultrastructuretypical of echinoderm sclerites are numerically dominant amongthe secondarily phosphatized fossils of the basal EmigrantFormation. Star- and shield-shaped, globular and irregularsclerites are present (Figs. 4A, 4B), but no specimen can beassigned to any particular taxon. Similar disarticulated echino-derm ossicles are common among small shelly fossil assem-

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blages from the Atdabanian and later Cambrian strata whereversecondary phosphatization of originally calcareous skeletonsoccurs.

Perforated spheresFig. 4C

Material: USNM 530278 and five additional specimens fromthe basal Emigrant Formation, Split Mountain, EsmeraldaCounty, Nevada.

Description: More or less spherical objects with a diameterof approximately 0.3 mm (Fig. 4C). All available specimensare hollow and their exterior has a rough, crystalline texture.The external surface also has a set of evenly distributed cir-cular perforations of almost uniform dimensions (19–24 µmin diameter).

Discussion: These spherical objects probably represent someform of biological structure. They show some similarities toOlivooides Qian, 1977 and other spherical fossils representing

metazoan eggs and embryos (see review in Steiner et al.2004) but differ from these fossils in that they have a perfo-rated surface. The external perforations recall the problem-atic fossil Aetholicopalla Conway Morris in Bengtson et al.(1990) from South Australia and France, but the specimensfrom the Emigrant Formation are much larger and have fewerperforations. The specimens are considered too poorly pre-served to allow detailed taxonomic assessment.

Chancelloria spp.Figs. 4D, 4E

MATERIAL: USNM 530279, USNM 530280, and hundreds ofadditional specimens from the basal Emigrant Formation,Split Mountain, Esmeralda County, Nevada.

DISCUSSION: Chancelloriid sclerites are usually represented inthe Emigrant Formation material by isolated and poorly pre-served individual rays. Among the few more complete scleritespresent, two morphologies are represented in approximatelyequal numbers (1 + 6 and 0 + 3). Larger sclerites have one

Fig. 3. Parkula esmeraldina n.sp. from the basal Emigrant Formation of Split Mountain, Nevada. (A, B) USNM 530272 (holotype)from sample CS990917-03 in (A) dorsal view and (B) oblique view from aperture. (C) USNM 530273 from sample CS990917-03,dorsal view showing prominent growth lines. (D–F) USNM 530274 from sample CS990917-03. (D) Exterior of operculum from cardinalshield. (E) From right-lateral side. (F) From above. (G–I) USNM 530275 from sample CS990917-01. (G) Interior of operculum frombelow. (H) Obliquely from conical shield. (I) From left-lateral side. Arrows in G and H indicate position of possible muscle scars.Scale bar represents 100 µm.

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vertical ray and six equal rays in the plane of the basal facet(Fig. 4D). The basal facet is a well-defined platform withcircular or teardrop-shaped basal foramina. Sclerites withoutcentral rays have three equal rays that diverge clearly fromthe plane of the basal facet (Fig. 4E). The two sclerite typespossibly belong to different genera and (or) species, butneither form is considered well enough preserved to allowtaxonomic assignment.

Girvanella sp.Fig. 4F

MATERIAL: USNM 530281 and 13 additional specimens fromthe basal Emigrant Formation, Split Mountain, EsmeraldaCounty, Nevada.

DISCUSSION: Irregular aggregates of thin phosphatized cyano-bacterial strands occur in the samples from the EmigrantFormation. The aggregates are up to 0.4 mm long, but largerspecimens may have been broken up during acid prepara-tion. Individual strands within the aggregates are unbranchedand are preserved as double tubes with outer and inner diame-ters of approximately 20 and 10 µm, respectively. Aggregatesof cyanobacterial strands from the Lower Cambrian havebeen referred to in the literature as Girvanella Nicholsonand Etheridge, 1878 (e.g., Daneilli 1981). Although the SplitMountain specimens are very similar to Girvanella sp. fromSouth Australia (Runnegar in Bengtson et al. 1990), the

morphology of the fossils is too generalized for meaningfultaxonomic comparison.

Hexactinellid spiculeFig. 4G

MATERIAL: USNM 530282 from the basal Emigrant Forma-tion, Split Mountain, Esmeralda County, Nevada.

DISCUSSION: Poriferans are represented in the acid residues bya single hexact spicule. All six rays are of approximatelyequal dimensions and diverge at right angles to each other.The specimen is probably a mold of the axial canal of acalcihexactinellid spicule. Similar fossils have previously beendescribed from different Cambrian small shelly fossil assem-blages (e.g., Hinz 1987; Skovsted in press).

Acknowledgments

J.S. Peel is gratefully acknowledged for generously providingfinancial and practical support in connection with the collec-tion, preparation, and SEM work on the Split Mountainmaterial. A.R. Palmer, who organized the Laurentia 99 fieldconference, is thanked for hospitality and for providing muchinformation on the geology and paleontology of the GreatBasin. G. Brock and my wife Anne Mette are thanked forvaluable comments on an early version of the manuscript.Comments by two external reviewers, E. Landing and

Fig. 4. Miscellaneous fossils from the basal Emigrant Formation of Split Mountain, Nevada. (A, B) Echinoderm sclerites. (A) USNM 530276from sample CS990917-03. (B) USNM 530277 from sample CS990917-03. (C) Perforated sphere, USNM 530278 from sample CS990917-03.(D, E) Chancelloria spp. (D) USNM 530279, sample CS990917-01, 6 + 1 sclerite viewed from basal facet. (E) USNM 530280,sample CS990917-02, 3 + 0 sclerite viewed from basal facet. (F) Girvanella sp., USNM 530281 from sample CS990917-03.(G) Hexactinellid spicule, USNM 530282 from sample CS990917-03. Scale bar represents 50 µm for (C) and 100 µm for all otherpictures.

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Skovsted 495

B. Runnegar, and the Associate Editor of the journal, J. Jin,greatly improved the manuscript.

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