The specimen described herein is preserved in thin-bedded, slightly bituminous shale from the middle part of the Upper Lochkovian Si Ka Formation of the Lung Cu–Ma Lé section (23° 21′ 07″ N; 105° 17′ 36″ E; altitude: 1416 m; Fig. 1). It has been prepared by etching away the bone with 5% hydrochloric acid. After having been photographed (Fig. 2B1), the posterior part of the internal, natural cast of the specimen has been removed with a needle, in order to expose the postbranchial wall and the ventral dermoskeleton of the abdominal division of the shield (Fig. 2B2). An elastomer cast stained in black was made and photographed after being whitened with magnesium oxide. The specimen has been obliquely distorted during fossilization (Fig. 2A). Its original shape and proportions have been restored by using the “skew” command of Photoshop, and bringing the commissural sensory line in a transverse position (Fig. 2C).

The Lower Devonian of eastern Bac Bo (northeastern Vietnam) is typically composed of a gradual succession of three formations, the Si Ka, Bac Bun and Mia Lé formations, the former two being gathered in the Song Cau Group [27]. The Si Ka Formation generally consists of reddish sandstone and siltstone, the Bac Bun Formation of siltstone, marl, shale and dolomite, and the Mia Lé Formation of marl, shale, sandstone and limestone. This formation series indicates, from base to top, a progressive installation of a more and more marine environment, in which the fish remains become abundant at the transition between the Si Ka and Bac Bun formations, along with the first marine invertebrates (notably the brachiopod Howittia wangi) in the middle part of the Bac Bun Formation [25], [26] and [27]. This sequence shows some local variations, notably the lack of the arenaceous Si Ka Formation and the predominance of bituminous shale in the Quan Ba area, north of Ha Giang [22] and [28], and further west, in the Lai Chau Province.

The specimen described herein comes from the siltstone beds of the middle part of the Si Ka Formation that crops out along the Ma Lé (Mia Lé)–Lung Co road, a classical Devonian section which is known since the early works of Mansuy [18] and Deprat [1] and [10]. It was found along with plant remains, large arthropod fragments, gastropods, rare lingulids and abundant remains of other fish groups. Among the latter are at least two other galeaspids (probably Polybranchiaspis sp. and a large indeterminate form), at least three antiarchs (Heteroyunnanolepis sp., Yunnanolepis cf. Y. spinulosa, Janvier and Ta Hoa, 1999 [10] and Chuchinolepis sp.), an indeterminate placoderm (arthrodire or petalichthyid) and at least one sarcopterygian (probably a Youngolepididae).

The specimen displays the overall morphology of the two galeaspid species currently referred to the genus Sanqiaspis and characterized by a transversely elongated median dorsal opening, elongated head shield, posteriorly directed cornual processes, anterior rostral process and numerous, transversely elongated branchial fossae (Fig. 2 and Fig. 3E–G). However, it differs from all other described species of the genus Sanqiaspis by its proportions and the shape of its rostral process and is therefore referred to a new species.

Subclass Galeaspida Halstead, Tarlo, 1967 [23]

Diagnosis: A Sanqiaspis species with short and distally expanded rostral process. The maximum breadth of the head shield is about thrice that of the median dorsal opening and the length of the cornual processes is about half the breadth of the median dorsal opening.

Type specimen: An incomplete and slightly distorted head shield (Museum of the Geological Survey of Vietnam, Hanoi, BT338) (Fig. 2A, B).

Type locality: Lung Cu–Ma Lé section, Lung Cu village, Dong Van district, Ha Giang Province, northern Vietnam.

Age: Lower Devonian (Lochkovian), middle part of the Si Ka Formation.

Referred material: The holotype only.

Remarks: Sanqiaspis vietnamensis clearly differs from the type species of the genus Sanqiaspis, Sanqiaspis rostrata, Liu, 1975 [14] (Fig. 3E) by its much broader head shield and much shorter rostral process, which expands distally at a short distance from its root, as in Sanchaspis magalorostrata Pan and Wang, 1981 [21] (Fig. 3B). Sanqiaspis vietnamensis is, however, more closely similar to S. zhoaotongensis, Liu, 1975 [14] (Fig. 3F), from which it differs by its slightly different sensory line pattern and shorter cornual processes [29]. A third species, Sanqiaspis sichuanensis, Pan and Wang, 1978 [20], was described on the basis of a very poorly preserved specimen and later regarded as a junior synonym of Sanqiaspis rostrata [16]. Zhu and Gai [36] provided the first extensive phylogenetic analysis of the Galeaspida, which resulted in three equally parsimonious trees, differing only in the position of the most inclusive (Silurian) galeaspid genera Hanyangaspis, Changxingaspis and Dayongaspis. These trees also yield much the same major clades as defined in earlier works [4], [5] and [8], namely the Eugaleaspidiformes and Huananaspidiformes, but show the Polybranchiaspidiformes as monophyletic (contra [5] and [8]). Sanqiaspis (Fig. 3E–G) is nested within the Huananaspidiformes, as the sister group of a clade comprizing seven genera (Nanpanaspis, Asiaspis, Huananaspis [Fig. 3H], Lungmenshanaspis, Qingmenaspis, Sinoszechuanaspis, Sanqiaspis and Macrothyriaspis). Apart from the question of the position of Antiquisagittaspis (Fig. 3A; a large but poorly known form [15]), the monophyly of the Huananaspidiformes is well supported and the members of the group are readily recognized by their generally long rostral process.

Thanks to acid preparation, the cast of Sanqiaspis vietnamensis shows a number of details that had never been observed in the previously described Sanqiaspis material from China. Galeaspids are generally depressed fishes and their endoskeletal head shield is weakly ossified. Moreover, their dermoskeleton is very thin and made up by small and poorly consolidated units [6], [12] and [31], hence the frequent distortion of their head shield. The entire dermoskeletal shield of Sanqiaspis vietnamensis is ornamented with small, pointed and costulate tubercles of variable size. The largest tubercles are located near the midline both on the dorsal surface and the dermal covering of the postbranchial wall and the smallest ones lie dorsal to the branchial region of both sides (Fig. 2A1). The head shield of Sanqiaspis vietnamensis displays the characteristic festooned pattern of the dorsal sensory-lines of all galeaspids [36], which were mainly located in grooves, but partly enclosed in canals, as in, for example, Polybranchiaspis [22], [26] and [28]. Surprisingly the dorsal sensory line pattern appears quite different from that of Sanqiaspis zhaotongensis (Fig. 3F) [34], otherwise very similar to Sanqiaspis vietnamensis by the overall shape of its head shield. By contrast, the sensory line pattern of Sanqiaspis vietnamensis resembles more closely that of more generalized Huananaspidiformes, such as Antiquisagittaspis or Sanchaspis (Fig. 3A, B) [15] and [29]. Like in all other Sanqiaspis (and most other Huananaspidiformes) species, the orbits of Sanqiaspis vietnamensis are opening laterally and there is no pineal opening (a variable character in galeaspids). Yet, the internal surface of the dermoskeleton shows the impression of a pineal recess (Fig. 2B3). The median dorsal opening is elliptic in shape and transversely elongated, as in the other Sanqiapis species. The cornual process is preserved only on the left side and shorter than in Sanqiaspis zhaotongensis, but similarly double, with a large anterior process and a smaller one, separated by an embayment (Fig. 2A2). The rostral process is incomplete, but its preserved distal extremity suggests that it was somewhat broadened distally, as is also the case in Sanqiaspis rostrata (Fig. 3E) and other generalized Huananaspidiformes, such as Sanchaspis and Gantarostrataspis (Fig. 3B, C). Moreover, and like in the latter genus, the margin of the rostral process bears a single series of enlarged tubercles.

The dermoskeletal postbranchial bar is generally regarded as lacking in galeaspids, except in the two plesiomorphic genera Xiushuiaspis and Changxingaspis [36]. This is probably because of the lack of preparation of the ventral side of the head shield of most galeaspids. However, Sanqiaspis vietnamensis shows an extensive postbranchial bar that covers ventrally the abdominal division of the head shield and bounds off posteriorly the oralobranchial fenestra, like in osteostracans [8] (Fig. 2B3). This agrees with the presence of an equally extensive endoskeletal postbranchial wall that closes posteriorly the oralobranchial chamber (Fig. 2B3). The endoskeletal roof of the oralobranchial chamber is poorly preserved but shows indications of at least 11 pairs of transversely elongated branchial fossae (Fig. 2B); that is, far less than the 17–19 pairs that are visible in Sanqiaspis rostrata [14] and [32]; their number in Sanqiaspis zhaotongensis is unknown. The posteriormost branchial fossae are slightly oblique relative to the midline and extend on the postbranchial wall, as in osteostracans, and there seems to be a single large median postbranchial opening. The lateral margins of the oralobranchial fenestra are embayed by branchial notches that are bordered by minute tubercles (Fig. 2B2).

The ventral counterpart of the specimen, before preparation of the postbranchial wall, showed the trace of the posteriorly tapering occipital region and, at the limit between the endoskeleton and dermoskeleton, an extensive subaponevrotic vascular network (Fig. 2B1).

The lateral part of the dorsal dermoskeleton that overlies the oralobranchial chamber shows, at any rate on the right side, a small gap (Fig. 2A2) that is suggestive of the “fenestrations” mentioned in various galeaspid taxa, notably Huananaspidifomes [36], and once regarded as either homologues of the “lateral cephalic fields” of osteostracans [17], or dorsal external branchial openings. Janvier [8] doubted both interpretations (except perhaps for Microhoplonaspis, in which these dorsal openings show a distinct margin lined with endoskeleton), and considered that these fenestrations are an artefact of preservation due to the fragility of the very thin dermoskeleton in this particular region of the head shield, where the roof of the branchial fossae is directly in contact with the dermoskeleton. Similar dorsal gaps are also found in some specimens of Polybranchiaspis, whose dorsal dermoskeleton is normally not fenestrate [22].

Despite its poor preservation, Sanqiaspis vietnamensis shows that preparation of galeaspids in the form of natural moulds and casts provides a wealth of hitherto unavailable detail information, in particular as to the ornamentation of the dermoskeleton. The discovery of a complete postbranchial wall in Sanqiaspis suggests that this feature is more widespread in galeaspids than previously thought and is yet another character shared by this taxon and the osteostracans.

The discovery of Sanqiaspis in northern Vietnam increases the distribution of this huananaspidiform genus in the Devonian of the South China Block. The two previous occurrences from China, in Jiangyou, Sichuan and Zhaotong, northeastern Yunnan suggested possible endemism [14], [28] and [30], until it turned up in the Pragian of Wenshan, Yunnan [34]. The type horizon of Sanqiaspis zhaotongensis was initially referred to the Late Lochkovian “Cuifengshan Formation” but later correlated with the Pragian Posongchong Formation of Wenshan and Guangnan of southern Yunnan, where Sanqiaspis rostrata also occurs [34] and [37], and which are the Chinese vertebrate-bearing localities that lie closest to Lung Cu in Vietnam (Fig. 1). The marine invertebrate fauna of the Bac Bun Formation that immediately overlies the vertebrate-bearing shale at Lung Cu is widely accepted as essentially Late Lochkovian in age (with a possibly Early Pragian top part) [27]. Therefore Sanqiaspis vietnamensis, which clearly comes from the underlying Si Ka Formation, is Lochkovian in age; that is, older than the Chinese occurrences of Sanqiaspis. So far, none of the other Lower Devonian vertebrate faunas hitherto recorded from Vietnam has ever yielded Sanqiaspis, despite the abundance of galeaspids in certain outcrops of the Bac Bun and Si Ka formations, notably Polybranchiaspis and Laxaspis, which also occur in the Lochkovian Xishancun Formation (Cuifengshan group) of Yunnan [11], [22], [24], [26], [28] and [33]. One must, however, consider with caution the age of these Lower Devonian vertebrate occurrences of the South China Block, which are rarely associated with unambiguously marine invertebrates. Sanqiaspis belongs to the “Sanchaspis–Asiaspis” assemblage III [35] and [37], which is known from the Posongchong, Xujachong and Nagaolin formations of Yunnan (Zaotong, Wenshan), Sichuan (Jiangyou) and Guangxi (Liujing), respectively. In China, this assemblage is dated as Pragian on the basis of associated invertebrate faunas, including conodonts and palynomorphs [33] and [35]. This contrasts with the Lochkovian age of the Vietnamese Sanqiaspis occurrence. Moreover, Sanqiaspis vietnamensis is found in association with taxa that are regarded as typical for the Late Lochkovian “Diabolepis–Nanpanaspis” assemblage II, such as the antiarch Chuchinolepis [35]. However, in the same beds also occur taxa, such as the antiarch Heteroyunnanolepis, which are referred to the Early Lochkovian “Polybranchiaspis liaojiaoshanensis–Laxaspis qujingensis” assemblage I [35]. Therefore, we suspect that the galeaspid occurrences are strongly facies-bound and the galeaspid taxa may either not be entirely reliable guide fossils, or some of the vertebrate assemblages defined in China are imperfectly dated. The four major Lower Devonian vertebrate faunas hitherto described from the Bac Bun Formation (or the transition between the latter and the underlying Si Ka Formation) of northern Vietnam (i.e., from Trang Xa, Ban Nhuan, Dong Mo and Tung Vai, respectively [10], [11], [12], [22], [25], [26] and [28]) all occur below the first Howittia wangi-bearing beds and yield taxa that generally belong to the Lochkovian assemblages I and II. A possible exception is Tung Vai [22] and [28], whose vertebrate fauna is dominated by Polybranchiaspis, Laxaspis, Minicrania and Heteroyunnanolepis but devoid of Chuchinolepis, and likely to be older than all other Early Devonian vertebrate faunas from Vietnam.