Cambrian rhynchonelliform nisusioid brachiopods: phylogeny and distribution

A comprehensive review and phylogenetic analysis of genera and species presently assigned to the rhynchonelliform superfamily Nisusioidea and family Nisusiidae suggests that this short‐lived but important group of brachiopods first appeared in peri‐Gondwana during the second half of the Cambrian Series 2, before going extinct by the end of Drumian times. Nisusiides achieved their maximum morphological disparity and geographical distribution during the Wuliuan Age, and Laurentia was probably the major centre of their dispersal. A new phylogenetic analysis suggests an early separation of the lineages of spinose and non‐spinose nisusiids. The non‐spinose nisusiids probably evolved in Laurentia by the end of Cambrian Series 4. The new nisusiid genus Bellistrophia is described. The new species Nisusia multicostata represents the first documented rhynchonelliform (kutorginide) brachiopod from the Miaolingian (Drumian) of the Alborz Mountains, Iran.

N I S U S I A is one of the most common early to mid-Cambrian rhynchonelliform brachiopods, with a stratigraphic range from the unnamed Cambrian Stage 3 to the Miaolingian (Drumian). Nisusia has been recorded from most major Cambrian palaeo-continents except Baltica, and there are only unconfirmed reports from North China. Although it is generally common, Nisusia and the entire Nisusiidae is in need of taxonomic revision due to the fact that most species were described more than half a century ago, and the most significant latest review of the genus is that of Walcott (1912). Mao et al. (2017) listed 39 species assigned by them to the genus, but in the present review only 19 species can be assigned to Nisusia with some degree of confidence, and 5 more are tentatively considered to be within the genus. Many species of Nisusia should presently be considered to be nomena nuda until they can be restudied.
The Nisusiidae is well suited for phylogenetic analysis. Although they have a relatively simple shell morphology, they exhibit a good set of distinctive morphological features which can be extracted even from relatively simple descriptions and basic illustrations. The main problem with the phylogenetic analysis is that it proved difficult to include quantitative characters, which could have improved the resolution (Wright & Stigall 2014). Nevertheless, the new phylogeny of the Nisusiidae is mainly compatible with the stepwise stratigraphic sequence of appearance of individual taxa and general patterns of biogeography of the family.
Recent palaeontological studies in eastern Alborz Mountains, northern Iran, have shown that rhynchonelliform brachiopods constituted an important component of Furongian benthic communities in eastern Gondwana (Popov et al. 2011, 2013. They often represent the most abundant type of skeletal debris in the extensive Furongian echinoderm-brachiopod carbonate shell beds. However, rhynchonelliform brachiopod occurrences in older rocks (Cambrian Series 2 and Miaolingian) are almost absent in Iran, with the exception of scattered occurrences of Diraphora and some possible Nisusia in the Kuhbonan Mountains, northern Kerman Province (Wolfart 1974). This is the first report of rhynchonelliform (kutorginide) brachiopods in the Drumian (Miaolingian Series) of the Alborz Mountains.

PHYLOGENETIC ANALYSIS
Our phylogenetic analysis is based on 28 morphological characters (Table 1) identified in 22 species of the Nisusiidae, assigned to four genera (Nisusia, Bellistrophia, Eoconcha and Narynella). Two species of the closely related Kutorginidae (see also Popov et al. 1996), including Agyrekia aff. obtusa Koneva, 1979 andKutorgina cingulata (Billings, 1861) were selected as the outgroup. The Kutorginidae are among the oldest known rhynchonelliform brachiopods and the earliest occurrences of Kutorgina (Ushatinskaya & Malakhovskaya 2006) predate the earliest occurrences of the Nisusiidae. Kutorgina is also the oldest known member of the rhynchonelliforms from the Maotianshan Chengjiang Lagerst€ atte of southern China, where soft-part preservation includes well preserved lophophores, digestive tracts and pedicles (Zhang et al. 2007(Zhang et al. , 2008Holmer et al. 2018a).

Method
The inferred phylogeny of the family Nisusiidae presented in the paper (Fig. 1) is based on the outcomes of phylogenetic analysis of 24 taxa, including 22 species assigned to the family, characterized by 28 unordered and unweighted characters (Tables 2, 3 A significant number of nisusiid species, including some used in the analysis, still require revision; nevertheless, the analysed matrix gives a satisfactory representation of the family and makes it possible to trace its evolutionary history from the first occurrence in the unnamed Cambrian Stage 2 to its extinction at the end of the Drumian. It gives also useful information for understanding the biogeographical dispersal of nisusiids during the early to mid-Cambrian.
Two main clades can be recognized within the Nisusiidae. The first, Node 3 ( Fig. 1), comprises the majority of Nisusia species (excluding Nisusia sulcata), which are characterized by having a ventral valve profile that is concave umbonally, convex anteriorly (18.1), and with maximum height at the umbo (19.0). The second clade, Node 18 ( Fig. 1), is distinguished only by having a subequally biconvex shell (4.0) and includes almost all nisusiids lacking spines along with the finely spinose Nisusia sulcata. At Node 19 ( Fig. 1) the non-spinose nisusiids are united only by the presence of a dorsal sulcus originating at the umbo (22.1), while the absence of hollow spines is not recognized as a synapomorphy in the analysis. This may be an effect of the apparent absence of spines in Nisusia? guizhouensis from the Kaili Formation (Mao et al. 2017) and while a secondary loss of spines is possible, the absence of spines in the Chinese species may also simply represent poor preservation, since it is preserved as internal/external moulds in an argillaceous matrix. In other characters, Nisusia? guizhouensis shows typical features of the spinose nisusiids and constitutes a sister taxon to Nisusia granosa (Node 8; Fig. 1), another species from the Kaili Formation. In the latter species, the bases of hollow spines were observed by Mao et al. (2017). Rowell & Caruso (1985) described Nisusia sulcata as lacking spines, but the presence of minute spines on silicified shells were described by Holmer et al. (2018b). Nevertheless, our analysis placed Nisusia sulcata within the group of nonspinose nisusiids, including Bellistrophia.
Eoconcha austini from the upper part of the Shady Formation, Austinville (VA), which contains a trilobite fauna characteristic of the Bonnia-Olenellus Zone (Resser 1938;Cooper 1951), is the earliest documented representative of the group including non-spinose nisusiids. The monotaxic Narynella from the Miaolingian of the Alai peri-Gondwana terrane (Popov et al. 2015) is a sister taxon to Bellistrophia sp. from the Miaolingian (Wuliuan) of the Chingiz-Tarbagatay island arc system which, according to Popov & Cocks (2017), was located in low southern latitudes in proximity of western margin of Australasian Gondwana during the Cambrian (Fig. 2). These are the only species of non-spinose nisusiids yet recorded outside Laurentia, and our phylogenetic analysis indicates that the nonspinose nisusiids first appeared and diversified in Laurentia early in the unnamed Cambrian Epoch 4. Their migration towards equatorial peri-Gondwana probably occurred at the beginning of the Wuliuan, which also coincides with a time of major turnover in the biogeographical patterns of linguliform brachiopods (Popov et al. 2015;Fig. 2).
The early divergence of the spinose nisusiid clade (Node 3; Fig. 1) is obscured by the inadequate early Cambrian fossil record of this group. There are some possible early records of nisusiids in the Fallotaspis Zone of Laurentia (Rowell 1977) as well as in the Pagetiellus anabaricus Zone of Siberia (Ushatinskaya 1986), both from the lower part of the unnamed Cambrian Stage 3. However, the taxonomic position of the possible nisusiid specimens in these two reports cannot be evaluated, because the cardinalia and umbonal part of the ventral valves, key for their taxonomic affiliation, could not be observed. Nevertheless, three of five nisusiid species documented from the unnamed Cambrian Series 2, including Eoconcha austini, Nisusia festinata and Nisusia ancauchensis, are from Laurentia, suggesting that this continent was probably the major centre of the early Cambrian nisusiid dispersal.
To date, the earliest documented species of the genus is Nisusia alaica from the Cambrian (probably the upper part of the unnamed Cambrian Stage 3) of the Alai terrane, where it occurs in association with unidentified archaeocyaths and a highly endemic brachiopod fauna including Chile, Naukat, Oina and Kutorgina (Popov & Tikhonov 1990). In the phylogenetic analysis it is placed close to the base of the major Nisusia clade together with F I G . 1 . Inferred phylogenetic relationships for species of Nisusiidae obtained from a single most parsimonious tree generated from TNT v. 1.5-beta (Goloboff et al. 2008). Numbered nodes (in circles) supported by character states listed in the text, and Tables 2 and 3. Geographical occurrences of analysed species are shown as: 1, Laurentia; 2, Siberia and associated terranes; 3, South China; 4, peri-Gondwana terranes and microcontinents; 5, Gondwana.
Nisusia multicostata from the Miaolingian (Drumian) of the peri-Gondwana Alborz platform appears to be an early offshoot that separated from the major Nisusia clade (Node 6; Figs 1, 2), before its major radiation in the unnamed Cambrian Epoch 4 had occurred. This group is characterized by a subequally biconvex shell (4.0), a catacline dorsal interarea (16.1) and an evenly biconvex dorsal valve sagittal profile with maximum height at mid-length (20.2).
A significant proportion of the Miaolingian Nisusia species from Siberia and Gondwana appears in the analysis as a monophyletic clade with Laurentian roots (Node 11; Figs 1, 2). An early Cambrian (unnamed Cambrian Epoch 4) Laurentian species Nisusia ancauchensis forms a sister taxon of that clade. It occurs in allochthonous setting within the Cuyania terrane at Argentinian Precordillera (Fig. 2). The Laurentian origin of that terrane is well supported by data given by Benedetto et al. (2009), including an associated trilobite assemblage of the Bonnia-Olenellus Zone with strong Laurentian signature. Another species at the base of the clade is Nisusia borealis, a Miaolingian species from the vicinity of Eagle, Alaska, where it occurs together with Arctohedra (Cooper 1936). As demonstrated by Bassett et al. (2002), the Cambrian protorthides and clitambonitoids (Arctohedridae) were restricted in their geographical distribution to Gondwana and associated terranes. Therefore, this Nisusia species is probably derived from an exotic peri- Gondwana terrane that was later accreted to Laurentia. Dispersal of that clade and its radiation outside Laurentia probably occurred sometime in the unnamed Cambrian Epoch 4; the Siberian Nisusia kotujensis occurs in deposits dated as pre-Wuliuan. In the analysis it appears as a derived member of the clade together with two other Siberian species including Nisusia minussensis and Nisusia paspelovi (Node 15; Fig. 1). The Australian species Nisusia grandis and its sister taxon Nisusia paspelovi constitute the most derived members of the clade. The Wuliuan nisusiids show a significant increase in morphological disparity with three genera documented. Among them, 11 species can be confidently assigned to Nisusia at present. Most of the species were confined to low-latitude palaeogeographical settings, including Laurentia ( During the Drumian nisusiids went through a considerable decline, with only six species known, while among non-spinose nisusiids only Narynella crossed the Wuliuan-Drumian boundary. However, they were present in Laurentia, Siberia and Australasian Gondwana (Fig. 2). The geographical distribution of nisusiids is confined almost exclusively to the low latitudes. They are unknown in Baltica and outside the Australasian segment of Gondwana. All nisusiids became extinct by the end of the Drumian time. Abbreviations. Lv, Ld, sagittal ventral and dorsal valve length; T, maximum thickness; W, maximum width.

Geological setting
The Iranian specimens of the Nisusiidae come from a single locality (Fig. 3) at the Mila-Kuh hill (MK-2015/1, at 53°48 0 45″E and 35°59 0 5″N coordinates), 6.5 km north of the highway connecting Semnan with Damgan, and about 6.5 km south-west of the village of Tuyeh. The Cambrian succession in Mila-Kuh was first described by St€ ocklin et al. (1964), who introduced the Mila Formation for the middle Cambrian to Lower Ordovician strata exposed in the area, which was subdivided it into five informal members. The Cambrian fossil faunas and trilobite-based biostratigraphy of the Mila-Kuh section were described by Kushan (1973Kushan ( , 1978 and Wittke (1984). The traditional views of the Mila lithostratigraphy in the Alborz Mountains were criticized by Geyer et al. The section studied (Fig. 3) is situated somewhat to the east of the section described by St€ ocklin et al. (1964). Here the lower boundary of the Mila-2 Member is faulted and only the upper part of the unit is preserved (Fig. 1). The brachiopod sample MK-2015/1 was collected 34 m below the base of the Mila-3 Member, which coincides with the upper part of Unit 4 (Kushan 1978, p. 14). It is located on the flank of a decimetre-thick echinoderm-rich bioaccumulation. The background deposits through this interval are represented by intercalating argillaceous limestones and calcareous argillites, which contain a monotaxic trilobite association with Iranolesia pisiformis (King, 1937). The presence of this trilobite allows an attribution to the Iranolesia Zone of Kushan (1973)  The studied specimens of Narynella ferganensis (Andreeva, 1962) are most probably paratypes (donated by Andreeva to the late Alwyn Williams, Glasgow). They were sampled from a Cambrian olistolith (probably Sdzuyella-Aegunaspis beds, Miaolingian Series) at Madygen, northern slopes of eastern Turkestan Range, Kyrgyzstan. No detailed locality data were presented in the original publication and the age of the sample was erroneously presented as 'Lower Cambrian'. It is likely that an allochthonous limestone block at Sauk Tanga gorge, Madygen, which represents the major source of the Cambrian (Miaolingian) fossils described by Geyer et al. (2014b), is the type locality of Narynella ferganensis. Remarks. Until recently Nisusia was regarded as a poorly defined 'waste basket' taxon, due to the fact that the morphology of many species described in the first half of the previous century are inadequately known. Here we restrict Nisusia to comprise only species with hollow spines. It is likely that most of the taxa (mainly from Laurentia) from the lower Cambrian (unnamed stages 3-4) that have been assigned to Nisusia, neither belong to Nisusia nor to Nisusioidea (see below Nisusia ferganensis Andreeva, 1962. This is the type species of Narynella discussed below. Nisusia fulleri Mount, 1981, from the Latham Shale Formation (unnamed Cambrian Series 3) of California. Generic affiliation of the species cannot be proved due to insufficient description and illustrations (no data on interareas, pseudodeltidium, foramen and interiors of both valves). The taxon should be considered to be a nomen dubium.
Nisusia hayasakai Sun, 1924, from the Changhsia Formation (Miaolingian) of Luanxian County, northeastern Hebei Province, North China. Taxon was not revised since its formal designations, although the type material (J.-Y. Rong, pers. comm. 2018) was probably lost. Until topotypes are available and revised, this taxon should be considered to be a nomen dubium.
Nisusia? howelli Shaw, 1957, from the lower Parker Slate (unnamed Cambrian Stage 4) of Vermont, USA. The taxon is known from a single imperfectly preserved ventral valve, while a very short description in the original publication does not provide necessary information for the generic discrimination of the species and it should be considered to be a nomen dubium.
Nisusia spinigera Walcott, 1924, from the Ottertail Formation (Furongian) of British Columbia. The species has an open delthyrium and the original report on a presence of spines looks unsupported. The taxon definitely does not represent Nisusia or Nisusioidea, while its generic affiliation cannot be defined without data on cardinalia, thus it is here considered to be a nomen dubium. Nisusia? sp., from the Campito Formation, Montenegro Member (unnamed Cambrian Stage 3, Fallotaspis Zone). According to Rowell (1977), specimens may belong to a new, as yet formally undescribed genus. No data on the interior and characters of the delthyrial opening were presented in the original description, although specimens apparently lack hollow spines. They are unlikely to belong to Nisusia or Nisusioidea.
Nisusia? sp., from the Pagetiellus anabaricus Zone (unnamed Cambrian Stage 3) west side of the Lena River near Sailyk village, Central Siberia, Russia (Ushatinskaya 1986). This unnamed taxon is known from a single dorsal valve showing straight hinge line and distinct radial ornament with possible bases of 12 spines on the ribs. While assignation of this shell to Nisusioidea is possible, in the absence of data on the interior and characters of posterior margin in both valves its generic attribution remains uncertain.
Nisusia (Jamesella) amii Walcott, 1905, from a boulder in the Saint-Nicolas Formation (Cambrian Series 2), eastern Quebec, Canada. The species is known from a single ventral valve which is high, subconical, with a high, planar interarea bearing a convex pseudodeltidium. The umbonal area was described by Walcott (1912) as broken, but it may represent a partly preserved foramen similar to those of chileides (e.g. Acareorthis).
Nisusia (Jamesella) argenta Walcott, 1908, from the Silver Peak Group (unnamed Cambrian Series 2) of Nevada. The species is characterized by an open delthyrium, paucicostate radial ornament and a lack of spines. Its generic affiliation is uncertain, although it does not represent Nisusioidea. The species should be considered to be a nomen dubium.
Nisusia concentrica Endo & Resser, 1937, from the Taitzu Formation (Miaolingian) of north-east China. The taxon is based on two dorsal valves and a single incomplete shell. Brief descriptions and inadequate illustrations do not provide sufficient information on the interior of either valve, characters of delthyrial opening, or the presence or absence of spines; therefore taxonomic affiliation to the family or genus level is impossible. The species name should be considered to be a nomen dubium.
Nisusia (Jamesella) erecta Walcott, 1908, from middle Cambrian of Nevada, USA. While the holotype definitely possesses a pseudodeltidium, it is characterized by radial ornament of simple coarse ribs unusual for Nisusia, although a presence of hollow spines was not reported and interior of both valves is unknown. The generic assignation of the species is uncertain and it should be considered to be a nomen dubium.
Nisusia (Jamesella?) kanabensis Walcott, 1908, based on a single incomplete ventral internal mould from the Tonto Group (Furongian) exposed at the mouth of Kanab Canyon, Grand Canyon of the Colorado, Arizona, USA. While the generic affiliation of the specimen is impossible to determine, it definitely does not belong to either Nisusia or Nisusioidea and should be considered to be a nomen dubium.
Nisusia (Jamesella) lowei Walcott, 1908, from Led Shale Member (formerly Spence Shale Member) of the Langston Formation (Miaolingian, Wuliuan), of Idaho, USA. Description and illustrations given by Walcott (1912) are not satisfactory to assign this taxon to either Nisusia or to Nisusiidae and it should be considered to be a nomen dubium.
Nisusia mantouensis Endo & Resser, 1937, from the Taitzu Formation (Miaolingian) of north-east China. The taxon is based on a single ventral valve which does not exhibit features helpful for its generic and species affiliation. The taxon should be considered to be a nomen dubium.
Nisusia (Jamesella?) nautes Walcott, 1905, from the Led Shale Member (formerly Spence Shale Member) of the Langston Formation (Miaolingian, Wuliuan), of Idaho, USA. In the absence of data on the foramen, pseudodeltidium and cardinalia, generic affiliation of the species cannot be proved. Also, it apparently lack spines and therefore does not represent a species of Nisusia; it is here considered to be a nomen dubium.
Nisusia orientalis Endo & Resser, 1937, from the Taitzu Formation (Miaolingian) of north-east China. No data on characters of the delthyrial opening or interior of either valve were presented; the specimens apparently lack spines. The species should be considered to be a nomen dubium.
Nisusia salebrosa Endo & Resser, 1937, from Taitzu Formation (Miaolingian) of north-east China. The taxon is based on a few fragmented shells, which are inadequate for determining family or generic affiliation; also interior of both valves remains unknown. The species should be considered to be a nomen dubium.
Nisusia (Jamesella) utahensis Walcott, 1905, from the Wuliuan of Utah. The taxon is known from two imperfectly preserved specimens and could not be identified outside the type locality until new material is available. It should be considered to be a nomen dubium.
Orthisina compta Tate, 1892 is based on a single specimen from the Kulpara Limestone (unnamed Cambrian Stage 3) of Yorke Peninsula. Brock (1998, p. 608), rejected it as a species of Nisusia. In view of the limited material presently available this taxon should be considered to be a nomen dubium.
Protorthis spencei Walcott, 1905, from the Led Shale Member (formerly Spence Shale Member) of the Langston Formation (Miaolingian, Wuliuan), of Idaho, USA. Species requiring revision. It was reassigned to Nisusia by Walcott (1912, p. 737). Bell (1941), who studied the types, noted the absence of spines and a general similarity to Nisusia (= Bellistrophia) desei, but the original description and illustrations are not satisfactory for its precise generic discrimination.
Description. Shell subequally biconvex, transverse, subrectangular to trapezoidal in outline, length about three-quarters of width, with maximum width at hinge line. Cardinal extremities slightly acute to almost rectangular; anterior commissure rectimarginate. Ventral valve lateral profile concave anterior to umbo, becoming almost straight in middle part and moderately convex in anterior third of valve length in mature individuals. Ventral umbo strongly raised, with small suproapical foramen. Ventral interarea high, slightly apsacline with convex pseudodeltidium occupying about one-third of interarea width. Dorsal valve moderately and evenly convex, depth slightly more than quarter of length, with small pointed beak facing posteriorly. Dorsal interarea high, catacline with broad, subtriangular open notothyrium. Radial ornament multicostellate with faint, rounded ribs increasing mainly by intercalation and separated by narrow interspaces; in total, 60-70 ribs with 9-11 ribs per 3 mm along anterior margin of mature individuals. Rib crests bearing faint, hollow spines. Ventral interior lacking distinct features. Dorsal interior not observed.
Remarks. Nisusia multicostata sp. nov. is distinct from most of the species assigned to the genus in having a finely multicostellate ornament with minute, densely placed spines on the rib crests, and in the complete absence of the ventral sulcus and dorsal median fold. In these characters, as well as in having a relatively low, slightly apsacline ventral interarea and maximum shell width along the hinge line it recalls Nisusia kotujensis Andreeva, 1962 from the unnamed Cambrian Series 3 of north-central Siberia, Russia, but can be distinguished in having a wider pseudodeltidium approaching one-third interarea width, a rectimarginate (not emarginate) anterior commissure, a sagittal profile of the ventral valve concave umbonally and evenly convex anterior to the umbo, and in the absence of the dorsal and ventral sulcus. Among other species of the genus documented from the unnamed Cambrian Series 3, Nisusia nasuta Nikitin, 1956 is another similar species, which has similar radial ornament and a rectimarginate anterior commissure; however, Nisusia multicostata can be readily distinguished from the former taxon in having a ventral valve sagittal profile with maximum height at the umbo, a catacline dorsal interarea and a complete absence of the ventral sulcus.
A very finely spinose shell is also characteristic for Nisusia sulcata Rowell & Caruso, 1985, from the Marjum Formation (Drumian) of western Utah, USA; however, Nisusia multicostata differs readily from that taxon in having a rectimarginate (not sulcate) anterior commissure, in the complete absence of ventral sulcus, maximum shell width at the hinge line, rectangular (not obtuse) cardinal extremities and fine radial ornament.
Nisusia multicostata differs from Nisusia metula Brock, 1998, from Murrawong Creek Formation (Drumian) of the southern New England Fold Belt, north-eastern New South Wales, Australia in having a subequally biconvex (not ventribiconvex) shell, maximum shell width at the hinge line and finely multicostellate ornament with minute spines, Nisusia multicostata differs from Nisusia grandis grandis Roberts & Jell, 1990, from the 'first discovery limestone' member of the Coonigan Formation (Wuliuan) of western New South Wales, Australia in having a subequally biconvex (not ventribiconvex) shell, rectimarginate (not uniplicate) anterior commissure, an absence of welldefined concentric lamellae, a narrow pseudodeltidium not exceeding one-quarter of maximum shell width, and a complete absence of the dorsal and ventral sulcus.
Nisusia multicostata differs from Nisusia paspelovi Aksarina, 1960, from the Mundybash Regional Stage (Wuliuan) of Gornaya Shoriya, Russia in having a maximum shell width at the hinge line, rectangular (not obtuse) cardinal extremities, a subequally biconvex (not dorsibiconvex) shell, a rectimarginate (not emarginate) anterior commissure, a narrow pseudodeltidium not exceeding one-quarter of maximum shell width, planar, not umbonally incurved ventral interarea, a concave umbonally, convex anteriorly sagittal profile of the ventral valve, and a complete absence of the dorsal and ventral sulcus.
Nisusia  Description. Shell slightly ventribiconvex, transverse, subrectangular in outline with hinge line slightly shorter than maximum shell width at mid-length. Cardinal extremities obtuse. Anterior commissure sulcate. Ventral valve lateral profile gently concave immediately anterior to umbo, then moderately convex, with maximum height slightly anterior to umbo. Pseudodeltidium broad, convex, slightly exceeding one-third of ventral interarea width. Umbo raised, pointed ventrally and terminated with circular foramen. Poorly defined median fold present anterior to mid-length of mature individuals. Dorsal valve evenly convex with maximum height between umbo and mid-length. Dorsal interarea apsacline with broad, open notothyrium. Radial ornament multicostellate with 6-9 rounded ribs per 3 mm along anterior margin of mature individuals. Ribs increasing in number both by intercalation and bifurcation and separated by interspaces slightly narrower than ribs. Ventral interior without distinctive characters. Dorsal interior with transverse socket plates bounding anteriorly narrow and shallow sockets, low notothyrial platform bisected medially weakly defined ridge and variably developed double septum bisecting weakly impressed adductor muscle field.
Remarks. The revised description of Narynella ferganensis presented here is based on the study of four specimens (most probably paratypes) donated to the late Alwyn Williams. The material is now housed in the National Museum of Wales, Cardiff, and illustrated types deposited in the CNIGR Museum (St Petersburg). In the original description of the species (Andreeva 1962) there is no precise data on the fossil locality at Madygen, while the 'lower' Cambrian age of the source rock was misleading as pointed out by Geyer et al. (2014b). The description of Andreeva (1962) unfortunately provided illustrations that are of poor quality, and no data on the interior of either valve was given. Andreeva's specimens of Narynella cf. ferganensis came from an allochthonous limestone block at the Sauk Tanga gorge. It is most probably the type locality and the illustrated specimens are topotypes. As shown by detailed geological mapping performed in the Turkestan and Alai ranges by the South Kyrgyz Geological Survey shortly before the collapse of the USSR, the Cambrian rocks in the area occur exclusively as olistoliths and olistostromes in the Silurian Pulgon Formation (Koren et al. 1993;Holmer et al. 2000), which were shown, probably erroneously, on the schematic map presented by Geyer et al. (2014b) as the 'Ordovician siliceous shale'. The Cambrian outcrops of the Sauk Tanga gorge were also briefly described by Repina et al. (1975), and this publication contains a description of Narynella ferganensis, which was erroneously identified by Aksarina as Nisusia nasuta var. ramosa Nikitin, 1956 (see also Geyer et al. 2014b). While these specimens came from a different fossil locality at Sulukty Gorge, they represent a useful addition to Andreeva's (1962) original description of the species, because the dorsal internal mould illustrated by Aksarina (1975, pl. 5, fig. 15) shows a distinct dorsal double septa. The specimens from Sulukty occur in the Sdzuyella-Aegunaspis Beds as defined by associated trilobites, so they are approximately contemporaneous with those from Madygen (Geyer et al. Diagnosis. Shell subequally biconvex to slightly dorsibiconvex, with hinge line shorter than maximum shell width at mid-length, variably emarginate anterior commissure and sulcus present in both valves. Ventral valve lateral profile moderately convex with maximum height between umbo and mid-length. Ventral interarea apsacline with broad, convex pseudodeltidium. Dorsal valve evenly convex with anacline interarea. Radial ornament multicostellate with rounded ribs increasing in number mainly by intercalation. Impressions of muscle scars undiscernible in both valves. Species assigned. In addition to the type species, the genus includes Nisusia montanensis Bell, 1941, from the Meagher Limestone, Ehmania Zone (Miaolingian, Wuliuan) of Montana, USA and Bellistrophia sp. from the Atei Formation (Miaolingian, Wuliuan) of the Chingiz Range, Kazakhstan.
Species questionably assigned. Protorthis spencei Walcott, 1905, from Led Shale Member (formerly Spence Shale Member) of Langston Formation (Miaolingian, Wuliuan), of Idaho, USA. Species is inadequately known. Its provisional generic affiliation is mainly due to assessment of Bell (1941) who recognized a general similarity of the taxon to Nisusia (= Bellistrophia) desei and reported a complete absence of spines.
Remarks. Bellistrophia differs from Nisusia Walcott, 1905, in the complete absence of hollow spines, as well as in having an apsacline ventral interarea and an emarginate anterior commissure. It differs from Narynella Andreeva, 1987 in having emarginate (not unisulcate) anterior commissure, apsacline (not catacline) ventral interarea, and in the absence of a ventral median fold and dorsal double septa; although Bellistrophia has an emarginate (not uniplicate) anterior commissure, a dorsal sulcus and multicostellate radial ornament unlike Eoconcha Cooper, 1951.