Asialepidotus shingyiensis, SU, 1959

Xu, Guang-Hui & Ma, Xin-Ying, 2018, Redescription and phylogenetic reassessment of Asialepidotus shingyiensis (Holostei: Halecomorphi) from the Middle Triassic (Ladinian) of China, Zoological Journal of the Linnean Society 184 (1), pp. 95-114 : 97-104

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ASIALEPIDOTUS SHINGYIENSIS SU, 1959

Asialepidotus shingyiensis Su, 1959: 205 , pl. III, figs. 1, 2. – Chang & Jin, 1996: 466. – Li & Jin, 2003: 796. – Chang & Miao, 2004: 538, fig. 3A (mislabeled as Sinoeugnatus kueichowensis in the figure legend). – Jin, 2009: 120, three figures (unnumbered). – Benton et al., 2013: 220, 221, fig. 20B. – Tintori et al., 2014: 400, fig. 8A. – Xu & Ma, 2017: 37, fig. 1.

Guizhouella analilepida – Liu et al., 2003: 350, pl. I, fig. 2; pl. IV, figs. 1, 2; pl. V, fig. 1; pl. VI, figs. 1, 2.

Guizhoueugnathus analilepida – Liu, 2004: 447.

Holotype: IVPP V2434, an incomplete, laterally compressed specimen, with most of the skull and caudal fin missing ( Fig. 1 View Figure 1 ).

Referred material: IVPP V 19005 View Materials , 19009, 19996–19999, 20673–20679, 22856–22869, 22995–23000, 23002– 23018; NIGP 136040 ( Fig. 2 View Figure 2 ), 136041; MES 3603 .

Locality and horizon: Dingxiao and Wusha, Xingyi, Guizhou Province; Jiyang hills, Fuyuan, and Changdi, Luoping, Yunnan Province; Zhuganpo member of the Falang Formation, Ladinian, Middle Triassic.

Emended diagnosis: A large-sized ionoscopiform distinguished from other members of this order by the following combination of features: frontal nearly three times as long as parietal; parietal rectangular, slightly longer than wide; supraorbital sensory canal contacting anterior pit-line in parietal; dermopterotic 1.3–1.4 times as long as parietal; three (two, occasionally) pairs of extrascapulars; two supraorbitals; dermosphenotic with canal-bearing innerorbital flange; five infraorbitals; two suborbitals; quadratojugal splint-like; supramaxilla single, nearly half of length of maxilla; maxilla with branch of infraorbital sensory canal; maxilla ending at level of posterior margin of orbit; 14 pairs of branchiostegal rays; median gular large and nearly triangular; 15 rays in each pectoral fin; 10–11 principal dorsal rays; 11–12 principal anal rays; 21–23 principal caudal rays; rhomboidal scales with serrated posterior margin; and scale formula of D25–26/P11–13, A22–24, C37–40/T43–45.

COMPARATIVE DESCRIPTION

GENERAL MORPHOLOGY AND SIZE

Asialepidotus has a blunt snout, a fusiform body and a moderately forked caudal fin ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). The dorsal fin inserts slightly posterior to the origins of pelvic fins. The anal fin is nearly equal to the dorsal fin in size. The great body depth lies midway between the posterior margin of the opercle and the origin of the dorsal fin. The smallest known specimen ( IVPP V23003 View Materials ) is 60 mm in standard length ( SL, the length from the tip of the snout to the posterior extremity of the caudal peduncle), and the largest specimen ( IVPP V23010 View Materials ) reaches a SL of 273 mm. The head length (measured from the tip of the snout to the posterior margin of the opercle), accounting for 30–37% of SL, is relatively larger in small specimens than in large specimens ( Table 1). The incompletely preserved holotype ( Fig. 1 View Figure 1 ) has a length of 125 mm from the posterior margin of the opercle to the posterior extremity of the caudal peduncle, and its SL is estimated to be ~ 190 mm. The outer surfaces of the cranial bones are ornamented with ganoine tubercles and ridges .

SNOUT

The canal-bearing bones of the snout consist of a median rostral and a pair of nasals and antorbitals ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). The rostral is narrow and nearly V-shaped, with a small posteriorly pointing median apex and a pair of lateral extensions. It overlies parts of the premaxillae anteriorly, and contacts the nasals posteriorly and the antorbitals laterally. The rostral houses the anterior commissure of the lateral line system, connecting the right and left sides to each other through the antorbitals. There is a sensory pore near the base of each lateral extension of this bone.

The paired nasals are elongate and taper posterodorsally. Each nasal contacts the rostral anteroventrally, the antorbital laterally and the frontal dorsally, and meets its counterpart along the middle line. The anterior margin of the nasal is slightly concave for the anterior nostril. The posterior nostril is located between the lateral margin of the nasal and the medial margin of the antorbital, resembling the condition in Panxianichthys ( Xu & Shen, 2015) . An anterior portion of the supraorbital sensory canal is enclosed in the nasal, indicated by several small pores on this bone.

The antorbitals are curved and elongate, having a tube-like anteroventral extension that contacts the rostral and an expanded posterodorsal portion that contacts the frontal and supraorbital. Posteriorly, it forms part of the anterior orbital margin, showing a condition as in Subortichthys (Ma & Xu, 2017) , Panxianichthys ( Xu & Shen, 2015) , Robustichthys ( Xu et al., 2014) , Oshunia ( Maisey, 1991) and Ophiopsis muensteri ( Lane & Ebert, 2012, 2015). In contrast, the antorbital is relatively short and does not reach the orbital margin in other ionoscopiforms. The conjunction of the ethmoid commissural canal and the infraorbital sensory canal is located at the anterior one-third portion of the antorbital.

SKULL ROOF

The skull roofing bones include a pair of frontals, parietals and dermopterotic, and three (two, occasionally) pairs of extrascapulars ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ).

The elongate frontals are the largest elements on the skull roof. Each frontal is nearly three times as long as the parietal, having a slightly excavated lateral margin above the orbit, where it contacts the supraorbitals. The frontal widens posteriorly and contacts the parietal and the dermopterotic with a sinuous suture. The supraorbital sensory canal enters the frontal from the nasal, runs longitudinally through this bone, and enters the parietal posteriorly.

The paired parietals contact each other in a strongly digitated suture ( Figs 3 View Figure 3 , 4 View Figure 4 ). Each parietal is nearly rectangular, longer than wide. There is a straight, narrow zone where the extrascapulars lap onto the parietal. Three pit-lines are present. The anterior pit-line runs anteriorly near the lateral margin of this bone and contacts the supraorbital sensory canal in the frontal ( Fig. 3 View Figure 3 ). The middle pit-line extends from the posterolateral portion of the parietal, runs laterally into the dermopterotic, and ends near the temporal sensory canal in this bone. The posterior pit-line originates slightly anterior to the level of the middle pit-line, extends posterolaterally, and ends near the posterior margin of this bone.

The dermopterotic is elongate, with a tapered anterior process that fits a notch at the posterior portion

BD, body depth; HL, head length; PAL, preanal length; PDL, predorsal length; PVL, prepelvic length; SL, standard length; TL, total length.

of the frontal ( Fig. 5 View Figure 5 ). The dermopterotic is 1.3–1.4 times as long as the parietal, to which it is sutured medially with a nearly straight margin. The lateral margin of the dermopterotic is excavated. The temporal sensory canal runs longitudinally through the dermopterotic near its lateral margin, and enters the lateral extrascapular posteriorly.

The extrascapulars are trapezoidal bones and vary in size and number. Most specimens have three pairs of extrascapulars ( Figs 2 View Figure 2 , 3 View Figure 3 ), as described by Liu et al. (2003). However, several specimens have only two pairs of extrascapulars ( Figs 4 View Figure 4 , 5 View Figure 5 ). This probably represents an intraspecific variation, because the other anatomical features are consistent in these specimens. The supratemporal commissure runs transversely through the extrascapulars, indicated by several small pores at the middle portions of these bones.

CIRCUMORBITAL BONES

There are two elongate supraorbitals. The anterior is 1.4–1.5 times as long as the posterior. Two supraorbitals are otherwise present in Panxianichthys and Robustichthys ( Xu et al., 2014; Xu & Shen, 2015), but other ionoscopiforms generally have three or more supraorbitals ( Bartram, 1975; Alvarado-Ortega & Espinosa-Arrubarrena, 2008; Machado et al., 2013; Lane & Ebert, 2015; Ma & Xu, 2017). Notably, Oshunia lacks any supraorbitals ( Maisey, 1991), representing a particular case in this order.

Five infraorbitals are present ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). The anteriormost or first infraorbital (= lachrymal) is large and cleaver shaped. It tapers anteriorly, having a triangular anteroventral portion that inserts between the anterorbital and the maxilla. The dorsal margin that borders the anterorbital is nearly straight, and the ventral margin is slightly rounded and borders the anterior half of the maxilla. The posterior margin of the first infraorbital is slightly concave. The infraorbital sensory canal passes longitudinally through the bone near its ventral margin, and has a branch running into the maxilla.

The second infraorbital is small and nearly trapezoidal, with a convex anterior margin and a straight posterior margin. The ventral border of this bone is slightly concaved and contacts the anterior portion of the supramaxilla ventrally. The infraorbital sensory canal runs longitudinally through the ventral portion of the second infraorbital, about two-thirds of the way down from its dorsal margin.

The third infraorbital is large and nearly pentagonal. It contacts the posterior portion of the supramaxilla with a concaved ventral margin and the ventral suborbital with a convex posterior margin.

The fourth infraorbital is small and quadrangular. It is sutured to the third infraorbital with a convex ventral margin. This bone tapers dorsally and contacts the fifth infraorbital with a straight dorsal margin.

The last, fifth infraorbital is also small, equal to the fourth infraorbital in size ( Figs 3 View Figure 3 , 5 View Figure 5 ). It is nearly quadrangular, 1.6 times deeper than wide, with a convex ventral margin and a posteriorly inclined dorsal margin. The sensory canal runs dorsoventrally through the fourth and fifth infraorbitals near the anterior margins of both bones.

The dermosphenotic has a broad dorsal portion and a narrow ventral, innerorbital flange. The dorsal portion is trapezoidal, ornamented with tubercles and ridges. The ventral innerorbital flange is tube-like and smooth, through which the dermosphenotic receives the infraorbital sensory canal from the last infraorbital.

The sphenotic (= autosphenotic of Rayner, 1948), not fused with the dermosphenotic, has a small portion participating in the skull roof, as in many other holosteans ( Olsen & McCune, 1991; Grande & Bemis, 1998; Grande, 2010; Cavin et al., 2013). This exposed portion is smooth on the surface. It is sutured to the dermosphenotic anterodorsally and makes up a posterodorsal portion of the orbital margin, resembling the condition in other ionoscopiforms ( Bartram, 1975; Grande & Bemis, 1998; Alvarado-Ortega & Espinosa-Arrubarrena, 2008; Xu et al., 2014; see Gardiner, Maisey & Littlewood, 1996, for discussion on this feature).

There are two suborbitals. The upper is large and trapezoidal, with its anterior margin slightly overlapped by the fourth and fifth infraorbitals. The lower is slightly smaller and sub-triangular, tapered ventrally. It bears two pit-lines: the dorsal originates at the middle portion of the suborbital, extends posteriorly, and ends near the posterior margin of this bone; the venteral originates just below the anterior tip of the horizontal pit-line, extends ventrally parallel to the posterior margin of the middle suborbital, and ends at the ventral corner of this bone. Compared with other ionoscopiforms, Asialepidotus has the fewest number of suborbitals; Subortichthys , Panxianichthys , Ophiopsis , Ionoscopus and Quetzalichthys commonly have three suborbitals ( Bartram, 1975; Alvarado-Ortega and Espinosa-Arrubarrena, 2008; Xu & Shen, 2015; Ma & Xu, 2017), Macrepistius has five suborbitals ( Schaeffer, 1960), and Teoichthys and Robustichthys have eight or more suborbitals ( Machado et al., 2013; Xu et al., 2014).

Two sclerotic bones are discernible near the orbital rim. They are proportionally narrower than those in C. furcatus ( Grande & Bemis, 1998) but resemble those of Panxianichthys in size ( Xu & Shen, 2015).

Given that the dermosphenotic is incorporated into the skull roof and posteroventrally sutured to the exposed portion of the sphenotic, Liu et al. (2003) misidentified the dermosphenotic as the ‘third supraorbital’ and the exposed portion of the sphenotic as a ‘dermosphenotic’. This kind of misidentification also occurred in Saint-Seine’s (1949) description of Ionoscopus and Schaeffer’s (1960, 1971) descriptions of Macrepistius , as already pointed out by Maisey (1991) and Gardiner et al. (1996), respectively. In addition, Liu et al. (2003) misidentified two suborbitals as part of the ‘infraorbitals’ in Asialepidotus .

VOMERS AND PARASPHENOID

The vomers are elongate, abutting the premaxillae anteriorly and the parasphenoid posteriorly. Teeth are present only on the anterior portion of each vomer ( Fig. 6 View Figure 6 ). They are conical, and slightly smaller than those on the premaxilla.

The parasphenoid is cross shaped, with a pair of small, laterally directed basipterygoid processes and well-developed ascending wings. A large patch of teeth with rounded tips covers most of the oral margin of this bone from the level of the ascending wings to that of articular surfaces for the vomers. There are no foramens for the internal carotid and/or afferent pseudobranchial arteries in this bone. The teeth on the anterior portion of the patch are notably larger than those on the posterior portion ( Figs 4–6 View Figure 4 View Figure 5 View Figure 6 ).

PALATINE, HYOID AND BRANCHIAL SERIES

The dermopalatines are small and elongate, bearing teeth similar to those on the vomers. The ectopterygoid, entopterygoid and metapterygoid are sutured to each other, and it is difficult to identify the boundaries between them in most specimens. Densely arranged teeth are present on their medial surfaces, most of which are styliform with rounded tips, except those near the lateral portion of the ectopterygoid, which are pointed. In size, the teeth on the entopterygoid and the anterior portion of the ectopterygoid are largest and those on the metapterygoid smallest ( Fig. 6 View Figure 6 ).

The quadrate is small and triangular, articulating with the lower jaw with a strong condyle( Figs 2–4 View Figure 2 View Figure 3 View Figure 4 ). Most of the quadrate is exposed beneath the ventral margin of the third infraorbital and the lower suborbital, showing a condition similar to Panxianichthys ( Xu & Shen, 2015) and Subortichthys (Ma & Xu, 2017) but different from Robustichthys ( Xu et al., 2014) and other ionoscopiforms, in which most of the bone is covered laterally by the infraorbital, suborbital and/ or maxilla.

The quadratojugal is splint-like, with a slightly expanded anteroventral portion and a tapered posterodorsal portion ( Figs 3–5 View Figure 3 View Figure 4 View Figure 5 ). It rests on the anterior edge of the ventral portion of preopercle, and medially, covers a short posteroventral portion of the quadrate (including part of the condyle) but does not take part in the jaw articulation, showing a condition similar to that in Subortichthys (Ma & Xu, 2017) , Panxianichthys (G-H Xu, personal observation on GMPKU P3116) and some ginglymodians ( López-Arbarello, 2012). A quadratojugal is absent in Robustichthys ( Xu et al., 2014) and more derived ionoscopiforms.

The hyoid arches are best exposed in IVPP V22856 View Materials ( Fig. 6 View Figure 6 ), including paired hyomandibulas, symplectics, hypohyals, and anterior and posterior ceratohyals. The hyomandibula is hatchet shaped, having a relatively long, posteriorly directed opercular process that articulates with the opercle. The depth from the base of the opercular process to the dorsal margin of this bone is about one-third the depth of the whole hyomandibula. The hyomandibula bears a foramen at the level of the opercular process, through which the hyomandibular branch of the facial nerve might have passed.

The symplectic is rode-like ( Fig. 6 View Figure 6 ), resembling that in Subortichthys (Ma & Xu, 2017) and Robustichthys ( Xu et al., 2014) . It has an expanded dorsal portion, a slightly constricted middle portion, and a strong ventral condyle that is partly exposed beneath the ventral margin of the preopercle and articulates with the lower jaw.

The hypohyal is nearly square, contacting its opposite medially. A foramen for the afferent hyoidean artery is absent in this bone, as in other holosteans