Introduction

The Paleozoic faunas of northern Vietnam have been documented since the time of the French Colonial Survey in the early 20^th Century (Mansuy, 1915). Recent efforts have been made to revise and refine aspects of Vietnamese lower Paleozoic biostratigraphy, in particular in the Lower Ordovician strata of north-east Vietnam (Rushton et al., 2017), and the Silurian strata of central and north-east (same as title) Vietnam (Williams et al., 2016; Saparin et al., 2020). Here we describe a new Late Ordovician graptolite and shelly fauna from rocks of the Phu Ngu Formation in the Na Ri District of north-east Vietnam (Figures 1, 2). The fossils include brachiopods, conulariids, graptolites, orthoconic nautiloids, trilobites, and ostracods, and represent an assemblage entirely new to this region. Graptolites are geographically and stratigraphically widespread throughout the Ordovician, Silurian, and Lower Devonian rocks of Vietnam and have been used to help establish a biostratigraphic scheme for the country (Nguyen, 2002; Tong and Vu, 2011; Tong et al., 2013). However, much of the existing work is based on specimens that have not been figured, making it difficult to assess the resilience of this biostratigraphic framework. Ostracods are not previously documented from the Ordovician of Vietnam, but elsewhere the group has proven very useful for establishing biogeographical patterns for that period (Schallreuter and Siveter, 1985; Vannier et al., 1989; Williams et al., 2003a).

We provide taxonomic notes for the various fossil groups identified, and a formal description of a new ostracod species. The new fossil data enable a revision of the stratigraphic range of the Phu Ngu Formation in northern Vietnam, which has generally been considered to belong to the Upper Ordovician and lower Silurian (Tong and Vu, 2011; Tong et al., 2013), but with an uncertain lower age constraint.

Figure 1.

Geographical and geological setting of the Phu Ngu Formation exposures examined in this study. A–C, geographic setting of the study area (localities 1 and 2) near Na Dam, Bac Kan Province, northern Vietnam. D, stratigraphic log through localities 1 to 2, with notable fossil occurrences at each locality. E–G, field photographs from localities 1 and 2 illustrating the exposures.

Figure 2.

Stratigraphic setting of the Phu Ngu Formation and correlated lithostratigraphical units within the lower Paleozoic succession of Vietnam (adapted after Tong and Vu, 2011, figure 3.2). The informal term ‘lower Silurian’ means strata of pre-Ludlow Series age.

Geological setting and previous work

The northern part of Vietnam was a component of the South China paleocontinent during the early Paleozoic (Tong and Vu, 2011; Torsvik and Cocks, 2013). Within this region the Bac Bo Province can be divided into three geological zones; the northernmost Eastern Bac Bo Zone contains the Na Ri District of the Bac Kan Province. The Eastern Bac Bo Zone is bound to the south and west by the Western Bac Bo Zone (separated by the Song Chay Fault), and to the east by the Quang Ninh Zone (Figures 1, 2; Tong et al., 2013). The lower Paleozoic Phu Ngu Formation (Figure 2) crops out regionally in the Eastern Bac Bo Zone and comprises a predominantly siliciclastic succession, approximately 2300 m to 2400 m thick at the type section, with three more argillaceous lower subunits and three sandier upper subunits, which incorporate mafic and felsic volcanic rocks, respectively (Tong and Vu, 2011; Tong et al., 2013). The Phu Ngu Formation is thought to have been deposited in a deep marine forearc basin setting on the slope of the Paleotethys Ocean during the Late Ordovician and Silurian (Tong and Vu, 2011; Tong et al., 2013).

In the Na Ri District, the Phu Ngu Formation crops out as approximately 250 m of grey and green-grey mudstones, black mudstones and sandy mudstones (Tong et al., 2013). However, accurately identifying the stratigraphic positions of localities within the Phu Ngu Formation in the Na Ri District is complicated by dense vegetation which limits exposure, and by the likely presence of faults between exposures. Tong et al. (2013) reported several Ordovician and Silurian graptolite taxa from the lower Phu Ngu Formation of the Na Ri District, namely: Climacograptus sp., C. cf. scalaris, Diplograptus, Glyptograptus, and Ptilograptus, but these graptolites were not figured. In addition, Tong and Vu (2011, p. 86) reported but did not figure Dictyonema and Climacograptus (= Amplexograptus according to Riva, 1987) latus from the Na Ri District exposures. The genera Climacograptus, Glyptograptus, and Monoclimacis were also reported from the stratotype section of the Phu Ngu Formation (Tong et al., 2013). Previously, Nguyen (2002) had reported two distinct assemblages from near Na Dam (sometimes referred to as ‘Nazam'), in the Na Ri District: (1) comprising ‘Diplograptus bohemicus’ (likely = Normalograptus ojsuensis in South China and hence in Vietnam, following Chen et al., 2005), ‘Diplograptus' (= Normalograptus) ojsuensis, Glyptograptus daedatus, G. elegantulus, and Normalograptus bicaudatus; and (2) comprising ‘Glyptograptus' (= Normalograptus) persculptus, G. gracilis, G. elegantulus, N. madernii, Dictyonema cf. compactum, and Thallograptus sp. None of these graptolites have been figured.

If the identifications are accurate, the strata bearing ‘Climacograptus’ (= Normalograptus) cf. scalaris and Monoclimacis are likely to be of early Silurian age (Tong et al., 2013). The first assemblage of Nguyen (2002) including N. ojsuensis may belong to the N. extraordinarius-N. ojsuensis Biozone (see Chen et al., 2005), and the second assemblage, including ‘Glyptograptus' (= Normalograptus) persculptus, may belong to the N. persculptus Biozone. These two assemblages would therefore be early and late Hirnantian (Late Ordovician) in age, respectively (Cooper et al., 2012), and these strata of the Phu Ngu Formation would therefore reflect depositional conditions during the icehouse climate of the Hirnantian. Although broadly Late Ordovician to Silurian in age, the precise stratigraphic range of the Phu Ngu Formation remains poorly constrained, as do the ages of individual outcrops of this formation across northern Vietnam (Nguyen, 2002; Tong et al., 2013). This hampers a broader understanding of its along-strike lithofacies variation and temporal evolution.

The fossil specimens examined in this study were collected from two roadside localities, separated by approximately 100 m, in the Na Ri District near the village of Na Dam (Figure 1), close to the ‘Nazam’ Section 25 of Nguyen (2002). At both localities, strata dip at approximately 40° to the east. Locality 1 comprises black mudstones interlaminated with lighter-coloured siltier and fine sand-rich layers, sandwiched between greenish-grey to grey mudstones (Figure 1). This locality yields graptolites, orthoconic nautiloids, and conulariids. Locality 2 (see Figure 1) is stratigraphically above Locality 1 and comprises beige-coloured sandy mudstones that yield graptolites, ostracods, orthoconic nautiloids, conulariids, brachiopods and trilobites. A 5 m stratigraphical interval with no exposure separates the two localities (Figure 1).

Materials, methods, and repositories

Figured specimens from this paper are curated in the collection of the Vietnam National Museum of Nature, Hanoi (Table 1). Graptolites from Locality 1 (Figure 3) are preserved as flattened specimens with a slight tectonic overprint discernible. Graptolite specimens from Locality 2 (Figure 4) are preserved either as flattened remains preserving the periderm, or three-dimensionally by iron hydroxide, likely after pyrite. There is no tectonic overprint on the specimens from Locality 2. The shelly fauna is preserved as moulds, the original shell material having decomposed.

Graptolites and other fossil specimens were examined using reflected light microscopy and, where necessary, were developed using a fine mounted needle. All specimens were photographed using a Canon EOS 5D DSLR camera which was mounted to a Leitz Aristophot apparatus with Leica 12 mm Summar lens for high magnification photography. Graptolite illustrations were drawn in pencil using a camera lucida attached to a Wild Heerbrugg M8 binocular microscope; the pencil drawings were traced in ink and digitised. Photographs and camera lucida drawings were used to make precise measurements of and identify the graptolite specimens. Graptolite biozones are established based on the known biostratigraphical ranges of key species, especially biozonal index species.

Ostracods are preserved as moulds on rock slabs, so silicone casts were made using ‘Silcoset 101'. Due to the heavily weathered rock surface and high porosity, the specimens were first consolidated using a solution of 1% ‘Paraloid B72' in acetone. To further prevent the silicone from adhering to the specimen ‘Ambersil HD’ silicone release agent was applied to the consolidated surface prior to applying the Silcoset with a syringe. Positive internal moulds on the surface of slabs could not be directly imaged with SEM due to the high porosity preventing the use of gold coating. Therefore, epoxy replicas of these specimens were made from the silicone casts, using a black epoxy resin placed in a pressure chamber at 2 bar overnight to reduce bubbles. The resulting silicone and epoxy casts were gold-coated and stereo-images taken using a Hitachi S-3600N environmental scanning electron microscope in the University of Leicester, School of Geography, Geology and the Environment.

Graptolite biostratigraphy

Previous work on the Phu Ngu Formation has identified graptolites of the Normalograptus extraordinarius-N. ojsuensis and N. persculptus biozones (latest Ordovician), as well as the definitively Silurian genus Monoclimacis (Nguyen, 2002; Tong and Vu, 2011; Tong et al., 2013). The graptolite-bearing section examined here is close to Section 25 of Nguyen (2002) who identified two different graptolite assemblages in the Phu Ngu Formation in Na Ri, both of Hirnantian (latest Ordovician) age. We also recovered two different graptolite faunas from the Phu Ngu Formation exposures in Na Ri, a stratigraphically lower fauna from the black mudstones of Locality 1 (Figures 1, 3), and an upper, more diverse, fauna from the sandy mudstones of Locality 2 (Figures 1, 4).

Normalograptids dominate the low-diversity assemblage of Locality 1 (Figure 3). The majority of these are identified as Normalograptus (formerly Glyptograptus) daviesi (Williams, 1982), though other Normalograptus species are present. Graptolites of the dendroid (acanthograptid) Dictyonema are also found at Locality 1, though they are exclusive to the silty and sandy laminae and therefore likely belong to an allochthonous fauna only brought deeper into the basin during events of rapid deposition (Figures 5, 6A, C–E). Species-level identification of these Dictyonema has not been possible, and their use for biostratigraphy is limited. The presence of N. daviesi is indicative of the Dicranograptus clingani Biozone (Zalasiewicz et al., 1995, 2009), which is divided into lower Ensigraptus caudatus and upper Dicellograptus morrisi subzones. Normalograptus daviesi has been used as a marker of the morrisi Subzone in the Southern Uplands of Scotland (Williams, 1982; Zalasiewicz et al., 2009), though a similar form may occur in the underlying caudatus Subzone at Whitland, Wales (Zalasiewicz et al., 1995).

Table 1.

List of figured specimens. Presence of an asterisk means that the specific fossil figured in the part cannot be identified on the counterpart.

Figure 3.

Camera lucida illustrations of graptolites from Locality 1, Phu Ngu Formation. A–C, Normalograptus daviesi (Williams 1982); specimens VNMN.0156c, VNMN.0156d, and VNMN.0161a, respectively; D, Normalograptus? sp. 2; VNMN.0156b, possibly comparable to N. brevis; E, Normalograptus sp. 1; VNMN.0156a. Scale bar: 1 mm.

The graptolite fauna of Locality 2 (Figure 4) has greater taxonomic diversity, though lower specimen abundance, than that of Locality 1. Alongside Climacograptus and Orthograptus species is a single Dicellograptus, tentatively identified as D. flexuosus Lapworth, 1876 that may be indicative of the morrisi Subzone (Zalasiewicz et al., 1995), though it is found in the underlying caudatus Subzone in southern Wales (Williams et al., 2003b). There are other biostratigraphically informative species at Locality 2, including Climacograptus dorotheus Riva, 1976 which is diagnostic of the morrisi Subzone in the Southern Uplands of Scotland (Zalasiewicz et al., 1995), but is found more widely throughout the clingani Biozone in Wales where the subzones are not well established (Williams et al., 2003b). The occurrence of a specimen likely referable to Orthograptus truncatus pauperatus Elles and Wood, 1907 provides further support for a clingani Biozone age, though as this species also occurs throughout both subzones in Wales and possibly also in Scotland (Williams, 1982; Williams et al., 2003b; Zalasiewicz et al., 2009) it does not help refine the graptolitic age estimate of these two localities. The co-occurrence of a low-diversity normalograptid-dominated assemblage and a higher diversity Dicellograptus-climacograptid-orthograptid assemblage may find a comparison in the Upper Ordovician strata of Scotland and Wales (Zalasiewicz et al., 1995; and Williams et al., 2003b, respectively).

Only tentative graptolite biozone assignations can be justified at this stage, but there is sufficient evidence to support revising the graptolite age of the lower part of the Phu Ngu Formation in the Na Ri District from the Normalograptus extraordinarius–N. ojsuensis and N. persculptus graptolite biozones (early and late Hirnantian, respectively) to the Dicranograptus clingani Biozone (early Katian). Although the assemblage of Locality 2 is most strongly associated with the Dicellograptus morrisi Subzone, further collecting is required to fully resolve the subzone affinities of these strata.

Regional correlation based on graptolites

Correlation of lower Paleozoic strata across Vietnam is hampered by a lack of well-documented graptolites and other fossils regionally (but see Figure 2). Although the Tan Mai and Co To formations are generally described as Upper Ordovician to Silurian, no diagnostic taxa younger than the Silurian are known (Tong et al., 2013). The same is true of the Song Ca Formation, farther south in the Viet Laos Region of central Vietnam (Tong and Vu, 2011). The Long Dai Formation in the Viet Laos Region preserves both Ordovician and Silurian graptolite faunas (Tong et al., 2013). Records of Early Ordovician graptolite-bearing strata referred to the Long Dai Formation need further investigation (Tong and Vu, 2011).

Figure 4.

Camera lucida illustrations of graptolites from Locality 2, Phu Ngu Formation. A, B, Dicellograptus flexuosus Lapworth, 1876; VNMN.0180a part and counterpart, respectively; C, Orthograptus truncatus pauperatus Elles and Wood, 1907; VNMN.0186a part; D–F, Orthograptus ex gr. truncatus; VNMN.0172b part, VNMN.0181c part, and VNMN.0175b part, respectively; G, P–R, Climacograptus sp.; VNMN.0188a, VNMN.0182c counterpart, VNMN.0177a, and VNMN.0179a, respectively; H–J, Climacograptus sp. 1; VNMN.0183a part (I–J) and counterpart (H); K, Climacograptus dorotheus Riva, 1976; VNMN.0188b; L–N, Orthograptus sp. 1; L, N VNMN.0182b counterpart, M VNMN.0182b part; O, possible Orthograptus; VNMN.0185a; S, graptolite sp. indet; VNMN.0176a. Scale bars: 0.5 mm (J, N); 1 mm (A–I, K–M, P–S); 2 mm (O).

Correlating these lower Paleozoic strata more widely across the South China paleocontinent is challenging. The Sandbian-Katian boundary in the Yangtze region lies within the non-graptolitic, mostly calcareous, Pagoda Formation (Chen et al., 2010). The Nemagraptus gracilis graptolite Biozone is known from the underlying Miaopo Formation, and the local Dicellograptus elegans Biozone (correlated to the Pleurograptus linearis Biozone) has been identified in the overlying graptolite-sparse Linxiang (Linhsiang) Formation. Chen et al. (2017a) also recorded a slightly higher upper Katian graptolite assemblage of the Dicellograptus complexus Biozone, correlated to the D. ornatus Biozone, from the Wufeng Formation, and the ornatus Biozone has also been recognised in the uppermost Linxiang and Wufeng formations in a drill core from the Yichang region of South China (Maletz et al., 2019). However, graptolites of the highest Sandbian and lowest Katian biozones have not been recovered from these successions in South China (Chen et al., 2010; Maletz et al., 2019). Therefore, strata of the Phu Ngu Formation provide an important early Katian graptolite age constraint on the South China paleocontinent.

Figure 5.

Graptolite taphonomy and sedimentology at Locality 1, Phu Ngu Formation. A, B, Modes of graptolite occurrence on the bedding planes; C–E, Vertical thin sections of black mudstone (F); F, G, Vertical cross-sections of black mudstone intercalating with very fine sandstone (V. F. S.) and coarse siltstone (C. S.) layers. Normalograptus (n) occur in the dark-coloured background mudstone. Dictyonema (d) occur in the lighter-coloured coarse siltstone laminae that are interpreted as distal turbidite deposits. The thicker, several millimetre- to centimetre-scale, beds of light-coloured very fine sandstone are also interpreted as distal turbidite deposits.

Figure 6.

Non-graptoloid fauna of Locality 1, Phu Ngu Formation. A, C–E, dendroid Dictyonema sp.; A, VNMN.0152a; C, VNMN.0151a part; D, magnified detail of E, showing dissepiments; E, VNMN.0150a part (black rectangle represents area magnified in D); B, conulariid, Conulariella?; VNMN.0155a. All scale bars are 10 mm.

Correlation with strata of the North China paleocontinent is more promising. Dicellograptus flexuosus is known from the Climacograptus spiniferus Biozone of North China, coming in above the Ensigraptus caudatus Biozone (Chen et al., 2017b). The caudatus and spiniferus biozones of Australasia are correlated with the caudatus and morrisi subzones of the Dicranograptus clingani Biozone of Britain (Cooper et al., 2012). This provides further support for the hypothesis that Locality 2 lies within the upper part of the clingani Biozone. However, species of this interval in the Australasian region, such as Amplexograptus praetypicalis Riva, 1987, Climacograptus (Diplacanthograptus) lanceolatus VandenBerg, 1990, and Climacograptus (Diplacanthograptus) spiniferus Ruedemann, 1912, have not been identified in our localities, so a firm identification with the spiniferus Biozone/morrisi Subzone cannot currently be supported.

Figure 7.

Orthoconic nautiloid, tentatively referred to Michelinoceras sp. from Locality 1, Phu Ngu Formation. A, VNMN.0209a; B, VNMN.0210c (silicone cast); C, VNMN.0210d (silicone cast), and D, VNMN. 0211a; E, mould of VNMN.0212a with remnant of siphuncle highlighted (black arrow). All scale bars are 10 mm.

Non-graptolite fauna of the Phu Ngu Formation

Brachiopods, conulariids, orthoconic nautiloids, ostracods, and fragments of trilobites have been recovered from the Phu Ngu Formation, in addition to graptolites. The graptolite and non-graptolite faunas of Locality 1 are similarly depauperate. There is a single conulariid specimen (Figure 6B) from the Dictyonema-bearing coarser-grained laminae, and abundant orthocone nautiloids that occur in both the sandier-siltier and mud-rich layers. The conulariid specimen is at least 62 mm long, missing the basal attachment, and is preserved as a flattened mould stained orange-brown, presumably iron hydroxides after pyrite. Although the state of preservation hampers robust identification of this specimen, it shares characteristics, such as a lack of nodes on the transverse ribs and no evidence of inflection of the ribs at the interradii, with the genus Conulariella which is known from Lower and Middle Ordovician strata in South China (Van Iten et al., 2013). The orthoconic nautiloids at Locality 1 (Figure 7A–E) likely belong to the genus Michelinoceras, though there is insufficient siphuncular detail visible to allow a more precise identification.

The non-graptolite fauna of Locality 2 is more diverse than that of Locality 1, and includes brachiopods, conulariids, orthocone nautiloids, ostracods, and trilobite fragments (Figures 8, 9). The orthoconic nautiloids (Figure 8D) are more poorly preserved than those from Locality 1 but might also belong to Michelinoceras. The conulariids from Locality 2 are smaller than the specimen from Locality 1, though again poor preservation hampers identification. The brachiopods from Locality 2 are preserved as moulds (Figure 8A–C). They are all rhynchonelliforms, two of which are strophomenids, possibly belonging to the cosmopolitan Ordovician genera Leptellina (Figure 8B) and Chonetoidea (Figure 8C; see e.g. Zhan et al., 2008; Popov and Cocks, 2014). A third specimen (Figure 8A) resembles an orthid brachiopod in gross morphology, though taxonomically useful features are obscured by an overlapping conulariid.

Figure 8.

Non-graptolite fauna of Locality 2, Phu Ngu Formation. A, Possible orthid brachiopod; VNMN.0181a part. B, Dorsal external mould of a strophomenid brachiopod, tentatively identified as Leptellina?, from its strongly concavo-convex profile, sub-semicircular outline, and unequal parvicostellate ornament; VNMN.0184a part. C, Ventral internal mould of a strophomenid brachiopod, possibly Chonetoidea?, from the radial ornament, weakly parvicostellate towards the margin, pronounced medium septum and radially arranged septules; VNMN.0182a part. D, Orthoconic nautiloid; VNMN.0184c part. E, F, Conulariid, black arrow (E) indicates the brachiopod enlarged in (A); VNMN.0181d part and counterpart, respectively. Scale bars: 5 mm (A, C–F); 1 mm (B).

The ostracods are podocopes. The fauna is notable for the presence of a new species of Kinnekullea (Figure 9A–C). This genus is known from several species of Late Ordovician age, especially in the Baltic region. Other ostracod taxa include Ordovizona? sp. (Figure 9D, F) and Laterophores sp. (Figure 9E).

Taphonomy and paleoecology of the fauna

Both dendroid and graptoloid graptolites are present at Locality 1, though they are preserved in different sedimentary layers. The graptoloids are densely packed on bedding planes in black mudstone layers (Figure 5). Their periderm has degraded and they are now preserved as mineral films, presumably of clay. These diplograptids sensu lato possess straight biserial rhabdosomes and locally show a preferred orientation, indicating some remnant current activity during deposition. In contrast, the dendroid Dictyonema occurs in the yellow-weathering, thin coarse silty, silty-sand, and fine sand-rich laminae (Figures 5, 6A, C–E), some of which may show low-angle cross- and parallel-lamination. The conulariid and the better preserved orthoconic nautiloids of this locality also occur in the silty and sandy laminae. Orthoconic nautiloids are also found in the black mudstone sedimentary layers, though they are not well-preserved. The Dictyonema are mostly complete and are preserved as carbonaceous remnants of the original periderm.

Figure 9.

SEM micrographs of ostracods from Locality 2, Phu Ngu Formation. All images are stereo-pairs and lateral views A, B, Kinnekullea gaia sp. nov.; A, holotype right valve (VNMN.0182f part); B, left valve (VNMN.0182e part); C, Kinnekullea gaia?, incomplete left? valve (VNMN.0172e part); D, F, Ordovizona? sp.; D, right valve (VNMN.0182i part), partly obscured anteriorly; F, juvenile valve, with possible velum developed ventrally (VNMN.0167b); E, Latephores sp. left valve (VNMN.0182l part). All specimens are silicone casts of external moulds. All scale bars are 100 µm.

At Locality 2, graptoloids occur sparsely in yellow-weathered mudstones and thin silty sandstones of turbiditic origin. Most of the graptoloids preserve a delicate carbonaceous layer, after the original periderm, and in some cases the three-dimensional structure is preserved by iron hydroxide, presumably after pyrite, filling the specimen cavities. Many graptoloids are nearly complete, though several miss their proximal end, which hampers firm identification. The shelly fauna at Locality 2 also includes weakly biomineralised phosphatic conulariids, which at both localities are preserved by iron hydroxide, presumably after pyrite. The originally calcareous taxa, including trilobites, brachiopods, orthoconic nautiloids and abundant ostracods, have lost their calcite shells and are moulds with or without a coating of iron hydroxide, presumably after pyrite.

The graptolite assemblages suggest a deep marine shelf or slope setting for the Phu Ngu Formation at Na Ri. Overall this area appears to have had limited connection to the open ocean as signaled by the low diversity of the diplograptid fauna in the mudstones at Locality 1. Similar, low-diversity diplograptid faunas of this age have been documented elsewhere in Ordovician marine shelf settings, for example in the Welsh Basin on the Avalonian microcontinent (Zalasiewicz et al., 1995; Williams et al., 2003b). The greater diversity of the graptolite fauna at Locality 2, which includes a single Dicellograptus, might signal greater oceanic influences (Williams et al., 2003b), but notably these graptolites are also associated with an ostracod assemblage – albeit likely transported – that includes palaeocopids that strongly suggest a marine shelf (cf. Vannier et al., 1989; Williams et al., 2003a). Mixed ostracod and graptolite assemblages are noted elsewhere in mud- and silt-rich lithofacies of the Late Ordovician, for example in the Girvan area of southern Scotland, where they occur in deep shelf or continental slope settings of the early Paleozoic Laurentian margin (e.g. Floyd et al., 1999; Mohibullah et al., 2011). A similar setting might be envisaged in Na Ri during the Late Ordovician and would be consistent with the forearc basin setting interpreted for the Phu Ngu Formation (Tong and Vu, 2011; Tong et al., 2013), with the conulariids, shelly fauna, and Dictyonema representing a more proximal benthic biota washed into this deeper marine setting by turbidity currents. Unfortunately, the absence of precise biostratigraphical data elsewhere in north-east Vietnam makes it difficult to correlate the lithofacies of the Phu Ngu Formation at Na Ri over a wider area. Subsequent work should attempt targeted and systematic collection for graptolites and may be fundamental for constraining the overall age of the Phu Ngu Formation, and for identifying facies changes across its basin of deposition.

Taxonomic notes on the graptolite and shelly fauna

Conclusions

This first report of Late Ordovician, Katian-age graptolites, ostracods and other shelly fauna from the Phu Ngu Formation, Na Ri District, north-east Vietnam, includes illustrations of the biostratigraphically important species. We revise the lower graptolite age constraint of the Phu Ngu Formation from the Normalograptus extraordinarius-N. ojsuensis Biozone (Hirnantian) to the Dicranograptus clingani Biozone (Katian). Ostracods from the Phu Ngu Formation include Kinnekullea gaia and species of Laterophores and possible Ordovizona, that are consistent with a Late Ordovician age. Our work suggests that, with further collecting, a more complete Late Ordovician through early Silurian succession may be identified within the Phu Ngu Formation.