Small Shelly Fossils from the Basal Emigrant Formation (Cambrian, Uppermost Dyeran Stage) of Split Mountain, Nevada

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Small shelly fossils from the basal Emigrant Formation (Cambrian, uppermost Dyeran Stage) of Split Mountain, Nevada

Christian B. Skovsted

Abstract: A latest Early Cambrian fauna of helcionelloid molluscs and small shelly fossils from the basal Emigrant Formation of Nevada is described. The fauna is the first of its kind to be described from trilobite-bearing strata in the Great Basin and is well preserved, but of limited diversity. At the specific level the assemblage is largely endemic, but it 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. Two additional 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 la base de la Formation d’Emigrant, au Nevada, est décrite. Cette faune, la première en son genre observée dans des strates à trilobites du Grand Bassin à faire l’objet d’une description, est bien préservée, mais sa diversité est limitée. Sur le plan des espèces, l’assemblage est principalement endémique, mais il renferme plusieurs genres dont la distribution au Cambrien précoce et moyen est globale. Costipelagiella nevadense n. sp., présent dans cette faune, est le plus vieux représentant de Costipelagiella Horný, 1964, un genre répandu, bien que peu connu. Deux autres nouveaux taxons sont également décrits : Anabarella chelata n. sp. et Parkula esmeraldina n. sp. [Traduit par la Rédaction] Skovsted 496

Introduction cussed by Geyer and Shergold (2000), Brock et al. (2000), and Gubanov (2002). Extensive and accessible outcrops of Lower and Middle In this report, an assemblage of helcionelloid molluscs Cambrian rocks are found in the Great Basin Region of the and small shelly fossils is described from the basal Emigrant western United States. Although trilobite and brachiopod Formation at Split Mountain in Esmeralda County, Nevada. faunas from these rocks are well known (e.g., Rowell 1977, This is the first such fauna to be described from the trilobite- 1980; Sundberg and McCollum 2000, 2003), other fossil bearing Cambrian strata of the Great Basin. The assemblage groups have received little attention. In other Cambrian is of limited diversity but is well preserved by secondary regions, notably Siberia, China, Australia, and the Avalonian phosphatization of originally calcareous shells. microcontinent, molluscs, hyoliths, and problematic fossils In recent years, attempts by the Cambrian Subcommission of unknown biological affinity (collectively, small shelly fossils) of the International Commission on Stratigraphy (ICS) to have been studied extensively (e.g., Rozanov et al. 1969; identify internationally recognizable Cambrian time units Landing et al. 1989; Qian and Bengtson 1989; Bengtson et have led to intensified research in the stratigraphy and pale- al. 1990). A few studies on Cambrian small shelly fossils ontology of potential stratotype sections around the world from western Canada have been published (Nowlan et al. (e.g., Fletcher 2003; Babcock et al. 2005; Geyer 2005). 1985; Voronova et al. 1987), and diverse assemblages of Sundberg and McCollum (2003) implied that the Split small shelly fossils have recently been documented from the Mountain section could be considered a candidate strati- Lower Cambrian of Greenland and the Taconic region of graphical type section for a potential Cambrian stage bound- eastern Laurentia (e.g., Landing and Bartowski 1996; Landing ary defined by the lowest appearance of the polymerid et al. 2002; Malinky and Skovsted 2004; Skovsted 2004, in trilobite Oryctocephalus indicus. The present investigation is press; Skovsted et al. 2004). The potential of small shelly confined to material from a narrow interval at the base of the fossils for biostratigraphy and paleogeography has been dis- Emigrant Formation, about 14 m below the O. indicus Zone, and adds considerable detail to our knowledge of the fauna associated with the last olenellid trilobites in the Great Received 11 August 2005. Accepted 6 December 2005. Basin. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 25 May 2006. Geological setting Paper handled by Associate Editor J. Jin. The Emigrant Formation of southern Nevada is a condensed C.B. Skovsted. Centre for Ecostratigraphy and Palaeobiology, uppermost Lower Cambrian to Lower Ordovician sequence Department of Earth and Planetary Sciences, Macquarie of pelagic and hemipelagic sedimentary rocks deposited in University, NSW 2109, Sydney, Australia (e-mail: an outer shelf setting (Sundberg and McCollum 2000; Waggoner [email protected]). 2003). A detailed stratigraphical analysis of the basal Emigrant

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

Formation and the underlying uppermost Mule Spring Lime- Uppsala University, Uppsala, Sweden. Insoluble residues were stone at the Split Mountain Locality was presented by Sundberg sieved (mesh dimensions 1, 0.5, 0.25, and 0.125 mm) and and McCollum (2003), who recognized four biozones in the screened for fossils. Selected fossils were gold coated and basal 20 m of the formation. The Mule Springs Limestone photographed with Phillips and JEOL scanning electron was deposited in shallow water, and the abrupt facies change microscopes (SEM). All figured specimens are housed at the to the basal Emigrant Formation may be related to eustatic US National Museum of Natural History (USNM) at the sea-level rise (Ketner 1998). The lowermost Emigrant For- Smithsonian Institution in Washington, DC. mation at Split Mountain contains Early Cambrian trilobites (Olenellus Zone), and the Lower–Middle Cambrian boundary (as conventionally defined by the disappearance of olenellid Split Mountain fauna trilobites) occurs -1.5 m above the base of the formation Acid-resistant residues from the Emigrant Formation contain (Sundberg and McCollum 2003). The overlying Eokochaspis abundant trilobite spines and phosphatic brachiopods in nodosa and Amecephalus arrojosensis Zones span -13.5 m. addition to a suite of secondarily phosphatized shells of The lowest appearance of O. indicus at Split Mountain is molluscs and other small shelly fossils. No major differences -15 m above the base of the Emigrant Formation (Sundberg in the fauna were observed between the three samples. Three and McCollum 2003) (Fig. 1). groups of organophosphatic brachiopod are present (actrotretids, acrotheloids, and paterinids). Brachiopods from Early–Middle Material and methods Cambrian transitional beds of the Great Basin were described by Rowell (1980) but are not dealt with taxonomically in this Three samples of -1 kg each were collected near the base paper. The assemblage of secondarily phosphatized fossils of the Emigrant Formation at the Split Mountain section at described in this paper is numerically dominated by disarti- Clayton Ridge in Esmeralda County, Nevada, during the culated echinoderm ossicles and chancelloriid sclerites, and Laurentia 99 field conference to the Great Basin of the Cam- phosphatic internal molds of one helcionelloid mollusc and brian Stage Subdivision Working Group of the ICS (McCollum one hyolithid are relatively common. Other components of and Sundberg 1999). Two samples of limestone (CS990917-01 the fauna are rare and are represented by a few specimens and CS990917-02) were collected close to the base of the only. Emigrant Formation, and a third sample (CS990917-03) was It is noteworthy that almost all secondarily phosphatized collected -1.4 m above the base of the formation (Fig. 1). specimens are small, rarely exceeding 1 mm in length, while The limestone samples were digested in 10% phosphate- associated brachiopods are represented by larger specimens buffered acetic acid at the Department of Earth Sciences, (up to 5 mm in diameter) in addition to juveniles. All taxa

© 2006 NRC Canada Skovsted 489 represented in the fauna are closely related to species known DISCUSSION: Anabarella differ from most helcionelloid molluscs to attain much larger size at other localities, and the small by its great lateral compression. Among genera with similarly size of the phosphatized specimens from Split Mountain is compressed planispiral shells it differs from Stenotheca Salter probably related to taphonomic or diagenetic rather than bio- in Hicks, 1872 by the more strongly coiled shell and convex logical factors. apertural margins (in lateral view); from Mellopegma The diversity of the Split Mountain fauna is limited, and Runnegar and Jell, 1976 by the more strongly coiled shell only three taxa are adequately preserved to allow detailed with overhanging apex, and from Planutenia Elicki, 1994 by descriptions (e.g., Anabarella chelata n.sp., Costipelagiella the small, cap-shaped apex. nevadense n.sp., and Parkula esmeraldina n.sp.). All three Anabarella chelata n.sp. species are new, but other species of the same genera are widespread in the Early or Middle Cambrian. Faunal com- Figs. 2A–2E parisons of the Split Mountain assemblage are further ham- HOLOTYPE AND TYPE LOCALITY: USNM 530265 from sample pered by a general lack of data on small shelly fossils from CS990917-03 of the basal Emigrant Formation, Split Moun- the latest intervals (Toyonian and equivalents) of the Early tain, Esmeralda County, Nevada (Fig. 2A). Cambrian (as conventionally defined). Although many small shelly fossil assemblages from around the world have been DIAGNOSIS: Species of Anabarella with low shell and strongly described, most are considerably older (Tommotian to Botoman overhanging apex reaching well beyond the subapical margin. or equivalent in age). This is true for all assemblages described from Laurentia, including those described from the Yukon ETYMOLOGY: Latin chela, meaning claw. Derived from the (Nowlan et al. 1985) and the Mackenzie Mountains (Voronova claw-like appearance of the fossils in lateral view. et al. 1987) of Canada. Within Laurentia, the only possible exception is a fauna associated with trilobites of the latest ADDITIONAL MATERIAL: USNM 530266 – USNM 530268 and Early Cambrian Bicella bicensis trilobite assemblage from 48 unfigured specimens from the basal Emigrant Formation, the Taconic allochthon at Ville Guay in Quebec (Landing et Split Mountain, Esmeralda County, Nevada. al. 2002). The Ville Guay fauna shows no clear ties to the DESCRIPTION: Narrow internal molds of a small and strongly Split Mountain assemblage and instead shows similarities to compressed univalve. The fossils are bilaterally symmetrical older small shelly faunas from other parts of the Taconic and are 0.5–0.8 mm long, 0.3–0.4 mm high, and about allochthon (Landing and Bartowski 1996), Greenland (Malinky 0.2 mm wide. The molds are coiled through one quarter to and Skovsted 2004; Skovsted 2004, in press), and western one half of a whorl and rapidly expanding in the plane of Newfoundland (personal observation, 2004). coiling. The apex is strongly overhanging and reaches well Thus, the Split Mountain assemblage cannot be readily beyond the subapical margin of the aperture. In lateral view compared to any other fauna of small shelly fossils described the aperture is gently and evenly convex, except at the in the literature. It is a very small fauna, derived from only subapical end where it is more strongly flexed towards the three samples, and it is the first of its kind to be described apex, forming a deep sinus (Figs. 2A–2C). The mold of the from the Great Basin. This study demonstrates that secondary subapical sinus is slightly expanding laterally. A less pro- phosphatization of calcareous shells did occur in the Great nounced sinus is present at the supra-apical end of the aper- Basin and that investigation of small shelly fossil assemblages ture (Figs. 2B, 2C). The mold of the protoconch is small and may contribute to our knowledge of the paleontology and globular, but not clearly delineated from the rest of the conch. biostratigraphy of this region, as well as other Cambrian Faint furrows radiating from the very apex are present in one regions of the world. specimen (Figs. 2B, 2E). The surfaces of the molds are other- wise ornamented by faint comarginal lineations. Systematic paleontology DISCUSSION: Anabarella chelata n.sp. is represented in acid Phylum Mollusca Cuvier, 1797 residues by internal molds only. The relatively tight coiling Class Helcionelloida Peel, 1991 and strong lateral compression of the molds are characteristic Order Helcionellida Geyer, 1994 of Anabarella Vostokova, 1962. The species differs from Family Helcionellidae Wenz, 1938 other species of Anabarella by the more rapid expansion of Genus Anabarella Vostokova, 1962 the conch, resulting in a lower, more elongated shell with a Type species Anabarella plana Vostokova, 1962 more strongly overhanging apex. Internal molds of A. australis Runnegar in Bengtson et al. (1990) from the Atdabanian to COMPOSITION: Type species, Anabarella australis Runnegar Botoman of South Australia (Parkhaev in Gravestock et al. in Bengtson, Conway Morris, Cooper, Jell and Runnegar 2001), Germany (Elicki 1996), and Greenland (Gubanov et 1990, Anabarella simesi MacKinnon, 1985, and Anabarella al. 2004) are highly variable and frequently exhibit an over- chelata n.sp. hanging apex, but not to the same degree as in A. chelata. Further, the faint comarginal ornamentation of A. australis is DIAGNOSIS: Small, bilaterally symmetrical univalve mollusc not reflected on the surface of internal molds. Anabarella with advolute or slightly involute, rapidly expanding and later- australis is a very variable species and a variant with a more ally compressed shell coiled through about one half to one distinctly overhanging apex (originally described separately whorl; the shell is smooth or weakly ornamented by closely as Anabarella argus Runnegar in Bengtson et al. 1990; see spaced transverse growth lines; protoconch is small and glob- discussion by Parkhaev in Gravestock et al. 2001) differs ular. from A. chelata by the higher and more tightly coiled shell.

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 apical region. (C) USNM 530267 from sample CS990917-02, lateral view. (D) USNM 530268 from sample CS990917-01, oblique dorsal view. (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 apertural view. Scale bar represents 100 µm for (A–D), 75 µm for (E), and 150 µm for (F–J).

Anabarella simesi MacKinnon, 1985 from the Middle Cam- A. chelata and the apex rarely projects beyond the subapical brian of New Zealand and New South Wales (Brock 1998) margin in lateral view. does exhibit faint comarginal lineations on internal molds, The maximum size of A. chelata is small compared with but the shell of this species is much higher than that of that of other species of the genus. As discussed previously,

© 2006 NRC Canada Skovsted 491 this applies to all secondarily phosphatized taxa from the Costipelagiella, but this cannot be assessed until specimens basal Emigrant Formation and may be a diagenetic feature. with a well-preserved aperture are found. Costipelagiella nevadense n.sp. OCCURRENCE: Uppermost Lower Cambrian of Nevada. Figs. 2F–2J Genus Costipelagiella Horný, 1964 Type species Costipelagiella zazvorkai Horný, 1964 HOLOTYPE AND TYPE LOCALITY: USNM 530271 from sample CS990917-01 from the basal Emigrant Formation, Split COMPOSITION: Type species, Costipelagiella kochi Peel, 1988, Mountain, Esmeralda County, Nevada. Costipelagiella sp. A (sensu Bischoff 1990), and Costipelagiella nevadense n.sp. DIAGNOSIS: Species of Costipelagiella with spiral side flat and an oval cross section of the last whorl. External ornament of DIAGNOSIS: Univalve, dextrally coiled shell composed of one gently curved fold-like ribs on both sides of the smooth dor- or two rapidly expanding whorls. Spiral surface flat, slightly sum. convex, or concave. The last whorl is wide with oval or subtriangular cross section. External ornamentation of coarse, ETYMOLOGY: From the location of the type locality in Nevada. comarginal ribs or folds. Additional material: USNM 530269, USNM 530270, and six unfigured specimens from the basal Emigrant Formation, DISCUSSION: Pelagiellids are the most widely distributed of all Split Mountain, Esmeralda County, Nevada. helcionelloid molluscs in the Cambrian Period. Most species are referred to Pelagiella Matthew, 1895, but a few forms DESCRIPTION: Small, dextral shells, coiled through about one from the Middle Cambrian have been referred to Costipelagiella whorl. The spiral side is nearly flat, and the umbilical side is Horný, 1964, based on a distinctive external ornament of more strongly convex. The cross section of the whorl is oval. comarginal ribs. The type species of Costipelagiella, The suture separating the last whorl from the apex is a deep, Costipelagiella zazvorkai Horný, 1964 from the Middle narrow furrow. The protoconch is globose, smooth, and Cambrian of the Czech Republic (Horný 1964), is only delineated by a distinct furrow. The aperture is not pre- known from a single specimen, and the same applies to served in any of the available specimens. Both the spiral and Costipelagiella kochi Peel, 1988 from the Middle Cambrian umbilical sides are ornamented by closely spaced (average of North Greenland (Peel 1988). Costipelagiella cf. zazvorkai density 43 ribs/mm), fold-like ribs and intervening narrow was described from the Middle Cambrian of New Zealand grooves that seemingly run parallel to the aperture. Close to by MacKinnon (1985), and closely comparable specimens the smooth dorsum the ribs curve slightly towards the apex from the Middle Cambrian of New South Wales were reported and become progressively subdued. Co-occurring internal molds as Costipelagiella sp. A by Bischoff and Pendergast (1987). have unornamented surfaces. Bischoff (1990) suggested that specimens from New Zealand All specimens exhibit traces of the activity of endolithic and Australia are conspecific. In Costipelagiella sp. A, the alga, either as hollow, vermiform borings through the shell external ornament is sometimes evident on internal molds, (Fig. 2I, top of specimen) or as narrow undulating molds of but this is not the case in the new species from the Great bacterial strands on internal molds (Fig. 2F). Basin. Without the preserved external shell, internal molds of C. nevadense n.sp. are indistinguishable from similarly DISCUSSION: All pelagiellid specimens recovered from the basal preserved Pelagiella. Emigrant Formation are assigned to Costipelagiella Horný, The best known species of the related genus Pelagiella, 1964 because of their distinct external ornament of fold-like Pelagiella subangulata (Tate, 1892) from the Lower Cam- ribs on secondarily phosphatized specimens. Costipelagiella brian of Australia and Greenland, exhibits a high degree of nevadense n.sp. differs from all previously known species of variability in shell shape and ornamentation (Parkhaev in Costipelagiella by the complete absence of the folds on the Gravestock et al. 2001; Skovsted 2004). The three previously dorsum. As all specimens of C. nevadense with external recognized species of Costipelagiella are distinguished by shell preserved are relatively small, the paucity in ornamen- differences in shell shape and ornamentation that are so subtle tation could be interpreted as a juvenile character. This is not that they would easily fall within the range of variability and considered likely, however, as the width of the smooth zone ontogenetical change of P. subangulata. As two out of three on the dorsum appears to increase towards the aperture. species are known from one specimen only, however, it is The latest Early Cambrian age of the basal Emigrant For- difficult to evaluate their interrelationships and potential syn- mation means that C. nevadense is the oldest known species onymy. of Costipelagiella. Other reports of the genus are exclusively Flexure of the ribs on the dorsum of Costipelagiella was from Middle Cambrian strata. In the Middle Cambrian, interpreted by MacKinnon (1985), Peel (1988), and Bischoff Costipelagiella occurs in different paleogeographic provinces (1990) to reflect a sinus on the aperture of the shell. Pelagiella (East and West Gondwana, Laurentia), indicating that the subangulata has an external ornament of fine ridges that genus was very widespread by this time. Its potential for converge to form an inverted V-shaped pattern on the dorsum biostratigraphy is hampered, however, by insufficient knowledge (e.g., Skovsted 2004, figs. 8b,8c) The aperture of P. subangulata on the morphology and variability of the described species. is planar and the V-shaped ridges on the dorsum are not Bischoff (1990) described fossilized remains of possible associated with any notch or sinus on the aperture. Thus, the digestive glands in two specimens of Costipelagiella sp. A external ornament in Pelagiella does not reflect growth incre- from New South Wales. No comparable structures were ments. The same may apply to the external ornamentation of observed in C. nevadense.

OCCURRENCE: Uppermost Lower Cambrian of Nevada. Co-ocurring opercula (Figs. 3D–3I) have a narrow sigmoidal cardinal shield that reflects the dorsal median sinus of the Phylum uncertain conch and poorly defined tectula. The conical shield is strongly Class Hyolitha Marek, 1963 convex with well-defined comarginal ribs. The cardinal pro- Order Hyolithida Syssoiev, 1957 cesses are short, but in lateral view project beyond the cardinal Family unassigned shield. In effect they are situated in front of the cardinal Genus Parkula Bengtson in Bengtson, Conway Morris, shield rather than under it. One pair of clavicles that ends in Cooper, Jell and Runnegar, 1990 blade-like projections is situated at the junction of the cardinal Type species Parkula bounties Bengtson in Bengtson, and conical shields. Conway Morris, Cooper, Jell and Runnegar, 1990 One operculum has a series of pits on the internal surface of the conical shield between the clavicles (Figs. 3G–3I). COMPOSITION: Type species and Parkula esmeraldina n.sp. One centrally placed pit just below the junction of cardinal EMENDED DIAGNOSIS: Straight conchs with lenticular cross processes and clavicles reflects the apex of the conical shield. section, faint dorsal median ridge, and semielliptical ligula. Situated slightly behind the apical pit and immediately below Surface sculpture of transverse striations and sometimes the mid-point of the clavicles is a pair of shallow furrows or irregular longitudinal wrinkles. Opercula with strongly con- elongated pits (indicated by arrows in Figs. 3G and 3H). The vex conical shield, short cardinal processes, and one pair of short furrows run approximately parallel to each other and clavicles ending in blade-like projections. the anterior–posterior axis of the operculum.

DISCUSSION: Parkula differ from Microcornus Mambetov, 1972, DISCUSSION: Parkula esmeraldina n.sp. differs from P. bounties in the very weak dorsal median ridge and the elliptical cross in the surface sculpture of rounded comarginal ribs on the section of the conch. The operculum of Microcornus is not conch without longitudinal ribs or wrinkles and in the position well known, although the holotype of M. parvulus Mambetov, of the cardinal processes in front of rather than under the 1972 is preserved with the operculum still in the aperture of cardinal shield of the operculum (as seen in lateral view; the conch. The operculum has a narrow cardinal shield and compare Fig. 3E with Figs. 150d, 150h, and 151c of Bengtson well-developed comarginal ornamentation, but its internal et al. 1990). Similar cardinal processes are present in two morphology is not known. The type species of Parkula, opercula described as Parkula? by Malinky and Skovsted P. bounties Bengtson in Bengtson et al. 1990, was originally (2004) from the Bastion Formation of northeast Greenland, described from the Lower Cambrian (Atdabanian to Botoman where they occur together with P. bounties. Parkula? sp. has equivalents) of South Australia but has also been reported a smooth conical shield with subdued comarginal ornamenta- from Antarctica (Wrona 2003) and northeast Greenland tion, however. (Malinky and Skovsted 2004). Macroscopic hyoliths have previously been described from the Lower Cambrian of the Great Basin (e.g., Babcock and Parkula esmeraldina n.sp. Robison 1988). No previously described hyolithid matches Figs. 3A–3I the conchs and opercula of P. esmeraldina, however. The HOLOTYPE AND TYPE LOCALITY: USNM 530272 from sample general problem of comparing macroscopic and microscopic CS990917-03 of the basal Emigrant Formation, Split Mountain, hyoliths was discussed by Malinky and Skovsted (2004). Esmeralda County, Nevada. The shallow pits on the internal surface of the conical shield in Parkula esmeraldina probably represent sites of DIAGNOSIS: Species of Parkula with well-developed ornamenta- muscle insertion. Muscle scars are known to occur in various tion of transverse striations on conch and operculum. Aperture positions in different post-Cambrian hyolithids, and muscles of conch has a shallow median dorsal sinus. Operculum with probably associated with the movement of the helens are short but well-developed cardinal processes projecting well commonly inserted on the conical shield (Marti-Mus and beyond the cardinal shield. Bergström 2005).

ETYMOLOGY: From the location of the type locality in Esmeralda OCCURRENCE: Uppermost Lower Cambrian of Nevada. County, Nevada. Miscellaneous fossils Additional material: USNM 530273 – USNM 530275 and Echinoderm sclerites 18 unfigured specimens from the basal Emigrant Formation, Figs. 4A, 4B Split Mountain, Esmeralda County, Nevada. MATERIAL: USNM 530276, USNM 530277, and hundreds of DESCRIPTION: Straight and slender conchs (angle of divergence additional specimens from the basal Emigrant Formation, 20°) up to approximately 1 mm long, but some specimens Split Mountain, Esmeralda County, Nevada. lack the apical portion. The transverse cross section is lenticular, with the dorsal side more strongly convex than the ventral DISCUSSION: Fossils with a preserved stereome ultrastructure side. The dorsal and ventral surfaces meet along the smoothly typical of echinoderm sclerites are numerically dominant among rounded lateral edges. The dorsal side has a very weak median the secondarily phosphatized fossils of the basal Emigrant ridge. The ligula is semielliptical and approximately two Formation. Star- and shield-shaped, globular and irregular times wider than long. The aperture has a well-defined dorsal sclerites are present (Figs. 4A, 4B), but no specimen can be median sinus, and the conch is ornamented by well-developed assigned to any particular taxon. Similar disarticulated echino- rounded comarginal ribs. derm ossicles are common among small shelly fossil assem-

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 cardinal shield. (E) From right-lateral side. (F) From above. (G–I) USNM 530275 from sample CS990917-01. (G) Interior of operculum from below. (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.

blages from the Atdabanian and later Cambrian strata wherever metazoan eggs and embryos (see review in Steiner et al. secondary phosphatization of originally calcareous skeletons 2004) but differ from these fossils in that they have a perfo- occurs. rated surface. The external perforations recall the problematic fossil Aetholicopalla Conway Morris in Bengtson et al. Perforated spheres (1990) from South Australia and France, but the specimens Fig. 4C from the Emigrant Formation are much larger and have fewer perforations. The specimens are considered too poorly pre- Material: USNM 530278 and five additional specimens from served to allow detailed taxonomic assessment. the basal Emigrant Formation, Split Mountain, Esmeralda County, Nevada. Chancelloria spp. Figs. 4D, 4E Description: More or less spherical objects with a diameter of approximately 0.3 mm (Fig. 4C). All available specimens MATERIAL: USNM 530279, USNM 530280, and hundreds of are hollow and their exterior has a rough, crystalline texture. additional specimens from the basal Emigrant Formation, The external surface also has a set of evenly distributed cir- Split Mountain, Esmeralda County, Nevada. cular perforations of almost uniform dimensions (19–24 µm in diameter). DISCUSSION: Chancelloriid sclerites are usually represented in the Emigrant Formation material by isolated and poorly pre- Discussion: These spherical objects probably represent some served individual rays. Among the few more complete sclerites form of biological structure. They show some similarities to present, two morphologies are represented in approximately Olivooides Qian, 1977 and other spherical fossils representing equal numbers (1 + 6 and 0 + 3). Larger sclerites have one

Fig. 4. Miscellaneous fossils from the basal Emigrant Formation of Split Mountain, Nevada. (A, B) Echinoderm sclerites. (A) USNM 530276 from 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 other pictures.

vertical ray and six equal rays in the plane of the basal facet morphology of the fossils is too generalized for meaningful (Fig. 4D). The basal facet is a well-defined platform with taxonomic comparison. circular or teardrop-shaped basal foramina. Sclerites without Hexactinellid spicule central rays have three equal rays that diverge clearly from the plane of the basal facet (Fig. 4E). The two sclerite types Fig. 4G possibly belong to different genera and (or) species, but MATERIAL: USNM 530282 from the basal Emigrant Forma- neither form is considered well enough preserved to allow tion, Split Mountain, Esmeralda County, Nevada. taxonomic assignment. Girvanella sp. DISCUSSION: Poriferans are represented in the acid residues by a single hexact spicule. All six rays are of approximately Fig. 4F equal dimensions and diverge at right angles to each other. The specimen is probably a mold of the axial canal of a MATERIAL: USNM 530281 and 13 additional specimens from calcihexactinellid spicule. Similar fossils have previously been the basal Emigrant Formation, Split Mountain, Esmeralda described from different Cambrian small shelly fossil assem- County, Nevada. blages (e.g., Hinz 1987; Skovsted in press).

DISCUSSION: Irregular aggregates of thin phosphatized cyanobacterial strands occur in the samples from the Emigrant Acknowledgments Formation. The aggregates are up to 0.4 mm long, but larger specimens may have been broken up during acid prepara- J.S. Peel is gratefully acknowledged for generously providing tion. Individual strands within the aggregates are unbranched financial and practical support in connection with the collec- and are preserved as double tubes with outer and inner diame- tion, preparation, and SEM work on the Split Mountain ters of approximately 20 and 10 µm, respectively. Aggregates material. A.R. Palmer, who organized the Laurentia 99 field of cyanobacterial strands from the Lower Cambrian have conference, is thanked for hospitality and for providing much been referred to in the literature as Girvanella Nicholson information on the geology and paleontology of the Great and Etheridge, 1878 (e.g., Daneilli 1981). Although the Split Basin. G. Brock and my wife Anne Mette are thanked for Mountain specimens are very similar to Girvanella sp. from valuable comments on an early version of the manuscript. South Australia (Runnegar in Bengtson et al. 1990), the Comments by two external reviewers, E. Landing and

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