Asialepidotus shingyiensis

REDESCRIPTION OF ASIALEPIDOTUS SHINGYIENSIS 105

Elements of the branchial arches are partly preserved in a few specimens, including four hypobranchials and five ceratobranchials on each side ( Fig. 6 View Figure 6 ). They are rod-like bones. Small tooth plates are discernible on the oral margins of some hypobranchials. and more primitive actinopterygians ( Gardiner et al., 1996; Grande & Bemis, 1998; Grande, 2010).

The anterior ceratohyal is elongate and large, ~60% the length of the lower jaw. It widens posteriorly and is proportionally broader than that in Amia . The posterior ceratohyal is small and subcircular.

JAWS

The upper jaw is composed of a premaxilla, a maxilla and a supramaxilla ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 , 7 View Figure 7 ). The premaxilla is relatively large, having a horizontally expanded oral region and a deep, posterodorsally directed nasal process. A small foramen for the palatine ramus of the facial nerve lies slightly above the oral margin of the premaxilla ( Fig. 5 View Figure 5 ). There is no foramen for the olfactory nerve in the nasal process of this bone, showing a primitive condition as in Watonulus ( Olsen, 1984) and Robustichthys (G-H Xu, personal observation). Eight teeth are present along the oral margin of the premaxilla. They are conical and nearly equal to those on the anterior portion of the maxilla in size.

The maxilla is elongate, having a notched posterior margin and a peg-like, medially directed anterior process, which extends between the premaxilla, the dermopalatine and the vomer ( Fig. 6 View Figure 6 ). The dorsal margin of the posterior part of the maxilla is concave, into which the supramaxilla fits. The posterior end of the maxilla is located at the level of the posterior orbital margin. As in other ionoscopiforms, the maxilla encloses a branch of the infraorbital sensory canal that enters this bone from the first infraorbital. The branch passes through the anterior three-quarters of the length of the maxilla, indicated by a series of small pores and pits near the ventral margin of this bone ( Fig. 3 View Figure 3 ).

The supramaxilla is elongate and has rounded tips. It is about half the length of the maxilla.

The lower jaw is elongate and strong, bearing a large, dorsally expanded coronoid process at its posterior half portion. The maximal height of the lower jaw is 36–38% of its total length.

The dentary, the largest element of the lower jaw, is wedge shaped. It deepens posteriorly, forming the major part of the coronoid process.

The supra-angular is small and plate-like, forming a posterior portion of the coronoid process. In size, it is proportionally equal to the supra-angular of Watsonulus ( Olsen, 1984) and smaller than that of Amia ( Grande & Bemis, 1998) .

The angular is trapezoidal and slightly over half of the length of the lower jaw, with its anterior portion laterally covered by a flange of the dentary. In lateral view, the suture between the angular and dentary is sinuous and that between the angular and supra-angular is nearly straight. Corresponding to the articulation for the quadrate, the angular has a large notch in the dorsal margin of its posterior portion. There is a dorsoventrally directed groove on the posterior part of the angular ( Figs 4 View Figure 4 , 5 View Figure 5 ), which represents the ‘posterior mandibular pit-line’ of Wenz (1967). A similar pit-line is also present in other ionoscopiforms (e.g. O. muensteri, Lane & Ebert, 2012 , 2015; Robustichthys, Xu et al., 2014 ). This pit-line does not connect the preopercular sensory canal, different from the vertical branch of the mandibular canal in Amia ( Grande & Bemis, 1998) .

The retroarticular is small, and sutured to the posteroventral corner of the angular ( Fig. 5 View Figure 5 ).

Medially, two coronoid bones, a prearticular, a coronomeckelian and an articular are discernible in each lower jaw. Both coronoids are small and elongate bones, bearing relatively large, conical teeth at their oral margins ( Fig. 5 View Figure 5 ).

The prearticular is large and V-shaped, medially covered by dense small teeth ( Fig. 6 View Figure 6 ).

The coronomeckelian is small and nodular-like, laterally contacting the middle portion of the angular ( Fig. 6 View Figure 6 ).

A small portion of the articular is partly exposed beneath the notch of the angular, but its complete shape is still unknown.

OPERCULAR SERIES

The preopercle is narrow and crescent shaped. The anterior margin of the preopercle contacts the suborbitals and quadratojugal, and its dorsal end is nearly in contact with the dermopterotic. The preopercular sensory canal runs dorsoventrally through the preopercle, with some posterior diverticulae at the ventral portion of the preopercle, indicated by a series of pores near the posterior margin of this bone.

Immediately posterior to the preopercle lie the rest of the opercular bones, including the opercle, subopercle and interopercle. The opercle is large and trapezoidal, 1.5–1.7 times deeper than long. The subopercle is relatively small, bearing a triangular anterodorsal process that inserts between the preopercle and opercle. This process is about one-third to half the depth of the opercle. The interopercle is small and triangular, tapered anteroventrally.

GULAR AND BRANCHIOSTEGAL RAYS

The median gular is elongate and nearly triangular, having a pointed anterior tip and a broad, convex posterior end. Its length is slightly over half the length of the lower jaw.

Fourteen pairs of branchiostegal rays are present ( Figs 3 View Figure 3 , 4 View Figure 4 ). They are elongate and plate-like, increasing in length and width posteriorly.

Liu et al. (2003) failed to identify the median gular, although this bone is present in their studied specimen ( Fig. 2 View Figure 2 ). Additionally, Liu et al. (2003) inaccurately estimated that Asialepidotus has ten pairs of branchiostegal rays.

PAIRED GIRDLES AND FINS

A posttemporal, a presupracleithrum, a supracleithrum, a cleithrum, three postcleithra and two clavicle elements are present at each side of the pectoral girdle.

The posttemporal is sub-triangular, tapered medially, with a round posterolateral corner. Each posttemporal contacts the extrascapulars anteriorly and the supracleithrum laterally.

The presupracleithrum is small and sub-circular, lying between the posttemporal, the opercle and the supracleithrum. Although a presupracleithrum is absent in the living bowfin, this bone is present in many primitive halecomorphs, such as Watsonulus ( Grande & Bemis, 1998) , C. furcatus ( Lambers, 1992; Grande & Bemis, 1998), Robustichthys ( Xu et al., 2014) and Panxianichthys ( Xu & Shen, 2015) .

The supracleithrum is deep and anteriorly inclined, with its anterior portion slightly overlapped by the opercle.

The cleithrum is large and sickle shaped, bearing a series of curved ridges on the lateral surface of this bone. Small denticles are densely arranged on these ridges ( Figs 4 View Figure 4 , 5 View Figure 5 ). The anterior arm of the cleithrum is shorter than the dorsal arm ( Fig. 5 View Figure 5 ), showing a primitive condition, as in Watsonulus and other ionoscopiforms. In contrast, the anterior arm of the cleithrum is significantly longer than the dorsal arm in the Amiiformes ( Grande & Bemis, 1998) .

There are three plate-like postcleithra associated with the cleithrum. The dorsal is largest, as deep as the supracleithrum; the middle is trapezoidal, nearly one-third of the size of the dorsal; and the ventral is smallest and triangular.

There are two clavicle elements ( Fig. 5 View Figure 5 ). They are small and elongate, bearing several rows of denticles similar to those of the cleithrum. Two clavicle elements are otherwise known in Ionoscopus ( Lambers, 1992) , some amiiforms ( Amia , Amiopsis , Solnhofenamia and Amblysemius ; Grande & Bemis, 1998) and gars ( Grande, 2010).

The pectoral fins are large, inserting low on the body. Each pectoral fin bears 15 distally segmented rays ( Fig. 8B View Figure 8 ). The first is unbranched, preceded by one or two basal fulcra, and a series of fringing fulcra are associated with the leading ray. The remaining rays are branched distally. Given that the pectoral fins were incompletely preserved in the holotype, Su (1959) inaccurately estimated that Asialepidotus has only seven rays in each pectoral fin. Liu et al. (2003) made no descriptions of the pectoral fins.

The pelvic girdles are not exposed. The pelvic fins insert at the 12th or 13th vertical scale row. Each bears nine distally segmented rays, preceded by two or three basal fulcra and a series of fringing fulcra ( Fig. 8C View Figure 8 ). The first ray is unbranched, and the others are branched distally. Su (1959) did not distinguish the basal fulcra from rays and miscounted that 11 rays were present in each pelvic fin.

MEDIAN FINS

The dorsal fin originates above the 25th–26th vertical scale row. It is triangular and composed of ten or 11 principal rays ( Fig. 8D View Figure 8 ). The first principal ray is distally segmented and unbranched, preceded by a rudimentary ray, three or four basal fulcra and a series of fringing fulcra; the remaining rays are branched distally. The rudimentary ray is about half of the length of the first principal ray.

The anal fin originates below the 22th–24th vertical scale row. It has 11–12 distally segmented principal rays ( Fig. 8F View Figure 8 ). The first ray is unbranched, preceded by one rudimentary ray, two basal fulcra and a series of fringing fulcra, and the remaining rays are branched distally. The rudimentary ray is relatively short, twofifths of the length of the first principal ray, and is composed of two segments, with the distal one inserting between two fringing fulcra.

The caudal fin is hemi-heterocercal, with a moderately forked profile ( Fig. 8E View Figure 8 ). It has 21–23 principal rays. The dorsal marginal principal ray is segmented and unbranched in most specimens. In IVPP V20676 View Materials , however, this ray is also branched, showing a condition similar to that in O. muensteri ( Lane & Ebert, 2012, 2015). The ventral marginal principal ray is segmented and unbranched, and its length varies considerably in different specimens; the ratio of the length of the ventral marginal principal ray to that of the adjacent branched ray varies from onehalf to one. The middle principal rays are segmented and branched up to four times. Additionally, nine or ten basal fulcra are present in the dorsal lobe, and two basal fulcra and three or four rudimentary rays in the ventral lobe. Fringing fulcra are present in both lobes.

Su (1959) correctly identified 11 anal fin rays in the holotype but did not describe the dorsal and caudal fins. Liu et al. (2003) correctly identified 11 dorsal fin rays and 22 principal caudal fin rays in his studied specimens, but he did not describe the anal fin.

SCALES

The body is fully covered with rhombic scales. The scales are arranged in 43–45 vertical rows along the main lateral line. In addition, 10–13 rows of scales extend into the epaxial lobe of the caudal fin. The scales in the anterior flank region are 1.5 times deeper than wide, and they gradually become shorter and smaller dorsally, ventrally and posteriorly. In addition to the main lateral line, there is an additional lateral line, indicated by a line of seven or eight small pores on the scales in the predorsal region. A similar condition is also present in several other ionoscopiforms (e.g. Ophiopsis procera, Bartram, 1975 ; Robustichthys, Xu et al., 2014 ; Subortichthys, Ma & Xu, 2017 ). Most of the scales, except those covering the epaxial lobe of the caudal fin, have a serrated posterior margin, with 2–14 acute projections. The surfaces of the scales are largely smooth except for small pores and ridges on some scales. Pegs and anterodorsal extensions are exposed on some scales in the anterior flank region.

The trunk was broken into two parts in the holotype, with a few vertical rows of scales missing; Su (1959) estimated that Asialepidotus has ~40 vertical rows of scales along the lateral line.