A New Genus of Late Ordovician—Early Silurian Pentameride Brachiopods and Its Phylogenetic Relationships

Protanastrophia repanda gen. et sp. nov. is a reef-dwelling parastrophinid brachiopod in the Lower Silurian (uppermost Telychian) Attawapiskat Formation of the Hudson Bay region of Canada. It is characterized by a small, quasi-smooth shell with gentle anterior costae, a tendency towards an asymmetrical, sigmoidal anterior commissure, and widely separate, subparallel inner hinge plates. Protanastrophia first appeared in the marginal seas of Siberia (Altai, Mongolia) during the Late Ordovician, retaining the primitive character of discrete inner hinge plates in the superfamily Camerelloidea, and preferred a carbonate mound depositional environment. It survived the Late Ordovician mass extinction and subsequently spread to Baltica and Laurentia during Early Silurian (Llandovery) time. Superficially similar asymmetrical shells of Parastrophina portentosa occur in the Upper Ordovician carbonate mound facies of Kazakhstan but differ internally from the new genus in having a septum-supported septalium. Phylogenetic analysis indicates that, within the Camerelloidea, asymmetrical shells with a sigmoidal anterior commissure evolved in Protanastrophia repanda and Parastrophina portentosa independently during the Late Ordovician as a case of homoplasy. The two species belong to separate parastrophinid lineages that evolved in widely separate palaeogeographic regions.


Introduction
The syntrophiidines constituted a suborder of pentameride brachiopods and underwent two major episodes of diversifi− cation during the Early and Middle Ordovician, respectively (Carlson 1996;Harper et al. 2004). The family Parastro− phinidae evolved relatively late among the syntrophiidines by late Darriwilian time, characterized by the development of alate plates in their dorsal valves. Parastrophinids survived the Late Ordovician mass extinction event but were rare and low in diversity in the Silurian, and became extinct during the Early Devonian. Despite their nearly cosmopolitan distribu− tion, parastrophinids usually occur only as a minor compo− nent of brachiopod faunas in Laurentia (Titus 1986). Notable exceptions include the Trentonian (early Katian) Parastro− phina−bearing beds in the Ottawa area and the latest Ordovi− cian (Hirnantian) Parastrophinella reversa (Billings, 1857) shell beds from the Ellis Bay Formation of Anticosti Island, eastern Canada (Wilson 1914;Copper 1997, 2000).
In some early studies (Schuchert and Cooper 1932;Ams− den and Biernat 1965), there was some confusion about the morphological differences between Parastrophina Schuchert andLeVene, 1929 andParastrophinella Schuchert andCoo− per, 1931 especially regarding the configuration of the septa− lium in Parastrophinella. The early interpretation of a sessile septalium with subparallel inner hinge plates (= outer plates of old usage) was followed by Amsden and Biernat (1965) in the Treatise on Invertebrate Paleontology and became widely ac− cepted subsequently. Later, Jin and Copper (1997) studied the type species of Parastrophinella, P. reversa (Billings, 1857), from the type locality of Anticosti Island and showed that its septalium is supported along its entire length by a low median septum, although the septum is buried in shell thickening api− cally to make the septalium appear sessile.
The early misinterpretation of a sessile septalium with subparallel inner hinge plates led to some misidentifications of Parastrophinella. Several Late Ordovician and Early Silurian parastrophinid species with discrete inner hinge plates, which superficially resemble a sessile septalium, were assigned to the genus (Amsden 1968;Rozman 1970aRozman , b, 1981Rubel 1970;Oradovskaya in Nikolaev et al. 1977;Severgina 1978;Oradovskaya 1983). Most of these taxa were described from the middle Katian (lower Ashgill, Upper Ordovician) of the microplates and island arcs (e.g., Northeast Siberia, Mongo− lian Altai, and Sayany−Altai of Russia) that probably consti− tuted an active margin of Siberia during the Ordovician. These Siberian and Mongolian species exhibit a wide range of varia− tion in external morphology (such as anteriorly developed costae and shell asymmetry), but invariably possess discrete, subparallel inner hinge plates. The forms described by these authors were not assigned to Anastrophia Hall, 1867, which has discrete and clearly separated inner hinge plates, because they lack the Anastrophia−type, sharply developed costae that extend from the apex to the anterior margin. As a result, after the revised diagnosis of Parastrophinella by Jin and Copper (1997), a group of species with relatively weak, anteriorly de− veloped costae and distinctly discrete inner hinge plates were left without a generic assignment.
During recent field work in the Hudson Bay region of Can− ada, the first author found more material of faintly costate parastrophinid shells, with discrete inner hinge plates, from the Lower Silurian (uppermost Telychian) Attawapiskat For− mation. This provided an opportunity to examine this group of parastrophinids in detail. The main objective of this study, therefore, is to evaluate the taxonomic position of these para− strophinids and compare them with similar forms from Sibe− ria, Kazakhstan, and Mongolia reported previously by brachi− opod workers of the former USSR.

Localities of the Hudson Bay material
A summary of the Palaeozoic Hudson Bay Basin and the Lower Silurian stratigraphy was given recently by Jin (2003Jin ( , 2005 and will not be repeated here. Suffice it to note that the parastrophinid shells, described in this paper as Protana− strophia repanda gen. et sp. nov. (Fig. 1), occur as a rela− tively minor component of a numerically rich, taxonomically diverse, reef−dwelling brachiopod fauna in the Lower Silu− rian Attawapiskat Formation. The brachiopod associations are characterized by a high density of individuals and com− monly high species diversity, and are dominated variously by pentamerids, gypidulids, leptocoelids, trimerellids, atrypids, and lissatrypids (Jin 2003(Jin , 2005. So far, P. repanda has been found in three outcrops (AK2, AK4, and AK8) of the Attawapiskat Formation on Akimiski Island of the Hudson Bay region. In the list below, the locality coordinates are given in Universal Transverse Mercator (UTM) grid.
Locality AK2.-Tidal flat outcrop along shoreline at first point southeast of camp site, low−relief reef knob just on northwest side of another knob (AK1). About 1.5 m of sec− tion of the Attawapiskat reef exposed along shoreline. AK2a: Collection near southern end of AK2 reef exposure, UTM 17 E0502919, N5883641; a brachiopod cluster dominated by Pentameroides Cooper, 1931 andGypidula Hall, 1867. AK2b: Collection from an area of 3 m 2 near AK2c, in central part of AK2 reef exposure, UTM 17 E0502915, N5883664; a brachiopod cluster dominated by relatively small shells. AK2c: Collection from an area about 3 m 2 , UTM 17 E0502916, N5883663; a brachiopod cluster dominated by Gypidula. AK2−01a: Collection from an area of 3 m 2 area on eastern side of AK2 outcrop, UTM 17 E0502818, N5883943; a brachiopod cluster dominated by Pentameroides.
Locality AK4.-Houston Point, a patch reef outcrop on tidal flat immediately northeast of a prominent beach ridge. About 1.5 m of section exposed at low tide and becomes almost com− pletely submerged at high tide. AK4b: Collection from an area of 4 m 2 immediately adjacent to a gravel beach ridge, near south end of AK4 outcrop, UTM 17 E0492180, N5894740; a brachiopod cluster dominated by Pentameroides.
Other species assigned.-Most species assigned to the new genus were described initially in publications that are not easily accessible. Thus a brief note on each species is pro− vided herein.
Parastrophinella indistincta Rubel, 1970, Silurian, lower to middle Rhuddanian, Estonia. Shell up to 12 mm long, dorsibiconvex; anterior commissure uniplicate, symmetrical or slightly asymmetrical; ventral sulcus, dorsal fold, and costae developed anteriorly; one or two costae in sulcus, two or three on fold, one to three on each flank; broad spondy− lium supported by low median septum extending to mid− length of valve; parallel inner hinge plates. Strophic or astro− phic condition of the shell cannot be determined from avail− able data.
Parastrophinella sp. (Nikiforova, 1978) Hall, 1867 andGrayina Boucot, 1975 in having consistently discrete, subparallel inner hinge plates, but the latter two genera of Wenlock-Pragian age have a symmetrical shell with strong costae that extend from the apex to the margin (Havlíček 1990;Carlson 2002). In the new genus, asymmet− rical fold and sulcus tend to become common in larger shells (greater than 8 mm long and 10 mm wide). Such shell asym− metry is rare in other genera of the family Parastrophinidae. In all other parastrophinid genera, the inner hinge plates are united basomedially to form a septalium, which may be pos− teriorly sessile or supported by a median septum along its en− tire length (Jin and Copper 1997 Material.-AK2a (5 specimens), AK2b (21), AK2c (21, in− cluding illustrated types), AK2−01a (2), AK4b (1) Table 1). Hinge line one−third to one−half of shell maximum width; maximum width of shell attained near midlength of shell. Ventral valve weakly convex posteromedially, becoming flattened towards lateral and anterior margins (Fig. 1A-D); ventral umbo low, inconspicuous; sulcus originating near midlength of shell, widening rapidly towards anterior mar− gin, symmetrical in smaller shells, becoming asymmetrical in relatively large forms. Dorsal valve three to four times deeper than ventral, with larger and more convex umbo ex− tending posteriorly beyond that of ventral valve; fold broad, gentle, developed corresponding to sulcus in position and in symmetrical or asymmetrical contours. Posterior and lateral portions of shell surface smooth; anteromedial part of shell varying from smooth, faintly costate, to moderately costate ( Fig. 1A 1 , B 1 , C 1 , D 1 ). Coarse, concentric growth lamellae usually present, best developed at late growth stage. Spondylium broadly triangular in transverse cross section (Figs. 1E and 3), sessile apically, raised slightly above valve floor anteriorly by short, low median septum. Spondylial comb structure (sensu Jin and Copper 2000) not observed. Inner hinge plates slender, high, subparallel to each other along their entire length (except at their apical origin); outer hinge plates rudimentary; alate plates present but not promi− nent (Fig. 3). Muscle field poorly impressed.
Diagnosis (emended herein).-Shell subpentagonal, dorsibi− convex, asymmetrical, with sigmoidal anterior commissure; ventral sulcus and dorsal fold originating at 7 mm or more from apex; costae subangular, 5-12 per valve, present in shells larger than 11 mm long. Spondylium V−shaped in transverse cross section, supported by medium septum along its entire length; septalium narrow, usually asymmetrical in cross section, supported by low median septum.
Discussion.-The species was initially thought to have a ses− sile septalium and assigned to Parastrophinella (Nikitin et al. 1996). Subsequent examination of paratypes by detailed serial sectioning shows that the species invariably possesses a low dorsal median septum that supports a septalium. Thus, the spe− cies is regarded herein as an aberrant form of Parastrophina, characterized by an asymmetrical shell and sigmoidal anterior commissure (Fig. 4), unlike other species of the genus. Its dor− sal cardinalia are similar to those of typical Parastrophina ex− cept for its asymmetrical cross section (Fig. 5). In Parastro− phina iliana Popov, Cocks, and Nikitin, 2002, from the Upper Ordovician Anderken Formation of the Chu−Ili Range, the adult shells also show a tendency towards an asymmetrical an− terior commissure, but their ventral sulcus and dorsal fold are well defined and the anterior commissure retains its uniplicate character.

Cladistic analysis
Outgroup selection and methods.-The morphological evo− lution of early pentamerides, especially camerelloids, is one of 226 ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008 the major features of the Ordovician biodiversification but re− mains poorly understood and in some cases confusing (Carl− son 1993(Carl− son , 1996. In particular, a spondylial structure of penta− merides is thought to have evolved through the merging of dis− crete dental plates (Carlson 2002 Williams and Harper (2000) to the Order Protorthida. Unlike other protorthides, however, Leioria has derived cardinalia, with dental sockets and brachiophores, and can be considered within primitive Syntrophiidina as was defined by Carlson (2002). It is thus likely that a free spondylium in pentamerides was a primitive and probably plesiomorphic character shared with protorthides and, in subsequent morphological evolution, it merged with the valve floor to form a sessile spondylium or became supported by a median septum. Moreover, recent study of the ontogeny of Apomatella and Gonambonites (see Vinn and Rubel 2000;Popov et al. 2007) also shows that spondylium simplex in the early clitambonitides (excluding polytoechioids) formed from a free muscle platform in juve− niles. This condition persists in the adult Mid Cambrian Arcto− hedra, that otherwise shows distinct clitambonitide−type car− dinalia. Protorthides have very primitive cardinalia devoid of brachiophores or sockets, but possess paired teeth and a free spondylium. For this reason, two protorthide taxa, Psiloria dayi Cooper, 1976 with a smooth shell and Glyptoria gul− chensis Popov and Tikhonov, 1993 with distinct radial orna− ments were selected for the present cladistic analysis. The chileides, which probably embrace the most primitive Cam− brian rhynchonelliformean brachiopods, were already proven useful for defining a polarity of some basic morphological fea− tures (Holmer et al. 1995). Therefore, Chile mirabilis Popov and Tikhonov, 1990 was included also in the analysis, as was the early strophomenate Billingsella aliena . Bohemiella romingeri (Barrande, 1848) and Wimanella seges  of Mid Cambrian age represent a variety of shell morphotypes within the early orthides.
The core taxa presently considered within the family Parastrophinidae is represented by 19 Ordovician and Silu− rian species and nine genera (see Appendix 1). A data matrix derived from 48 unordered, unweighted, taxonomically sig− nificant characters with 186 character states was analyzed cladistically using the software PAUP 4.0 (Swofford 2002;Appendices 2, 3). A heuristic search resulted in six shortest trees, 439 steps long, with a consistency index of 0.3189, homoplasy index of 0.6811, rescaled consistency index of 0.2069, and retention index 0.6486. A strict consensus tree is shown in Fig. 6 (see also . Character selection.-The number of diagnostic features available for camerelloids is rather limited mainly due to their simple external morphology and inadequate knowledge of their dorsal cardinalia and mantle canal system in most of the taxa. Potentially important differences in the morphology of spondylium, especially its subdivision into spondylium simplex and spondylium duplex, can hardly be applied to the camerelloids unless shell structure and ontogeny are studied and ontogenetic development of the ventral median septum is understood. The spondylium duplex is considered one of the diagnostic characters of the Family Parastrophinidae (Carlson 2002), although this view is not accepted by Sapelnikov (1985) who considered spondylium duplex to be characteristic only of the Pentameridina. In this study, 48 characters are used for the cladistic analysis of a relatively large number of taxa (see Appendix 2).

Results and implications for parastrophinid phylogeny and palaeobiogeography
Results of the present analysis should be viewed as a prelimi− nary attempt to understand the phylogenetic relationships of the taxa within the Superfamily Camerelloidea and, in partic− ular, genera presently included into the family Parastro− phinidae. The high level of homoplasy in character transfor− mations resulted in low values of consistency index. This, combined with a limited number of informative features that can be applied to the large number of analyzed taxa, do not allow the cladistic results to be considered as a firm basis for revising the camerelloid systematics. However, the possible plesiomorphic nature of the alate plates in septalium−bearing parastrophinids and their possible secondary loss in the camerellids makes the alate plates a weak basis for defining the Family Parastrophinidae. Results of the analysis also put more weight on the presence of the septalium and the dorsal median septum. As a consequence, genera of the Family Camerellidae appear as a sister stock of early septalium−bear− ing parastrophinid genera Eoanastrophia and Plectosyntro− phia. Nevertheless, all analyzed camerellid taxa are strati− graphically older, and independent acquisition of the septa− lium within the group of early camerelloid genera initially lacking the alate plates cannot be excluded from consider− ation.
The cladogram shows five species of Protanastrophia (Fig. 6, node 13) characterized by such a synapomorphic character as shell asymmetry in juveniles. The presence of alate plates and subparallel inner hinge plates seems to be symplesiomorphic at the base of camerelloid clade (node 10). They also constitute a sister group of the Family Paralle− lelasmatidae, which differs from Protanastrophia in the ac− quisition of an astrophic shell and in the absence (secondary loss) of alate plates.
Another outcome of the analysis is that the Silurian ge− nus Bleshidium, originally assigned to the Camerellidae (Havlíček and Storch 1990), should be reassigned to the Parallelelasmatidae. The presence of the long, discrete, subparallel inner hinge plates, together with the absence of alate plates, a septalium and a dorsal median septum, is in accordance with the revised diagnosis of the family (Carl− son 2002). The presence of a septalium and dorsal median septum appears to be synapomorphic feature shared by advanced camerellids and parastrophinids (Fig. 6, node 18), whereas the astrophic shell was acquired by camerellids (Fig. 6, node 23) and advanced parastrophinids (Fig. 6, node 26) inde− pendently. The cladistic analysis suggests that the smooth shells of Liostrophia Kindle, 1936 andIlistro− phina Popov, Cocks, andNikitin, 2002 are the most ad− vanced state of shell morphology within the parastrophinids.
In addition to the paraphyletic nature of the Parastro− phinidae, the cladistic analysis shows a clear separation of derived parastrophinids with a septalium from the taxa with discrete inner hinge plates (Anastrophia, Protanastrophia, and the Parallelelasmatidae), notwithstanding the presence or absence of alate plates. Interestingly, there is a concomi− tant biogeographical separation between these two groups of parastrophinids.
In Laurentia and peri−Gondwana, parastrophinids with a septalium first appeared near the Mid−Late Ordovician boundary. They coexisted with the family Parallelelasma− tidae, a sister group of Protanastrophia according to the cladistic analysis of this study. In these palaeogeographic regions, Anastrophia? kurdaica Sapelnikov and Rukavish− nikova, 1975 is the only species known to have discrete in− ner hinge plates during the Ordovician. In Laurentia, para− strophinids usually constituted an insignificant component of the benthic faunas (Titus 1986) whereas in tropical peri− Gondwana, particularly in North China and the Chu−Ili microplate, they experienced rapid diversification early in the Late Ordovician Fu 1982) as a dis− tinct component of the benthic assemblages in the carbon− ate mound biofacies. Some of these taxa show a tendency towards an asymmetrical shell with a sigmoidal anterior commissure, such as in Parastrophina portentosa. In exter− nal morphology, P. portentosa shows some degree of su− perficial similarity to Protanastrophia gen. nov. and espe− cially to Protanastrophia repanda sp. nov., but it differs in− ternally by having a septalium supported by a median sep− tum (Figs. 1,4,5). The morphological differences in the dorsal cardinalia of Parastrophina portentosa and Protana− strophia repanda are shown by the cladistic analysis to be of phylogenetic significance.
The first occurrence of Parastrophina in the Oandu Re− gional Stage (early Katian) of Baltica  postdates its first occurrence in tropical peri−Gondwana (North China, Chu−Ili) by about one chronostratigraphical stage, suggesting a delayed faunal migration of Parastro− phina between the two tectonic regions. A probable scenario is that septalium−bearing parastrophinids evolved some− where in tropical peri−Gondwana and subsequently spread outside Gondwana in two dispersal events, the first being an invasion to Laurentia during the late Darriwilian, and the sec− ond to Baltica during the early Katian. A similar migration pattern in Late Ordovician brachiopods was observed also for the trimerellides and early atrypides (Popov et al. 1997(Popov et al. , 1999. During the Late Ordovician, Eoanastrophia was con− fined largely to the temperate and high−latitude regions of Gondwana (Mélou 1990;Villas 1985).
The palaeobiogeographic pattern of Protanastrophia was remarkably different. All the Late Ordovician occurrences of the genus were confined to the marginal seas of Siberia (Altai−Sayan, Mongolia, and Northeast Siberia), probably the most remote and isolated continent during the Early Palaeozoic (Bassett et al. 2002;Fortey and Cocks 2003). Morphologically, the dorsal cardinalia of Protanastrophia were relatively primitive, especially in terms of the widely separated, subparallel inner hinge plates, without any ten− dency to develop into a septalium. This suggests an early separation of Protanastrophia from the main parastrophinid lineage. It is notable that septalium−bearing parastrophinids are absent from Siberia and its adjacent microplates, except for the Altai−Sayan region. Specimens referred by Kulkov and Severgina (1989) to Parastrophina bilobata Cooper, 1956 from the Karasinskaya Formation (upper Darriwilian) do not yield adequate information on their cardinalia and thus their parastrophinid affinity is questionable. Anastro− phia aff. kurdaica from the Khankharinslaya Formation (lower Katian) lacks alate plates, which precludes its affinity to the family Parastrophinidae. Parastrophina orlovensis Severgina, 1978 andEoanastrophia transversa Severgina, 1978 from the Orlovskaya Formation (upper Katian) may be true septalium−bearing parastrophinids, although their cardi− nalia remain poorly known.
Despite the incomplete fossil record of Ordovician para− strophinids from Siberia, the predominance of parastrophi− nids with relatively primitive cardinalia (i.e., widely discrete inner hinge plates) suggests that the Siberian parastrophinids evolved in prolonged palaeogeographic isolation, following an early separation from the main camerelloid stock. Palaeo− geographic expansion of Protanastrophia towards Laurentia, Siberia and probably terranes of Central Asia in the Llando− very took place under a different palaeogeographic and palaeoenvironmental situation as a result of the Hirnantian mass extinction and the closure of the Iapetus Ocean.
Another outcome of the analysis is a possible link of the early pentamerides to protorthides. Leioria and Palaeostro− phia are placed close to the base of the pentameride clade, whereas Huenella, which typifies the family Hunellidae, ap− pears more derived.