Luoxiongichthys hyperdorsalis, Wen & Zhang & Hu & Zhou & Xie & Huang & Chen & Benton, 2012

Wen, Wen, Zhang, Qi-Yue, Hu, Shi-Xue, Zhou, Chang-Yong, Xie, Tao, Huang, Jin-Yuan, Chen, Zhong Qiang & Benton, Michael J., 2012, A new basal actinopterygian fish from the Anisian (Middle Triassic) of Luoping, Yunnan Province, Southwest China, Acta Palaeontologica Polonica 57 (1), pp. 149-160 : 151-158

Luoxiongichthys hyperdorsalis sp. nov.

Figs. 2–4 .

Etymology: From Greek hyperdorsalis , over—of the back, the species name is derived from its unique feature, the elevated “hump” in front of the dorsal fin.

Type material: Holotype: LPV−10144, a nearly complete specimen ( Fig. 2 ). Paratype: LPV−10120, a 170 mm−long specimen, showing the major part of the skull in dorsal view ( Fig. 3B ).

Type horizon: Member II, Guanling Formation, Middle Triassic ( Nicoraella kockeli Zone , late Pelsonian, middle–late Anisian).

Type locality: Daaozi Village , Luoxiong Town , Luoping County, Qujing City, Yunnan Province. China ( GPS: N 24 ° 46247.6; E104 ° 19240.7"; Fig. 1 ) .

Material.—LPV−10625, a specimen lacking most of the dorsal fin and the whole anal fin ( Fig. 3A ); LPV−6868 A and LPV−6868B, part and counterpart of the same fossil showing

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the body outline; LPV−11817, complete fin system with scattered skull bones.

Diagnosis.—Middle−sized fishes, laterally compressed. The outline is triangular, with the apex of the triangle formed by a pointed “hump” in the dorsal region. Opercular series comprising opercle, subopercle, interopercle, and preopercle; subopercular has long ramus anterior to opercular; subopercular a little less than half the size of the opercular; preopercular almost vertically oriented, with slender dorsal limb; three square−like suborbitals; at least eight branchiostegal rays with tubercles on the surface and comb−like ornament on their anterior margin; strong lower jaw has coronoid process; pointed grasping teeth in upper jaw, lower jaw, parasphenoid and entopterygoid; articulation of jaw immediately below posterior margin of orbit; cleithrum is strong, with ridge ornament on surface of upper part; supracleithrum is large, nearly half the depth of the cleithrum, the lower end is somewhat pointed; two postcleithra lie behind the cleithrum; clavicle present; hemiheterocercal tail slightly forked; each radial supports one lepidotrichium in unpaired fins; all rays segmented from middle part and bifurcated distally; fringing fulcra on anterior margin of all fins; enlarged fulcra before anal fin and caudal fin; the whole body is covered with ganoid scales with tuberculate ornament; scales contact each other by peg−and−socket structure.

Description

Luoxiongichthys hyperdorsalis has many characteristics of Neopterygii. For example, many cartilages become ossified during ontogeny, the maxilla is no longer hinged with the preopercular, the mandible has a coronoid process, the dorsal branch of the preopercular is slender, but it is also almost vertical, the interopercular is present ( Hurley 2007), each radial supports one lepidotrichium, the tail is hemiheterocercal (= externally almost symmetrical), and the scales are ganoid ( Gardiner et al. 2005). Among those characters, the

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compound coronoid process, vertical suspensorium, mobile maxilla, interopercule present, and the unpaired fin rays about equal in number to their supports are considered as synapomorphies of neopterygians ( Olsen and McCune 1991). However, the new fish is distinguished from all other neopterygians by its unique high dorsal region, double jaw joint and thick tuberculous scales all over its body.

The total length is almost four times the head length, and body depth is twice the head length.

Skull.—The skull is triangular in lateral view ( Fig. 2 ). The roof of the skull is poorly preserved in the holotype, but is better seen in LPV−10120 and LPV−10625 ( Fig. 3 ). The elements include a long frontal and an oblong parietal. The parietals are rectangular and almost twice as long as wide, and they suture anteriorly with the frontals, and laterally with the oblong dermopterotics. The frontals are three times as long as the parietals. The narrowest part of the frontal is just above the orbit, and anterior and posterior parts are slightly widened to a similar extent. The dermopterotic bones are relatively long, approximately rectangular, and extend anteriorly lateral to the frontals. They are about 2.7 times longer than deep and about 1.2 times longer than the parietals. The extrascapular bones are poorly preserved and their number is unknown.

The opercle ( Fig. 3 ) is large, the ventral side a little longer than the dorsal. It is an oblong element, 1.8 times as deep as long. At the anteroventral corner of the subopercle there is a long and narrow triangular interopercle in LPV−10625 ( Fig. 3A ). The preopercular is elongated, with a slender dorsal ramus, curving ventrally. A sensory canal runs along the posterior margin of the preopercle. There are five branchiostegals in LPV−10625 ( Fig. 3A ). The hyomandibular is well preserved in the holotype ( Fig. 2 ): it is a triradiate element, comprising a dorsal head, a ventral shaft, and a prominent opercular process. There is an isolated hyomandibular in LPV−11817. The triangular symplectic lies right beside the hyomandibular. As in Parasemionotidae , the symplectic is very large and bears a head for the articular. In LPV−10625, the quadrate lies right behind the mandible, indicating that both the symplectic and quadrate contributed to the jaw joint. This is characteristic of Halecomorphi . The two splint−like bones probably are fragments of quadratojugal ( Fig. 2B ). In front of the margin of the shorter bone, there is a small bone bearing five pencil−shaped teeth. There are three small pores distributed on the anterior margin. These traits indicate that the small bone is a fragment of the premaxilla.

The gape is relatively small. The articulation of the lower jaw is below the posterior margin of the orbit. The dentary has a gently elevated coronoid process. The strong mandible comprises dentary and angular. Both of these participate in the coronoid process. The articular cannot be seen in the holotype. The dentary bears powerful pointed teeth. The upper jaw comprises maxilla and premaxilla, but in the holotype ( Fig. 2 ), only an impression of the premaxilla is preserved. The parasphenoid is well preserved in both LPV−10144 and LPV−10625, and it is 30 mm long in LPV−10144. It has a very broad end. Pencil−like teeth are also seen on the entopterygoid and parasphenoid in the holotype. In LPV−10625 ( Fig. 3A ), the maxilla also has pointed grasping teeth. A line of pores appears along the dentary and continues to the angular, representing the mandibular canal.

Large nasal bones can be seen in LPV−10120. The right nasal is not complete, and has moved to contact the left nasal ( Fig. 3B ). The rostral bone is also not complete; it is a V−shaped element, but the right one has eroded away.

There are three supraorbital bones bordering the dorsal margin of the orbit ( Fig. 3B ), but these cannot be counted in other specimens. The anteriormost supraorbital is triangular and contacts three infraorbital bones ventrally, closing the circumorbital series. There are four infraorbitals, and two additional infraorbitals forming the anterior border of the orbit. The first three infraorbitals are deeper than long. The postero−ventral corner is occupied by a very large infraorbital. Its dorsal margin is slightly curved. The outlines of the last two infraorbitals are not so clear. The antorbital is approximately triangular, narrowing anteriorly, like a slim tail. The dermosphenotic takes part in the circumorbital system and occupies the posterodorsal corner of the orbit. There are three suborbital bones on both sides, and they are not in their original places. They all differ in shape: the lower two suborbitals are roughly square in shape, and they are almost of the same size, but the uppermost one is larger on the left side and smaller on the right. The shapes of the suborbitals are cautiously taken as characteristic for the genus. On the left side of the paratype, the entopterygoid and ectopterygoid can be seen.

The opercle is high, up to 1.8 times as high as long. It has a round anteroventral corner, in front of which there is a well−developed long ramus of the subopercle. The dorsal ramus is narrow and relatively high, almost the same height as the subopercle. The subopercle is a little less than half the size of the opercle. Ventrally, the opercle overlaps the dorsal margin of the subopercle, which seems to be dorsally concave. The triangular interopercle articulates anterior to the subopercle and is lightly overlapped by the ventral portion of the preopercle. There are at least four branchiostegal rays in the holotype ( Fig. 2 ), five in LPV−10625, and eight in LPV−10120 ( Fig. 3 ). They are strong and covered by tubercles and comb−like ornamentation on the anterior margin.

The preopercle is slender, and the slim dorsal ramus appears to be vertically oriented. The lower portion is curved ventrally. Its posterior margin is slightly convex and contacts the opercle and subopercle. The anterior margin of the preopercle is overlapped by suborbital bones. The sensory canal runs along the posterior margin of the preopercle.

Pectoral girdle: The elements that can be observed in the pectoral girdle of the holotype include the cleithrum, supracleithrum, and postcleithrum ( Fig. 2 ). The cleithrum is the largest and longest paired element in the pectoral girdle, with many tubercles. Its lower end is bent forward and expanded a little. The upper part is ornamented with ridges, but is fragile and overlaps the supracleithrum a little. The supracleithrum is relatively large, nearly half the depth of the cleithrum, and the lower end is somewhat pointed. Two postcleithra lie behind the cleithrum. In LPV−10120, based on the presence of three pores of the sensory canal, one of the extrascapulars can be identified. It is oval in shape. The sensory canal runs along the lower margin. Behind the extrascapular is the large posttemporal. The sensory canal crosses the posttemporal and extends to the trunk ( Fig. 3B ).

It seems that there is a prominent triangular plate that lies immediately anterior to the cleithrum in LPV−10625 ( Fig. 3A ), LPV−11817 and 6868(A), and it is especially prominent in the first two, but not well preserved in the last. If this triangular plate is a separate bone, its position indicates it might be the clavicle. However, it is strange that this bone shows no ornamentation on its surface. It is clear that this condition is original and not a result of preparation damage. As a result, it is much more like a process for articulation of the clavicle, which still means that the clavicle was still present in this taxon. In LPV−11817, the situation is the same. This is a primitive character, as seen in Watsonulus eugnathoides ( Olsen 1984) , but unusual in what otherwise seems to be a neopterygian.

Axial skeleton.—Owing to the rather heavy scale covering, the axial skeleton is poorly known, but some information is available from the caudal region. The vertebrae are not ossified. The vertebral column is surrounded by neural and haemal arches. About 31 vertebrae can be counted. We can see three neural and four haemal spines in the region between the dorsal fin and anal fin due to the displacement of the scales. In the caudal region of LPV−6688, the neural spines bend sharply backward and the haemal spines are expanded to support the caudal fin.

Paired fins.—Little can be said about the pectoral fins in the holotype, which shows seven lepidotrichial rays. However, in LPV−10120 and LPV−11817 the pectoral fins are well pre−

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served, and they are seen to insert rather low. There are more than ten lepidotrichial rays in LPV−11817, and 13 in LPV−10120, the first of which bears numerous fringing fulcra. These rays begin to be segmented from the middle part and are bifurcated distally.

The pelvic fins are only preserved in LPV−11817, which shows their relative position and points of insertion. The pelvic fins are very small, with an unknown number of lepidotrichia, and placed closer to the anal fin than the pelvic fins.

Unpaired fins.—The dorsal fin lies in a very posterior position, opposite the anal fin ( Fig. 2 ). The dorsal fin originates posterior to the pelvic fins and anterior to the anal fin, and is markedly larger than the latter, the end of the dorsal fin being slightly anterior to the end of the anal fin. In the dorsal and anal fin, each lepidotrichium is supported by one radial, a synapomorphy of Neopterygii. The lepidotrichial ray is ornamented by two longitudinal rows of tubercles. Both dorsal and anal fins are segmented and bifurcated distally. The dorsal fin contains about 28 lepidotrichia, the anal fin about 12 in the holotype.

The caudal fin is hemiheterocercal, with an equilobate outline. It is incompletely preserved in all available specimens, so it is reconstructed here (Fig. 6) from all specimens we have. The upper lobe of the caudal fin is covered by ganoid scales. The lepidotrichial ray is still segmented and bifurcated distally.

Scales.—The whole body is covered by finely tuberculate scales ( Figs. 2A , 4 ). The scale−type is 60 14/17. The scales differ from one another in size and shape in different regions. They are deeper than wide and vary from quadrangular to rhombic in shape, as is usual in basal actinopterygians. There appear to be 50–60 scale rows between the cleithrum and dorsal fin, about 35 scale−rows anterior to the dorsal fin, and 38 in front of the anal fin.

In the uppermost region between the skull and dorsal fin, the scales ( Fig. 4C, G ) are largest, and they are 2.5 times as deep as wide: these cover the characteristic dorsal “hump”. Scale proportion decreases downwards and backwards. Dorsally and in the caudal region the scales are smallest, and rhombic in shape ( Fig. 4A, E ).

There are two basal fulcra respectively preceding the dorsal fin and anal fin. Many slender fringing fulcra are well−developed on all fins. In front of the dorsal lobe of the caudal fin, there is a large unpaired fulcra ( Arratia 2009) on the holotype ( Fig. 2 ) and LPV−11817. These rhombic fulcra are six times larger than other caudal peduncle scales. Dorsal ridge scales are not well−developed in our specimens.

Scales in the ventral region are smaller ( Fig. 4B, F ). In front of the anal fin, there is a large fulcrum in LPV−11817 ( Fig. 4D ), with a protuberant ridge in the middle.

A trace of the lateral line is visible on about 60 scales in both LPV−10144 ( Fig. 2 ) and LPV−10625. The median surface of the lateral line scales is longitudinally traversed by a groove, and the scales are excavated posteriorly. They bend sharply downwards in the dorsal fin region.

The outer scale surface is decorated by round or oval tubercles with pectinate ornament on the posterior margin. At the level of the anal fin and in the peduncle region, the serration is reduced. These serrations are very delicate and difficult to see. A process extends from the antero−dorsal side of the scale. As the scale deepens, the spine lengthens, and the spine is as long as the depth of the scale. The inner surface of the scale is smooth, with a peg−and−socket structure, and this articulation also exists in the scales of the caudle peduncle ( Fig. 4H ).

Remarks.—Several characteristics demonstrate that Luoxiongichthys is a neopterygian, for example, the presence of an interopercular bone, supraorbitals, the shape and separation of the maxilla from the preopercular, the presence of a coronoid process on the lower jaw, widely separated fin rays, and the hemiheterocercal tail fin ( Hurley et al. 2007).

Further, the new taxon appears to be a member of the clade Halecomorphi identified by Gardiner et al. (1996), based on the V−shaped rostral, squared parietal, and double jaw joint ( Li 2009). Luoxiongichthys also shows the classic halecomorph apomorphy ( Grande and Bemis 1998) of an elongated symplectic forming part of the double articulation with the lower jaw, but the other two features noted by those authors, a notch or concavity in the posterior margin of the maxilla and the single supramaxillary bone, cannot be determined.

However, Luoxiongichthys has some unique features that differentiate it from other halecomorphs. Owing to poor preservation of the skull, the family cannot be determined, but it may be identified as a new genus and species. The head length of the new species is about 50% the length of its body depth and 25% the length of total length. The new genus has a “hump” anterior to the dorsal fin, which is similar to that of the Triassic pycnodontid fish Gibbodon , but they differ from each other in teeth and dermal bones of the skull. For example, Gibbodon has bifid teeth and its opercular system is not complete ( Tintori 1981).

The circumorbital bone is of semionotid type ( López−Arbarello 2008). According to the characteristics of the opercular system and suborbital, this new genus resembles Lepidotes , because Semionotus only has one suborbital, but Lepidotes has more than one suborbital ( McCune 1986; Olsen and McCune 1991; Qian and Zhu 1997). However, many differences between Luoxiongichthys and Lepidotes can be summarized in the preopercular, fulcra, clavicle, antorbital, and scales ( McCune 1986; Jin 1987) ( Table 2).

The preopercular of Luoxiongichthys resembles that of Asialepidotus in being nearly vertical, curving ventrally, and with the ventral ramus slightly forward, a primitive structure ( Su 1959). The relative positions of the opercle and subopercle are also very like those in Asialepidotus . However, a marked difference between the two genera lies in the scales: Luoxiongichthys has scales with numerous tubercles on the surface, and fulcra are not well developed, with only a few fulcra in front of the dorsal lobe of the caudal fin, whereas the scales of Asialepidotus are smooth and the posterior margin is very finely pectinate, and there are small fulcra in front of all fins. Moreover, Lepidotes jurongensis also has smooth scales, which are quite different from those of Luoxiongichthys hyperdorsalis ( Qian and Zhu 1997) .

Judging from the characteristics of the scales, there are some similarities between Luoxiongichthys and Neolepidotes . The scales are large and rectangular in the trunk region. They decrease gradually in size backwards and become rhombic ( Zhang and Zhou 1974). In Neolepidotes liaodongensis , the scales are rhombic and smooth. The flank scales unite with a

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peg−and−socket articulation, and with their overlapped margin extended forwards at the antero−dorsal margin as prongs ( Jin 1987; Su 1996). The scales of Luoxiongichthys also have the same structure. In the holotype, some isolated scales can be seen. The peg−and−socket structure is prominent in the ventral region, but seems not to be typical in the dorsal region. Here, the scales are more “teleost−like” in that they articulate with each other by overlapping, rather than by a peg−and−socket structure. In Neolepidotes , the extended spine is as long as the depth of the scale, and as the scale becomes deeper, the spine lengthens. Further, the scales of Neolepidotes liaodongensis are smooth.

Because Luoxiongichthys is from the Anisian, it is no surprise that both the number of suborbitals and the ornamentation of the scales are different from Lepidotes , which is a Jurassic–Cretaceous taxon. Lepidotes from the late Mesozoic has more suborbitals.

The dermal shoulder girdle of Luoxiongichthys hypdorsalis has a clavicle like Watsonulus eugnathoides , a parasemionotid fish from the Early Triassic ( Olsen 1984; Gardiner et al. 1996). But Watsonulus eugnathoides has a preopercular with a typical broad dorsal limb ( Olsen 1984), which is different from Luoxiongichthys hypdorsalis . Another difference between the new genus and Parasemionotidae is the shape of the rostral: it is tube−like in the former, and V−shaped in the latter ( Lombardo 2001).

Geographical and stratigraphical distribution.— Type locality and horizon only.

Figures

Fig. 2. Holotype of the basal actinopterygian fish Luoxiongichthys hyperdorsalis gen. et sp. nov., LPV−10144, Guanling Formation, Anisian, Middle Triassic; Daaozi Quarry, Luoping, Yunnan Province, Southwest China; in lateral view (A), interpretive drawing (B).

Fig. 3. Skull of the basal actinopterygian fish Luoxiongichthys hyperdorsalis gen. et sp. nov., Guanling Formation, Anisian, Middle Triassic; Daaozi Quarry, Luoping, Yunnan Province, Southwest China. A. LPV−10625 in dorso−lateral view. B. LPV−10120 in dorsal view.

Fig. 4. Scales of the basal actinopterygian fish Luoxiongichthys hyperdorsalis gen. et sp. nov., Guanling Formation, Anisian, Middle Triassic; Daaozi Quarry, Luoping, Yunnan Province, Southwest China. Scales in the dorsal region: inner (A) surface. Scales in the Ventral region: outer (B) and inner (C) surface. Large fulcrum scale from in front of the dorsal fin (E) and anal fin (D). Rhombic scale from the caudal peduncle (F). Scale bars 1 mm.

Fig. 1. A. Lithological sequence of the Daaozi Section showing detail of Member II, with bed numbers, fossiliferous beds are shaded red. B. Location map (modified from Zhang et al. 2009). C. A photograph of fossiliferous beds 33–44.