Diandongichthys ocellatus, Xu, Guang-Hui & Ma, Xin-Ying, 2023

Xu, Guang-Hui & Ma, Xin-Ying, 2023, A new basal ginglymodian fish (Holostei: Neopterygii) from the Middle Triassic (Anisian) Luoping Biota, Yunnan, China, Zoological Journal of the Linnean Society 201 (3), pp. 1-14 : 2-9

publication ID	https://doi.org/ 10.1093/zoolinnean/zlad144
DOI	https://doi.org/10.5281/zenodo.13314214
persistent identifier	https://treatment.plazi.org/id/A66B87C5-FFE0-1024-57AC-F887130BFC9E
treatment provided by	Plazi
scientific name	Diandongichthys ocellatus
status	sp. nov.

Treatment

Diandongichthys ocellatus sp.nov.

lsid: zoobank.org:act: 816D8D59-941A-4B25-9CE5-55FF9EA 7E324.

A new ginglymodian fish from China • 3

Etymology: Ŋe specific epithet ‘ cyclotus ’ refers to the nearly rounded profile of the caudal fin.

Holotype: IVPP V25610, a complete, laterally compressed specimen from Luoping, eastern Yunnan.

Referred material: IVPP V 20616, 24229, 24369–24372, 25611– 25614, 25699, and 27623.

Locality and horizon: Luoping, Yunnan, China; Second (Upper) Member of Guanling Formation, Pelsonian (∼ 244 Mya), Anisian, Middle Triassic (Zhang et al. 2009).

Diagnosis: A small-sized ginglymodian distinguished from other members of the clade by the following combination of features (autapomorphies, those unique among ginglymodians, identified with an asterisk): frontal about three times as long as parietal; parietal nearly square; large pores for supraorbital sensory canal in frontal situated very close to lateral margin of this bone (*); three supraorbitals; eight infraorbitals; three suborbitals;

quadrate exposed in lateral view (uncovered by cheek bones); operclular length nearly equal to its depth (*); seven or eight pairs of branchiostegal rays; presence of median gular; absence of supramaxillary process in maxilla; supracleithrum, postcleithra and scales with serrated posterior margin; posterior margin of axial body lobe with incomplete row of two or three scales (*); about 10 pectoral fin rays; six pelvic fin rays; 11 or 12 principal dorsal fin rays; six principal anal fin rays; 13 principal caudal fin rays; caudal fin with rounded posterior profile; and pterygial formula of D17-18/P9-10, A16-17, C28/T32 (*).

Description

General morphology and size Represented by 13 specimens, Diandongichthys is a small-sized ginglymodian that has a blunt snout, a fusiform body, and an abbreviated heterocercal caudal fin ( Figs 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). Ŋe holotype (Fig. 1A) has a standard length (SL) of 35. 2 mm and a total length of 45. 5 mm. Ŋe largest known specimen (Fig. 3A) reaches a SL of 41.0 mm, and the smallest one (Fig. 5A) has a SL of only 27. 4 mm. Measurement data (e.g. head, predorsal and pre-anal lengths) for relatively completely

4 • Xu and Ma preserved specimens are provided in Table 1 View Table 1 . Ŋe greatest body depth occurs at the midway between the head and the origin of the dorsal fin, accounting for 30–33% of the SL. Ŋe head length (from the tip of the snout to the posterior extremity of the opercle) is slightly larger than the body depth. Ŋe outer surfaces of the cranial bones are ornamented with ganoine tubercles and ridges ( Figs 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). Ŋe reconstruction of the skull and pectoral girdle (Fig. 6) is mainly based on the holotype (Fig. 1) and several other specimens ( Figs 2–4 View Figure 2 View Figure 3 View Figure 4 ). Ŋe vertebral column is not visible due to the squamation in situ.

Snout Ŋere is a median rostral and a pair of nasals and antorbitals in the snout region. Ŋe rostral is short and tube-like, contacting the nasals posteriorly and the antorbitals laterally ( Figs 1C View Figure 1 , 2C View Figure 2 , 4C View Figure 4 ). Ŋe ethmoid commissure traverses the rostral, indicated by three small sensory pores on this bone. Ŋe nasals are elongate and slightly curved, having the maximal width at its middleventral portion where the nasals contact each other medially. Ŋe nasal slightly tapers posterodorsally and meets the frontal

with its dorsal tip (Fig. 2C). Ŋe antorbital is L-shaped; the horizontal branch of the antorbital, slightly shorter than the dorsal branch, extends forwards and transfers the ethmoid commissural canal into the rostral medially. Ŋe dorsal branch of the antorbital, inserts between the nasal and the first infraorbital. A distinct, small notch is present at the posterior margin of the junction between the horizontal and dorsal branches of the antorbital (Fig. 4C). Notably, the posterior end of the antorbital does not reach the anterior margin of the orbit, as commonly in other ginglymodians ( Cavin 2010, Grande 2010, López-Arbarello 2012, Xu et al. 2018, 2019).

Skull roof Ŋe skull roof includes a pair of frontals, parietals, dermopterotics, and extrascapulars. Ŋe sutures between frontals, parietals, and dermopterotics are distinct in most specimens ( Figs 1 View Figure 1 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 ), and based on them, we describe the shape and size of the skull roofing bones below. However, these sutures are not discernible in several other (relatively large) specimens (Fig. 2), in which, the frontals are probably fused with the

A new ginglymodian fish from China • 5

parietals and dermopterotics. Similar conditions are present in some small-sized stem neopterygians and stem teleosts ( Bürgin 1992, Arratia 2013, Xu 2021), but they are rarely known in other ginglymodians; the fusion of paired frontals into a median bone is present in some species of Eosemionotus (López-Arbarello et al. 2019).

Ŋe frontal is nearly trapezoidal, about three times as long as the parietal. Ŋe anterior half of the bone is relatively narrow; it widens posteriorly and reaches its maximal width at the level of the posterior margin of the orbit. Posteriorly, it narrows rapidly and contacts the parietal posteriorly and the dermopterotic posterolaterally. Ŋe less frontal contacts the right one along a strongly curved sagiưal suture. Ŋe paired parietals are nearly square, and the median suture between them is slightly curved (Fig. 3C). Ŋe supraorbital canal enters the frontal from the nasal, runs through the frontal parallel to the lateral margin of this bone, and ends at the anterior portion of the parietal. Ŋe sensory pores for the canal are relatively large, including three on the nasal, six to eight on the frontal, and two on the parietal.

Ŋree pit-lines are present on the parietal; the anterior and posterior ones are short, and the middle one is much longer, extending laterally into the dermopterotic ( Figs 3C View Figure 3 , 4C View Figure 4 ).

Ŋe dermopterotic is elongate, 1.5 times as long as the parietal. It widens posteriorly, contacting the parietal and the posterior portion of the frontal in a curved medial suture ( Figs 1C View Figure 1 , 3C View Figure 3 , 4C View Figure 4 ). Ŋe trapezoidal extrascapular tapers medially and contacts its counterpart at the median line of the skull. Ŋe supratemporal canal extends through the dermopterotic and posteriorly enters the extrascapular, indicated by a longitudinal line of three to five pores parallel to the lateral margin of the dermopterotic ( Figs 1C View Figure 1 , 4C View Figure 4 , 5C View Figure 5 ).

Circumorbital bones A supraorbital is discernible along the anterior part of the orbital margin of the frontal in the holotype (Fig. 1C), and two supraorbitals the posterior part of the orbital margin of this bone in IVPP V24371 (Fig. 4C). Ŋus, a complete series of three supraorbitals is reconstructed on each side of the skull (Fig. 6). Ŋey are rectangular; the middle supraorbital is

6 • Xu and Ma the longest, the anterior is slightly shorter than the middle, and the posterior is the shortest, being about two-thirds of the length of the middle.

Ŋere are eight infraorbitals between the antorbital and the dermosphenotic ( Figs 1C View Figure 1 , 4C View Figure 4 ). Ŋe first (anteriormost) is relatively narrow and slightly inclined posteriorly; the dorsal tip of the bone does not contact the supraorbital bone, and consequently the orbital rim is not closed. Ŋe second is relatively large and trapezoidal; the maximal depth is nearly equal to its length. Ŋe posterodorsal margins of the first and second infraorbital bones contribute the anterior margin of the orbit. Ŋe third to fissh infraorbitals are relatively small and rectangular or trapezoidal, forming the ventral margin of the orbit. Ŋe sixth, located at the posteroventral corner of the orbit, is large and nearly pentagonal with a convex posterior margin. Ŋe seventh is small and trapezoidal, having a depth 1.5 times its length. Ŋe last (eighth) infraorbital is slightly deeper than long, being the smallest one of the infraorbital series. Ŋe infraorbital sensory canal, marked by several pores, runs longitudinally through the anterior two infraorbitals near their ventral borders, traverses the third to fissh at their middle portions; posteriorly, it extends obliquely upwards and passes dorsoventrally through the remaining infraorbitals, and finally enters the dermosphenotic.

Ŋe dermosphenotic is sub-triangular, tapering anteriorly ( Figs 2C View Figure 2 , 4C View Figure 4 ). Ŋe sphenotic [=autosphenotic of Rayner (1948)], not fused with the dermosphenotic, has a small exposed portion on the skull roof (Fig. 2C); this condition is also present in many other holosteans ( Olsen and McCune 1991, Grande and Bemis 1998, Grande 2010, Cavin et al. 2013, Xu and Ma 2018).

Ŋere are three suborbitals between the last (posterior) three infraorbitals and the preopercle. Ŋe upper suborbital is nearly rectangular, twice longer than deep; the middle one is broad and trapezoidal; and the lower one is also trapezoidal, half of the size of the middle one.

Jaws Ŋe upper jaw consists of a premaxilla, a maxilla, and a supramaxilla. Ŋe premaxilla has a horizontally expanded oral region and a deep, posterodorsally directed nasal process ( Figs 1C View Figure 1 , 5C View Figure 5 ). Ŋe depth of the nasal process is nearly equal to the length of the oral margin of the bone. A small foramen at the base of the nasal process probably represents the palatine ramus of the facial nerve; there is no large foramen for the olfactory nerve in the nasal process, showing a primitive condition as in other basal holosteans (Olsen 1984, López-Arbarello et al. 2016, Xu 2019). Six or seven teeth are present along the oral margin

A new ginglymodian fish from China • 7

of the premaxilla ( Figs 3C View Figure 3 , 5C View Figure 5 ); they are conical and similar in size. Ŋe maxilla is elongate with a peg-like, medially directed anterior process. Ŋe oral margin is nearly straight, and the dorsal margin is slightly concave at the posterior portion of the bone to accommodate a supramaxilla (Fig. 3C). Ŋe maxilla extends posteriorly and ends below the posterior half of the orbit, having a rounded posterior margin. Ŋe supramaxilla is slim and

elongate, tapering at both ends ( Figs 2C View Figure 2 , 3C View Figure 3 ). Ŋere are 11 or 12 conical teeth along the oral margin of the maxilla, and their sizes are slightly smaller than those in the premaxilla.

Ŋe wedge-shaped dentary is the largest bone of the lower jaw. It becomes deeper posteriorly, forming the major part of the prominent coronoid process (Fig. 2C). Ŋe ventral margin of the dentary is slightly concave; the posterior margin of the bone is distinctly notched and receives the anterior process of the angular. Ŋe angular is nearly trapezoid in lateral view, being about one-third the length of the lower jaw ( Figs 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). Ŋe sensory pores associated with the mandibular canal include a longitudinal series of large pores extending from the dentary into the angular, as well as some small pores near the oral margin at the anterior portion of the dentary ( Figs 1C View Figure 1 , 4C View Figure 4 ).

Ŋe supra-angular is elongate and plate-like, forming the posterior portion of the coronoid process. A possible retroarticular is presented as a small bone at the posteroventral corner of the lower jaw in IVPP V24371 (Fig. 4C). Additionally, a slim and elongate coronoid bone is discernible near the anterior portion

8 • Xu and Ma of the dentary; it bears a row of five conical teeth. Other elements (e.g. pre-articular and articular) at the medial surface of the lower jaw are not exposed.

Palatal elements and suspensorium Ŋe parasphenoid and vomers are partly discernible through the orbit (Fig. 3C). Ŋe parasphenoid has a relatively broad anterior stem with a well-developed ascending process at its posterior portion. Ŋe small vomer is expanded anteriorly and sutured to the ventral margin of the anterior portion of the parasphenoid posteriorly. At least three small teeth are present at the anterior portion of the ventral margin of the vomer.

Ŋe dermopalatine is relatively short, contacting the elongate endopterygoid and crescent-shaped ectopterygoid posteriorly ( Figs 3C View Figure 3 , 5C View Figure 5 ). Four conical teeth are discernible at the ventral margin of the dermopalatine (Fig. 5C), and three relatively small teeth at the anterior portion of the ventral margin of the ectopterygoid (Fig. 3C). Ŋe metapterygoid is partly visible at the posteroventral portion of the palate; it is large and nearly trapezoidal (Fig. 5C).

Ŋe hyomandibula and sympletic are partly exposed in several specimens ( Figs 1C View Figure 1 , 3C View Figure 3 , 4C View Figure 4 ). Ŋe former is hatchet-shaped, bearing a large foramen for the hyomandibular branch of the facial nerve near the centre of the bone; the laưer is rod-like,

having a dorsal end more massive than its ventral end (Fig. 3C). Additionally, the quadrate is nearly fully exposed in several specimens ( Figs 1–4 View Figure 1 View Figure 2 View Figure 3 View Figure 4 ); it is fan-shaped with a well-developed condyle articulating with the lower jaw. Ŋe quadratojugal is a splint-like bone that contacts the posterolateral surface of the ventral portion of the quadrate. It tapers posterodorsally and rests on the anterior edge of the preopercle ( Figs 1C View Figure 1 , 2C View Figure 2 , 3C View Figure 3 ).

Operculo-gular series Ŋe preopercle is narrow and crescent-shaped with its dorsal tip nearly contacting the lateral margin of the dermopterotic ( Figs 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). Ŋe preopercular sensory canal runs dorsoventrally through the entire length of the preopercle, having a series of pores parallel to the posterior margin of the bone. Ŋe trapezoidal opercle is relatively long and dorsoventrally short, with a rounded posterior margin; the length of the bone is nearly equal to its depth. Ŋe subopercle is sickleshaped, bearing a short, triangular anterodorsal process that inserts between the preopercle and the opercle ( Figs 2 View Figure 2 , 4 View Figure 4 , 5 View Figure 5 ). Ŋe interopercle is small and triangular, tapering anteroventrally.

Ŋere are seven or eight pairs of branchiostegal rays ( Figs 1C View Figure 1 , 2C View Figure 2 ). Ŋey are elongate and plate-like, becoming longer posteriorly. Ŋe median gular is nearly elliptical, slightly over half the length of the lower jaw, with some concentric striae on the surface (Fig. 7B).

Paired girdles and fins Ŋe paired post-temporals are large and sub-triangular, with a rounded posterolateral margin ( Figs 1–5 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). Each post-temporal tapers medially, contacting its counterpart medially. Ŋe supracleithrum is rhomboidal and nearly as deep as the opercle. Ŋe lateral line pierces the lateral portion of the post-temporal and extends posteriorly through the dorsal portion of the supracleithrum. Ŋe large cleithrum has a dorsal arm longer than its anterior arm; the complete shape of the bone is still unknown because of the lateral coverage of the subopercle and branchiostegal rays. Ŋere are two postcleithra associated with the cleithrum; the upper is trapezoidal, slightly deeper than the supracleithrum, and the lower is short and nearly rhomboidal. Ŋe posterior margins of the supracleithrum and postcleithra are serrated ( Figs 2 View Figure 2 , 4 View Figure 4 ).

Each pectoral fin is composed of 10 distally segmented rays, preceded by two basal fulcra and a series of fringing fulcra (Fig. 7A). Ŋe first ray is unbranched, and other rays are branched distally.

A pair of pelvic plates (basipterygia) is exposed in IVPP V 24371 (Fig. 7D). Ŋey are hourglass-like bones. Ŋe pelvic fins are located at the 10th or 11th vertical scale row. Each is consisted of six distally segmented rays, preceded by two basal fulcra (Fig. 7B). Several fringing fulcra are distributed along the anterior margin of the first ray.

Dorsal and anal fins and pterygiophores Ŋe dorsal fin originates above the 18th vertical scale row and ends at the 26th vertical scale row. Ŋe fin is large with a rounded posterior margin. It is composed of 12–13 distally segmented rays (including a procurrent ray), preceded by five or six basal fulcra. Ŋe procurrent ray and the first principal ray are unbranched, and the remaining rays branched distally (Fig. 7D). A series of leaf-like fringing fulcra is associated with the anterior two rays. Ŋere are in total 13 proximal radials exposed in IVPP V24372 (Fig. 7C); the anterior two support the basal fulcra, and each of the posterior radials supports a ray. Additionally, tiny dot-like bones between the proximal radial and the ray are identified as middle radials. Ŋe distal radials appear unossified.

Ŋe anal fin originates below the 15th or 16th vertical scale row and terminates below the 20th or 21st vertical scale row. It is small and nearly trapezoidal, including seven distally segmented rays (a procurrent ray plus six principal rays). Ŋe procurrent ray is preceded by three basal fulcra (Fig. 7C). Additionally, several leaf-like fringing fulcra are associated with the anterior two rays. Ŋe anal pterygiophores are not exposed.

Caudal fin Ŋe caudal fin is abbreviated heterocercal with a rounded posterior profile, including 13 principal rays (Fig. 7F). Ŋe dorsal and ventral marginal principal rays are segmented and unbranched, and each is preceded by one or two shorter procurrent rays; other principle rays are branched distally. Ŋe middle principle ray close to the lateral line scale has a dorsal bulge near its arrow-like proximal head (Fig. 7E). Additionally, there are nine to 12 epaxial basal fulcra along the dorsal edge of the caudal peduncle; the anterior two are median, and others are paired. Moreover, four or five basal fulcra are present along the ventral edge of the caudal peduncle. A series of fringing fulcra is associated with the dorsal and ventral procurrent rays and the last (ventral most) principle ray.

Scales Ŋe body is fully covered with rhomboid scales in adult individuals ( Figs 1A View Figure 1 , 2A View Figure 2 , and 8 View Figure 8 ). However, in some juvenile specimens ( Figs 4A View Figure 4 , 5A View Figure 5 ), the body is naked in a long and narrow region close to the base of the dorsal fin. Ŋe scales are smooth with a serrated posterior margin, arranged in 33 vertical rows between the pectoral girdle and the caudal inversion. Posterior to the caudal inversion, there are two additional scales along the lateral line. In the holotype, 12 scales are present on each side of the body in the 9th vertical row, including four and seven scales above and below the lateral line, respectively. Seven or eight rows of inverted scales are present posterior to the hinge line in the caudal region; the last row of inverted scales consists of only three small scales (Fig. 7F). Ŋe lateral line scales in the anterior flank region are deepest with a ratio of depth-to-length being about 2. Ŋe body scales gradually become shorter dorsally, ventrally, and posteriorly. Five to six well-developed serrations are present on the posterior margins of anterior scales, and the counts gradually reduce in those of posterior scales. Scales along the dorsal ridge anterior to the dorsal fin are nearly heart-shaped with no distinct posterodorsal spine. Moreover, there is an enlarged dorsal scute anterior to the first epaxial basal fulcrum of the caudal fin. Ŋe peg-and-socket articulations between scales are present, as commonly in other early holosteans.