LATE SILURIAN MARINE SHELLY FAUNA OF CENTRAL AND NORTHERN VIETNAM Th a nh TONG-DZUY, Ar t h ur J BOUCOT, J i a -y u RONG & Z o n g -j i e FANG TONG-DZUY T., BOUCOT A.J., RONG J.Y & FANG Z.J 2001 Late Silurian marine shelly fauna of Central and Northern Vietnam [Une faune coquillière marine du Silurien supérieur du centre et du nord du Vietnam] GEO BIOS, 34, 3: 315-338 Villeurbanne, le 31.07.2001 Manuscrit déposé le 20.07.1999; accepté définitivement le 10.11.2000 ABSTRACT -The Late Silurian (late Ludlow-Pridoli) brachiopods and bivalves from North and Central Vietnam are described and illustrated from the Kien An (North Vietnam) and My Due (Central Vietnam) localities The biostratigraphic relations and age of the fauna, the so-called Retziella fauna, are discussed The My Due fauna is the same as that at Kien An, indicating that during the Late Silurian both North Vietnam and Central Vietnam, situated on the South China and Indochina Plates respectively were probably fairly close geographically to each other The Late Silurian is globally a time of moderately high, although not highest, provincialism as regards marine benthic inver tebrates A new brachiopod species, Nikiforovaena vietnamensis B o u c o t & Rong, and three new bivalve species Schizodus kienanensis Fang, S ? myducensis F ang, and Modiomorpha paracrypta F an g are described © 2001 Édi tions scientifiques et médicales Elsevier SAS KEYWORDS: BRACHIOPODS, BIVALVES, NEW TAXA, LATE SILURIAN, BIOGEOGRAPHY, VIETNAM RESUME - Les brachiopodes et les bivalves du Silurien supérieur (Ludlow term inal-Pridoli) des localités de Kien An (Nord Vietnam) et de My Duc (Centre Vietnam) sont décrits et figurés L’âge de la faune Retziella et les relations biostratigraphiques sont discutés La faune de My Duc est la même que celle de Kien An, indiquent que pendant la fin du Silurien le Nord Vietnam et le Sud Vietnam, situés respective m ent sur les plaques Sud-Chine et Indochine, étaient probablement assez proches l’une de l’autre La fin du Silurien est globalement une période de provincialisme marqué, bien que n’é ta n t pas le plus accentué, pour les invertébrés benthiques m arins Une nouvelle espèce de brachiopodes, Nikiforovaena vietnamensis B o u c o t & R o n g , et trois nouvelles espèces de bivalves Schizodus kienanensis F a n g , S ? myducensis F a n g , et Modiomorpha paracrypta F a n g sont décrites © 2001 Editions scientifiques et médicales Elsevier SAS MOTS-CLÉS: BRACHIOPODES, BIVALVES, NOUVEAUX TAXONS, SILURIEN SUPÉRIEUR, BIOGÉOGRAPHIE, VIETNAM INTRODUCTION The Retziella fauna has been known for some time from South China, North China, C entral Asia, and Australia, based on specimens w eathered out of cal careous shales for the m ostpart The occurrences in northern Vietnam have not previously been illus trated or described in detail The Vietnamese bra chiopods described here occur as casts and molds which substantially supplem ent the earlier des criptions of the interiors based mostly on serial sec tions The paleoecological significance and overall biogeographic significance of the Retziella fauna were considered by Rong Jia-yu et al (1995) The bivalves accompanying the Retziella fauna brachio pods have not previously received m uch attention GEOLOGICAL SETTING Silurian rocks w ith Retziella weberi have been known in N orth and C entral Vietnam for some time First, Duong Xuan Hao and'Nikiforova (1963) published on the My Due (Quang Binh Province, Central Vietnam; 17°14'N, 106°41'E) occurrence of Retziella weberi, and later occurrences were found in the downstream region of the Da River, and at Kien An (20°48'N, 106°35'E) near Hai Phong in North Vietnam The Retziella weberi faunas described in this paper were collected by Tong-Dzuy T hanh and A.J Boucot in 1994 a t My Due (Quang Binh Province, Central Vietnam) and a t Kien An near Hai Phong (North Vietnam) (Figs 1A-D) KIEN AN AREA Patte (1926, 1927) first described the Paleozoic faunal assemblage a t the Tien Hoi, Xuan Son and Phu Lien M ountains of the Kien An area, some five kilo meters, as the crow flies, from H Phong City, as Late Devonian w ith Spirifer cf ziczac R o e m e r and Rhynchonella sp A I Jam oida (in Dovjikov 1965) discovered brachiopods in the Xuan Son M ountain th a t Duong Xuan Hao and Nikiforova (1963) iden tified as Retziella weberi, Eospirifer cf lynxoides, and corals (Nipponophyllum sp., Xiphelasma sp.) 316 - A Map showing loca tions within tie Kien An area (Xuan Son, Phu Lien, Kho Lam and Cuu Vien Mountains) The fauna from the Xuan Son Mountain is described in this paper B Stratigraphie column for the Xuan Son M ountain locality, indicating where the Retziella weberi fauna was col lected th at is described in this paper C Map showing loca tions within the My Due area, Central Vietnam, and the spot where the Retziella weberi fauna described in this paper was collected D Stratigraphie column for the My Due area, indicating where the Retziella weberi fauna was collected th at is described in this paper All figured specimens are from My Due The specimens described in this paper are deposited at the Hanoi Geological Museum, Collection Number BT 178 All figured specimens of brachiopods in Figs 2-5 are from My Due The others are noted where appropriate in the figure legends A Carte des localités dans la région de Kien A n (Montagnes de Xuan Son, Phu Lien, Kho Lam et Cuu Vien) La faune de la montagne de Xuan Son est décrite dans cet article B Colonne stratigraphique de la localité de Xuan Son mon trant la position de la faune Retziella weberi C Carte des affleurements de la région de My Duc, Vietnam central et localisation du gisem ent Retziella weberi D Colonne stratigraphique de la région de My Duc avec le positionnement de la faune Retziella weberi Tous les spécimens figurés pro viennent de My Duc Les spéci mens décrits dans cet article sont déposés au Musée Géolo gique de Hanoï, sous le numéro de collection BT178 Tous les brachiopodes des figures 2-5 proviennent de My Duc Les autres localités sont indiquées dans les légendes des figures Fig u r e B -100 m Ash-gray limestone bearing Famennian Foraminifers, Amphipora Ị axepeđorata Marl, dark gray limestone lenses: / Uesotavositessp., Holmophyllum sp„ Mpponophyllumsp., Retziella weberi, Nikiforovaena ferganensis Intercalation of quartz sandstone and mudstone: Cu Bai Formation Form ation Dai Giang Formation Long Dai Formation W4 -400 m Howellella ett bragensis -120 m Calcareous mudstone, limestone lenses, thin beds of shale: Microplasma sp., Nikiforovaena fergana, (Retziella weberi in Phu Lien Ml) Dark gray limestone Marl at base of section, Stromatoporoids, Corals, BrachioDOds Sandstone, red or wine colored 300 rn mudstone with Unaula Sandtone, mudstone, marl and 400 limestone at upper of section 450 m Brachiopods Tribbites, Corals, Vertebrates 450 Sandstone, mudstone 500 m Brachiopods (first Retziella weberi) Long Dai Formation (Upper Ordovician-Lower Silurian) mudstone and datfc gray Dai Giang Formation (Silurian) Tan Lam Formation intercalation of mudstone and (Lower Devonian) sandstone with mart tenses sandstone and red and limestone at base or wine colored mudstone U r o e r 'l£ S ) t a S limestone, intercalation of , t(W1 ^ ri limestone mudstone siliceous mutjSj0| le ’ ‘ Sandstone, shale Graptolites Pliocene-Quaternary Basalt Nguyen Quang Hap (1967) nam ed these rocks the Kien An Suite (Formation), while Hoang Ngoc Ky et al., and Ngo Quang Toan et al (Vu Khuc & Bui Phu My 1989) renam ed this u nit the Xuan Son Formation Priority rests w ith the term Kien An Formation In the Xuan Son M ountain the tripartite rock sequence is as follows: Lower p art consists of bluegray m arl, m udstone and yellowish-gray sandstone bearing Eospirifer cf lynxoides and indeterm inate rugose corals The thickness is about 120 meters; following are gray sandstone and quartz sandstone with dark purplish-red m udstone interbeds having about a 320 m eter thickness These mudstones yiel ded Retziella weberi, Nikiforovaena ferganensis and I H Quaternary and recent alluvium ^ F°m' a‘i°n (Permo-Carbomferous) 9fay limestone ! T Fossil tocalitv Howellella sp The upper p a rt of the Section on the N orth slope of Xuan Son M ountain consists of thick bedded dark gray limestone with shale and m arl interbeds in the upperm ost layers, where we collec ted an abundant Retziella weberi assemblage; the thickness of this unit us about 100 m At the same level on the north slope of Tien Hoi M ountain the following brachiopods were noted: Retziella weberi, Eospirifer cf lynxoides (= Nikiforovaena ferganen sis), Howellella bragensis, Howellella sp (Vu Khuc & Bui Phu My 1989) Additionally, corals are abun dant, including Favosites admirabilis, Xiphelasma su., Nipponophyllum sp.; and recently Mesofavosites was identified by Tong-Dzuy Thanh Commonly the upper pa rt of the Kien An Form ation consists of 317 dark gray, thick-bedded limestones w ith some ashgray shale and m arl interbeds, reaching a thick ness of about 320-400 meters 1) Lower p art consisting of interbedded sandstone and mudstone which reach a thickness of 550 meters At Phu Lien M ountain the cross-section is shorter, w ith only two beds of the formation present, as fol lows: brownish-yellow sandstone, siliceous shale and yellowish, purple-red, cross bedded mudstone at the top, w ith a thickness of about 200 m eters, yielding poorly preserved brachiopods including Retziella weberi, Nikiforovaena ferganensis, Eospirifer ? sp., Howellella sp., and Camarotoechia sp ( unidentified rhynchonellid) Intercalations of yello wish sandstone and gray m arl w ith calcareous mudstone having a thickness of about 50 m eters, with poorly preserved fossils (Retziella weberi, Eospirifer ? sp., Howellella sp and rhynchonellids) have been noted (Vu Khuc & Bui Phu My 1989) 2) A middle p art consisting of dark gray, thin bed ded m arl and calcareous m udstone with some limestone interbeds, w ith a thickness of about 220 450 meters The majority of the fossils cited above indicate a Late Silurian age, but some are close to the Lower Devonian, for example, Favosites admirabilis No conodonts have been found Based on the brachio pods described here the Kien An area Retziella weberi bearing rocks are dated as Late Silurian, Late Ludlow or Pridoli THE DOWNSTREAM DA RIVER BASIN In the downstream Da River Basin, w estern North Vietnam, the Retziella weberi assocation has been found in the Bo Hieng Formation This formation consists of m arl, calcareous shale and thin bedded, dark gray limestone w ith a thickness of about 500 to 900 m eters It lies between the Sinh Vinh Formation (Ordovician-Silurian) and the Song Mua Formation (Early Devonian) Like the fossil assem blage in the Kien An Formation, the Retziella webe ri association in the Bo Hieng Formation consists mainly of Silurian brachiopods (Retziella weberi, Tadschikia xuanbaoi, Lissatrypa sp (probably an Atrypoidea), Fardenia ? sp.) and bivalves (Modiomorpha brevis and others) In addition, some corals ordinarily considered to be of Devonian age such as Favosites kunjakensis, Squameofavosites ex gr cechicus (S q f bohemicus), Tryplasma ex gr karcevae and others are present, although their age here is Late Silurian ' MY DUC AREA In the My Due area (Quang Binh Province, Central Vietnam) the Retziella weberi association was col lected from the upper p a rt of the Dai Giang Formation A.M Mareishev and T ran Due Luong (Dovjikov ed 1965) first described the Dai Giang Formation, and then Nguyen Xuan Duong et al (1996) studied it in detail The formation consists mainly of fine-grained sandstone, m udstone and shale, which crop out widely in Quang Tri and sou thern Quang Binh Provinces, C entral Vietnam (Fig 1) In 1977 Nguyen Xuan Duong described the best section of this formation, along the Dai Giang River Three parts of the formation are cited here from the eight members described by Nguyen Xuan Duong 3) An upper p art consisting of m udstone with sand stone interbeds w ith a thickness reaching 250 meters The total thickness of the formation is more than 1,000 meters Abundant fossil associations have been collected from different horizons a t different localities Among them the most characteristic are graptolites (Monograptus sp., Bohemograptus bohemicus, Monoclimacis sp., Pristiograptus ludlovensis), brachio pods (Retziella weberi, Nikiforovena ferganensis, Howellella nucula), trilobites (Metacalymene sp., Cromus beaumonti, Encrinurus sinicus) and corals (Multisolenia cf formosa, Nipponophyllum anmaense), indicating a Silurian, mostly Late Silurian age In the My Due area, near An Ma M ountain and the Cam Ly Reservoir, west of the My Due railway sta tion (Fig 1C), the sequence consists of a lower member which is composed of fine-grained sandsto ne and mudstone, th a t becomes coarser upwards, with a thickness of about 450-500 m eters, and an upper member which is largely arenaceous, but becomes somewhat carbonaceous in its uppermost part, w ith a thickness of about 400-450 m eters (Fig ID) It is possible th a t these two members corres pond to the above mentioned lower and middle parts of the formation They were formerly dated as Middle Devonian (Fromaget 1927; Dovjikov ed 1965) Later, thanks to the collection of Retziella weberi assemblage brachiopods, such as Retziella weberi, N ikiforovaena ferganensis, Howellella nucula, trilobites (E ncrinurus cf sinicus) and corals (Multisolenia cf formosa, Nipponophyllum anmaense) these beds in the My Due area were assigned to the Late Silurian Dai Giang Formation (Tran Van Tri et al 1977; Vu Khuc & Bui Phu My 1989; Nguyen Xuan Duong et al 1996) The age of the Dai Giang Form ation has been the subject of debate in the Geology of Central Viet Nam Dovjikov ed (1965) and T ran Van Tri et al (1977) assigned it to the Silurian, whereas Nguyen Xuan Duong et al (1996,1977, in lit.), Vu Khuc and Bui Phu My (1989) dated it as Late Silurian-Early Devonian The Late Silurian-Early Devonian assi gnment was supported by the ‘majority of fossils collected in the Dai Giang Formation which indica te a Late Silurian age; but apart from these Late S ilurian fossils, in some cross-sections Early Devonian fossils are also noted’ (Vu Khuc & Bui Phu My 1989) The authors did not provide a list of ‘Early Devonian fossils’, while all the fossils cited in their description, including graptolites, trilobites and brachiopods not support an Early Devonian age for even the top of the formation It is notable th at in the top of some cross-sections the collected fossils indicate a Late Silurian age only including, for example, trilobites (Encrinurus cf sinicus), 318 graptolites (Bohemograptus bohemicus, Pristiograptus ludlovensis), and brachiopods (Retziella weberi and others) Thus, the Retziella weberi bea ring rocks of Central Vietnam m ust be correlated with those in N orth Vietnam (Kien An area and the downstream p a rt of the Da River Basin); all yield Late Silurian fossils AGE Although no conodonts are found in the studied areas in Vietnam, based on regional stratigraphic correlation and the range of the brachiopod species, in particular, Retziella weberi, R alaica ‘Howellella’ lynxoides, and Nikiforovaena, our conclusion for the age of this Vietnam ese fauna is th a t it is of late Ludlow to Pridoli age Additionally, the presence of earlier Ludlow grap tolites below the Retziella fauna in the My Due area is consistent w ith a later Ludlow regional shallo wing, i.e., upward replacem ent of the deeper water graptolitic facies by the BA 2-3 shelly facies in coar ser clastics, followed in the earlier Devonian by nonm arine beds In the Kien An area graptolitic beds, unfortunately, are unknown, but to the nor theast in the offshore islands the upper parts of the Co To Formation have yielded earlier Ludlow grap tolites, with all of this being consistent w ith the assignm ent of the Kien An and My Due shelly fau nas to the la te r Ludlow-Pridoli The Co To Formation, w ith citations to it's Ludlow graptolites, has been described by Nguyen Huy Mac and Pham The hien (1972) and Tran Van Tri et al (1972) No conodonts have yet been found from the Retziella-bearing stra ta in Central Asia The most abundant species in the fauna in North Vietnam is Retziella weberi N i k i f o r o v a which occurs in Ludlow to Pridoli rocks in C entral Asia, although the type specimens came from th e Isfara Horizon of Pridolian age In the western p a rt of South China Retziella-bea ring stra ta are well developed, and their age was determined as late Ludlow to early Pridoli by Rong and Yang (1980, p 264) chiefly based on the study of the brachiopods from the Miaokao Formation, Qujing, eastern Y unnan, Southw est China It should be pointed out th a t there is conodont evi dence; Ozarkodina crispa and others were found in the upper K uanti, M iaokao, and Y ulungsuu Formations indicating late Ludlow to early Pridoli age (Wang 1980, 1981; W alliser & Wang 1989) REMARKS The occurrence of the Retziella weberi fauna a t the three localities noted in this paper, two (Kien An area and downstream p a rt of the Da River Basin) are north of the Song Ma Suture on the South China Plate, or Guangxi-Yunnan-North Vietnam Block (Wu 2000), and one (My Due area) south of the Song Ma suture on the Indochina Plate, argues for these two areas having been reasonably close to each other during the Late Silurian The situation for the Early Devonian is more difficult to inter pret N orth of the Song Ma Suture w ithin North Vietnam there is a characteristic South China Region, Old World Realm fauna of m arine benthos, as well as vertebrates sim ilar to those present in South China However, south of the Song Ma Suture there is no biogeographically useful infor m ation concerning m arine invertebrates, although the information about the vertebrates does feature taxa distinct from those of the South China Region in both N orth Vietnam and South China TongDzuy T hanh has discussed the relation of the known Vietnamese Early Devonian faunas with those p resen t in th e R henish-B ohem ian and Tasm an regions (Tong-Dzuy Thanh et al 1985, 1996, 1997) PALEOGEOGRAPHY Trying to work out the paleogeographic and lithofacies relations of the Chinese and Vietnamese Retziella fauna localities is difficult owing to lack of truly compelling data However, we will attem pt to provide some insight into the varied possibilities First, it needs to be recognized th a t lateral move m ents on the Song Ma Suture and the Red River F ault Zone may well have significantly displaced the My Due region, belonging to the Indochina Plate, relative to the Vietnamese and Chinese loca lities to the north th a t all belong to the South China Plate We will first discuss the gross lithological sequence a t the localities In the My Due region, as discussed above, the Retziella fauna is from the relatively shallow w ater, upper p art of the Dai Giang Formation, which is underlain by Silurian graptolitic shales and overlain by upperm ost siliciclastics th a t have yielded Silurian trilobites, which are overlain in tu rn by relatively unfossiliferous siliciclastics of Old Red Sandstone aspect (Tan Lam Formation) th a t probably represent the Early Devonian in la r gest part, overlain themselves by younger Devo nian, fossiliferous limestones In the Kien An area, near Hai Phong, there are Silurian graptolitic shales in the small islands nor theast of Hai Phong, and earlier Devonian nonm a rine beds to the south of the city However, the com plex structural relations in the Hai Phong region make the working out of a detailed lithostratigraphic sequence difficult In the Da River Basin, well to the west of H a Noi the Bo Hieng Formation w ith its Retziella fauna, is overlain by typical South China Region, fully m ari ne, probably Benthic Assemblage 3, fossiliferous, Early Devonian beds of the Song M ua Formation, and underlain by the relatively unfossiliferous Ordovician-Silurian Sinh Vinh Formation In northern Vietnam, in the Lang Son region, non m arine Early Devonian strata, followed by m arine Early Devonian, lie unconformably on Ordovician beds, w ith fossiliferous S ilurian unrecognized 319 These relations are sim ilar to those to the nor theast in the nearby Liujing area, Hengxian Coun ty, Guangxi where the m arine Early Devonian, richly fossiliferous, Benthic Assemblage through L ianhuashan Formation lies unconformably on Cam brian strata Silurian beds w ith m arine fossils are unrecognized in this region F ar to the north in the Qujing area of eastern Yunnan, the Retziella fauna of the Miaokao Form a tion is overlain by the Pridoli, m arine Yulungsuu Formation, followed by paralic Early Devonian stra ta (Wang Nian-zhong 1997, concludes th a t the lower p art of these beds may be of late Pridoli age), and underlain by the early-middle Ludlow K uanti Formation of m arine aspect, which also yields a very low diversity Retziella fauna Trying to tie these very scattered bits of informa tion together paleogeographically suggests the pre sence of an Early Devonian land area from Qujing on the north, south to Liujing and nearby Lang Son, w ith an intervening region of m arine environ m ents present further south in N orth Vietnam, while in the My Due area there is another land area during the E arly Devonian In Late Silurian, Retziella fauna tim e there was shallow sea everyw here except in the Liujing and adjacent Lang Son areas where land may have been present Aul of this is complicated, of course, in term s of how much lateral movement may have taken place on the Song Ma Suture and the Red River Fault Zone The occurrences of the Retziella weberi fauna noted in this paper are located in three localities Two (Kien An area and downstream p art of the Da River Basin) are north of the Song Ma Suture, or in other words, are p a rt of the South China Plate, whereas the third (My Due area) is located on the Indochina P late The reasonably endem ic Late S ilurian Retziella weberi fauna occurs on both sides of the im portant Song Ma S uture The shelly Early Devonian (including the highly endemic Dicoelostrophia faunas of N orth Vietnam and adjacent Guangxi Province, so characteristic of the South China Region; Hou & Xian 1975; W ang & Rong 1986; Wang et al 1987) and Eifelian, earlier Middle Devonian faunas north of the Song Ma Suture are of South China Region, Old World Realm aspect (Benthic Assemblage 5, deep w ater, Eifelian faunas were collected in 1997 a t Na Ri in N orth Vietnam; they are sim ilar to faunas of the same age in sou thw estern Guangxi, near Nanning, and are now being studied) Until rich shelly Early Devonian and Eifelian m arine faunas are discovered and col lected south of the Song Ma Suture we will be uncertain of the earlier Devonian situation MATERIAL The Late Silurian fossils described here all occur as casts and molds in heavily w eathered, arenaceous mudstone They came from collections made a t two localities The first includes 95 pedicle valves and 125 brachial valves of species of brachiopods from the Xuan Son Formation in a quarry near the town of Kien An, near Hai Phong, northern Vietnam The second, a much larger collection, contains 1777 pedicle valves and 1391 brachial valves, belonging to brachiopod species from the Dai Giang Form a tion of the same age a t a locality near My Due, north-central Vietnam 679 specimens were collected near Kien An, with 221 brachiopods (32.7 % of the entire association) Among the brachiopods (including one specimen w ith conjoined valves), the commonest species is Retziella weberi N i k i f o r o v a (62.6 % of the brachio pod total) The other brachiopods are ‘Howellella’ lynxoides (N i k i f o r o v a ) (18.5 %), Nikiforovaena vietnamensis B o u c o t & R o n g , nov sp (9.5 %), and Retziella alaica N i k i f o r o v a (9.5 %) The brachio pods are associated w ith m any bivalves (total 389 specimens, 57.3 %), the commonest taxon in this association, a distinctive feature of this collection Some gastropods (64 specimens, 9.4 %), trilobites (2 pygidia and free check, 0.044 %), and orthoceratid (0.015 %) are also present No other fossils were noted In the My Due area, 3459 fossils were gathered Among them there occur 3203 brachiopod valves (including 35 specimens w ith conjoined valves), 94.13 % of the entire association W ithin the bra chiopods, Retziella weberi is again the most abun dant species, including 43.9 % of the entire bra chiopod association, much less th an in the Hai Phong area collection The other two abundant taxa are Nikiforovaena vietnamensis B o u c o t & R o n g , nov sp (23.8 %) and ‘Howellella’ lynxoides (N i k i f o r o v a ) (21.0 %), higher percentages than present in the Hai Phong area Retziella alaica N i k i f o r o v a (9.8 %) is fourth, and the percentage of this species is close to th a t in the Kien An area collection Another five species are rare, each less than 30 valves w ith Atrypoidea (0.9 %), Morinorhynchus (0.1 %), Reticulatrypa, Spirinella ?, and gen et sp indet (each 0.03 %) None of these rare species has been found at the Kien An area locality The My Due area brachiopods occur w ith various other m arine benthic invertebrates, including bivalves (2.54 % of the whole association), gastropods (2.25 %), tabulates (0.03 %), rugose corals (0.12 %), trilo bites (0.61 %), bryozoans and ostracodes (each 0.14 %), pelmatozoan debris, and vertebrates (0.52 %), and a few other undeterm inable fossils (0.06 %) These last forms are relatively rare in number The following fossils are recorded from the locality near Kien An, northern Vietnam Taxa Brachiopoda Retziella weberi N i k i f o r o v a Retziella alaica N i k i f o r o v a ‘Howellella' c f lynxoides ( N i k i f o r o v a ) Nikiforovaena vietnamensis B o uc o t & Ro n g , nov sp Brachial valves Conjoined valves 95 51 10 21 125 86 11 20 1 13 Pedicle valves Bivalves 389; this count includes broken and incomplete specim ens The relatively complete specim ens, those used in the taxonomic study numbe red 81 Actinopteria mansuyi Grabau Pterinea sp aff P dianensis Guo Schizodus kienanensis Fang, nov sp 14 50 320 Sanguinolites sp Sphenotus antecedens G r a b a u Goniophora dianensis Guo 12 Gastropods 64 Trilobites Orthoceratid Total 679 The following fossils are recorded from the My Due area in central Vietnam Taxa Brachiopods Morinorhynchus sp Atrypoidea sp R eticu latrv p a sp Retziella weberi N i k i f o r o v a Retziella alaica N i k i f o r o v a ‘Howellella’ cf lynxoides Nikiforovaena vietnamensis B o u c o t & R o n g nov sp Spirinella ? sp G en.and sp indet Pedicle valves B rachial valves Conjoined valves 1777 16 688 175 451 1391 13 35 663 145 234 35 441 335 Bivalves 88; this count includes broken and incomplete specimens The relatively complete specimens, those used in the taxonomic study numbe red 44 Pterinea sp aff P dianensis Guo Schizodus ? myducensis F a n g nov sp Sphenotus antecedens G r a b a u Modiomorpha paracrypta F a n g nov sp Goniophora dianensis Guo Gastropods Trilobites Tabulates Rugose corals Ostracodes Bryozoans (?) Orthoceratids Vertebrates Indet fossils Total 10 i 27 78 21 5 18 3459 SYNECOLOGIC ANALYSIS The Late Silurian brachiopods studied in this paper belong to the Retziella Fauna which is widely dis tributed in east-central Asia and eastern A ustralia (Rong et al 1995) It m ust be emphasized th a t at many localities in which R weberi N i k i f o r o v a or R uniplicata (G r a b a u ) occur, each of these species is very abundant and dom inant, and usually make up more th an 30-70 %, or even more, of the total asso ciation There are two examples of this situation in which the last two species were recorded 1) R weberi with very abundant individuals on the same bedding plane was recorded from the Late Silurian rocks of the northern slope of the Alay Mountains, southwest Kirghizistan, Central Asia (Nikiforova 1937, pi 12, fig 8; also see Rong et al 1995, Appendix 1) More recently Kim and Sapelnikov (in Sapelnikov et al 1999) recorded Retziella weberi Community from the type locality (Havalbet Mountain, Severnyi N uratau Range, southern Tienshan) This is a high dominance, low diversity unit consisting almost entirely of the eponymous taxon and a BA or inner position lias been suggested 2) Very abundant specimens of R uniplicata were recorded with Striispirifer sinensis R o n g & Y a n g , and Atrypoidea foxi J o n e s in the lower p art of the K uanti Formation of the Qujing area, eastern Yunnan, Southwest China (see Rong & Yang 1980; Jones & Rong 1982), a low diversity, high domi nance brachiopod association, previously named the Atrypoidea Community The three species are all abundant, and the horizon in which they occur is not far from the base of the formation which is underlain disconformably by Middle Cam brian rocks The local paleogeography and the paleoecology indicate a nearshore, normal, shallow, warm w ater, level-bottom BA to inner position for this association R weberi from the My Due area locality, north-cen tra l Vietnam, herein described, also is the most abundant taxon in the collection The other bra chiopod genera associated with R weberi are not as numerous, being parts of a relatively low diversity fauna This probably indicates a similar, nearsho re, normal, shallow w ater environment for the widely distributed Retziella fauna in the LudlowPridoli This fauna may have inhabited a BA to inner BA position We will discuss in detail the environm ental evidence for each locality The fossils from both Vietnamese localities are almost in situ This is chiefly based on the following evidence 1) Preservation of fossils is excellent although the m atrix is relatively coarse; even micro-ornamentation is sometimes well preserved in this coarse matrix; brachiopod shells are always complete, w ith no broken valves or signs of abra sion, indicating no strong w ater currents; 2) Relatively equal percentages of pedicle and bra chial valves within most species (with little dispa rity) among the brachiopods At the locality near My Due the Retziella weberi collection contains 688 pedicle valves and 663 brachial valves; a t the Kien An area locality the ‘Howellella’ cf lynxoides collec tion contains 21 pedicle valves and 20 brachial valve, but things are different for this species in the My Due area where there are 451 pedicle valves and 234 brachial valves of this species; 3) The fos sil associations we found in these two localities appear to be relatively in situ, without any eviden ce of mixing of components of one community with those from another; 4) There is a complete size spectrum, large to small, for the relatively abun dant taxa are all preserved in the place where they inhabited with an overall range in shell width from 2-4 mm to more than 20 mm for the species (R et ziella weberi, Retziella alaica, Nikiforovaena vietnamensis nov sp., and ‘Howellella’ cf lynxoides) The same thing characterizes the other fossils, including the bivalves and gastropods Based on the statem ents above, it seems th a t there is no evi dence for significant net directional transportation in these collections Of course, this does not mean th a t there were no w ater currents present, because most of the shells are disarticulated The fossils from the Kien An area, indicate a shal lower environm ent th an those from the My Due area One of the evidences for dem onstrating this conclusion, in particular, is th a t the bivalves from the My Due area (2.56 %) are much less abundant than those from the Kien An area (57.3 %), indica 321 ting a shallower w ater environm ent for the latter A higher proportion of the gastropods (9.4 %) in the Kien An area th an those in the My Due area (2.27 %) again supports this conclusion A lower diversi ty (5 different higher taxa of fossils, and only spe cies of brachiopods) and high dominance of bivalves and Retziella weberi, which make up 90 % of the association in the Kien An area again suggests th a t the locality represents a nearshore, very shallow w ater environm ent (most likely BA 2) The My Due area locality, on the other hand, repre sents a relatively deeper, normal m arine environ m ent than th a t in the Kien An area, but still is considered a shallow w ater situation This is m ain ly based on the higher diversity of the entire faunal association (10 higher taxa), higher brachiopod diversity at the generic level, far fewer bivalves and gastropods, much more abundant brachiopod indi viduals, and presence of some normal m arine, shal low w ater benthic invertebrates [such as rugose corals, bryozoans (?), tabulates, ostracodes and others which are lacking in the Kien An area] in the My Due area than in the Kien An area Therefore, an assignm ent to inner BA for the col lection from the My Due area is suggested This is consistent w ith all the records eisewhere in Asia and A ustralia for the Retziella Fauna which has been ascertained to have no deep w ater representa tion in BA 4-5 (Rong et al 1995) The new V ietna mese data furth er support the conclusion th a t the Retziella Fauna is typicaliy a nearshore, shallow w ater benthic fauna, mainly BA to shallower BA position, a good environm ental indicator for investigation of brachiopod communities in the Sino-Australian Province during the Late Silurian The overall abundance of bivalves a t Kien An, as stated above, suggests a very nearshore environ ment, permissively in agreem ent w ith w hat the ecologically better known brachiopods suggest The taxonomic composition of the bivalve assemblage is sim ilar to th a t a t My Due, w ith the relative abun dances also being not too different from each other (Goniophora and Schizodus are the abundant taxa, with Actinoptena absent a t My Due) with a possible occurrence in North Korea), South China Plate (eastern Yunnan, eastern Guangxi, northern Vietnam, w estern Sichuan, and w estern Qinling), Tarim Plate (including southern Tienshan, C entral Asia), Indochina Plate and A ustra lian Plate in the Late Silurian In addition, there are some possible or doubtful occurrences of the genus Retziella in C entral Pam ir, E astern Afgha nistan, E astern Iran, and the northw est corner of South Island, New Zealand (Rong et al 1994) All of the definite areas are characterized by the pre sence of the shallow w ater, norm al m arine, Retziel la Fauna, indicating a possibly close biogeographic relationship among them in the Late Silurian, and a Sino-Australian Province has been proposed for the above regions th a t include the fauna (Rong et al 1995) Both the Retziella Fauna and the Tuvaella Fauna have been recognized as two major biogeographic units w ithin the Uralian-Cordilleran Region (Boucot 1975; Rong et al 1995) As one result of this study it has become apparent th a t those howellellids w ith one or more plications in the sulcus and corresponding grooves on the fold, a group of species th a t we suspect will end up in the genus Nikiforovaena (see taxonomic discussion of this genus), provides one more piece of evidence strengthening the concept of the Sino-Australian Late Silurian Province by providing a better understanding of the distribution and morphology of th a t genus 1) Relation to Central Asia It is rem arkable th at Central Asia and Indochina possess the same spe cies group of Retziella with the most abundant, com mon species, R weberi The more interesting situa tion is th a t R weberi is associated with R alaica N i k i f o r o v a [= Retzia (Retziella) weberi var alaica N ik i f o r o v a , 1937] in both Central Asia (Nikiforova 1937) and Vietnam (this paper) Moreover, both regions have sim ilar constituents not only at the generic, but also at the specific levels for the bra chiopod fauna in which Retziella occurs BIOGEOGRAPHIC RELATIONSHIPS According to Nikiforova (1937, p 7), Retziella webe ri is one of the most im portant species of the Isfara fauna (Pridolian) of C entral Asia in which is Eoreticularia tsehernyschewi N a l i v k i n (probably Spirinella), Nikiforovaena ferganensis N ik i f o r o v a , and Lissatrypa camelina B u c h (should be Atrypoidea) As mentioned above R weberi is associated with R alaica in Central Asia where diversity of the fauna is changeable: only a single species (R weberi) (sec Nikiforova 1937, pi 12, fig 8) at one extremity, or at the other extrem ity more th an taxa as exemplified by an association at outcrop num ber 1552 collected by Weber in 1910, including R weberi N i k i f o r o v a , Eoreticularia tschernyschewi m attschensis N i k i f o r o v a (probably Spirinella), Eospirifer cf togatus (B a r r a n d e ), Atrypoidea came lina (B u c h ), and Atrypa aspera aff squamosa (Sow e r b y ) (probably Gotatrypa) in 1552 m, and Schizophoria (Eoschizophoria) ferganensis N i k i f o r o v a in 1552i, and Nikiforovaena ferganensis (N i k i f o r o v a ) in 1552 (with no ‘i’ or ‘m’ m arked after ‘1552’) (Nikiforova 1937) The Retziella Fauna occurs in the N orth China Plate (Central Jilin and southern Inner Mongolia, The majority of these genera in the fauna in Central Asia are known from the My Due area on The very rare, disarticulated vertebrate plates (Thanh et al 1997) found in the My Due area pro bably were transported into the Retziella weberi Community there, although w hether from a m arine or nonm arine fauna elsewhere is unknown The My Due area fauna is assigned to the Retziella weberi Community, but the Kien An area fauna belongs to a bivalve-Retziella weberi unit, which might represent a m ixture between a somewhat deeper w ater, i?etete//a-dominated fauna and a shallower water, nearer shore, bivalve-dominated fauna; in the absence of more samples from other localities we leave the Kien An area fauna unna med 322 the Indochina Plate, and some are known in the Kien An area of the South China Plate Both Cen tral Asia and Vietnam have R weberi and R alaica; ‘Howellella’ cf lynxoides (N i k i f o r o v a ); Nikiforovaena vietnamensis nov sp from the Kien An area and the My Due area are also very sim ilar to H lynxoides and N ferganensis respectively from Central Asia It should be kept in mind th a t the lat ter four species are m ost abundant in the collec tions from the Kien An area and the My Due area Moreover, Tadschikia, a distinctive genus of rhynchonelloid in C entral Asia, has also been recorded in Vietnam (Duong 1980, p 68, pi 34, figs 8a-d) Although some genera (such as Eospirifer, Eoschizophoria, Atrypa) occurring in Central Asia have not been found in Vietnam, and some [such as Morinorhynchus, Retieulatrypa and an undeterm i ned taxon (gen and sp indet in this paper)] pre sent in Vietnam have not been discovered in Cen tra l Asia, this probably reflects different lithofacies or collection bias It seems to the authors th a t the sim ilarities between these two regions are more im portant th an the differences 2) Relation to South China There are two types of Retziella in eastern Yunnan, South China, one with a single rib, and another w ith two or more ribs in the pedicle sulcus, nam ely R uniplicata (G r a b a u ) and R minor (H a y a s a k a ) [= R plicata (M a n s u y ) and m any other synonyms] respectively Remarkably, they are sim ilar to R weberi and R alaica respec tively from C entral Asia and Vietnam, but they are different evolutionary stocks w ithin the genus This led us to conclude th a t the Retziella fauna descri bed in this paper from Vietnam is closer to th a t of Central Asia th an to South China, although, R weberi and R uniplicata are very sim ilar to each other if they are not conspecific, even though diffe ren t tectonic plates are involved while keeping in mind th a t the Tarim Plate m ay represent a former northw estern portion of w hat is now the South China Plate w ith the Indochina Plate close to both This conclusion is further supported by the follo wing evidence There are genera (Eoschizophoria, Aesopomum, Atrypa, Protathyris, and Striispirifer) in South China th a t have not been found in Viet nam, and Retieulatrypa and an undeterm ined taxon (gen and sp indet in this paper) from Viet nam th a t have not been discovered from South China Additionally, abundance of some common genera is different in the two regions: Atrypoidea and Spirinella are common in South China but very rare in Vietnam Moreover, specific consti tuents of the fauna in Vietnam are evidently diffe rent from those of South China All of this suggests th a t the discrim ination between them may have been related to slightly different environments, chiefly different substrates on which they lived We are aware th a t both regions have m any genera in common (Morinorhynchus, Atrypoidea, Retziella, ‘Howellella’, Nikiforovaena and Spirinella), indica ting th a t they belonged to the same biogeographic province in the Late Silurian, even to the same sub province (Rong et al 1995), in spite of occurring in different lithofacies Most recently, Rong et al (1995) recorded an occur rence of a very low diversity, high dominance R et ziella Fauna with a single species of Retziella from Late Silurian rocks a t Chengxi, eastern Guangxi, geographically closer to the Kien An area of Vietnam, but considered to be w ithin the Yunkai block (or terrane) rath e r th an in the South China Plate, far from the eastern Yunnan region in the Late Silurian It is notable th a t the bivalve species from both Kien An and My Due have much in common (4 out of and out of respectively) with those from the Qujing area of eastern Yunnan, which further rein forces the evidence provided by the brachiopods The other species are either new, not known from elsewhere, or not identifiable specifically 3) Relation to eastern Australia Since Molongia elegans capricornae M c K e l l a r from Queensland, New South Wales, Victoria and the A ustralian Capital Territory has been moved to Retziella (Rong et al 1994), the occurrence of the Retziella F auna in eastern A ustralia has been confirmed It should be pointed out th a t R capricornae (M c K e l l a r ) possessing a single m edian rib in the sulcus is very sim ilar to R weberi or R uniplicata if they are not conspecific Besides, the A ustralian and Vietnamese faunas have some common genera including Retziella, Morinorhynchus, Atrypoidea, Atrypa, ‘Howellella’, Nikiforovaena, and Spirinella All these facts suggest a faunal relationship bet ween Vietnam and eastern A ustralia during the Late Silurian However, the relation of Vietnam to eastern A ustralia was significantly less th an th at to Central Asia and South China This is because m any genera present in eastern A ustralia have not been discovered in Vietnam, including Aeginia, Pholidostrophia, Mitchellella, Protochonetes, Aliconchidium, Notoconchidium, Molongia, and S trii spirifer The overall abundance of Notoconchidium in the Tasm an region and its absence in Asia emphasizes the difference between these two geo graphic entities The occurrence of some endemic forms led Rong et al (1995, p 48) to establish a new subprovince w ithin the Sino-A ustralian Province for eastern Australia A separate biogeo graphic unit for the A ustralian Retziella faunas m ay eventually be needed Iu summary, we position the Indochina Plate, including the My Due area, between the Tarim (including Central Asia) and South China Plates, and closer to the Tarim Plate th an to South China particularly in term s of the same species group of Retziella in both the Tarim and Indochina Plates Genera* Eoschizophoria ‘Leptostrophia' Aesopomum Morinorhynchus Gypidula (Gashaomiaoia) Stegerhynchus Linguopugnoides Tadschikia Atrypoidea 10 Retieulatrypa 11 Atrypa 12 Retziella 13 Molongia Central Asia X S Inner Mongolia X X ? Eastern Yunnan Kien An Vietnam My Due Eastern Vietnam A ustralii X X X X X X X X X X X X XXX XXX X XXX X XXX X X X X XXX X XXX X 323 14 Protathyris 15 Eospirifer 16 Striispirifer 17 Howellella 18 Nikiforovaena X X X X X X X 19 Spirinella X X X X X X X X X X X X X *The data came from Nikiforova (1937) for C en tral Asia; Rong et al (1985) for south ern In n er Mongolia; Rong & Yang (1980) and F ang et al (1985) for e a stern Y unnan; th is p aper for V ietnam ; and M cK ellar (1969), S tru sz e t al (1984) for A ustralia Some d a ta are from Rong e t al (1995) O ther gene ra, such as Aegiria, Pholidostrophia, M itchellella, Protochonetes, A liconchidium , and Notoconchidium , w hich only occur in A ustralia in th e L ate S ilurian, are not included here XXX m eans the m ost common species in th e collection 1) Morinorhynchus sp.; 2) Atrypoidea sp.; 3) Reticulatrypa sp.; 4) Retziella weberi N i k i f o r o v a , 1937; 5) Retziella alaica N i k i f o r o v a , 1937; 6) ‘Howellella’ cf lynxoides (N i k i f o r o v a , 1937); 7) Nikiforovaena vietnamensis B o u c o t & R o n g nov sp.; 8) Spirinella ? sp.; and 9) Gen and sp indet BRACHIOPODA Order STROPHOMENIDA Superfamily ORTHOTETOIDEA Waagen, 1884 Family CHILIDIOPSIDAE Boucot, 1959 Genus M orinorh yn ch us H a v l i c e k , 1965 Morinorhynchus sp REGIONAL RELATIONS The regional paleogeographic and lithofacies rela tions of the Retziella Fauna are complex The South China Plate includes northern Vietnam, and proba bly the Kien An locality A central, eastern Chinese locality yielding a very low diversity Retziella assemblage in eastern Guangxi (Chengxi) is consi dered to be within the Yunkai block (or terrane), ratier than in the South China Plate in the Early Paleozoic It m ight have been related to the Indochina Plate, but this rem ains to be decided In southern Guangxi, and the Qujing area of eastern Yunnan, one finds Late Silurian or Early Devonian beds resting w ith angular unconformity on the Cam brian or Ordovician The Retziella F auna occurning in eastern Guangxi and in eastern Yunnan is interpreted to be present in ‘bays’ exten ding inland into an overall nonm arine region The Early Devonian faunas of this region are present in the Old Red Sandstone facies, locally interpreted as being nonmarine, and overlain by m arine beds canying the Emsian, Early Devonian Rostrospirifer tonkinensis Fauna In none of these areas is graptolitic earlier Silurian recognized either above or below the angular unconformity In the Hai Phong region, which includes the Kien An area, on the contraly, there is good evidence in the islands pre sent to the northeast for graptolitic pre-Retziella Fauna, with nonm arine Devonian also occurring in this region To the south, in the My Due area, cen tral Vietnam region on the Indochina Plate, one also finds graptolitic, older Silurian overlain by the shal lower water Retziella Fauna, overlain in tu rn by Old Red Sandstone facies beds th a t have flot yet yielded dated fossils The overall faunal similarities between the Kien An area Retziella Fauna and th at to the south in the My Due area suggests th a t they were not far apart geographically during the later Ludlow-Pridoli A unifying them e here is th a t latest Silurian Pridoli, Downtonian) to Early Devonian Old Red Sandstone facies is present almost everyw here Fig 2.1-2, 15 M aterial - Only specim ens consisting of pedicle valves w ith in tern al and external molds and brachial valve w ith both molds D escrip tion - Shell small, 2.5-6.4 mm in length and 3.6-9.7 mm in width, transversely subrectangular in outline, gently biconvex in profile; shells not always symmetrical Pedicle valve interarea high, catacline or slightly hypercline; brachial valve interarea anacline, much lower th an pedicle valve Ornam ent of fine costellae, increasing m ain ly by insertion, costellae sharp, finer than inter spaces; a few concentric growth lam ellae developed near the anterior margin Pedicle valve interior w ith short and very thin den tal plates, widely divergent Brachial valve interior w ith short socket ridges, widely divergent a t about 120 degrees; they connect with cardinal process which is bilobed, separated and extending evidently posteriorly above the hingeline; no muscle scars seen M ea su rem en ts - Dim ensions of th e figured specimens of M orinorhynchus sp (in mm) № of specimen Internal pedicle mold Internal pedicle mold In tern al brachial mold Length W idth 2.6 3.6 5.6 9.7 3.9 6.4 Height of interarea 1.5 ? ? Com parison- The assignm ent of this undeterm i ned species to Morinorhynchus is considered to be better th an to assign it to Aesopomum since it pos sesses dental plates, although they are thin and short, and its cardinalia are much more like those of Morinorhynchus This species is also characteri zed by rath er small individual size, whereas other species attributed to the genus are larger th an the valves described here O rder ATRYPIDA Superfamily LISSATRYPOIDEA Twenhofel, 1914 Fam ily LISSATRYPIDAE Twenhofel, 1914 Genus A try p o id e a M i t c h e l l & D u n , 1920 SYSTEMATIC PALEONTOLOGY R ep ository - The specimens figured and described here are deposited a t the Hanoi Geological Mu seum, Collection Number: BT 178 brachiopod species are described in this paper Atrypoidea sp Fig 2.9-14, 18-19 M aterial - 16 in tern al pedicle valve molds and 13 in tern al brachial valve molds 324 325 D escrip tion - Shell small, w idth and length m ea sured less th an 10 mm (see below), elongately oval in outline, gently dorsi-biconvex in profile; pedicle valve shows a very shallow m edian depression near the anterior m argin of the shells; shell surface smooth No dental plates in the pedicle valve B ra chial valve interior with separate hinge plates and widely divergent socket ridges Muscle scars in both valves not observed M e a s u re m e n ts - D imensions of th e figured specimens of Atrypoidea sp in mm № of specimen Internal pedicle mold Internal pedicle mold Internal pedicle mold Internal brachial mold Internal brachial mold In tern al brachial mold Length Width 6.1 ? 7.2 3.9 6.1 9.3 5.9 7.0 7.9 4.2 6.6 9.9 R em arks - This taxon is assigned to Atrypoidea based on its smooth surface, its cardinalia, and the absence of dental plates There are m any species which have been attributed to Atrypoidea up to date It is difficult to compare our Vietnamese spe cimens w ith those of other species since 1) there are probably some synonyms w ithin the genus and even different species which possess sim ilar shell shape a t the early stages; 2) there are no large indi viduals of Atrypoidea sp in the collection we stu died herein for comparison Superfamily ATRYPOIDEA Waagen, 1881 Fam ily ATRYPIDAE Waagen, 1881 Genus R e tic u la tr y p a S a v a g e , 1970 Reticulatrypa sp Fig 2.3,4 M a te ria l - Only one pedicle valve w ith both external and in tern al molds D escrip tion - Shell small, 4.8 mm long and 5.4 mm wide, subcircular, pedicle valve gently convex w ith a m edian crest like a low fold; ornam ent of fine costeliae w ith about regularly spaced concen tric lamellae Pedicle valve interior w ith thin, short, divergent (about 80 degrees) dental plates, and muscle scars discernible R e m a rk s - M any features, such as outline, convexity and ornam entation of th e shells, of th is undeterm inable species are consistent w ith those proposed by Savage (1970) for th e genus Reticulatrypa We not give a species nam e for the m aterial in th is paper because of th e presence of only a single pedicle valve Order ATHYRIDIDA Boucot, Johnson & Staton, 1964 Superfamily RETZIELLOIDEA Modzalevskaya, 1996 R e m a rk s - The retziellid group was assigned to Athyrisinacea in the 1965 Treatise on Invertebrate Paleontology Brachiopoda Volume which was followed by many paleontologists Recently, Rong et al (1994) thought th a t it is essentially different from the athyrisinids which are sim ilar to athyridids and they attributed it to the Meristelloidea Waagen, 1883 Most recently, Alvarez, Rong and Boucot (1998) have further pointed out th at the retziellids are peculiar both in th eir external and internal struc tures and agree with Modzalevskaya (1996) in promoting this group to superfamily rank There is only one family, Retziellidae, involved in this superfamily It includes five genera: Gissarina M e n a k o v a & N ik if o r o v a , 1986; Metathyrisina R o n g & Y a n g , 1981; Molongia M i t c h e l l , 1921; Qinlingia R o n g , Z h a n g & C h e n , 1987; Retziella N i k if o r o v a , 1937 The biogeographic significance of th is peculiar group h as been discussed in Rong et al (1995) and its fu rth e r im plications are evaluated above Family RETZIELLIDAE Rhzonsnitskaya, 1974 (nom transl Rong, Strusz, Boucot, Fu, M odzalevskaya & Su, 1994 ex R etziellinae R zhonsnitskaya, 1974, p 54) (= M etathyrisinidae W a n g , R o n g & Y a n g , in Rong & Yang, 1981, p 245) Genus R e tz ie lla N ik if o r o v a , 1937 T y p e S p e c ie s - Retziella weberi N ik i f o r o v a , 1937, p 57, pi 12, figs 8, pi 14, figs 1-4; from U pper Isfara Horizon (Pridolian), Isfara River, n o rth ern slope of Alay M ountains, southw est K hirghizistan, C en tral Asia The holotype speci men of th is species h as been re-illu strated by Rong et al (1994) E m ended D iagn osis - Retziellidae with variably developed ventral sulcus and dorsal fold, both usually plicate (ribbed), w ith to (a few even or 5) plicae in the sulcus and to on each flank, with thin, short dental plates and very narrow lateral cavities in the pedicle valve, and w ith a well-deve loped apical septalium supported by a short, thin median septum in the brachial valve R e m a rk s - Having studied the internal structures, Rong et al (1994) concluded th at Protathyrisina Z h u , 1974, from South China, Gannania Fu, 1982 and Stegospira Fu, 1982 from Western Qinling should be regarded as junior subjective syno nyms of Retziella Some species formerly assigned to Athyrisina and Molongia should also be attributed to Retziella All species of the genus Retziella have been listed in Rong et al (1994) Retziella can be distinguished from Molongia in having a rib bed pedicle sulcus and well developed septalium supported by F ig u r e - 1-2,15 Morinorhynchus sp 1, ventral external impression; 2, ventral internal impression; 15, dorsal internal impression; all specimens x 3, Reticulatrypa sp., ventral external and internal impressions, x Spirinella ? sp., ventral internal impression, x 6-8, gen et sp indet 6, ventral internal impression 7-8, ventral external and internal impressions, x 9-14, 18-19 Atrypoidea sp., 9-11,19, internal dorsal impressions, x 12-14 (11, 13, same specimen), 18,19, internal, external, ventral impressions, x 16-17, 20 35 Retziella weberi N ik if o r o v a , 16-17, two dorsal internal impressions, x 20, 23, 27-28, steinkern, anterior, lateral, ventral, dorsal views, x 21, 29-31, steinkern, anterior, dorsal, lateral, ventral views, x 22, ventral internal impression, x 24, dorsal internal impression, x 25, dorsal external impression, x 26, ventral internal impression, x 32-34, three dorsal internal impressions, x 35, dorsal external impression showing coarse, concentric growth lamellae, x 1-2,15 Morinorhynchus sp 1, moule externe ventral; 2, moule interne ventral; 15, moule interne dorsale; tous x 3, 4, Reticulatrypa sp moules externe et interne ventraux x 5, Spirinella ? sp moule interne ventral 6-8, gen et sp indet 6, moule interne ventral; 7, 8, moules externe et interne ventraux 9-14, 18-19 Atrypoidea sp 9-11, 19, moules internes dorsaux x 12-14 (11, 13, même spécimen); 18, 19, moules interne et externe ventraux x 16-17, 20-35 Retziella weberi N i k i f o r o v a 16, 17, deux moules internes dorsaux x 20, 23, 27-28, moule interne en vues antérieure, latérale, ventra le et dorsale x 3; 21, 29-31, moule interne en vues antérieure, dorsale, latérale et ventrale x 3; 22, moule interne ventral x ; 24, moule interne dorsal X ; 25, moule externe dorsal x 2; 26, moule interne ventral x 3; 32-34, trois moules internes dorsaux x 3; 35, moule exter ne dorsal montrant les fortes lamelles de croissance concentriques x 326 327 a m edian septum , w hereas Molongia has a sm ooth pedicle su l cus, and no septalium in th e brach ial valve A ge an d D istr ib u tio n - L ate W enlock to Pridoli; K irghi zistan and T adzhikistan, C en tral Asia, N o rth ern Tarim , S o u th C hina, N o rth C hina, N o rth K orea (?), n o rth e rn V ietnam , C entral V ietnam and E a stern A ustralia Retziella weberi N i k i f o r o v a , 1937 Figs 2.16-17, 20-35; 3-18-20, 24-25 Retziella weberi N i k i f o r o v a , 1937, p 57, pi 12, fig 8, pi 14, figs 1-4; Nikiforova 1949, p 19, pi 7, fig 4; R zhonsnitskaya 1974, p 61, pi 11, fig 8; Duong 1980, p 68, pi 34, figs 4-7; Rong et al 1994, p 566, pi 2, figs 9, 12-19, 21-27; pi 4, fig 12; Rong et al 1995, p 41, Fig 1, L,P,Q,V M aterials - All specimens of th is species are preserved in a coarse sandy m atrix as external an d in tern al molds 1777 pedicle in tern al molds and 1391 brachial in tern al molds from the My Due area and 51 pedicle an d 86 brachial molds from the Kien An area Most of them are d isarticulated, b u t 35 conjoined valves w ere recovered The in tern al stru ctu res of both pedicle an d brachial valves are well preserved w ith th e exception of spiralia We only obtained a single specimen w ith spiralia directed laterally, indicating its assignm ent to th e rib bed, im punctate athyridids D escrip tion - Shell medium to large in average size (the m inimum width of the shell only attains mm, and the maximum more than 15 mm); chan geable in outline, usually round to roundly penta gonal, wider th an long, and vice versa as well; gent ly subequally ventri-biconvex in lateral profile; ventral beak prominent and relatively high, w ith a mesothyrid foramen and conjoined small deltidial plates underneath the foramen; shallow ventral sulcus and low brachial fold present, the sulcus expanded widely anteriorly O rnam ent of promi nent, usually rounded plications well preserved, interspaces between two plications wider th an the latter; usually plications, sometimes and on the flank area, only a single plica present in the sulcus and on the fold; all plications s ta rt from the beak of the shells; widely located, strongly concentric lam ellae well developed on the whole shell surface, in particular on the anterior h alf of the shells, one specimen (lenght 11.5 mm, width 12.5 mm) w ith 16 concentric lamellae, the lamellae crowded near the anterior m arginal area Pedicle valve interior w ith thin, short, widely diver gent dental plates, attaining 1/7-1/10 of shell leng th; lateral cavities very narrow and small; muscle scars usually discernible, possibly lim ited to the pedicle chamber area Brachial valve interior w ith thinner outer hinge plates supported dorsally by a pair of plates conver gent onto a thin m edian septum to form an apical septalium which is very small and shallow (see dis cussion below); the m edian septum short, exten ding anteriorly, gradually thinning, and attaining about 1/5-1/4 the length of the shells; spiralia orien ted laterally M ea su rem en ts - D imensions of th e figured specimens of Retziella weberi N i k i f o r o v a , 1937 (in mm) Abbreviations are length (L), w idth (W), thickness (T), num ber of lateral plica tions (LP) and num ber of plications in th e sulcus (SP) or fold (FP) № of specimens Internal brachial mold Conjoined valves Internal brachial mold Internal brachial mold Conjoined valves Internal brachial mold Internal pedicle mold Internal pedicle mold Internal pedicle mold 10 In tern al brachial mold L W 3.1 7.4 9.5 9.8 10.0 10.8 12.9 12.2 12.6 16.1 3.2 7.8 10.7 14.2 11.7 14.0 13.1 12.3 12.0 13.2 T 2.9 7.0 LP 4 3-4 4-5 3-4 4-5 SP 1 FP 2 2 2 1 Com parison - Two species of the genus Retziella, R weberi Nikiforova, 1937 from the Pridolian rocks of Central Asia and R uniplicata (Grabau, 1931) from late Ludlow to early Pridoli strata of south west China, are very sim ilar to each other Externally they have the same shape and size range, plications well developed on flanks, and only plication in the pedicle sulcus and on the bra chial fold, and the latter two variably developed in both species They have great variation in shell shape and size in each population For example, in addition to the shape of the shells, numbers of pli cations are also variable in R weberi: we have mea sured both pedicle and brachial valves of 1) the holotype of the species (Nikiforova 1937), 2) two topotype specimens housed in the Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing (Rong et al 1994), and 3) 13 specimens of this species from C entral Asia housed in the U.S National Museum, Washington, altogether 18 speci mens with 36 valves: 19 valves with plications (including the holotype and all other topotype speci mens), with 4-5 plications, with plications, with plications, and one each w ith 3, 3-4, and 5-6 But, we can see th a t the majority of the species pos sess plications on each flank But, for R uniplica ta, we measured the neotype (NIGP 46666: length 9.0, width 9.1, and thickness 5.7 mm) and another - 1-3, 6-7,12-15, 21-23, 26-27 Retziella alaica N ik if o r o v a , 1, dorsal internal impression, x 2-3, two ventral internal impres sions, x 6-7, two ventral internal impressions, x 12-13, two ventral internal impressions, x 14, dorsal internal impression, x 15, ventral internal impression, x 21, dorsal internal impression showing septalium, x 22, dorsal internal impression, x 23, ven tral external impression showing fine filae, x (on sam e specimen as bivalve holotype) 26-27, ventral external impressions showing fine flac, x 18-20, 24-25 Retziella weberi N ik if o r o v a , 18-20, 24, steinkern, dorsal, lateral, ventral, anterior of same specimen, x 25, dorsal internal impression, x 4-5, 8-11, 16-17 ‘Howellella’ cf lynxoides (N ik if o ro v a ), 4, ventral internal impression, x 5, rubber replica of ventral external impression, x 8, 9, two dorsal internal impressions, x 10, dorsal external impression, x 11, ventral external impression, x 16, 17, rubber replica of ventral external impression, x -3 , 6-7, -1 , -2 , -2 Retziella alaica N i k i f o r o v a 1, moule interne dorsal x 3; -3 , deux moules internes ventraux x 3; 6-7, deux moules internes ventraux x 2; -1 , deux moules internes ventraux x 2; , moule interne dorsal x 2; , moule interne ventral x 2; , moule interne dorsal x 3; , moule exter ne ventral m ontrant les fines filae x (sur le même échantillon que Vholotype); -2 , moules externes ventraux montrant les filae x 8 -2 , -2 Retziella weberi N i k i f o r o v a -2 , , moule interne, vues dorsale, latérale, ventrale et antérieure x 3; , moule interne dorscil -5 , -1 , -1 ‘Howellella’ cf lynxoides (N i k i f o r o v a ) , moule interne ventral x 3; 5, empreinte en latex d ’un moule externe ven trale x 3; 8, , deux moules internes dorsaux x 3; , moule externe dorsal x 3; 1 , moule externe ventral x 3; , 17, empreinte en latex d ’un moule externe ventral x F ig u r e 328 topotype (NIGP 46667: length 12.5, width 12.9, and thickness 8.0 mm) which have and in each of the valves This feature can be used for distinguishing this species from R uniplicata Moreover, the inter spaces between plications in R weberi are usually wider than those in R uniplicata The Vietnamese specimens identified as R weberi were originally described briefly and illustrated by Duong (1980) and are studied in detail in this paper They are more like R weberi of Central Asia than R uniplicata from southw est China We have m easured lateral plications on the flanks of 10 figu red specimens (two conjoined valves, pedicle val ves, and brachial valves) w ith the following results: and 3-4 plications on valves, plica tions on valves, 4-5 plications on valves and plications on one valve The m ajority of the valves have 3-4 plications on each flank, a feature diffe rent from R uniplicata Moreover, the interspaces between two plications are wider in the Vietnamese specimens than in the Chinese specimens of R uni plicata Retziella weberi N i k i f o r o v a is also sim ilar to R minor (H a y a s a k a , 1922) in m any characters, inclu ding general shell size, shape, profile, and plica tions on the lateral areas R minor can be differen tiated from R weberi by having or more (3-5) pli cations in the pedicle sulcus, whereas Retziella weberi possesses only a single plica in the sulcus, and moreover R weberi has wider rib interspaces than R minor Retziella alaica N i k i f o r o v a , 1937 Fig 3.1-3, 6-7, 12-15, 21-23, 26-27 Retzia (Retziella) weberi v a r alaica N i k i f o r 14, figs 5-7 ova , 1937, p 58, p i M aterial - 175 pedicle in tern al valve molds and 145 brachial internal molds from th e My Due area, and 10 pedicle and 11 brachial in tern al molds from th e K ien An area D escription - Shell small to medium (the m ini mum width of the shell only attains mm, and the maximum more th an 16 mm); outline changeable, round to roundly pentagonal, wider than long or vice versa; usually gently subequally biconvex; ven tral beak relatively high, pedicle sulcus shallow, narrow, or sometimes discernible during the whole ontogeny, and brachial fold low or sometimes indis tinct Ornam ent of prom inent, usually low, rounded plications, interspaces between two plications prom inantly wider th an the latter; usually 14-16 pli cations on the whole shell surface, or more plica present in the sulcus and or more on the fold; all plica originate a t the beak; num erous fine concen tric fila well developed on the whole shell surface, a few strongly concentric growth lamellae only seen at the anterior margin Pedicle valve interior w ith a pair of thin, very short, widely divergent dental plates, lateral cavi ties very narrow; muscle scars usually discernible, possibly limited to the pedicle chamber area Brachial valve interior w ith outer hinge plates sup ported dorsally by a pair of plates convergent onto a thin, short m edian septum to form an apical sep- talium which is very small and shallow; the median septum attaining about 1/5 the shell length M ea su rem en ts - Dimensions of th e figured specimens of Retziella alaica N i k i f o r o v a , 1937 (in mm) A bbreviations are length (L), w idth (W), num ber of lateral plications (LP), and num ber of plications in the pedicle sulcus (SP) or brachial fold (FP) № of specimens Internal pedicle mold Internal brachial mold Internal pedicie mold Internal pedicle mold 5.Internal brachial mold Internal pedicle mold In tern al pedicle mold L W LP SP 9.6 10.2 10.4 15.6 15.6 16.1 16.4 11.3 10.8 10.5 13.8 15.4 14.7 14.5 4-5 4-5 4-5 5 (4) 3 (3) 2 FP C om parison - We promote Retzia (Retziella) webe ri var alaica N i k i f o r o v a , 1937, to specific level because alaica can be easily distinguished from R weberi by the following characters: finer and more costae, much less developed (usually almost discer nible) sulcus and fold, and more than costa in the sulcus and more th an on the fold Moreover, we examined the Vietnamese specimens of R alaica studied in this paper which possess very fine, concentric fila on the whole shell surface, which are absent in R weberi, and the concentric lamellae well developed in R weberi are almost lacking or only located at the anterior m arginal area of the shells in R alaica Retziella m inor (Ha y a s a k a ) from the Miaokao Formation (late Ludlow-early Pridoli) of the Qujing area, southwest China, also has plicae in the sul cus, a character similar to R alaica They differ from each other in the following: R minor pos sesses smaller shell size on average, much more developed pedicle sulcus, and much less wide inter spaces between two plicae than R alaica Regar ding the synonymy question of species assigned to Retziella from South China there is a complicated story What Grabau (1931) identified as Athyrisina plicata (Ma n s u y ), Rong & Yang (1980) as Protathyrisinaplicata, and Rong et al (1994) as Retziella plicata, can be differentiated from minor chiefly by having plications in the sulcus However, in Chinese specimens, those with or plications (or even more) in the sulcus occur in the same popula tions, and may be better regarded as different phe notypic forms of the same species Based on the same reasoning, those with plications in the sul cus and identified as Retziella quadriplicata (Gr a ba u ) from the Miaokao Formation of the Qujing area (Zhu in Fang & Zhu 1974; Rong & Yang 1980) should also be assigned to Retziella minor (Ha y a sa k a ) It would be better to consider those forms with 2, 3, or plications in the sulcus of various forms of the genus Retziella from South China as different phenotypes of the same species occurring in the same population However, it should be pointed out that the holotype of Retzia plicata Ma n s u y , 1922, came from an unknown hori zon at Lunan, Central Yunnan and is lost Since its exact locality and horizon are unknown the topoty pe specimens of this species cannot be restudied It is also uncertain whether or not the internal struc tures of Retzia plicata Ma n s u y are the same as those in Retziella minor Meanwhile, the holotype 329 specimen of Athyrisina quadriplicata G r a b a u , from the Middle Devonian W angjia Beds of southern Gansu, N orth China (Grabau 1931), cannot be investigated as well because the holotypes have not been located Therefore, it is also unknown w hether or not A quadriplicata G r a b a u and th a t identified as A quadriplicata G r a b a u (in Rong et al 1974) and Protathyrisina quadriplicata Z h u (in Fang & Zhu 1974) from the Miaokao Formation of the Qujing area are conspecific F u rth er investigation of these specimens is w arranted Order SPIRIFERIDA Waagen, 1883 Superfamily DELTHYRIDOIDEA Phillips, 1841 Family DELTHYRIDIDAE Phillips, 1841 Genus H o w ellella K ozL O W S ia, 1946 ‘Howellella’ cf lynxoides (N ik if o r o v a , 1937) Fig 3.4-5, 8-11, 16-17; 4-6, 18, 22-28 cf Spirifer (Eospirifer) lynxoides N a l i v k i n nom en nudum , in Nikiforova, 1937, p 50, pi 10, figs 7-1 non Eospirifer lynxoides N a l i v k i n , Duong, 1980, p 68, pi 34, figs 3a-d M aterial - 21 pedicle and 20 brachial in tern al molds from the Kien An area, and 451 pedicle and 234 brachial valve molds from th e My Due area All preserved as in tern al and external molds D escrip tion - Small to m edium in size, transverse rhomboidal in outline, subequally biconvex in late ral profile; wider th an long, the maximum width at the hingeline, w ith acute extremities Pedicle valve gently convex, interarea low, beak not higher, ove rhanging the hingeline very much, umbo gently curved, not swollen; sulcus well developed, expan ding anteriorly widely, occupying approximately 2/5 of the shell width Brachial valve also gently convex, the most convex p art of the shell near the mid line, m edian fold carinate, prom inently above the flanks of the shells Megascopic-ornament of about 3-5 (mostly or 5), occasionally 6, simple, strong plications on the flanks, and a short, m edian rib in the pedicle sulcus w ith a corresponding median furrow on the brachial fold, in the sm aller specimens the rib is absent, in a single specimen examined in the collection there are two weak, low plications present in the sulcus, in addition to the median one, there is another one lateral to the la t ter, indicating an irregular phenomenon; micro ornam ent is not usually well preserved in the col lection, but a few specimens of this species clearly show concentric growth lam ellae upon which are superimposed capillae th a t become fim briate a t the anterior m argin of the growth lamellae Pedicle valve interior w ith short, thin, narrowly divergent dental plates, disposed ju st a t the outsi de of the two strongest plications delimiting the sulcus, the la tte r extending anteriorly to expand widely and relatively deep; no muscle field seen Brachial valve interior w ith a pair of widely diver gent sockets, subparallel to the hingeline, crural plates almost lacking, striated cardinal process located at the end of small delthyrial platform; muscle fields discernible M ea su rem en ts - D imensions of th e figured specimens of ‘Howellella’ cf lynxoides ( N i k i f o r o v a , 1937) (in mm) Abbre viations are length (L), w idth (W), num ber of lateral plications (LP), and num ber of plications in th e pedicle sulcus (SP) or brachial fold (FP) № of specimen Internal pedicle mold Internal pedicle mold Internal pedicle mold Internal pedicle mold Internal pedicle mold Internal pedicle mold Internal brachial mold Internal brachial mold Internal brachial mold 10 Internal brachial mold 11 In tern al brachial mold L 5.5 6.4 5.8 8.2 11.7