1993d the devonian vertebrates (placodermi, sarcopterygii) from central vietnam

Journal of Asian Earth Science Vol 15 № 45 pp 393-406 997 1997 Elsevier Science All rights reserved m i c iu in m u im u u n iL Printed in Great Britain I II: S I -9 120(97)00020-:» j 357.912097 S1700 + 000 IVramon T he De vonia n ve rte brate s (P la c o de r mi, Sar copte r ygii) fr o m Ce ntr a l Vie tna m and the ir be ar ing on the De vonian palae oge ogr aphy o f Southe a s t As ia P h ilip p e Ja n v ie r ,* T o n g - Dz u y T h a n h ,+ T a H o a P h u o n g ,t Doa n Nhat T ruongJ * U R A du C N R S , L ab orato ire tie Paleontologie, rue Bullon 75005 Paris France tN a lio n a l U niversity o f V ietnam in H a n o i, Departm ent o f G eology, 90, Nguyen T i Sir T h a n h X u a n , H a n o i, V iet N a m ^Institute o f G eology and M in e l Resources, T hanh X u a n , H a n o i Viet N am (R cceivcd 28 O cto b er 1996; accepted f o r p u b lic a tio n Ja n u a ry 1997) Abstract— N ew vertebrate remains are described fro m the terrigenous lacies o f the G ivetian Dong T h o F o rm a tio n in Q uang Binh Province, central Vietnam They are referred to three placoderm groups, the A n tia rc h i, A n h ro d ira and A c anlh otho raci, and to a sarcopterygian T h e antiarchs are represented by a new form , V uklutclcpis lylioacn.us n.g n.sp, which is regarded here as closely related to the Yunnanolcpididac an endemic group from the Low er Devonian o f the South China Block A d d itio n al m aterial o f the phlyctaeniid arthrod ire L y lio a le p is d u ckh o u i T o n u -D /u y Thanh, Janvier and D o an N h iit T ru o n g is described T he sarcoptcrygian is regarded here as a dipnom orph and may be closely related to the Low er M id d le Devonian Voungolepididae o f the South China Block This m aterial conlirm s the close ties between the Indochina and South C hina blocks as early as the M id d le Devonian 1997 Elsevier Scicnce Ltd Intr o duc tio n Numerous vertebrate remains have been recorded from the Lower Devonian o f northern Vietnam (В а с Bo) during the past decade (T o n g -D zu y Thanh and Janvier l l)87, 1990, 1994; T o n g -D zu y Thanh el (tl 1995; Janvier i't at 1993, 1994), and show very clear allinities with the Lochkoviun and Pragian vertebrate faunas o f southern China (principally eastern Yunnan and Guangxi), currently referred to as the “ galeaspid yunnanolepid” vertebrate fauna This fauna characterizes a faunal province restricted 10 the South China (Y a n g lzi) Block, and therefore referred to as the “ galeiispid yunnunolepid province” (Y o un g 1981, 1990, 1993) The lale M iddle Devonian (G ivetian ) vertebrate fauna o f the D o Son Sandstone Formations in the D o Son Peninsula, south o f H Phong (Figs I A , 2; Janvier e t i l l 1989; Long cl ill 1У90; Tông D uy Thanh c l III I l)94) is less clearly linked with the South Chinese endemic vertebrate faunas, since the antiarch Vietmtmuspis Long, Burrell, Pham K im Ngan and Janvier and other unnamed taxa are unique to this locality, and Botliriolepis is a widespread (axon (L ong et III 1990) U ntil recently, no significant Devonian vertebrate remains had been recorded from the part o f the Indochinese peninsula that extends south o f the Song Hong (Red River), apart from the mention o f some plucoderm and sarcopterygian fragments found along with conodonts in residues from the G ivetian Quy D a l l orm ation ( = M u c Bai Form ation, see Fig 2), and chondrichthyan and actinopterygian scales trom the Kamennian X o m N Form ation (F ig 2), both o f Central Vietnam (Long 1993) T w o m ajor discoveries have recently filled this gap in our knowledge Pham K im Ngun and Nguyên Cônu Ltionii (1995) discovered a small vertebrate fauna ill the base o f the Nam Pia Form ation along the Sòng D a (Black River), and this fauna turned out to include galeaspids, yountỉ olepidids and various placoderms which all had clear alliniiics with the galeaspid-yumumolepid fauna o f the South China Block (Janvier et til 1996) Then, farther south, placoderni remains referred to the phlyctaeniid arthrodire Lyhotepix tlttck Itotti have been described from the Givetian Dong T h o Form ation at Ly H oa, Quang Binh Province, in central Vietnam (To ng -D zu y Tlutnh et (it 1994) This discovery was soon followed by that o f other vertebrate taxa ill a second locality o f the same area (T o ng -D zu y Thanh Ơ I ill 1996a), and which are here described ill detail The Ly H oa fauna is particularly im portant, as it is the first occurrence o f Devonian vertebrates south o f the Sônn M a suture: thill is, 011 I he Indochina Block proper, and also as il shows clear allinities with the “ galeaspid yiinnunolepid" fauna o f the South Chinn Block (T o ng -D zu y Tluinh el til 1996a) Ge o lo gic a l s e tting Silurian and Devonian exposures are frequent in the Quang Binh Province o f centrál Vietnam , east o f Cape Ly H oa and south o f the G iiinh River (Sòng G ia n li) (Fig IB ), in the southern part o f the Tru on g Son anticlinorium The Late Silurian (W e n lo c k -P rid o li) Dai G iang Form ation consists o f line-grained sandstones and shales which yield brachiopods, graptolites and vertebrate remains (T o ng -D zu y Thanh el at 1997) Four Devonian formations are recognized in this area: 393 394 Philippe Janvier ei (il A ip* 104* 10ô -21* -I* ã 17* -15* 9* ES Dai Qiang Formation (Upper Silurian - Lower Devonian) It I ll Rao Chan Formation M 'JC d Dong Tho Formation (Glvetlan - ?Lower Frasnian) ea Carboniferous (Lower Devonian) E m Ban Glang Formation (Eifelian) Bai Formation (Glvetian) En sO Triassic □ I + Prô ã Lower Carboniferous granite Quaternary l-ig I Locality limp A Location o f the Ly I Ion and D o Son areas and o f sonic m ajo r Icctoilic boundaries in Vietnam U simplified geological map o f the l.y llo a area I vertebrate localities referred to in the text and figure captions as Lyhoa I and respectively (F ro m T o n g -D m y sponuy bone; r l r ventrolateral riduc; '.‘rlrc c possible remnant o f the wall o f the ventrolateral recess Yunnanolepididac in which the internal surface o f the A V L plale is know n In Yunnanolcpix c h ii and Y p o riic there arc tw o divergent anterior internal Iransvcrsc crests (Zh ang 1978 see Fig B; Z lu i 1996 see Fig E) The anterior crcst is oblique, relative lo the lateral margin o f llic plate, and corresponds to the postbranchial lam ina, whereas the poslcrior one is perpendicular to the lateral margin o f the plate In the chuchinolepid ( = procondylolcpiform ) C ln icliin o lcp ix there seems lo be a single, oblique, postbranchial lamina (T o n g -D zu y T hanh and Janvier 1990, sec Figs 21 B) In contrast, the yunnanolcpidid I ’ h y n io lc p is , the euantiarch M in ic n ia and (lie Sinolepidac display a single, sharp, transverse anterior internal crest (Z h u and Janvier 1996: Ritchie c l at 1992; Zhu 1996, see Fig I IF.) W hen considering the condition in other placoderms in particular acanlholhoracids one may infer lhat the single transverse crest is a derived condition, which results from (lie fusion o f the postbranchial lamina proper and the transverse internal crest (Janvier and Pan 1982) T he sub-cephalic, o r sub-branchial, lamina o f the A V I plalc o f I lylioacnxix (i.e the portion o f the plate tlial extends anteriorly to the transverse internal crcst; sbrl see Fig 5B) is large, concave, almost rectangle-shaped, and il is devoid o f ornam entation (contrary lo the condition in C ln icliinolcpix', T o n g -D /u y T hanh and Janvier 1990 see Fig B) There is no clear evidence for a ventrolateral recess (T o n g -D zu y T hanh and Janvier 1990 see Fig 15 ' ‘ reel "), which is a characteristic feature o f the Yunnanolepididac How ever, a minute internal recess o f the internal surface o f the lateral lam ina may suggest such a structure C’vlrec see Fig 5B) The external ornam entation o f the A V L plalc consists o f small, scattered tubercles, which arc arranged ill parallel rows along the ventrolateral ridge A n terio r median dorsal plale ( A M D ) : an isolated anterior median dorsal plate (Fig 4B, 6A ) is referred here lo 1' lylioacnxix, on the basis o f its si/e and proportions, which are com patible w ith the holotype A V I plale described above The bone o f this plale is extremely thin and poorly preserved, ami Ihcrc is no clear external ornam entation, apart from a lew minute tubercles in its postcrolnlcral portion As often occurs in various Low er and M id d le Devonian anliarchs principally from C hina, the external surface o f the A M D plale is marked w ith broad ridges which radiate from the centre o f the plalc There are also m inor ridges which arc parallel lo the main ones, in particular the posterolateral main ridges T he outline o f the A M D plate is roughly pentagonal in shape, as in mosl Yunnanolepididac (Zhang 1978 sec Fig T o n g -D zu v Thanh and Janvier 1990, sec fig s 9, 10 15) allhough il is unclear whether its anterior pari is pointed and overlapped by the A V I plates, as in all Yunnanolepididac, or shows a narrow anterior tree margin As in all Yunnanolepididac, there is lo be a double overlap for the posterior dorsolateral plate (o a P D L see Fig A ) w ith the A M D overlapping the P D L anteriorly and the P D L overlapping the A M D posteriorly This feature, however, is known lo occur also in some euanliarchs {S icg o le p ix liyxxa ca n lln ix R cin if;o lcp is) w ith a high rale o f individual variation, and Zhu (1996) regards il as a prim itive condition for all anliarchs The internal surface shows a low median ridge, which is hollowed by an elongated median groove and a pit in the centre o f the plate, right beneath the tergal angle (am vp nivgr sec Fig 6A 2) as in the Yunnanolcpidoidci (Zh ang 1978 see Fig 3: T o n g -D /u y Thanh and Janvier 1990 sec Figs 15 19) This plale however, dillers from that o f mosl yunnanolepidoids by its relatively short anil broad proportions A second, smaller and incomplete A M D plate dillers from lhe preceding one in being ornamented with numerous, closely-set tubercles and showing no marked radiating ridges Its ventral surface, however, clearly shows an elongated median groove or pit (am vp sec Fig 6C ) The two overlap areas for the anterior dorsolateral ( A D I ) plates (o a A D L sec Fig 6C 2) arc convergent but not meet anteriorly, and leave The Devonian vertebrates from Central Vietnam m oaAMD Fig t u k ln u lf/iis lyluHicnsis n g ,‘ n.sp„ G iv e iia n D ong T lio F o rm a lio n Lyhou (locality 2) Q uang Binh Province, Vietnam A A M B plate in tlorsal ( A I ) a ml ventral (A ) view (reconstruction based on BT203b) B I’ M I ) plate in ventral view (reconstruction based on BT2(Bc): C com bined reconstruction o f the A M D and P M D plates brought to ihe same scale (based on reconstructions in A and B) C camera lucida draw ing o f a fragm entary A M D plate d ou btfu lly referred to I' in dorsal ( C l ) and ventral (C ) view (B T I) Scale bars: I 111111 w n r p , anterio r median ventral pit: m rg r , median ventral groove; oa.-l 1)1,, A M D , / ’ / ) / P M D , overlap areas lo r the A D L A M D , P D L and P M D plates; p im p , posterior median ventral process and pit a narrow , free anterior m argin This m argin, however, is not broader than that o f the A M D plate o f C ln u liin o le p is am ong the Yunnanolcpidoidci (T o n g -D /.u y Thanh and lanvicr I ‘M il see Tig IS); Z h u 1996, see Tig 21) It is not clear w hether this A M D plate belongs to the same species as Ihe preceding one, owing to the dillerenee in ornam entation Posterior median dorsal plate (P M D ): the P M D plate is only know n by a poorly preserved specimen (T'ig C , B), Ihe si/e o f which broadly agrees with the most complete A M D just described By bringing the si/e o f its overlap area for the A M D to the si/e o f Ihe corresponding area o f Ihe A M D described above allows a reconstruction (F ig D ) which matches the proportion in the Yunnanolcpidoidci, i.e w ith a P M D plate which is about h a lf the length o f Ihe A M D plate T h e ventral surface o f this P M D plate is quite sim ilar to that o f the Yunnanolcpidoidci (Z h a n g 1978, sec l-igs 4, 15, 19; Z h u , 1996 see l-igs 71:, I IB , I5 C , 251:); that is, it has broad overlap areas for the posterior dorsolateral ( P D L ) plate (o a P D L see Tig (>B) and a large posterior median internal process hollowed by a pit (pm vp, see Fig B) U n fo rtu n a te ly , the specimen is too poorly preserved to show w hether the posterior transverse internal crest passes anterio rly to the posterior median internal process (a diagnostic feature o f the Yunnanolcpididac; Z h u 1996) The external surface o f the plate shows no grooves or crests and is ornam ented with relatively large, rounded tubercles, as in most yunnanolepidoids where the tubercles are larger 011 this plate than 011 the rest o f the arm our Paranuchal plate: a poorly preserved plate is tentatively referred to as a left paranuchal plate (T'ig A ) Its external surface is ornam ented with closely-set tubercles, but shows 110 distinct sensory-line grooves Its ventral surface, although much weathered shows a thickening which may be interpreted as the posterolateral region o f the plate Pectoral lin elements: one specimen may be interpreted as elements o f the pectoral lins because it is more or less tube-shaped and cannot be referred to any other plate o f the a rm o u r(F ig s F „ 7B) and because it is ornam ented w ith minute, closely-set tubercles, sim ilar to those o f most other antiarclian plates o f locality Its overall shape would accord w ith the proxim al part o f an appendage-like pectoral lin although it shows no trace o f sutures between the various platelets which, in o ilie r antiarehs, cover the pectoral lin O nly an overlap area seems lo be present o f its presumed distal part (oa see Fig 7B) A cantho lho raci Stcnsio 1947 A thick derm al bone fragment from locality I (Figs F , ) is referred to as an A canlh oih oraci 011 the basis o f its 400 Philippe Janvier et al chiiractcrislic ornam entation, which consists o f large, elongated, and strongly costulated tubercles (Fig B) It bears a longitudinal crest and shows an asymm etry which suggests that it is part o f a lateral plate or scute It may be the ventrolateral edge o f the derm al shoulder girdle Its internal surface is slightly concave and rugose T he ornam entation is clearly different from that o f L vh m ife p is and is suggestive o f that o f some Early Devonian acanthothoracids from Japan (O ku and G o to 1992) and Australia ( W vvjaspvraspis W hite 1978; M w riiu lu la s p i.s Long and Y oung 1988) Interestingly, this type o f ornam entation, w ith strongly costulatc and elongated tubercles, is unknown in N o rth Am erican and European acanthothoracids and seems to be restricted to Australian and East Asian forms The Canadian acantholhoracid R a m m itliiui (0 r v ig 1975) is ornamented w ith stellate tubercles, but these arc never elongated in shape Placodermi gen et sp indcl An impression is referred with doubt to a spinal plate o f some placodcrm (Fig 3C): Uy its ornam entation and length, it certainly docs not belong to l.y lio a lc p ix but could belong to either a different phlyctaeniid or a pctaliehlhyid It is strikingly sim ilar to the spinal plate o f C httn gyim o p h yio n liiipcicn.sc S/e from the Late Devonian Huangehiateng F orm ation o f Hupei China (P'an 1962) and which is regarded as either a pctaliehlhyid or more likely, a grocnlandaspidid arlhrodire A fragment o f a spinal plalc (Fig G ) showing large medial denticles and an ornam entation o f parallel ridges, may be referred lo as an A rlh ro d ire but il is too slender to belong to LyluH ilcpix which has very short spinal plates Osteichlhycs Huxley 1871 Sarcopterygii Rom er 1955 ,'Dipnom orpha Ahlberg 1991 A number o f derm al bone fragments are covered with a cosniine laser w ith large and closely-scl pores (Fig 411 I) This type o f cosmine is suggestive o f certain D ip no m o rp and in particular Y oitn a o lcp h from the Early Devonian o f Yunnan (Cluing and Yu 1981; Chang 1982) None o f them shows I - J Fig A canlhothoraci gen el sp iiulel G ivelian Dong I ho F orm atio n Lyhoa (locality I ) Quang Hinli Province Vietnam A camera lucida draw ing o f a fragm entary dermal shoulder girdle in external view (B T I) w itli two transverse sections at the indicated levels (scale bar: 10 mm); B detail o f the ornam entation (scale bar I mm I Fig Vuklm t lcpi.s lyhiw ensis n.g n.sp G ivctian Dong Tho F orm atio n Lyhoa (locality 2) Quang Binh Province Vietnam A ,’paranuchal plalc in internal ( A t ) and external (A ) views (camera lucida drawing based on BT203g); B proxim al portion o f the pectoral (in (camera lucida draw ing based on BT2()Je) and transverse section at the indicated level Scale bars: I mm oa ? overlap area for adjacent lin plates W cstoll-lines thereby ruling out the D ip no i None o f these fragm entary bones can be identified, since they show no sensory-line canals or pit-line grooves, and no clear natural margins Howev er, their si/e and thickness (about I mm I agrees with the si/e o f the average individuals o f YЧ 12 pi 5:10, I I ) from the "S ilu ria n '' (in fact, the D evonian) ol S i-Y an g-1 ang in eastern Yunnan There are however, doubts as to w hether Sinucaris actuallv is a cruslacan or bivalve (K o lle l % ‘>) A nother possible arthropod specimen is a m inuie tube-shaped element, which is ornam ented w ith sharp ridges but is not made up o f bone (I ig 10) It could be part o f a crustacean appendage I ig 10 '? C rustacea gen, et sp inilel G ivctian D ong Tho I orm alio n Lyhoa Quang Binh Province Vietnam Possible fragment o f an appendage (locality 2: H 1203s) and section at the indicated level Scale bar: mm Well-preserved plant remains occur in the D ong Tho F orm ation near M in h Le northwest to Ly llo a (T o n g -D /u y T hanh and C'ai 14>S>5) but the two vertebrate-bearing localities only yield rare plant fragments A lew indeterm inable stem fragments are seen in Lv llo a and their relatively large si/e accords with the si/e o f the lycophvle stems from M in li I.e Dis c us s io n Although rare anil poorly preserved, a consideration o f the vertebrate remains from Ly Hoa bears on three ilillerent but related questions Firstly, the phylogenetic and biogeographical allinities o f the laxa with those met with in comparable Devonian faunas in other areas Secondly, ihe environmental conditions o f this fauna and, finally, its bioslraligraphical constraint I ’liy hif’ciictic Hint hiof’ coi’ rtipliicdl ichilittiiships 1'ig .У '.’ Cruslacea gen et sp indct., G ivctian D o ng T h o fo rm a tio n Lyhoa, Q uang Binh Province, Vietnam A , almost complete carapace in lateral aspect (locality I, В Т ц ) (X 1.5): В presumed anterior portion o f the carapace (loca lity 2, B T 203r) (X ) The lirsi vertebrate discovered at Ly llo a was Lyhottlcpis (litclJiiHii, a phlyctaeniid arthrodire charac­ terized by a trunk arm our which is deeply embayed posterolalerally and in which no suture is visible between the dermal plates (T o ng -D zu y Thanh cl nl IW ) Besides these unique characters, Lylioulcpis did not clearly display any feature uniquely shared with one other particular phlyctaeniid taxon In this respect, it must be regarded as itwertae satis within the Phlyctaenii until more inform ation is obtained, in particular on the head arm our In contrast, Vukhmlepis lyhoacnsis clearly shows closest resemblances to the Yunnanolepidoidei and other non-euantiarchan antiarchs from the South China Block N ow the question is about the meaning o f this 402 Philippe Janvier vĩ al resemblance 11' it only indicated primitive, or general, antiarchan characters (absence o f pectoral articular process, small P M D plate, double overlap between A M D and P D L plates), then it may he non-inform alive and only tells us that V lylioavnsis is an anliarch So far, there arc only two characters in V lyhoavnsis that sucucst reliably that this species belongs to the Yunnanolcpidoidci or even the Yunnanolepididae, as defined by Zlui (1 % ) These arc (he anteriorly pointed A M D plate and the possible presence o f a vcntroltilcral ICCCSS Whatever the prccisc relationship o f Viiklmvlvpiw this type o f presumably generalized antiarch m or­ phology was to date, unknown outside the South China B lo c k ' The same line o f ìéasoniim CÍII1 apply to the younuolepidid-like dermal hone fragments and scales from Lv Hoa None o f the unique characlcrs defining YoiiiiỊ ỉ olvpix (paltcrn o f tile palatal dentition, slruclurc o f lhe braincase; Chang and Y u 1981; Chang 1982) can be observed in this poorly preserved material, and ils overall resemblance to YouiiỊ ỉ olvpis as lo the aspect o f ihc cosmine and ornamentation o f the scales may be reuarded as uencral dipnomorph characters thill also occur yet lo a lesser; extent, in other primitive ilipnomorphs, such as (lie porolcpiforms Poiolvpis and llv im v n ia IVom the Lower ami M iddle Devonian o f Furope (0 rv ig 1969) Nevertheless, one must concede lhal such a scalc ornamentation entirely consisting o f distinct sinuous ridiies as round in the Ly Hoa material, is unknown in typical porolepiforms hut widespread ill tho Lower Devonian South Chinese and northern Vietnamese )'(>í«jtiíf>//.v-bciiriniĩ localities Finally the type o f ornamentation o f the acanlholhoracid plate fraumcnl would place il in ail ensemble o f forms unrecorded outside eastern Asia anil Australia From the above consideration o f the systematic position o f the Lv Hoa vertebrate taxa it follows lliat two o f them, Vuklinclcpis Ivhoavnsis and the unnamed dipnomorph sarcoptervizian shows closest bioiieoizraphical aliinilies to I he South China Block and the acanthothoracid lo eastern Asia and Australia All oilier taxa ciled ( l.yhoalvpis and the doubtful petaliehlhyid) arc o f undecided allinities Ijirironm vnt Tlie'cnv'ironmcnt o f the two vertebrate localities o f Ly llo a is clearly deltaic, bill with some marine influence indicated by litre inarticulate brachiopods iiiul possible pti\llocaritl remains Tilts kind o f environment is frequently encountered in Devonian vertebrate-bearing localities and within southeast Asia, bears a close litholoilic rcscinblancc to the D o Son Sandstone Form ation (l-ic Kinh M o n area) o f (lie D o Son Peninsula, soulii o f Hai Phoim (Jiinvicr VI a t 1989, 1994: I one V I at I WO; Tone Duv Thanh VI al 1994; T one-D zuv Thanh VI ill 1996a) The tectonic relation­ ship o f the Do Son Form ation is slill obscure, due to the extensive Ccnozoic coverin'’ which surrounds the Do Son Peninsula, bill it is assumed lo be situated north of lhe Red River Fault Zone and is clearly north o f the Soils: M il Suture (Fii> I A ), and w ould (Inis belong to the South China Block When consideritm the extent (if I he eastward shift o f peninsular Indochina aloni: the Red River laull complex since (lie Laic Mesozoic (estimated as about 700 km + 200 km: Leloup (’I ill 1995), the Dong T ho Form ation would come not too I'ar from the D o Son Sandstone Form ation in pre-extrusion times, given that the Song M a suture was already closed (see below) A t this stage, no conclusion can be drawn about a possible correlation between these two formations, nor about their possible belonging to the shoreline o f the same landmass Their remarkable lilhologic similarity (and approximately similar age) is however, an important thread to be followed for future palaeogeographical reconstructions o f Southeast Asia S lr a lig r a p li iv a l c o n s tra in ts The most intriguing feature o f the Ly Floa vertebrate fauna is the discrepancy between the widely accepted Givctian age o f the Dong T ho Form ation and the current stratigraphical distribution o f higher taxa to which are referred or compared L y lio a lv p is V tik ln iv lv p is the unnamed acanthothoracid and the unnamed sarcoptervgian respectively The most reliable age hitherto obtained lor the Dong T ho formation rests on the plant remains ( L v p iilo ilv n d r o p s is P m to lv p u lo d c n dron) and spore assemblage which suggest a Givctian age (T ong-D zuy Thanh and C'ai 1995) M oroever if the Dong Tho Form ation actually is a lateral facies o f the M uc Bai Form ation, the latter provides additional stratigraphical inform ation in the form o f a rich marine invertebrate fauna (see Geological setting) which also indicates a Givctian age with a possible extension into the Lower Ftasnian In contrast the vertebrates all belong to higher taxa which are mostly or exclusively known from the Lower Devonian, in particular the l.ochkovian and Pragian Although phlyctaeniid arthrodircs occur in the M iddle and I ale Devonian, they are most abundant in the Farly Devonian (after the Fifelian, they are essentially represented by a single family, the Groenlandaspididae to which l.y lio a lc p is certainly does not belong) Forms with strongly sutured plates, such as l.y lio a lc p is , are mainly known in the Lochkovian and Pragian (Goujct 1984) In this sense, the occtirrcnce o f l.y lio a lc p is in the Dong Tho Form ation is surprising, yet not strikingly anachronistic, since phlyctaeniids are still known in the F.msian and Fifelian o f Canada, but none o f them possess such an entirely sutured dermal arm our The yunnanolepidoids as well as all other non-euantiarch taxa o f antiarchs are restricted to the Lower Devonian o f China and northern Vietnam, with some possible Silurian occurrences in China All M iddle and Late Devonian antiarchs known to date are euantiarchs and sinolepids (if the latter are not included in the euantiarchs) The presence o f the yunnanolepidoid-like antiarch V u k lm c lv p is in the Dong T ho Form ation thus sounds clearly anachronistic Acanthothoracids are essentially Lochkov ian Pragian in N orth America Europe and Siberia, but extend into the 1-msian in Australia The first Silurian acanthothoracid is recorded from the Wcnlock Pridoli Dai Giang Form ation o f M y Due not loo far from Ly llo a area (Tong-D zuy Thanh v i a l in press) N o acanthothoracid has hitherto been recorded from the M iddle Devonian The same remark can be made lor the Ly llo a sarcopterygian since Y o in i^ o lv p is is known exclusively from the Lower Devonian o f China and northern Vietnam and the latest known cosminc-covcred porolepilorm , / / ( ’//wmW is Fifelian in age (Orvig 1969) Am ong post-F.ifelian dipnomorphs only dipnoans The Devonian vertebrates IVoni Central Vietnam remain cosmine-covcred nnlil ihe end o f the Devonian I Urn ever, as we have seen above llie dipnom orph dermal bones and scales IVom Ly I loa are unlikely to belong to a dipnoan, since they show no evidence lor Westoll-lines on the surface o f the cosmine Despite these lour anachronistic taxa, il is improbable that the Dong T h o Form ation extends into the Lower Devonian Firstly, the few plant remains found in association with the lishes, although indeterminable, are relatively large and accord with si/e o f the large lycophyte stems and leaves found in the sandstones o f the Dong T ho I ’orm ation at M inh Le Secondly, the Dong T ho I'orm ation is underlain by the Ban Ciiang Form ation dated as liilelian on the basis o f an abundant invertebrate fauna (see Geological selling) This discrepancy as lo the age o f the Ly Hoa vertebrate fauna can be explained by the fact that yunnanolepidoids and Y o w ig o ie p is have not been found in the Ciivelian o f C hina, either because ihey have not been looked for, or because comparable deltaic or marginal facies are rare As we shall see below, it may also be explained by endemism due to a temporary geographical isolation IIw l y llm t /a n n a am i S o u th e a s t A s ia D e v o n ia n l> a l(ic t> t;c tii!ra /> liy The Southeast Asian peninsula is currently regarded by geologists as being composed o f two m ajor blocks, the Indochina and Sibumasu blocks, and a number o f smaller "m icroplates" or terranes, such as the Simao, Song Da and Phu llo a l lerranes (G atinsky and I lutchison 1987; Hutchison 1989; Metcalfe 1988, 1990, 1993 1995; W u e l a t 1995; M o u rei 1994; Tran Van T ri 1995) These units are limited by sutures, the ages o f which still are a matter o f debate The current theory (e.g., Metcalfe 1996) is that all these lerranes originate from the northeastern margin o f the Gondwana, and have drifted northward during the Palaeozoic to become accreted lo the South China (or Yangtzi, or Cathaysia) Block, which is also assumed lo have detached earlier from the Gondwanan margin There is now fairly good evidence that the Sibumasu (or Shan-Thai) terrane came into contact with the Indochina (or K on lu m , or A nnam ia) Block in Late Permian limes, through the closure o f the Palaco-Telhys The question o f the age o f the accretion o f the Indochina Block to the South China Block is far less resolved Since the mid-1980s, two roughly parallel suture zones between these two blocks have been considered; ihe Song Da suture and the Song M a suture (Sengor 1984; Sen go r and Hsii 1984; see for review, M etcalfe 1996) A small terrane, the Song Da terrane, was envisaged between these two suture zones The age o f these two sutures has long been debated The Son D a suture was daled from the Silurian to ihe Triassic (Sengor and Hsii 1984; M ouret 1994) and the Song M a suture from the Lower Carboniferous to the Triassic (Sengor and Hsii 1984, Hutchison 1989) Nowadays, the Song M a suture is unanimously accepted a's a m ajor suture The Song D a "suture” was probably a rift zone in the Late Permian (Phan Van Quynh 1995; Phan C'u Tien 1993), but there are strong palaeontolog­ ical evidence that the area referred to as the “ Song D a ” terrane was already part o f the South China Block in Early Devonian times (Janvier e l a l 1996) Other presumed suture zones, such as the “ Da N ang Line” in southern Vietnam (Fig IA ), are o f debated age although il has been suggested that they may be Devonian The question o f the palaeogeographical relationships between the Indochina Block (i.e south o f the Song M a suture and easl o f the N a n -U tta d it suture) to the South China Block is thus complex As for the Devonian, some marked dilVerences have long been noticed between the invertebrate faunas o f northern and central Vietnam (Tong Duy Thanh e t a l 1996; Tong Duy Thanh 1993; Ton g -D zu y Thanh e l a l 1996b) The Lower Devonian brachiopod communities o f the Bac Bo (e.g the l l o w i t t i a w a n g i and I ' iir y s p ir if e r to n k in e n s is communities o f the Bac Bun and M ia Le formations respectively) are widespread north o f the Song M a suture, as well as in the Lower Devonian o f Yunnan and Guangxi in China, but never occur south o f the Song M a suture Conversely, the M iddle Devonian l- 'n u iiiu e lla ro n e n s is community o f central Vietnam never occurs in the Bac Bo This has led some authors (e.g., Metcalfe 1995) lo suggest a biogeographical barrier between the two areas, at any rate in Devonian times It is noteworthy that this launal dilVerence is well-marked in the Lower Devonian, but less so in the Eifelian; alter the G ivetian it fades away and most invertebrate taxa are ubiquitous, yet o f palaeotethyan aflinity This suggests that if any oceanic barrier existed between the Indochina and South China blocks in Devonian times, it could not have been very large The discovery o f the Ly llo a vertebrate fauna, which includes at least two reputedly South Chinese endemic taxa, is evidence in favour o f a close connection between the Indochina and South China blocks in Devonian limes (T o n g -D /u y Thanh e i a l 1996a) C ontrary to most Devonian invertebrates previously considered, these vertebrates are closely bound to clastic, terrigenous, marginal, and deltaic facies The fact that yunnanolepidoid antiarchs or galeaspids, for example, never dispersed to neighbouring blocks (other than the Indochina Block), such as the eastern Gondwana or Kazakhstan, is enough to demonstrate that they were unable to cross major oceanic barriers and can practically be treated as continental organisms for palaeobiogeographical recon­ structions This distribution o f the yunnanolepidoid or yunnanolepidoid-like antiarchs and the youngolepidids exclusively in the South China and Indochina blocks thus provides strong support for some continental connection between these two blocks in Devonian limes (T ong-D zuy Thanh e t a l 1996a) This is also strongly supported by the similar spore assemblage in the M iaoko u Form ation o f Yunnan and the D ong Tho Form ation (To ng -D zu y Thanh and Cai 1995) The age discrepancy between the yunnanolepidoids o f the South China and Indochina blocks remains a riddle, but may be explained by endemism on the Indochina Block One may imagine that the two blocks were united in the Silurian and Early Devonian, allowing a wide distribution o f these vertebrate taxa, and that a separation (in the form o f a rift in the Song M a area) occurred by the end o f the Early Devonian or M iddle Devonian, accounting for the dilVerence in invertebrate faunas, whereas yunnanolepidoids and youngolepidids survived as relics on the Indochina Block, until a final suture occurred in the Early Carboniferous, as evidenced by the discovery o f a South; Chinese (and Laurasian) land flora in the Indochinese part o f Thailand (Laveine Philippe Janvier c l cil 404 e l a t 19 4) T h is w o u ld also agree w ith M e tc a lfe 's (1 9 ) the D e v o n ia n e n d e m ic ta x a o f the S o u th C h in a B lo ck th e o ry o f a S ilu ria n to E a r ly D e v o n ia n n o r th w a r d d r if tin g o f the S o u th C h in a B lo c k , fo llo w e d by a s im ila r d r iftin g o f the In d o c h in a B lo c k , w ith a c o llis io n b etw een b etw een th e G iv e tia n age o f th is fo r m a tio n a n d the g e n e lly accep ted s lra tig r a p h ic a l d is trib u tio n o f the H o w e v e r, it is p e c u lia r because of the d is c rep a n c y these tw o b lo c k s w ere first u n ited w h ile still p a rt east G o n d w a n a n m a rg in , in E a r ly P a la e o zo ic N o w , w e k n o w th a t b o th the S o u th an d C h in a b lo c k s , as w e ll as the T a r im , a lre a d y an e n d e m ic v e rte b te fa u n a (w ith galcaspids v e rte b te ta x a it c o n ta in s , w h ic h a re e s s en tia lly o r u n iq u e ly L o w e r D e v o n ia n e lsew h ere S ince the G iv c lia n age o f the D o n g T h o F o r m a tio n is w e ll-c o n s tra in e d by p la n ts , spores, a n d u n d e rly in g in v e rte b te fau n as , this d is c re p a n c y is p ro v is io n a lly a ttr ib u te d to a te m p o ry e n d e m is m on the In d o c h in a B lo c k T h e m ost s trik in g a ffin itie s w ith the S o u th -C h in e s e D e v o n ia n v e rte b te a n d p ro b a b ly y u n n a n o le p id o id -lik e a n tia rc h s ) as e a rly as the L la n d o v e ry (W a n g 1993; L iu 1995) T o date,, non e fau n as a re s h o w n by the y u n n a n o le p id o id a n tia rc h I 'u k ln tc lc p is an d a s a rc o p tc ry g ia n w h ic h resem bles o f these ta x a y o u n g o le p id s O t h e r species h av e c ith e r East A s ia n and A u s tr a lia n a ffin itie s (A c a n llio t h o r a c i) o r u n d ecid ed In d o c h in a a n d S o u th C h in a in the L a te D e v o n ia n o r E a r ly C a rb o n ife ro u s H o w e v e r , a c c o rd in g to M e tc a lfc 's m o d e l, o f the tim es N o rth d is p la y occu rs in A u s tr a lia o r elsew h ere T h is m eans th a t these b lo c k s w e re a lre a d y s e p a rate d fro m the G o n d w a n a n m a rg in in E a r ly S ilu ria n tim es L a te S ilu ria n v e rte b te s are n o w k n o w n fro m the In d o c h in a B lo ck ( D a i G ia n g F o r m a tio n ; T o n g - D z u y T h a n h e l a l b io g e o g p h ic a ffin itie s ( L y lu n ilc p is ) g p h ic a lly m ost s ig n ific a n t D e v o n ia n The b io g c o - v e rte b te s are g e n e lly associated w ith c lastic, m a r g in a l, terrig e n o u s 1997) b u t non e o f th e m seems to be p a r tic u la rly re la te d facies, w h e re v e r they a re fo u n d , a n d these are k n o w n to to a S o u th -C h in e s e e n d e m ic , w ith the possible e x c ep tio n d is p la y s tro n g ly e n d e m ic b io g e o g p h ic a l p attern s Y u n n a n o le p id o id a n tia rc h s in p a r tic u la r, w ere k n o w n o f a d o u b tfu l y o u n g o lc p id id s a rc o p tc ry g ia n N e v e rth e ­ less they are fo u n d in a s s o c ia tio n w ith a b c h io p o d fa u n a w h ic h is w id e ly k n o w n in n o rth e rn V ie tn a m a n d w estern Y u n n a n T h e fact th a t y u n n a n o lc p id o id s and y o u n g o le p id id s a rc E a rly D e v o n ia n (a n d possibly S ilu ria n ) in C h in a suggests th a t an In d o c h in a S o u th fro m such facies o f S o u th C h in a , the la te E a rly D e v o n ia n , b ut fro m the S ilu ria n a p p a re n tly never dispersed to w a rd the m a jo r n e ig h b o u rin g c o n tin e n ta l b lo cks (K a z a k h s ta n , S ib e ria A u s tr a lia ) T h e ir o c c u r­ rence on the In d o c h in a B lo ck thu s im p lie s very C h in a c o n tin e n ta l c o n n e c tio n m u st h ave existed p r io r to the e x tin c tio n o f these la x a in S o u th C h in a , th a t is the latest E a r ly D e v o n ia n T h e absence o f g aleaspid re m a in s close g e o g p h ic lin k s betw een the la tte r a n d the S o u th C h in a B lo ck in llie D o n g T h o F o r m a tio n , a lb e it n eg a tiv e e viden ce, suggests th a t g alcaspid s e ith e r n ever e n te re d the In d o c h in a B lo c k , o r b ecam e e x tin c t th e re b e fo re (lie G iv c tia n N e v e rth e le s s , o ne m u st keep in m in d th a t, even Acknowledgem ents ■ -T h e present study has been made thanks to the financial support o f the Program o f Fundam ental Research in N a tu ral Sciences o f Vietnam and the grant nr 50S9 93 o f the N atio n al Geographic Society The authors are grateful to Daniel G o i'jet (Paris) for his knowledgeable advice on placodcnns P h o t o g r a p h s were taken by D Serretle Paris and maps were compiled by I I I.avina Paris in S o u th C h in a , g alcaspid s a re e x tre m e ly re a fte r the P g ia n O n a c c o u n t o f these new d a ta , o n e m a y p ro p o se the fo llo w in g scenario; REF E RE N CE S In F.arlv D e v o n ia n o r L a te S ilu ria n tim es, o r even e a rlie r, the In d o c h in a B lo ck w as a m e re s o u th w e s te rn p ro lo n g a tio n o f the S o u th C h in a B lo c k , b e a rin g ro u g h ly the sam e e n d e m ic v e rte b te fa u n a ; By the end o f the E a r ly D e v o n ia n , som e b a r rie r o c c u rre d b etw een the tw o areas, in the fo r m o f e ith e r an e p ic o n tin e n ta l sea o r, m o re lik e ly , a m o d e te ly b ro a d rift (a l the level o f the S o ng M a s u tu re ) w h ic h isolated the c o astal v e rte b te fa u n a o f the In d o c h in a B lo c k T h is b a r rie r m a y h ave p ro te c te d it fro m the c o n c u rre n c e o f o th e r g ro up s (e g , e u a n tia rc h s ) w h ic h , in S o u th C h in a , possibly o u lc o m p e te d the g aleaspid y u n n a n o le p id la u n n T h is m a y e x p la in the s u rv iv a l o f y u n n a n o le p id s y o u n g o le p id id s a n d a c a n th o lh o c id s u n til the G iv c lia n in In d o c h in a , an d the d e v e lo p m e n t o f e n d e m ic p h ly c ta e n iid s such as l.v h o a le p is A t this stage, this h yp o th es is is sisten t w ith the p a la e o g e o g p h ic a l re c o n s tru c tio n p ro p o se d by Seotese an d M c K e r r o w (1 9 ) fo r the M id d le L a te D e v o n ia n b o u n d a ry : In the F a t ly C a rb o n ife r o u s , this b a r rie r d is a p p e a re d a n d the In d o c h in a B lo ck was c o n n e c te d a n e w to the S o u th C h in a B lo ck at the level o f the S o n g M a sutu re C M M (1982) The brainease o f )'oungolcpi\ a Low er Devonian crossoptcrygian fro m Y unnan south-western Chin«» Ph.D thesis D epartm ent o f G eology S tockholm U n iversity S tockholm ( hang M M am i Y u X IV (1981) A new crossoptcrygian )'t>itngoh pi\ praecursor gen et sp nov fro m the Low er D evonian o f Lastern Y unn an C hina Scicntia Sinira 24 89 97 G a tin sky Y G a ml I lutehison C S (19X7) Cathaysia G o iu lw a n a la m l am i the Paleotcthys in the e vo lu tio n o f tinental Southeast Asia Hull ( i t' o l Soc ;Malaysia 20, 179 199 G o 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(1444) The early from Trang Х а (В а с Thai, Devonian Viel N am ) T ilin g Bo Viel Nam [Stratigraphy o f the Devonian and the Upper Devonian Lower Carboniferous in the lower p a il o f Central