Oldest known Dicoelosia and Epitomyonia, deep water brachiopods from the Beiguoshan Formation (Middle Katian, Upper Ordovician), Shaanxi, north China



Abstract:  A diverse brachiopod fauna from a relatively deep water carbonate facies of the Upper Ordovician Beiguoshan Formation (uppermost Caradoc – lower Ashgill, middle Katian) is characterized by small shells and contains the oldest known Dicoelosia and Epitomyonia, two diagnostic taxa of deep water brachiopod palaeocommunities during the Late Ordovician and Silurian. Three new species are recognized: Dicoelosia cordiformis sp. nov., Dicoelosia perbrevis sp. nov. and Epitomyonia fui sp. nov. These pioneer forms of the family Dicoelosiidae show a relatively high degree of morphological plasticity. The shells of Dicoelosia from the Beiguoshan Formation range from the typical slender-lobed form with a concavoconvex profile to the strongly equibiconvex, fat-lobed morphotype that was not known previously until the late Silurian. The Beiguoshan dicoelosiids point to an important attribute of the deep water brachiopods: small generalists with high morphological plasticity, which make them ideal candidates as progenitors for the evolution of shallow water brachiopod faunas in shelf and platform depositional environments.

Dicoelosia and Epitomyonia, together with some other small-shelled brachiopods (e.g. Skenidioides and Jonesea) were diagnostic taxa of deep water marine shelly benthos, particularly during the Silurian Period (Johnson et al. 1973, 1976; Boucot 1975; Zhang 1989; Musteikis 1991; Jin and Chatterton 1997; Rong et al. 1999, 2006). Since the early study on the detailed shell morphology of Dicoelosia (e.g. Wright 1968a; Rubel 1971; Musteikis and Puura 1983; Lenz 1986), there has been a revived interest in these deep water brachiopods with regard to their functional morphology, taxonomy and evolution (Jin and Copper 1999; Watkins et al. 2000; Chen et al. 2008; Chen and Jin 2010). Despite numerous early investigations, the origin and early evolution of Dicoelosia and Epitomyonia during the Late Ordovician remain poorly understood, partly because of their much less common occurrences compared to their fossil record in the Silurian. The following questions remain unanswered:

  • 1What was the immediate ancestor of Dicoelosia and related forms? Although there is some circumstantial evidence that the ancestral stock of dicoelosiids lies within the superfamily Dalmanelloidea (Wright 1968a; Chen and Jin 2010), there has been a sore lack of intermediate forms between the subcircular morphotype of typical dalmanelloids and the characteristic dicoelosiid bilobate morphology.
  • 2Did these predominantly deep water taxa originate in deep water or shallow water environments? The answer to this question has significant implications for our understanding of onshore vs. offshore benthic faunal evolution and expansion during the early Palaeozoic (Jablonski et al. 1983; Sepkoski and Sheehan 1983; Watkins et al. 2000; Zhan and Jin 2008).
  • 3What palaeoecological factors contributed to the confinement of the dicoelosiids to deep water facies but periodic invasion into shallower water shelves and platforms during the Silurian? Investigations into this problem can potentially shed some light on whether deep water species in general served the role of a steady precursory stock for the evolution of shallow water brachiopod faunas.
  • 4Evolutionary responses. The Late Ordovician was marked by major marine transgressions and frequent oceanic anoxia events, punctuated by cooling episodes and terminated in the Hirnantian glaciation (Fortey and Cocks 2005; Cherns and Wheeley 2007; Page et al. 2007; Trotter et al. 2008). It is assumed that deep water brachiopod faunas would have been most sensitive to the environmental changes during oceanic anoxia, whereas during the Hirnantian glaciation the deep water benthic regime could serve as refugia for the marine shelly benthos (Rong 1984; Rong et al. 2007). If so, what were the evolutionary responses of the deep water brachiopods during the Late Ordovician?

The study of other Late Ordovician deep water brachiopods, especially the Foliomena Fauna, suggests that the water depth range that was tolerable to a fauna could shift between deep water and shallow water settings (Rong et al. 1999; Zhan and Jin 2005), probably as a dynamic faunal response to water temperature, oxygen content, turbulence level and other related palaeoecological factors. Similar investigations into the functional morphology of Dicoelosia and Epitomyonia have been attempted recently by Chen et al. (2008) and Chen and Jin (2010), with a focus on the abundant deep water forms of Silurian age. The investigation into the Late Ordovician species of these two genera has been hampered by the much less common and usually less well preserved material.

In this study, we report the earliest known occurrence of Dicoelosia and Epitomyonia in the Beiguoshan Formation (uppermost Caradoc – lower Ashgill, middle Katian), Shaanxi Province, north China. The Beiguoshan dicoelosiids are approximately coeval with Dicoelosia sp. (Koskarasu Beds, middle Angrensor Formation, uppermost Caradoc, middle Katian) and Epitomyonia cf. glypha (Odak Beds, upper Angrensor Formation, lower Ashgill, upper Katian) reported from central Kazakhstan (Nikitin et al. 2006). A detailed study of the morphology and taxonomy of these pioneer forms will help to better understand the origin and early evolution of these two genera, which became predominantly deep water taxa by early Wenlock time during the Silurian.

Geological and stratigraphical settings

The Beiguoshan Formation (Che 1963) is best exposed in the border area between Longxian County, western Shaanxi Province and Pengyang County, Ningxia Autonomous Region (Text-fig. 1). The formation occurs as far east as the Tongchuan–Hancheng area, more than 50 km north and north-east of Xi’an (Fu 1977, 1982). Palaeogeographically, the outcrop area of the Beiguoshan Formation was located on the south-western margin of the north China palaeoplate.

Figure TEXT‐FIG. 1..

 A, map of study area, with location of the fossil locality marked by a light grey rectangle. B, area details of the fossil locality at Beiguoshan, Longxian County, Shaanxi Province, north China. Outcrop of the Beiguoshan Formation indicated by a solid triangle (35°4′38.93″N, 106°42′14.71″W; elevation 1468 m).

During much of the Ordovician Period, the north China palaeoplate was located in the tropical region, just south of the palaeoequator (Boucot et al. 2009). The Lower Ordovician succession of the study area comprises mainly thick-bedded dolostone and blocky limestone with chert nodules. The Middle Ordovician is represented by the Sandaogou Formation consisting of limestone and marls in the Longxian-Pengyang area. Eastwards, the Sandaogou Formation changes into thick-bedded dolomitic limestone (the Majiagou Formation). The underlying Pingliang Formation of early Late Ordovician age is characterized by greenish grey to black shale, with some interbedded limestone in the lower part. The change from limestone to black shale probably corresponded to the Caradoc (Sandbian to earliest Katian) greenhouse episode and marine transgression of global scale. The black shale facies changes eastwards into the light grey, blocky limestone of the Yaoxian Formation in the Tongchuan–Hancheng area, probably corresponding to a change from anoxic, deeper water environment to an oxygenated, shallow water setting onto the north China Platform.

Further upsection in the study area, the Beiguoshan Formation comprises a thick succession of thick-to-massive-bedded carbonates ranging from micritic mudstone to wackestone, punctuated by local development of microbial mudmound facies, with thicknesses exceeding 600 m. The typical lithofacies of the formation covers an area of 30 km2 in Beiguoshan and its vicinity in Longxian, characterized in the outcrops as vertical cliffs. The massive carbonate unit usually forms a distinct cap over the underlying shales of the Pingliang Formation. A recessive unit of calcareous mudstone and nodular argillaceous limestone occurs in the middle part of the Beiguoshan Formation and yields the dicoelosiids of this study, as well as a rich and diverse brachiopod fauna (Text-fig. 2).

Figure TEXT‐FIG. 2..

 Stratigraphical section of the Beiguoshan Formation at Beiguoshan, Longxian County, Shaanxi Province, north China.

The type section of the Beiguoshan Formation is located at Beiguoshan Mountain near the village of Lijiapo, about 30 km north-west of the Longxian county town, Shaanxi Province (Che 1963). Here, the formation is dominated by grey to light brown, thick-bedded limestone, with local development of mudmound facies, and contains minor interbeds of nodular limestone, calcareous shale and mudstone. It has an exposed thickness of 640 m and is unconformably overlain by thick-bedded sandstone of Triassic age.

The lower part of the Beiguoshan Formation has a maximum thickness of 280 m and consists mainly of grey, thick-bedded to blocky limestone, with minor partings of calcareous shale, as well as interbeds of nodular limestone near the top of the member. The trilobites and nautiloids of the lower member are similar to those in the upper Pagoda and Linhsiang formations of the Yangtze Platform of south China palaeoplate (Chen et al. 1984).

The middle part of the Beiguoshan Formation is approximately 160 m thick and comprises mainly light grey limestone, interbedded with reddish brown limestone, with a rich and diverse brachiopod fauna in the interbeds of calcareous mudstone and marls (Fu 1977).

The upper part of the Beiguoshan Formation is dominated by massive carbonate mudmounds, with a cumulated thickness exceeding 200 m. No collections of shelly fossils have been made from this unit due mainly to its massiveness and vertical cliffs.

A preliminary study of new collections made from the middle part of the formation indicates the presence of nearly 50 genera (based mainly on identifications during the 1995 field season), with Skenidioides, Epitomyonia, Altaethyrella, Antizygospira, Cyclospira (to be reassigned to PectenospiraPopov, Nikitin and Sokiran, 1999) being the most abundant constituents. The two species of Dicoelosia reported in this paper make up only a minor component of the brachiopod fauna.

In addition to its high diversity, the brachiopods of the Beiguoshan Formation are characterized by their notably small size. Except for a few taxa (e.g. Altaethyrella, Ovalospira and Phaceloorthis) that attain a length or width greater than 10 mm, the brachiopod shells are typically minute, in the range of 4–8 mm (e.g. Skenidioides, Dicoelosia, Epitomyonia, Ptychopleurella, Leptellina, Synambonites and Antizygospira), with a few forms smaller than 3 mm (such as Dicoelosia perbrevis sp. nov. and Skenidioides sp.). The predominance of small, articulated and well-preserved shells and the occurrence of such typical deep water taxa as Skenidioides, Epitomyonia, and Dicoelosia suggest a low energy, deep water depositional environment equivalent to BA4 or slightly deeper (see, for example, Zhang and Boucot 1988; Zhang 1989; Jin and Chatterton 1997; Jin and Copper 1999; Chen et al. 2008; Chen and Jin 2010).

The age of the Beiguoshan Formation is confined broadly as middle Katian (uppermost Caradoc to lower Ashgill). Graptolites from the top strata of the underlying Pingliang Formation belong to the Diplacanthograptus spiniferus Biozone (X. Chen, pers. comm. 2010) and confine the base of the Beiguoshan Formation as uppermost Caradoc (middle Katian) or younger. Recent examination of the conodonts from the Beiguoshan Formation, which were previously reported by Wang and Luo (1984) and An and Zheng (1990), revealed the presence of Taoqupognathus beiguoshanensis (Yu and Wang, 1986). This species is morphologically similar to Taoqupognathus tumidusTrotter and Webby, 1994, which is known to be of uppermost Caradoc–lower Ashgill (middle Katian) age in north China and Australia (Zhen 2001). Trilobites of the lower unit of the Beiguoshan Formation are characterized by Remopleurides, Parisoceraurus, Scotoharpes, Trinodus, Geragnostus, Pseudosphaerexochus, Pliomerina, Sphaerexochus, and Brontocephalina. The upper unit yields such trilobites as Sphaerexochus, Heptabronteus, Brontocephalina, Remopleurides, Trinodus, Amphilichas and Parisoceraurus. The occurrence of Heptabronteus is of significance, as the genus has been regarded as a junior synonym of Eokosovopeltis which is known from Caradoc (middle Katian) strata of Australia (Edgecombe et al. 2004), and from the Caradoc–middle Ashgill (middle to upper Katian) of Kazakhstan and south China (Zhou and Zhou 2008; Z. Zhou, pers. comm. 2010).

Implications for the origin and early evolution of dicoelosiids

Despite their diagnostic bilobation, the origin of the dicoelosiids has been an evolutionary puzzle for the lack of an obvious ancestor or intermediate forms within the orthide superfamily Dalmanelloidea, to which they are assigned (Wright 1968a; Harper 2000; Chen and Jin 2010). The morphological characters of the Beiguoshan dicoelosiids, particularly of the two species of Dicoelosia, provide some clues for the origin and early evolution of the bilobate shells of the dicoelosiid family. In previously known species of Dicoelosia from the Late Ordovician (e.g. Wright 1968a; Potter 1990), the shells already have the full-fledged characteristics of this taxonomic group, including a well-defined anteromedian notch (emargination) and a concavoconvex lateral profile; the ribs on each lobe showing a bilaterally symmetrical pattern, with a strong costa marking the medial axis of each lobe, and weak to faint ribs in the ventral and dorsal sulci. In comparison, the Beiguoshan forms of Dicoelosia retained certain aspects of the normal dalmanelloid shell morphology, as will be discussed below.

Shell shape.  The transversely elliptical shell outline of Dicoelosia cordiformis sp. nov. (Text-figs 3–4) resembles that of a dalmanellid, especially in its relatively short hinge line, rounded cardinal extremities and rounded lateral margins. The biconvex lateral profile can be traced to the type species and some other forms of Dalmanella (see Rong and Li 1999; Jin and Bergström 2010), although the dorsal valve of normal dalmanellids is not as strongly convex as in D. cordiformis. Also, some dalmanellids may develop a strong medial furrow in the dorsal valve and a faint anteromedian notch, such as ‘Onniellameeki (Miller, 1875) from Richmondian strata of the Cincinnati region (Text-fig. 3; see also Hall 1962). Therefore, the shallow and gentle anteromedian emargination of D. cordiformis can be regarded as an early stage of bilobation from dalmanellids to dicoelosiids. Thus, only the ventral sulcus of Dcordiformis can be regarded as a synapomorphic character that separate the dicoelosiids from other dalmanelloids.

Figure TEXT‐FIG. 3..

 Postulated evolutionary pathways of major species groups of Dicoelosia.

Figure TEXT‐FIG. 4..

 A–O, Dicoelosia cordiformis sp. nov. from the Beiguoshan Formation, Shaanxi, north China. A–E, holotype, NIGP 152506 (from University of Western Ontario collection, temporary catalogue number W2970, to be formally reposited at NIGP after this study), dorsal, ventral, lateral, posterior and anterior views. F–J, paratype, NIGP 152507, dorsal, ventral, lateral, posterior and anterior views of partly damaged shell. K–O, paratype, NIGP 152508, dorsal, ventral, lateral, posterior and anterior views of relatively small but strongly biconvex shell. P–T, Dicoelosia varica (Conrad, 1842), dorsal, ventral, lateral, posterior and anterior views of silicified shell, Waldron Shale (Wenlock), Indiana, CMC-IP 35709a.

Shell ribbing pattern.  The ribs (including both costae and costellae) are largely uniform across the shell surface in Dcordiformis, especially in the posterior half of the shell that represents the early growth stage. There is not yet a clear development of the bilaterally symmetrical pattern within each lobe, which characterizes all later forms of Dicoelosia. The ribs in the sulcus show a reduction in size only in the anterior half of the shell (Text-fig. 4A–E). In Dperbrevis sp. nov., the ventral sulcus bears one strong costa (plus one costella or two costellae) of similar strength as the normal ribs on each lobe. This can be regarded as a plesiomorphic character derived from a dalmanelloid ancestor, and it contrasts distinctly with the fully evolved forms of Dicoelosia (especially those of Silurian age) that usually have fine, faint, much reduced ribs in the sulcus.

When plotted against the generalized spatiotemporal trend of morphological change in Late Ordovician and Silurian species of Dicoelosia, as proposed by Chen and Jin (2010), the Beiguoshan species Dperbrevis falls within the range of the Late Ordovician morphotype, characterized by a transversally extended shell outline (length/width ratio less than 1) and a low emargination index (0.23–0.28). Such emargination index values fall within the lowest range among all forms of Dicoelosia compiled in Chen and Jin (2010) and are in sharp contrast to the index value of 0.67 in the deep water Silurian species with long lobes (e.g. Ddiversifrons). This implies that the Beiguoshan species most likely lived in moderately deep water in mid-shelf or equivalent platform setting (lower BA3 or BA4), but not as deep as the depositional environments occupied by the slender, long-lobed forms (such as Ddiversifrons) found commonly in BA5–6 settings during the Silurian (Johnson et al. 1976; Zhang 1989; Jin and Chatterton 1997). Therefore, the Beiguoshan fossil evidence suggests a possibility that the early Dicoelosia originated in relatively deep water, outer shelf environments and subsequently invaded even deeper waters during the Silurian, although the hypothesis is subject to the test of future fossil discoveries of more intermediate forms between normal dalmanellids and dicoelosiids.

With the morphological data available, it would be premature to conduct any vigorous quantitative phylogenetic analysis of the early dicoelosiids because (1) internal structures are not clearly known for some species, especially of Dicoelosia, either because of inadequate material or because of traditional emphasis on external morphology; (2) it is difficult to quantitatively code continuously variable characters within individual species; (3) even multivariate analyses based on biometric measurements of individual shells of different species met only limited success in delineating species clusters (see Chen and Jin 2010).

In terms of qualitative investigation into evolutionary pathways, available fossil data indicate that the morphotype of Dcordiformis (biconvex shell with an incipient anteromedian emargination) was not represented, after its first appearance, until Wenlock time at the earliest. The other Beiguoshan form, Dperbrevis, with a weakly concavoconvex shell, relatively wide lobe angle and shallow emargination, is probably at the root of the main branch of Dicoelosia evolution, as most Ordovician species share one or more of these morphological traits. The latest Katian species from the Boda Limestone, DtransversaWright, 1968a and Dindenta (Cooper, 1930), for example, have wide lobe angles and shallow to moderately deep emarginations (Text-fig. 3). Dicoelosia sp. 1 Potter, 1990 from Ashgill rocks of Klamath Mountains may even have an Epitomyonia-like shell outline. Despite its broad lobes resembling that of the Dcordiformis and Dvarica, the latest Katian species DanticipataWright, 1968a from the Pirgu strata of Estonia has a concavoconvex shell.

By early Silurian time (Llandovery and early Wenlock), several species exhibited a trend towards low lobe angles, with some approaching subparallel lateral margins, as visible in DosloensisWright, 1968a from the Oslo District, DdauphinesisJin and Copper, 1999 from Anticosti Island, DbalticaMusteikis and Puura, 1983 from Estonia, Dverneuiliana (Beecher, 1891) and DparalataBassett, 1972 from Gotland. This species group, characterized by a shallow-to-moderate emargination and weakly concavoconvex profile, occurs predominantly in platform/shelf carbonate settings (BA3–4).

Recent examination of a large collection (hundreds of well-preserved specimens, Cincinnati Museum, IP-53961) of a hitherto poorly known species, Dicoelosia acutiloba (Ringueberg, 1888) from the Wenlock Rochester Shale, shows a wide range of infraspecific variation, spanning morphotypes of several species, including Dbiloba, Dverneuiliana, Dparvifrons, etc. (Text-fig. 3). It is conceivable that the deep water forms (e.g. Ddiversifrons and Dparvifrons of BA5 settings or deeper) either derived from the Dacutiloba stock or formed its sister group. Ddiversifrons with long and slender lobes occupied predominantly shelf margin to slope environments during the Wenlock and Ludlow (Johnson et al. 1976; Zhang 1989; Jin and Chatterton 1997; Chen and Jin 2010).

From Wenlock onward, various species of Dicoelosia in epeiric sea and mid-shelf environments showed a trend towards a planoconvex to ventribiconvex shell profile and a biconvex cross-section of individual lobes, as is typified by the Ludlow DoklahomensisAmsden, 1951. This broad trend of morphological transition has been confirmed by several authors (e.g. Amsden 1968; Wright 1968a; Musteikis and Puura 1983; Chen and Jin 2010). Amsden (1968) postulated that the fat-lobed, biconvex shells of Dvarica (Conrad, 1842) and DvaricaformisJohnson, 1973, known then only from the Lower Devonian, evolved from such broad-lobed, planoconvex shell as Doklahomensis. Subsequent knowledge of such similar morphotypes as Dparalata and D. ‘varica’ of Wenlock age (see Chen and Jin 2010), as well as Dcordiformis described in this study, now makes Amsden’s hypothesis less certain because it is possible that the Dvarica lineage may have rooted in the Late Ordovician (Text-fig. 3).

Systematic palaeontology

Repository.  Repositories of figured specimens: Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China (NIGP); University of Western Ontario, London, Ontario (W); and Cincinnati Museum Center, Cincinnati, Ohio (CMC-IP).

Order ORTHIDA Schuchert and Cooper, 1932
Suborder DALMANELLIDINA Moore, 1952
Superfamily DALMANELLOIDEA Schuchert and Cooper, 1932
Family DICOELOSIIDAE Cloud, 1948

Genus DICOELOSIA King, 1850

Type species. Anomia bilobaLinnaeus, 1758. Type locality and type stratum uncertain; probably from lower Wenlock strata of either Salop, UK or Gotland, Sweden. As determined by Wright (1968a; see also Bassett and Cocks 1974), this species was first described in 1758, which predates Linnaeus, 1767 cited in both editions of the brachiopod Treatise.

Dicoelosia cordiformis sp. nov.
Text-figures 4–5

Figure TEXT‐FIG. 5..

 Principal component analysis of broadly bilobate shells of Dicoelosia cordiformis sp. nov. from north China and Dvarica (Conrad, 1842) from the Silurian Waldron Shale (Wenlock) of Indiana and the Early Devonian Helderberg Formation (Lochkovian) of New York.

2010 Dicoelosia sp. Chen and Jin, p. 6, fig. 5F–J.

Derivation of name.  Latin, cordis, heart; cordiformis refers to the heart-shaped shell of the new species.

Holotype.  NIGP 152506 (Text-fig. 4A–E); calcareous shell preserved in articulation. This specimen was initially donated by Fu Lipu to the UWO collection and registered as field collection W2970 (see Chen and Jin 2010, p. 7) but will be formally reposited at NIGP after this study.

Paratypes.  NIGP 152507 and 152508, calcareous shells preserved in articulation.

Type locality.  Beiguoshan, Longxian County, Shaanxi, north China.

Type horizon.  Soft, recessive unit, Beiguoshan Formation, uppermost Caradoc – lower Ashgill (middle Katian).

Diagnosis.  Shell relatively large for the genus, cordate in outline, bisulcate, subequally biconvex in overall shell curvature and in cross-section of each lobe. Hinge line short, slightly more than half width of shell. Shell lobes short and broad, forming shallow anteromedian emargination. Sulcus of each valve narrow, shallow posteriorly, becoming sharp anteriorly, bearing slightly finer ribs than in other parts of shell.

Description.  Shell small, with maximum length 5.4 mm, width 7.3 mm and thickness 3.6 mm; shell outline cordate, with rounded lateral margins and shallow anteromedian emargination; overall shell curvature subequally biconvex in lateral view, with ventral valve slightly deeper than dorsal; hinge line short, attaining 49–58 per cent of maximum width of shell, with rounded cardinal extremities and without ears. Maximum width of shell located slightly anterior of mid-length. Anterior margin bearing shallow and gentle medial notch; anterior commissure rectimarginate.

Ventral umbo relatively narrow, prominent, with suberect beak. Interarea high, apsacline, longitudinally curved (Text-fig. 4C, H, M); delthyrium open. Medial furrow starting at apex, widening slightly but deepening noticeably towards anterior margin, with slightly finer ribs than in shell flanks.

Dorsal umbo weakly convex; interarea inconspicuous, about one-quarter height of ventral interarea, nearly orthocline. Medial furrow in umbonal area widening and deepening slowly to become clearly delimited sulcus in anterior half of shell, bearing similarly fine ribs as in ventral sulcus.

Overall shell ribs (costae and costellae) strong with rounded crest; number of ribs increasing mainly by asymmetrical bifurcation, with weaker new costella branching out from medial side of parental costa. Fine growth lines and coarser growth lamellae irregularly developed.

Cardinal process platy, bearing chevron-like crenulations on myophore. Other internal structure not investigated because of limited material available for study.

Remarks.  Despite the limited number of specimens, the unique morphology of the Beiguoshan shells, interpreted here as an intermediate between the families Dalmanellidae and the Dicoelosiidae, warrants the status of a new species. In its subequal biconvexity, short and broad lobes, and shallow emargination, the new species has some superficial resemblance to Dicoelosia varica and D. varicaformis from the Ludlow and lower Devonian of North America (Text-fig. 4P–T; see also Hall and Clarke 1893–1895; Harper et al. 2010, p. 134; Niemeyer et al. 2010, p. 306; Amsden 1968; Johnson 1973). The Beiguoshan species, however, has normal ribbing in the ventral and dorsal sulci and Dalmanella-like ventral umbo and interarea, being higher and more longitudinally curved than what is normal for Dicoelosia. The ribs are either absent or faint in the sulcus of Dvarica and Dvaricaformis. The overall shell shape of the new species can be produced simply from a dalmanellid shell by inserting a sulcus on each valve and a shallow anteromedian notch. The bilaterally symmetrical ribbing pattern (usually marked by a strong costa along the medial axis) of each lobe, which is characteristic of most species of Dicoelosia, had not yet developed in the new species. Also, most species of Dicoelosia typically have a straightened lateral margin, which contrasts to the smoothly rounded (Dalmanella-like) lateral margin of the new species. Besides the morphological differences discussed above (some of which are difficult to quantify), multivariate analysis based on measurable biometrics also corroborates the notion that there is only minimal morphological overlap between the new species and Dvarica (Text-fig. 5).

It is puzzling that this fat-lobed and subequally biconvex morphotype first appeared in the latest Caradoc but is unknown subsequently in the diverse forms of Dicoelosia of later Katian to early Wenlock age. This is either a case of evolutionary convergence within the Dicoelosiidae or of a single lineage with a major gap in its fossil record (Text-fig. 3).

Dicoelosia perbrevis sp. nov.
Text-figure 6

Figure TEXT‐FIG. 6..

Dicoelosia perbrevis sp. nov. from the Beiguoshan Formation, Shaanxi, north China. A–E, holotype, NIGP 152509, dorsal, ventral, lateral, posterior and anterior views. F–J, paratype, NIGP 152510, dorsal, ventral, lateral, posterior and anterior views. K–O, paratype, NIGP 152511, dorsal, ventral, lateral, posterior and anterior views of very small shell.

Derivation of name.  Latin, perbrevis, very short, denoting the short lobes and shallow anteromedian notch of the new species.

Holotype.  NIGP 152509 (Text-fig. 6A–E).

Paratypes.  NIGP 152510 and 152511; seven paratypes not figured (mostly minute immature shells or fragments, down to 1 mm in size).

Type locality.  Beiguoshan, Longxian County, Shaanxi, north China.

Type horizon.  Soft, recessive unit, Beiguoshan Formation, uppermost Caradoc – lower Ashgill (middle Katian).

Diagnosis.  Small, concavoconvex shells of Dicoelosia, with strong medial costa and one or two costellae in ventral sulcus; lobes diverging at relatively wide angle, and each lobe ventribiconvex in cross-section.

Description.  Shell small, wider than long, with maximum length 2.9 mm, width 3.7 mm and thickness 1.4 mm. Ventral valve approximately three times as deep as dorsal valve; overall lateral profile concavoconvex but with gentle curvature; each lobe ventribiconvex in cross-section. Hinge line short relative to total width of shell; maximum width of shell located near anterior ends of lobes, which diverge from each other at lobe axial angle of 68 degrees. Lateral margins mostly straight. Anteromedian emargination broad and relatively shallow, with depth not exceeding 1 mm in largest specimen (or 28 per cent of shell length).

Ventral interarea high, apsacline, with longitudinally curved surface; delthyrium open (Text-fig. 6A, C, H, I, M). Ventral umbo moderately and evenly convex, with small, suberect beak. Sulcus beginning immediately anterior of umbo, becoming wider and deeper towards apex of anteromedian emargination, bearing one strong costa, plus one costella or two costellae (Text-fig. 6B, G, L).

Dorsal interarea much lower than ventral, nearly orthocline. Dorsal umbo inconspicuous. Sulcus beginning anterior of umbonal area, widening and shallowing anteriorly towards apex of emargination, bearing three well developed ribs.

Ribs (costae and costellae) on each lobe generally strong, rounded; costa along longitudinal axis of each lobe being strongest in both valves, and ribs on each side of lobe axis somewhat weaker and bilaterally symmetrical within each lobe. Bifurcation of ribs infrequent.

Internal structure not investigated.

Remarks.  The shells of the new species have a maximum width greater than length, which is typical of the Late Ordovician forms of Dicoelosia (Chen and Jin 2010). Shell elongation does not become apparent until late Llandovery time during the early Silurian. As noted by Wright (1968a) and Amsden (1968), the cross-section of each shell lobe tends to be concavoconvex in pre-Wenlock species, but becomes increasingly biconvex from Wenlock to Early Devonian time. Thus, the ventribiconvexity of the lobe cross-section in Dperbrevis is somewhat unusual when compared with the general morphological trend of genus. The strong ribs in the ventral sulcus are distinct for the small shell size of Dicoelosia perbrevis and separate it from other congeneric species of Ordovician and Silurian age. Dicoelosia sp. reported by Potter (1990) from the Kangaroo Creek Formation (Ashgill) of eastern Klamath Mountains, northern California, may also have relatively strong ribs in the ventral sulcus, but it differs from Dperbrevis in having shell outlines that are unusual for Dicoelosia (some Epitomyonia-like) with a very shallow anteromedian emargination. Other Ordovician species with small shells (e.g. DjonesridgensisRoss and Dutro, 1966, DsimulataMitchell, 1977) lack the strong ribs in the ventral and dorsal sulci.

Dicoelosia perbrevis has a bilobate shell typical of the genus. Internal structures are rarely used for classification and not studied because of the lack of separated valves and the difficulty of serially sectioning minute shells.

Genus EPITOMYONIA Wright, 1968b

Type species. Epitomyonia glyphaWright, 1968b, Boda Limestone Formation, uppermost Ordovician, Sweden.

Remarks.  Recently, Chen et al. (2008) carried out a global review of various species assigned to Epitomyonia and recognized a total range of the genus from upper Katian to Wenlock. New faunal data suggest that Epaucitropida established by these authors, on the basis of material from locality S-2, Mackenzie Mountains, north-western Canada, is of Hirnantian age. Thus, the oldest forms of Epitomyonia previously known were Ashgill (predominantly middle Ashgill, or latest Katian) in age from Sweden and Kazakhstan (Wright 1968b; Nikitin et al. 2006).

Epitomyonia fui sp. nov.
Text-figures 7–9

Figure TEXT‐FIG. 7..

Epitomyonia fui sp. nov. from the Beiguoshan Formation, Shaanxi, north China. A–E, paratype, NIGP 152513, dorsal, ventral, lateral, posterior and anterior views of relatively small shell with clear anteromedian emargination. F–J, paratype, NIGP 152514, dorsal, ventral, lateral, posterior and anterior views of relatively large shell. K–O, paratype, NIGP 152515, dorsal, ventral, lateral, posterior and anterior views. P–T, holotype, NIGP 152512, dorsal, ventral, lateral, posterior and anterior views.

Figure TEXT‐FIG. 8..

 Serial sections of Epitomyonia fui sp. nov., paratype, NIGP 152516, Beiguoshan Formation, Shaanxi, north China. Numbers in the lower right corner of each section indicate distance (mm) from apex. Ventral valve on top.

Figure TEXT‐FIG. 9..

 Photomicrographs of serial sections Epitomyonia fui sp. nov. imprinted on acetate peels, paratype, NIGP 152516 (same specimen as for Text-fig. 8). A, peel section at 2.7 mm from apex. B–C, peel section at 3.1 mm from apex, with further local enlargement (C), showing dorsal median septum, transversal ridges and coarse punctae.

Derivation of name.  After Fu Lipu, in recognition of his contribution to the study of Palaeozoic brachiopods of north China.

Holotype.  NIGP 152512 (Text-fig. 7P–T).

Paratypes.  Four figured paratypes: NIGP 152513, 152514, 152515, and 152516 (the last serially sectioned, see Text-figs 8–9); 23 paratypes not figured.

Type locality.  Beiguoshan, Longxian County, Shaanxi, north China.

Type horizon.  Soft, recessive unit, Beiguoshan Formation, uppermost Caradoc – lower Ashgill (middle Katian).

Diagnosis.  Shell transversely extended at early growth stage becoming weakly elongate at late growth stage. Ribs (costae and costellae) strong in flanks and sulcus of both ventral and dorsal valves. Median septum in dorsal valve flanked by relatively low, simple, transversal ridges located near mid-length of valve.

Description.  Shell small, with average length 3.9 mm, width 4.6 mm and thickness 2.0 mm; largest shell 6.2 mm long, 6.0 mm wide and 2.8 mm thick; subtrapezoidal in outline, concavoconvex. Hinge line about two-thirds width of shell, forming rectangular ears. Ventral umbo strongly convex, with small, suberect beak. Interarea moderately high, reaching 0.6 mm in relatively large shells, strongly apsacline; delthyrium open. Dorsal umbo flat, with poorly defined beak; dorsal interarea low, about one-quarter height of ventral interarea, approximately orthocline. Sulcus of ventral valve well delimited but usually marked by gentle slopes in transverse section, beginning immediately anterior of umbo, bearing strong medial costa along its entire length (Text-fig. 7B, G, L, Q). Sulcus of dorsal valve poorly defined, beginning as narrow, shallow furrow immediately anterior of umbonal area, then broadening rapidly into a triangular, shallow depression, bounded on each side by gentle ridge along axis of each lobe; medial line marked by interspace. Anteromedian emargination shallow, with rounded apex. Ribs consistently strong over entire shell, with steep slopes and rounded crest, and infrequent bifurcation. Growth lines or lamellae not clearly visible because of calcareous matrix and cement on shell surface.

Ventral interior with vestigial dental plates (Text-fig. 8); teeth large and strong for shell size, with hinge fossettes on inner surfaces. Muscle field not clearly impressed.

Dorsal interior with simple, shallow hinge sockets sitting directly on valve floor. Cardinal process small, weakly bilobate. Crural plates (= inner hinge plates) thin, high, blade-like, extending for some distance anterior of hinge line (Text-figs 8–9). Median septum of dorsal valve beginning immediately anterior of hinge line, terminating near anterior margin, with maximum height in mid-part of shell, touching floor of ventral valve when shell is closed. Transversal ridge relatively low, simple (without crenulations), located on each side of median septum near mid-length of valve (Text-figs 8–9). Shell microstructure coarsely endopunctate (Text-fig. 9).

Remarks.  Several occurrences of Katian forms of Epitomyonia are known from Kazakhstan. For example, Epitomyonia sp. reported by Popov and Cocks (2006) from the Degeres Member of the Dulankara Formation (middle Katian), Chu-Ili Range of Kazakhstan, is represented by poorly preserved material, without information on internal structures, and a dorsal valve that does not seem to have the characteristic curvature of Epitomyonia. They differ from the new species from the Beiguoshan Formation in having finer, more frequently branching ribs, which are weak or absent in the posterior portion of the sulcus. Similarly, the conjoined shells of E. cf. glypha described by Nikitin et al. (2006) from the Odak Beds (upper middle Katian), upper Angrensor Formation, central Kazakhstan have a subcircular outline with finer and more numerous ribs and, judging by the serial sections, very weak transversal ridges. These authors also reported the presence of Dicoelosia sp. from the Koskarasu Beds (lower middle Katian), middle Angrensor Formation but did not show their internal structures, although the shell shape and ribbing pattern of their illustrated specimen (Nikitin et al. 2006, p. 257, fig. 23.1a–e) resembles Epitomyonia fui sp. nov. (compare with Text-fig. 7A–E, which was initially identified also as Dicoelosia).

The new species can be distinguished from other Late Ordovician and early Silurian species of Epitomyonia by having consistently strong, infrequently bifurcating ribs, including a strong medial costa running along the entire length of the ventral sulcus. The transversal dorsal ridges of Epitomyonia fui are simple, low, located near mid-length of the valve, as in EpaucitropidaChen, Jin and Lenz, 2008 from the uppermost Ordovician strata of Mackenzie Mountains, but the latter has a consistently larger, more prominently concavoconvex shell, with somewhat weaker ribs of uneven strength. Epitomyonia relicinaPotter, 1990 from Ashgill rocks of Klamath Mountains, northern California, may have a similar shell size and shape (with shallow-to-relatively deep anteromedian emargination) to the new species, but Erelicina tends to have a notably deeper emargination to make the shell Dicoelosia-like, and much higher transversal ridges in the dorsal interior. The serial sections of EglyphaWright, 1968b from the Boda Limestone revealed higher and more complex transversal ridges than the Beiguoshan species. Other species – such as E. triseptataLenz, 1977 from the lower Silurian (Llandovery to Wenlock) strata of north-western Canada (see also Jin and Chatterton 1997), E. pachytriseptataZhang and Boucot, 1988 from the uppermost Llandovery of Arctic Canada, and some shells of EjiangshanensisZhan and Cocks, 1998 from the uppermost Katian of south China – have a pair of longitudinal ridges flanking the median septum in the dorsal valve.

Acknowledgements.  Fu Lipu of Xi’an Institute of Geology and Mineral Resources generously made his specimens available for this study. Xu Hankui of Nanjing Institute of Geology and Palaeontology (NIGP) assisted in the 1995 field work at Beiguoshan. Pengfei Chen helped with the measurements of some specimens. Wang Guangxu and Liang Yan (NIGP) helped with drafting the map and stratigraphical column. Chen Xu and Zhou Zhiyi (NIGP) provided unpublished biostratigraphical information on the age of the Beiguoshan Formation. This study is funded by Chinese Academy of Sciences (KZCX2-YW-Q05-01), the Ministry of Science and Technology of China (2006FY120300-5), and the National Natural Science Foundation of China (40825006), and the Natural Sciences and Engineering Research Council of Canada.

Editor. Fernando Alvarez