1993a a new early devonian galeaspid from bac thai province

Its transversely elongated median dorsal opening, broad posterior m argin o f head shield and posterolaterally directed m ain lateral-line are also regarded as primitive galeaspid charac

A N E W E A R L Y D E V O N I A N G A L E A S P I D F R O M

BAC T H A I P R O V I N C E , V I E T N A M

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

A b s t r a c t A new large galeaspid, Bannhuanaspis vukhuci gen et sp nov., is described from the top part o f the

Si K a Form ation or the base o f the Bac Bun F orm ation (Early Devonian, Late Lochkovian or Early Pragian)

in the Phu Luong District, Bac Thai Province, northern Vietnam The overall shape o f its head shield is suggestive o f the ‘Polybranchiaspidiform es’, but this m orphology is regarded as a primitive feature for the Galeaspida Its transversely elongated median dorsal opening, broad posterior m argin o f head shield and posterolaterally directed m ain lateral-line are also regarded as primitive galeaspid characteristics However, it shares with the ‘Polybranchiaspidiform es’ and H uananaspidiform es a large num ber o f gill openings, and with the Galeaspidiformes very short and rounded cornual processes.

T h e Galeaspida, a group o f Silurian and Devonian jawless vertebrates endemic to China and Vietnam, were first described by Y H Liu (1965), but it is now clear that the small fragments of exoskeleton referred to by M ansuy (1915, pl 1, figs 2-5) as ‘ostracoderm e indéterm iné’, from the

Si K a Form ation o f N orthern Vietnam (Lung Co-Si K a section) is the earliest known record o f this

group Y H Liu (1975) restricted the name Galeaspida to the genus Galeaspis, and considered this

group as allied to the Osteostraci, within the Cephalaspidom orphi The other jawless vertebrate

genus he recorded from the Devonian o f China, Polybranchiaspis, was thus placed in a group of its

own, the Polybranchiaspida, which he regarded as related to the Heterostraci (Y H Liu 1975) Halstead Tarlo (1967) lumped the two groups into the Galeaspida, and he was later followed by

Janvier (1975) and all subsequent writers Galeaspis turned out to be preoccupied and was replaced by Eugaleaspis (Y H Liu 1980), but the change o f Galeaspida into Eugaleaspida or

Galeaspidiformes into Eugaleaspidiformes (Y H Liu 1980) was unnecessary, since the rules of nom enclature do not apply to taxa above the family-group level, and since these higher taxa were not preoccupied Therefore, the names Galeaspida and Galeaspidiformes Liu are retained here Besides the fragments collected by J D eprat around 1910, and described by M ansuy in 1915, the first evidence o f determ inable galeaspids from Vietnam dates back to 1973, when three incomplete head shields collected by Ta-Thanh Trung in the Si K a Form ation of Tong Vai, Q uan Ba district,

H a Giang Province, were sent to China for identification These have been determ ined as

Polybranchiaspis' nov sp ’, close to P liaojaoshanensis Liu (Ta-Thanh 1978 ; Pan Jiang, unpublished

report to the Institute o f Geology and M ineral Resources, Hanoi, 1978) Tông-Dzuy and Janvier

(1987), on the basis of photographs, suggested that they might rather belong to P gracilis Cao,

1986, which, however, may well be a mere individual variation o f P liaojaoshanensis The three

specimens, quoted as lost by Tông-Dzuy and Janvier (1987), have now been found to be deposited

in the Geological Institute of the Academia Sinica, Beijing

Fragm ents, scales, or incomplete shields of galeaspids have also been recorded from more southerly localities in Vietnam, namely in Trang Xa (Bac Thai Province: Tông-Dzuy and Janvier 1987), and Dong M o (Lang Son Province: Tông-Dzuy and Janvier, 1990)

The present description o f a new and unusually large galeaspid from Vietnam is based on m aterial collected in 1991 in the locality o f Ban N huan, Phu Luong district, Bac Thai Province The specimen belongs to the collection of the D epartm ent of Geology o f the University o f Hanoi (U H D G , VND 50-52)

298 P A L A E O N T O L O G Y , V O L U M E 36

t e x t - f i g 1 Locality map, showing the distribution o f the Song Cau G roup and the M ia Le, N a Q uan and

H a Lang Form ations across the Song Cau river The galeaspid locality near Ban N huan is indicated by a black

triangle.

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

Early Devonian vertebrate faunas o f Vietnam occur exclusively in the Si K a and Bac Bun

F orm ation of the Bac Bo (formerly the Tonkin), both united as the Song Cau G roup (Tong-Dzuy 1980) Their age was first believed to be Eifelian (Long 1967), but recent re-examination o f the associated or overlying invertebrate faunas (in particular brachiopods and corals) has shown that

they were rather Late Lochkovian or Early Pragian in age (Tong-Dzuy 1980; Tong-Dzuy et al

1986; Tong-Dzuy and Janvier 1990) This new dating is also supported by faunal com parisons with the Early Devonian vertebrate-bearing localities of southern China, in particular Qujing, Yunnan, and Liujing, Guangxi The fish-bearing parts o f the Si K a and Bac Bun Form ations could thus be correlated with the lower p art o f the Cuifengshan F orm ation o f Y unnan or the uppermost part of the Lianhuashan F orm ation o f Guangxi (Pan and Dineley 1988), that is the ‘Siegenian’ (Upper Lochkovian-Low er Pragian) in the sense o f S T W ang (1991) Previous work (Pham -D inh 1967),

as well as field investigations carried out since 1985 by the present authors, suggests that there are

J A N V I E R E T AL : G A L E A S P I D F R O M V I E T N A M 299

several fish horizons in the Si K a and Bac Bun Form ations, and that, despite similarities in the higher taxonom ic com position, differences at the specific or generic level may be due to slight differences in age rather than to differences in environm ental conditions (Tong-Dzuy and Janvier, submitted) The new galeaspid described here from Ban N huan, for example, has never been observed in any o f the m ajor fish localities of the Bac Bo (Tranx Xa, Dong Mo), even in the form

o f exoskeletal fragments

The locality of Ban N huan is situated 18 km north east o f the town of Du, in the Phu Luong District, approxim ately 30 km north o f Thai Nguyen, on the southern margin o f a large Palaeozoic anticlinorium (Text-fig 1) There, the Si K a and overlying Bac Bun F orm ation (Song Cau G roup) outcrop at the base of the hills, their top being generally m ade up o f the limestone o f the M ia Le and N a Q uan Form ations and the siliceous shales of the H a Lang Form ation All these form ations are intersected by the Song Cau river The best exposures occur along the path leading from the Song Cau river to Ban N huan, about one kilometre before arriving at the village There, several bone-beds are clearly visible within the massive dolomitic sandstone o f the top of the Si K a Form ation The large galeaspids described herein occur in a very fine-grained dolomitic sandstone

at the top of the form ation, probably just below the upperm ost bone-bed This type of sediment corresponds to a low-energy environm ent which perm itted the preservation of the extremely thin and fragile galeaspid exoskeleton Fragm ents o f exoskeleton with a similar structure occur also in the bone-beds, in association with num erous fragments of plates and scales o f yunnanolepid antiarchs and youngolepid sarcopterygians From the lithology, these fish-bearing beds can be placed near the boundary between the clastic Si Ka F orm ation and the dolomitic Bac Bun Form ation

These large galeaspid shields are associated with some antiarch plate fragments, one of which

could be determined as an anterior ventrolateral plate of Yunnanolepis sp.

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

Class g a l e a s p i d a Liu, 1965

O rder and Family undetermined Genus b a n n h u a n a s p i s gen nov

Derivation o f name After Ban N huan, the type locality o f the type species.

Type species Bannhuanaspis vukhuci sp nov.

Diagnosis As for the type species

Bannhuanaspis vukhuci sp nov.

Text-figs 2-6

Derivation o f name The species is dedicated to D r D ang Vu Khuc, Geological M useum, Hanoi.

Diagnosis A very large polybranchiaspidid-like galeaspid, with orbits situated close to the shield

margin and a transversely elongated m edian dorsal opening The maximum breadth o f the shield

is situated in its rearm ost part

Holotype An incomplete head shield (University o f H anoi, D epartm ent o f Geology, V N D 50; Text-figs 2, 3 a),

part and counterpart.

Type locality Ban N huan, N orth East o f D u, Phu Luong District, Bac Thai Province, Vietnam (Text-fig 1) Type horizon U pperm ost part o f the Si K a Form ation or lowermost part o f the Bac Bun F orm ation, Lower

Devonian, Late Lochkovian to Early Pragian.

300 P A L A E O N T O L O G Y , V O L U M E 36

t e x t - f i g 2 Bannhuanaspis vukhuci gen et sp nov Lower D evonian (Late Lochkovian to Early Pragian), top

part o f Si K a F orm ation or base o f the Bac Bun F orm ation; Ban N huan, Phu Luong district, Bac Thai Province, V ietnam ; holotype (VND 50); interpretive scheme o f head shield, a , dorsal p art o f incomplete derm al head shield in ventral view, showing the canals o f the sensory-line system, b , counterpart o f the latter specimen, showing part o f the m arginal region and ventral rim o f the dermal head shield in dorsal view; traces

of perichondral bone from endoskeleton dotted Scale bar = 10 mm A bbreviations: cp, cornual process; ibr, trace o f interbranchial ridges; iol, infraorbital portion o f main lateral line; Itl 1-6, lateral transverse lines; mdo,

m edian dorsal opening; mil, m ain lateral-line; orb, orbit; orn, oral n o tc h ;pi, pineal foram en; sol, supraorbital

line; tel, transverse commissural line.

J A N V I E R E T AL : G A L E A S P I D F R O M V I E T N A M 301

t e x t - f i g 3 Bannhuanaspis vukhuci gen et sp nov Lower Devonian (Late Lochkovian to Early Pragian), top

pan o f Si K a F orm ation or base o f the Bac Bun F orm ation; Ban N huan, Phu Luong District Bac Thai

the holotype; scanning electron micrographs o f the external surface, x

J A N V I E R E T AL.: G A L E A S P I D F R O M V I E T N A M 303

Referred material Posterior median dorsal part o f the head shield (VND 51, Text-figs 4, 5 a ), isolated fragment

o f ventral rim o f the shield (VND 52, Text-fig 3 b , Text-fig 5 a ) isolated scales in association with the latter specimens (Text-fig 3 c - d ).

Remarks Bannhuanaspis vukhuci is one o f the largest known galeaspids, together with the primitive

Silurian genus Hanyangaspis (N Z W ang 1986), Dongfangaspis major (Y H Liu 1975) and

Antiquisagittaspis (Y H Liu 1985) Its closest overall resemblance is to Polybranchiaspis, from

which it differs however by the m ore lateral position o f the orbits, the more posteriorly placed and more transversely elongated median dorsal opening, the broader posterior limit o f the head shield,

and its larger size Although the sensory-line pattern is broadly similar to th at o f Polybranchiaspis,

it differs from the latter in th at the posterior part o f the m ain lateral-line is posterolaterally - and

Hanyangaspis and Xiushuiaspis (N Z W ang 1991).

The question of the systematic position of Bannhuanaspis can only be answered in the context of

the question o f the monophyly of the ‘Polybranchiaspidiform es’, which will be briefly discussed below

Description The holotype V N D 50 is a slightly distorted and incomplete head shield, the dorsal aspect o f which

is known from the ventral surface o f the dorsal exoskeleton (Text-figs 2 a , 3 a ) The canals o f the sensory-line system, which, in galeaspids, lie against the basal surface o f the exoskeleton (Janvier 1990), are thus clearly

visible The m edian dorsal opening (mdo, Text-fig 2 a ) is transversely elongated, roughly rectangular in shape, with rounded angles, and situated relatively far behind the anterior m argin o f the shield One o f the orbits is

visible on the left side, although slightly distorted by crushing (orb, Text-fig 2 a ) A pineal foram en is present

(pi, Text-fig 2 a ).

The posterior m edian dorsal shield fragm ent V N D 51 (Text-figs 4, 5 a ), which was used for com pleting the reconstruction in Text-figure 5 b , exhibits the dorsal surface o f the exoskeleton Therefore, no sensory-line canal

is visible in external aspect, beside a minute series o f slits by which these canals open to the exterior (Text-fig 6) The pattern o f these canals in this specimen could, however, be traced on a radiograph (Text-fig 4 b ) This specimen, which is similar in size to the holotype, and which was found in the same block, could be assembled

to the latter, thanks to the position o f the posterior m argin o f the shield and o f the transverse commissural

sensory-line canal (tel, Text-figs 2 a , 4 b ) Clearly, there is only one commissural canal, a condition which is

regarded as a synapom orphy o f all the galeaspids, apart from Dayongaspis, Xiushuiaspis and Hanyangaspis

(Janvier 1984; N Z W ang 1991) There are probably six lateral transverse sensory-lines (Itl 1-6, Text-figs 2 a ,

4 b ) The infraorbital portion o f the m ain lateral-line (iol, Text-fig 2 a ) sends off num erous side-branches tow ard

the shield margin, and is connected anteriorly with the distal p art o f the supra-orbital line (sol, Text-fig 2 a )

A lthough no sensory-line canal has ever been reported in the ventral exoskeleton o f the Chinese galeaspids, this specimen clearly shows such a canal extending on the lateral part o f the derm al postbranchial bar and o f the

ventral rim (vl, Text-figs 4 b , 5 b 1) It displays a zig-zag pattern which differs from th at o f the dorsal lateral-line canals.

The posterior m argin o f the shield is rem arkably broad, with only shallow em bayments on either side o f a median dorsal process There is no distinct m edian dorsal crest, but a slight m edian elevation in the rearm ost part o f the shield Laterally, the posterior margin o f the shield is produced into a slight lobe, which may have

extended beyond the level o f the body and may represent an incipient cornual process (cp, Text-fig 2 a ) The ventral surface o f the head shield is known from the counterpart o f the holotype, which displays a slight

oral notch (orn, Text-figs 2 b , 5 b ), and from the ventral side o f isolated median shield fragment, which shows

part o f the ventral rim o f the oralobranchial fenestra (vr, Text-fig 5 a ) W ith this specimen, there is also an isolated portion o f ventral rim detached from another shield (Text-figs 3 b , 5 a ) These latter two specimens

clearly show the series o f branchial notches (brn, Text-fig 5 a - b ), which are quite num erous and suggest thus

a condition com parable to that in Polybranchiaspis or Duvunolepis, though the precise num ber o f these notches

t e x t - f i g 4 Bannhuanaspis vukhuci gen et sp nov Posterior m edian part o f head shield (VND 51), same

locality and level as the holotype a , specimen in dorsal view, showing the external aspect o f the exoskeleton, and some sensory-line canals o f the ventral exoskeleton on the right side, b , distribution o f the sensory-line

canals, based on a radiograph o f the specimen Scale bar = 10 mm A bbreviations: Itl 2-5, lateral transverse

lines; mil, m ain lateral-line; tel, transverse com missural line; vl, ventral sensory-lines.

P A L A E O N T O L O G Y , V O L U M E 36

J A N V I E R E T AL.- G A L E A S P I D F R O M V I E T N A M 305 remains unknow n (Text-fig 5 b 1) The dermal postbranchial bar does not seem to be complete (Text-fig 5 a , b1) The dermal covering o f the oralobranchial fenestra is unknown.

Some slight traces o f perichondral bone from the endoskeleton are visible in the counterpart o f the holotype, within the sediment which fills the oralobranchial fenestra A series o f transverse strands o f perichondral bone

The organization o f the exoskeleton is quite similar to th a t described in Polybranchiaspis sp by Janvier (1990)

and N Z W ang (1991) It consists o f loosely assembled, minute derm al units, each o f which bears a single tubercle covered with a shiny hard tissue (possibly an enameloid) The exoskeleton is completely recrystallized and its m icrostructure cannot be studied.

Num erous scales occur in the sediment in association with the shields, sometimes arranged into parallel series which suggest th at they retain their original position, as described by S F Liu (1983) in Eugaleaspis All

the scales are m inute rounded units (Text-figs 3 c - d , 5 d - e ), quite similar in shape and structure to the individual units o f the derm al head shield They have no pulp cavity and bear a single boss, or tubercle (Text- fig 5c) covered with a shiny hard tissue (Text-fig 3 d ).

D I S C U S S I O N

The phylogenetic interrelationships o f the Galeaspida have been briefly discussed by Janvier (1984),

S F Liu (1986) and in more detail by N Z W ang (1991) The first question th at arises in this connection is th at of the sister-group o f the Galeaspida, which may serve as an out-group to evaluate character-state polarities The Galeaspida have been regarded as the sister group of the Osteostraci (Janvier 1975; Halstead 1982; Young 1991), the Osteostraci + G nathostom ata (Forey 1984; Janvier 1984; Maisey 1986), the G nathostom ata alone (N Z W ang 1991), or also in a trichotom y with the Osteostraci and G nathostom ata (Young 1991, as a second possibility) The current classification o f the Galeaspida comprises four orders: the H anyangaspidida:

(Hanyangaspis, Xiushuiaspis, Dayongaspis); the Polybranchiaspidiformes (Polybranchiaspis, Dongfangaspis, Diandongaspis, Laxaspis, Damaspis, Siyingia, Cyclodiscaspis, Duyunolepis, Para- duyunaspis, Neoduyunaspis)-, the Huananaspidiform es (Huananaspis, Asiaspis, Nanpanaspis, Lungmenshanaspis, Sanqiaspis, Sanchaspis, Wumengshanaspis, Sinoszechuanaspis); and the

Galeaspidiformes (Eugaleaspis, Sinogaleaspis, Yunnanogaleaspis, Meishanaspis, Tridensaspis) In addition, there are some genera incertae sedis, based on too poorly preserved material

(Antiquisagittaspis, Kwangnanaspis, Qingmenaspis).

The Hanyangaspidida and the Polybranchiaspidiformes are most probably paraphyletic

Hanyangaspis is now regarded as being the sister-group o f all other Galeaspida (Janvier 1984) The

Polybranchiaspidiformes cannot be defined on the basis o f a unique derived characteristic The num ber o f characters available to reconstruct galeaspid phylogeny is quite limited because

of the generally poor preservation o f the specimens They are: (1) the shape and position of the

m edian dorsal opening, (2) the position o f the orbits, (3) the overall shape and proportions of the head shield, (4) the pattern o f the sensory-line canals, and (5) the num ber o f gill openings or corresponding gill com partm ents Features o f the internal anatom y and ventral dermal covering of the oralobranchial cham ber are so rarely preserved, and apparently so homogeneous throughout the entire group (apart from the m ore or less sinuous course of the dorsal jugular vein or the extension of the dorsal wall o f the abdom inal cavity), that they are not considered at the m om ent

to be useful for unravelling the relationships within the Galeaspida However, there are probably more characters to be found in the ventral dermal covering of the head shield, but this remains poorly known

t e x t - f i g 5 Bannhuanaspis vukhuci gen et sp nov a , drawing o f the ventral side o f the specimen in Text- figure 2, showing part o f the ventral rim o f the derm al head shield, as well as a fragm ent o f the ventral rim o f a presumably different specimen o f the same species (VND 52) b , reconstruction o f derm al head shield in ventral

( B l ) and dorsal ( b 2 ) aspect, with the pattern o f the sensory-line canals reconstructed on the basis o f the specimens in Text-figures 2 and 4 c, reconstruction o f an isolated body scale in lateral ( c l) and external (c2) view Scale bar: a - b = 10 mm, c = 1 mm A bbreviations: brn, branchial notches; orn, oral notch; i t , ventral

rim.

306 P A L A E O N T O L O G Y , V O L U M E 36

t e x t - f i g 6 Bannhuanaspis vukhuci g e n e t s p n o v , r e c o n s t r u c t i o n o f t h e h e a d a n d a n t e r i o r p a r t o f t r u n k

squam ation in dorsal view Scale bar = 10 mm.

Out-group com parison does not tell us much of the plesiomorphic state o f most of these characters The median dorsal opening is apparently unique to the Galeaspida Although Janvier (1981, 1984) regarded it as hom ologous to the nasopharyngeal duct o f hagfishes and to the presumed prenasal sinus o f the H eterostraci In this case, the closer this median dorsal opening is

to the anterior shield margin, the m ore plesiomorphic it is Hanyangaspis, in which this opening is

almost term inal in position, would thus show the m ost generalized condition for the Galeaspida

M oreover, a transversely elongated opening (as in Hanyangaspis) would be plesiomorphic relatively

to an oval, elliptic, rounded, heart- or slit-shaped opening, if evaluated by reference to the

Heterostraci Conversely, the rounded opening o f Dayongaspis would be plesiomorphic if assessed

by reference to hagfishes

The generalized position of the eyes for the vertebrates is a lateral position, and the dorsally placed eyes o f some galeaspids is thus presumably derived Therefore, the m ore laterally placed the

eyes are, the m ore plesiomorphic is the condition This condition is again m et with in Hanyangaspis and possibly in a few other galeaspid taxa (Cyclodiscaspis, Sanqiaspis, Huananaspis).

The overall shape and proportions o f the head shield are quite diverse in galeaspids, and the