1995b lower devonian biostratigraphy and vertebrates of tong vai valley

liaojaoshanensis are provided on the basis o f this material, with special reference to the structure and ornam entation o f the exoskeleton... Thamnopora polyforata Thin-bedded, dark-g

L O W E R D E V O N I A N B I O S T R A T I G R A P H Y A N D

V E R T E B R A T E S OF T H E T O N G VA I V A L L E Y ,

V I E T N A M

b y T O N G - D Z U Y T H A N H , p J A N V I E R , T A H O A P H U O N G

a n d D O A N N H A T T R U O N G

Ab s t r a c t A new vertebrate assemblage is described from the base o f the K hao Loc F orm ation at Tong Vai,

D ong Van district, H a Giang Province, Vietnam It includes the galeaspid Polybranchiaspis liaojaoshanensis, two acanthothoracid placoderms, and the sarcopterygian Youngolepis praecursor This assemblage is quite

similar to th at o f the X itun Form ation o f Y unnan (Late Lianhuashanian to Early Nagaolingian) and can also

be correlated with the vertebrate faunas which occur a t the base o f the Bac Bun F orm ation o f the Bac Bo in

Vietnam New d ata on the m orphology o f P liaojaoshanensis are provided on the basis o f this material, with

special reference to the structure and ornam entation o f the exoskeleton

THE T ong Vai valley is situated n e a r the Chinese V ietnam ese b o r d e r , west o f the Q u a n Ba ham let,

on the H a G ia n g -Y e n M in h m ain ro a d in the D o n g V an district (Text-fig 1) F ro m Q u an Ba, the ro a d to T o n g V ai ru n s th ro u g h a pass in a m o u n tain o u s area o f lim estone an d sericite-bearing shales T he distance betw een Q u an Ba an d the T ong V ai valley is directly a b o u t 10 km (18 km by road)

T he Palaeozoic rocks o f the T ong V ai valley an d its surro u n d in g s were considered by D ep rat (1915) to be L ate C am b rian to E arly O rdovician in age (see also the geological m ap o f the M a Li

Po area in this w ork) Y assilevskaya {inD ovjikov 1965) regarded the ‘L uong K h o L im esto n es’ o f the T ong V ai valley as O rdovician, on the basis o f po o rly preserved b rach io p o d s an d ostracodes o f

‘ O rdovician-S ilurian aspect ’

In 1973, T a T h a n h T ru n g an d H o an g A n h T ru o n g were the first to collect early D evonian fossils

from this area These included som e b rach io p o d s e.g Lingulella dussaultiP atte) a n d a specim en o f

the galeaspid fish Polybranchiaspis sp (T a T h a n h T ru n g 1978) H o a n X u an T in h (1976), chief engineer o f the G eological M ap p in g T eam f o r the B ao Lac sheet, correctly described, a p a rt from

a few inaccuracies, the stratig rap h ical sequence o f the E arly D evonian in the T ong Vai valley, and his description w as later referred to in the ‘ S tratig rap h y o f V ie tn a m ’ (Y u K huc a n d Bui P h u M y 1990) O f the five m em bers he described, the first tw o m ay n o t belong to the D evonian, b u t ra th e r

represent terrigenous beds th a t V assilevskaya (in D ovjikov 1965) referred to the L ate C am brian,

an d D e p ra t (1915) to the O rdovician The description o f the Polybranchiaspis-beanng levels in

H o an g X u an T in h ’s (1976) p ap er is quite different f r o m the one m ade later by T a T h a n h T ru n g (1978), w ho collected the galeaspids from ‘d a rk grey carb o n ate-b earin g terrigenous d e p o sits’ O n

the co n trary , H o a n g X u an T in h (1976) depicted his Polybranchiaspis-bearing th ird m em ber o f the

L ow er D ev o n ian as a succession o f opalescent, yellowish q u artzitic sandstones, siltstones and

m udstones, w hich he referred to as the ‘Bac B un S u ite ’ T o this au th o r, the ‘ Bac B un S u ite ’ com prised the Si K a an d Bac Bun fo rm atio n s first described by D è p ra t (1915) a n d later reviewed

by T o n g D zuy T h a n h (1967, 1982) an d T o n g D zuy T h a n h et al.(1986) A ccording to H o a n g X u an

T h in h ’s description, his th ird m em ber m ay be attrib u te d to the Si K a F o rm a tio n , alth o u g h , as will

be m entioned below , such coarse terrigenous rocks do n o t seem to occur in the L ow er D evonian

o f the T ong V ai valley

[Palaeontology, Vol 38, Part 1, 1995, pp 169-186, 3 pis.] © The Palaeontological Association

170 P A L A E O N T O L O G Y , V O L U M E 38

t e x t - f i g 1 Locality M ap; 1 4 location o f invertebrate and vertebrate-bearing exposures o f m ember 3 o f the

Tong Vai section

G E O L O G I C A L S E T T I N G

In sum m er 1991, one o f us (T H P ) m ade a field trip to the T o n g Vai valley an d recorded several fossiliferous localities w hich have since been investigated, in spring 1993, by T ong-D zuy T h a n h , T a

H o a P h u o n g an d D o a n N h a t T ru o n g in the fram e o f the project K T 04.6.1.1 o f the V ietnam ese

F u n d am en tal R esearch P ro g ram in N a tu ra l Sciences

Description

T he eastern slope o f the T ong V ai valley consists o f sericitized shales an d d a rk grey lim estones,

d ated, w ith reservations, as L ate C a m b rian (D ovjikov 1965) T he rest o f the area consists m ainly

o f lim estones a n d in terbedded m arls T he com plete stratig rap h ical colum n in T o n g Vai, from the

U p p e r C am b rian to the D evonian, is still u n k n o w n because o f tectonic com plications H ow ever, six successive m em bers can be distinguished in the D evonian, w ith o u t any break s (Text-fig 2) These are, from base to t o p :

1 T he basal m em ber consists o f light grey, relatively thin -b ed d ed (200 m m ) and som etim es opalescent, striped lim estones They resem ble the U p p e r Palaeozoic lim estones w idespread in the

n o rth o f V ietnam Som etim es, they display a schistosity to various degrees The co n tact betw een these lim estones and the underlying U p p er C am b rian is n o t clear and the thickness o f this m em ber

c a n n o t be estim ated precisely A thickness o f only c 300-350 m can be observed, b u t the sequence

m ay be thicker They have yielded only scolecodonts and sm all ro u n d ed m asses o r organic m atter (F P aris, pers com m )

T O N G - D Z U Y T H A N H E T AL : D E V O N I A N V E R T E B R A T E S 171

Grey, recrystallized limestone with numerous

indetermined remains of Amphipora and tabulate corals Thamnopora polyforata

Thin-bedded, dark-grey limestones Tabulate corals belonging to the

Euryspirifer tonkinensis -fauna

Thin-bedded, dark-grey limestones and mudstones with plant remains

Thin-bedded, dark-grey limestones with interbedded marls and calcareous shales with

Polybranchiaspis liaojaoshanensis

[ and Howittia wangi

Thin-bedded, dark-grey, recrystallized limestones with thin siliceous interbeds in the lower part

j 1 I ■L>r) Light-grey, striped limestones

Sericitized shales Lower Paleozoic

TEXT-FIG 2 Stratigraphical section o f the Early D evonian o f the Tong Vai valley

2 The second m em ber begins w ith cherts an d dark-grey, recrystallized lim estones an d dolom ites

F u rth e r up, the cherts d isap p ear an d the u p p er p a rt o f the m em ber consists only o f recrystallized

lim estones an d dolom ites The thickness o f this m em ber is c 200 m It has yielded only scolecodonts

(F P aris, pers com m )

3 The th ird m em ber consists o f m arls w ith in terb ed d ed d a rk grey lim estone an d m udstone layers Locally, lenses o f calcareous shales occur, in p artic u la r in the m iddle p a rt o f the m em ber, and these w eather to a pin k colour A b u n d a n t v erteb rate rem ains occur a b o u t 50 m above the base o f this m em ber T hey are associated w ith ostracodes an d occur in d a rk grey calcareous siltstones (see below fo r fau n al list)

40 m upw ards, on the ro a d from the T o n g Vai valley to B an T h an g (1, 2, Text-fig 1), some

b rachiopods were collected in m arls T hey are referred by D u o n g X u an H ao an d Le V an D e (1980)

to Howellella ex gr crispa (H isinger) an d H ysterolites wangiformis Z uong The la tter species is

H ow ittia wangi (Orientospirifer wangi H o u o f Chinese au th o rs) O ther b rach io p o d s occur n e a r the

to p o f this m em ber, on a sm all hill on the roadside close to L u ong K h o village (3, Text-fig 1) and

172 P A L A E O N T O L O G Y , V O L U M E 38

were referred by D u o n g X u a n H ao to H ysterolites wangiformis (H ow ittia wangi) an d Tadschikial aff xuanbaoi Z uong The latter is sim ilar to the type m aterial from the low erm ost Low er D evonian

o f the low er D a R iver basin (northw estern V ietnam ) F ro m T a T h a n h T ru n g ’s (1978) description,

his Polybranchiaspis sp an d Lingulella dussauld (Sam ple 2808/1) were certainly also collected in this

m em ber The to ta l thickness o f this th ird m em ber is c 200 m.

4 T he fo u rth m em ber consists o f thin-bedded black lim estones intercalated w ith calcareous shales an d m udstones, som e o f w hich are coal-bearing It has yielded som e u ndeterm ined p lan t

rem ains w hich were collected from the m udstones It is c 50 m thick.

5 The fifth m em ber consists o f thin-bedded, d a rk grey lim estones an d m arl lenses, which

co n tain ta b u late corals (in p articu lar, a b u n d a n t Favosites kolim aensis R ukhin) o f the Euryspirifer tonkinensis-fauna Its thickness is c 80 m.

6 T he u p p erm o st m em ber consists m ainly o f light grey recrystallized lim estones w ith a b u n d a n t traces o f ram ifo rm stro m ato p o ro id s These lim estones are very sim ilar to the M iddle D evonian

Am phipora lim estones form erly described by F ren ch geologists (‘C alcaires a A m p h ip o ra '; Saurin

1956) T he to p o f m em ber 6 ca n n o t be observed in the area o f T ong V ai valley, because o f faulting

Its observed thick n ess is c 250 m.

Discussion

F ro m H o an g X u an T in h ’s (1976) account, one o f us (T ong-D zuy T h a n h 1982; T ong-D zuy T h an h

et al 1986) referred the Polybranchiaspis-bearing beds o f the T o n g V ai valley to the Si K a

F o rm atio n The new field observations presented in this p ap er suggests a rein terp retatio n o f the

D ev o n ian o f this area The fau n a o f the th ird m em ber un q u estio n ab ly belongs to the H ow ittia wangi

assem blage, w hich defines the B acbunian regional stage in the Bac Bo (n o rth ern V ietnam , form erly

called the T onkin) Its m ajo r representatives are H ow ittia wangi an d Howellella ex gr crispa, and

the v ertebrates are quite sim ilar to those in the corresp o n d in g stratig rap h ical level o f D o n g M o and

T ran g X a (T ong-D zuy T h a n h an d Janvier 1990) T he only, m inor, difference is the presence o f the

b rach io p o d T adschikial aff xuanbaoi, sim ilar to the type m aterial from no rth w estern V ietnam

(D u o n g X u a n H ao an d Le V an D e 1980) T here is som e difference betw een the T ong Vai vertebrate fau n a an d th a t o f m ore southernly localities, such as T ran g X a an d D o n g M o (T ong-D zuy T h an h

an d Janvier 1987, 1990) A lth o u g h Youngolepis is present in b o th , no ac a n th o th o ra cid m aterial has

been recorded from the la tte r tw o localities M oreover, there is a m ark ed difference in the structure

a n d o rn a m e n ta tio n o f the exoskeleton o f the galeaspid Polybranchiaspis from T o n g Vai (see below)

a n d those o f the p o o rly preserved specim ens from D o n g M o referred to by T ong-D zuy T h a n h and

Janvier (1990, fig 4) as ‘Polybranchiaspis s p ’ In the latter, the o rn a m e n ta tio n consists o f simple,

ro u n d ed tubercles devoid o f a b asal recess, w hich are aligned into ridges along the shield m argin

T herefore it is p ro b ab le th a t the D o n g M o galeaspid, alth o u g h a p olybranchiaspidiform , does n o t

belong to the genus Polybranchiaspis, b u t to a form w hich is closer to Bannhuanaspis (Janvier et al

1993) in exoskeletal structure

A ccording to the observations o f one o f us (T D T ), the fo u rth m em ber o f the T ong V ai section

is quite sim ilar in lithology to the base o f the K h a o Loc F o rm a tio n in the B an H in h -K h ao Loc section, w hich is situated n o t far S o u th o f T ong Vai It can th u s be suggested th a t the lim estone o f

E X P L A N A T I O N O F P L A T E 1

Figs 1-3 Polybranchiaspis liaojaoshanensis Liu, Pragian, K hao Loc F orm ation, Tong Vai, H a G iang Province,

Vietnam 1, BT 170, head shield in dorsal view, photographed in immersion to show the pineal foram en (a) and elastomere cast o f its incomplete counterpart (b); note the ostracodes surrounding the specimens 2, BT 171, right side o f a headshield in dorsal view, elastomere cast o f natural impression 3, BT 172, left side o f a headshield in internal view, elastomere cast o f the internal surface o f the exoskeleton and the ornam entation

o f the posterior wall o f the m edian dorsal duct All x 2

PLATE 1

TO NG -DZUY THANH et a l, Polybranchiaspis

174 P A L A E O N T O L O G Y , V O L U M E 38

m em ber 4 an d u pw ards can be attrib u te d to the K h ao Loc F o rm a tio n (P ragian-G ivetian), w hich

is w idespread in the N o rth w est o f H a G iang Province (Text-flg 2) T he lim estones o f the u p p erm o st

m em ber o f T o n g Vai (m em ber 6) can be co rrelated w ith the u p p er p a rt o f the K h a o Loc F o rm a tio n and the B an P ap F o rm atio n The la tte r fo rm atio n is widely d istrib u ted in the N o rth o f V ietnam

This co rrelatio n is su p p o rted by the ab u n d an ce o f Amphipora, a guide fossil fo r the M iddle

D ev o n ian lim estone in the N o rth o f V ietnam

The red beds o f the Si K a F o rm a tio n do n o t occur in the T ong V ai area Instead, below the

B acbunian faunal assem blage (fishes and H ow ittia wangi), there is a thick series o f lim estones

(m em bers 1 an d 2), w hich are devoid o f stratigraphically significant fossils (only scolecodonts are found) They m ay be a lateral equivalent o f the Si K a F o rm atio n

T he co rrelatio n o f the Bac B un an d overlying M ia Le fo rm atio n s o f the Bac Bo w ith the N ak ao lin g (N agaoling) an d Y u k ian g fo rm atio n s (or stages) or so u th ern C hina have been p ro p o sed in o u r

form er pap ers (T ong-D zuy T h a n h 1982; T ong-D zuy T h a n h et al 1986, 1988a, b; T ong-D zuy

T h a n h an d Jan v ier 1987, 1990) on the basis o f b o th v ertebrate an d invertebrate faunas It is fu rth er

su p p o rted by the new m aterial described herein

T he B acbunian vertebrates in n o rth eastern V ietnam are frequently fo u n d in association w ith

invertebrates o f the H ow ittia wangi assem blage o r in beds w hich im m ediately underlie this

assem blage By co m p ariso n w ith the d a ta provided by S T W ang (1991), the B acbunian vertebrate assem blage (in D o n g M o, T ran g X a, T ong Vai and o th er V ietnam ese localities) is very sim ilar to

th a t o f the X itu n F o rm a tio n o f the C uifengshan G ro u p in eastern Y u n n an (C hina) M oreover, the

L ow er D ev o n ian succession in the n o rth e a ste rn Bac Bo, from the Sika to Bac B un an d M ia Le

fo rm atio n s is closely sim ilar to th a t from the L ia n h u ash an to N ak ao lin g an d Y uk ian g form ations o f

G uangxi, C hina (Y ang et al 1981) This striking resem blance is seen in b o th the lithology an d the

fau n al assem blages As a result o f the greater fau n al diversity in n o rth e rn V ietnam , these form ations can be precisely d ated, in p a rticu lar the M ia Le F o rm a tio n , w hich is clearly P rag ian in age (Tong-

D zuy T h a n h 1982; T ong-D zuy T h a n h et al 1988a) T his has been recently confirm ed by the discovery o f d acryoconarids o f the N ow akia zlichovensis an d N barrandei zones, an d a rich

co n o d o n t assem blage o f the Perbonus-zone (determ ined by P h am K im N g an , H anoi), in the base

o f the lim estones w hich overlie the M ia Le F o rm a tio n in the D o n g V an - M a L u section (H a G iang

P rovince, n ear the C hinese-V ietnam ese border) H ere, in the u p p erm o st beds o f the M ia Le

F o rm atio n , one o f us (T H P ) discovered new d acryoconarids am ong w hich is the w ell-know n

P rag ian species N ow akia arcuaria (H L ardeux, pers com m ).

In conclusion, these d a ta suggest th a t: (1) the Bac B un F o rm a tio n , w hich underlies the M ia Le

F o rm a tio n an d co n tain s the vertebrates described below, m ay be L ate L och k o v ian to E arly P rag ian

in age; (2) the Bac Bo area o f n o rth e rn V ietnam an d the Y u n n a n -G u a n g x i areas o f so u th ern C hina belong to the sam e p alaeobasin, characterized by endem ic fish fa u n a s ; (3) the B acbunian v ertebrate and inv erteb rate faunas o f n o rth e rn V ietnam display m ixed features o f the Y u n n a n and G uangxi assem blages; an d (4) they co rresp o n d to a foreshore to near-shore palaeoenvironm ent F u rth e r south, in the P h u L uong an d T ran g X a area, the larger a m o u n t o f detritic sedim ents in the Sika and Bac Bun F o rm atio n s suggests a n even m ore n ear-shore to deltaic type o f environ m ent

E X P L A N A T I O N O F P L A T E 2

Figs 1-3 Polybranchiaspis liaojaoshanensis Liu, Pragian, K hao Loc F orm ation, Tong Vai, H a G iang Province,

Vietnam 1, BT 173, incomplete headshield in dorsal view, elastomere cast o f the specimen (a, x 2), close-

up view o f the m edian dorsal opening, lit from the left (b, x 3), and S.E.M photograph o f the elastomere cast o f the anterior wall o f the median dorsal opening (c, x 20; d, x 15), to show the denticles on the anterior wall o f the duct 2, BT 172 (same specimen as PI 1, fig 3), S.E.M photograph o f an elastomere cast o f the ornam entation on the posterior wall o f the m edian duct, partly folded against the internal surface o f the exoskeleton, x 15 3, BT 174, incomplete headshield in ventral view, elastomere cast showing the ventral rim

o f the derm al headshield and the internal surface o f the dorsal exoskeleton, x 2

PLATE 2

TO NG -DZUY THANH et al., Polybranchiaspis

176 P A L A E O N T O L O G Y , V O L U M E 38

t e x t - f i g 3 Polybranchiaspis liaojaoshanensis Liu a , reconstruction o f the headshield in dorsal view (based on several specimens from Tong Vai); b , distribution o f the sensory-line canals Abbreviations: iorb, infraorbital canal; mdo, m edian dorsal opening; mil, m ain lateral-line; orb, orbit; pif, pineal foram en; sorb, supraorbital

canal; tcom, transverse commissural canal; 1/1-4, transverse lateral canals.

t e x t - f i g 4 Polybranchiaspis liaojaoshanensis Liu a , reconstruction o f the exoskeleton around and inside the median dorsal opening; b , reconstructed sagittal section through the median dorsal opening and d u c t; c, vertical section through two tubercles o f the exoskeleton (combined from several thin sections); d , vertical section

through the exoskeleton and a sensory-line canal (combined from several thin sections) Abbreviations: brec, basal recess o f exoskeleton; ctb, central tubercle; fp t, forward pointing tubercle o f median dorsal duct; Itb, lateral, or secondary tubercle; md, m edian dorsal duct; mdo, median dorsal opening; pb, perichondral bone;

sbap, subaponeurotic vascular canals; sic, sensory-line canal.

S Y S T E M A T I C P A L A E O N T O L O G Y

The v ertebrate m aterial from the T o n g Vai valley consists m ainly o f well preserved galeaspid headshields, as well as isolated placoderm plates an d the cosm ine-covered derm al bones an d scales

o f a sarcopterygian All the specim ens com e from the m arls an d shales o f m em ber 3, an d are associated w ith sm ooth-shelled ostracodes The m aterial described herein is registered in the

t e x t - f i g 5 Polybranchiaspis liaojao-

shanensis Liu, reconstruction o f the

exoskeletal headshield a , ventral view;

B, lateral view A bbreviations: brn,

branchial notch; n, notch; orn, oral

notch

T O N G - D Z U Y T H A N H E T AL.: D E V O N I A N V E R T E B R A T E S 177

collection o f the G eological M useum (Bao T an g D ia C h at, here abbreviated BT), 6 P h am N g u Lao Str., H an o i C asts are deposited in the collection o f the L ab o rato ire de P aléontologie, M uséum

N a tio n a l d ’H istoire N aturelle, Paris)

Class g a l e a s p i d a H alstead T arlo, 1967

O rd er p o l y b r a n c h i a s p i d i f o r m e s Liu, 1965

F am ily p o l y b r a n c h i a s p i d i d a e Liu, 1965

G enus p o l y b r a n c h i a s p i s Liu, 1965

The genus Polybranchiaspis was erected by Liu (1965) fo r the species P liaojaoshanensis Liu, 1965 (erroneously spelled as P liaojiaoshanensis by Liu 1975 an d several subsequent a u th o rs) from the

C uifengshan an d X itu n fo rm atio n s o f Y u n n a n (C hina) Polybranchiaspis now com prises nine

species (including the type species), all from Y unnan

Polybranchiaspis liaojaoshanensis Liu, 1965

Plates 1-2, Plate 3, figures 1 , 2; Text-figures 3-5

Type specimen An alm ost complete headshield (Institute o f Vertebrate Palaeontology and Palaeo-

anthropology, Beijing, N o V.3027; Liu 1965, pi 3, fig 1), from the Cuifengshan G roup at Qujing,

Yunnan A relatively large hypodigm is now also known from this locality Some other Polybranchiaspis species, e.g P gracilis Cao, 1985, P yunnanensis Cao, 1985, P rhombicus Cao, 1985 and P sinensis Cao, 1985,

described from the same locality and form ation, are probably reflections o f intraspecific variation within

P liaojaoshanensis.

Material The m aterial from Tong Vai consists o f five more or less complete headshields (BT 170-175) and

num erous exoskeleton fragments (not numbered)

178 P A L A E O N T O L O G Y , V O L U M E 38

Locality and horizon All the specimens described are derived from the four fish-bearing exposures o f the Tong

Vai valley (1^4, Text-fig 1), which correspond to the same shaly horizon in the basal part o f the third member

o f the Tong Vai section (second m ember o f the K hao Loc Form ation proper; Text-fig 2)

Description The headshield o f the Polybranchiaspis species from Tong Vai is indistinguishable from th at of

P liaojaoshanensis Liu from the Cuifengshan Form ation o f Y unnan (Liu 1965, 1975) O n the basis o f the

photographs o f the incomplete headshields discovered by T a T hanh T rung (now deposited in the Geological

Institute, Beijing), Tong-Dzuy Thanh and Janvier (1987) referred the Vietnamese specimens to P cf gracilis

Cao, recorded from the same form ation by Cao (1985) The latter was said to be characterized by posterolateral

orientation o f the foremost lateral transverse sensory-line canal (til, Text-fig 3 b ) However, exam ination of

large populations o f P liaojaoshanensis from Y unnan now suggests that P gracilis lies within the range of variation o f P liaojaoshanensis.

The exoskeleton o f the Polybranchiaspis specimens from Tong Vai is well preserved (in contrast to previously

described Chinese material) and has yielded new inform ation about its structure and ornam entation M ost of the specimens have been prepared as impressions, by removing the exoskeleton with hydrochloric acid, and

m aking elastomere casts (PI 1, figs lb , 2 - 3 ; PI 2 , fig 1; PI 3 , fig 1)

Ornamentation In external aspect, the ornam entation o f the exoskeleton o f P liaojaoshanensis shows relatively

large but low, star-shaped tubercles (PI 1, figs lb , 2 - 3 ; PI 2 , fig la ; PI 3 , fig la ; Text-figs 3 a , 4 c - d , 5 ) These are smaller in the anterior part than in the posterior part o f the dorsal surface o f the shield Also, in the posterior p art o f the shield, particularly on the m edian dorsal crest and along the lateral margins, they tend

to become elongated, and even spine-shaped (Text-figs 3 a , 5 b ) The tubercles on the ventral rim o f the shield are very small (PI 1, fig 3; PI 2 , fig 3; Text-fig 5 a ) They are irregular in shape, with a large m edian elevation,

or central tubercle (ctb, Text-fig 4c), and four or five ‘branches’, each o f which is made up by two or three

smaller, lateral tubercles (PI 3 , fig la ; Itb, Text-fig 4c) These ‘branches’ may unite one tubercle with

neighbouring ones A lthough the sensory-line canals are closed over most o f their course, their pattern can be traced as a result o f the presence o f double rows o f smaller tubercles (PI 1, figs lb , 2 ; PI 2 , fig la ; Text-fig

3 a ) In internal view, each o f these tubercles is hollowed by a shallow depression, or basal recess (PI 1, fig 3 ;

PI 3, fig lb ; brec, Text-fig 4c), which often leaves a more or less polygonal impression on the surface o f the

internal natural m ould o f the exoskeleton The perichondral layer o f the endoskeleton, when still present, closes

these polygonal recesses basally (PI 1, fig 3; pb, Text-fig 4c posteriorly to the orbit) This pattern has, for a

long time, given the impression th at the galeaspid exoskeleton was m ade up o f small tesserae, like th at o f

osteostracans (Halstead et al 1979) Janvier (1981) also regarded this polygonal pattern as evidence for a

honeycomb-like structure to the galeaspid exoskeleton, and com pared it with the similar structure o f the heterostracan exoskeleton Both interpretations appear now to be incorrect A vertical thin section through the

exoskeleton o f Polybranchiaspis (Text-fig 4 c - d ) displays basically the same histological structure as in the

D ong M o ‘ Polybranchiaspis sp.’, Bannhuanaspis (Tong-Dzuy Thanh and Janvier 1990, pi 1; Janvier 1990; Janvier et al 1993) and Xiushuiaspis (Changxingaspis, N Z W ang 1991), that is, an acellular, aspidine-like

structure with horizontal incremental lines There is no evidence for any type o f dentinous tissue and one cannot distinguish any histological discontinuity between the tubercles The walls o f the basal recesses are made

up o f the same kind o f lam inar hard tissue as the tubercles

The relation o f the structure in Polybranchiaspis to th at in Bannhuanaspis (where there is no basal recess and

where each tubercle seems to correspond to one exoskeletal unit, in particular in the posterior p art o f the shield)

is unclear If each o f the star-shaped tubercles o f Polybranchiaspis, with its basal recess, is regarded as a single

derm al unit, then it may be regarded primitive, and com parable to, for example, a thelodont scale with its pulp

cavity Conversely, one may consider th at the star-shaped tubercles o f Polybranchiaspis are in fact com pounds

o f much smaller units, represented by the central tubercle and the adjacent cusps on the radiating ridges Then,

each o f these ‘ prim ary ’ tubercles would correspond to one single unit o f Bannhuanaspis The form er hypothesis

could be supported by the fact th at a similar pattern (stellate or costulated tubercles with a large basal recess)

occurs also in the Silurian galeaspid Hanyangaspis (N Z W ang 1986), which was regarded by Janvier (1981)

and N Z W ang (1991) as the m ost generalized galeaspid on the basis o f several other characters The latter

hypothesis could be supported by the fact th at the structure o f the exoskeleton o f Bannhuanaspis is rem arkably

simple and passes progressively to the body squam ation Also the latter structure (small units, each corresponding to a single, simple tubercle) seems to be th at seen in m ost other galeaspids, in particular the Eugaleaspidiformes N o m ajor conclusions concerning the polarity o f the character states in the galeaspid exoskeleton can reasonably be draw n from such sparse data, and a review o f the exoskeletal structure in all other galeaspids is urgently needed