Type species: Isanichthys palustris Cavin and Suteethorn, 2006; Phu Nam Jun, Upper part of the Phu Kradung Formation, probably Berriasian in age.

Emended diagnosis.—Ginglymodian fish with skull roof bones strongly ornamented, with no continuous ganoin cover; frontals slightly narrower anteriorly than posteriorly; ratio of frontal length to parietal length less than 2.5; ratio of skull length to orbit length more than 6; closed orbital ring; two anterior infraorbitals not in contact with the orbit, anteriormost infraorbital deeper than long; two supraorbitals, the anterior one generally elongated with anterior margin contacting the first and/or the second infraorbital; cheek region completely covered by bones; quadrate lies below the orbit; preopercle slightly curved; maxilla formed by a posterior rounded platelike part and a thin anterior part, which precedes the articular process; one supramaxilla; epiotic with a short and simple posteriorly directed process; posttemporal fossa present; intercalar absent; basisphenoid present; presence of an oral sensory canal; 25 rays in caudal fin, 12 in upper lobe; 50 to 53 rows of ganoid scales along the flank and approximately 20 scales in the transverse row at the deepest level of the body.

Etymology: The species is named after Lertboos Gongtong, former chief of Kam Muang District, honouring his important contribution to the systematic excavation at the type locality.

Holotype: KS36-2 (Figs. 2, 3). Subcomplete specimen with most of the dermal bones preserved, with partially preserved trunk and fins in right lateral view, standard length (SL) 800 mm.

Type locality: Phu Noi, Tambon Din Chi, Kam Muang district, Kalasin province, northeastern Thailand.

Type horizon: Lower part of the Phu Kradung Formation, probably Late Jurassic in age.

Referred material.—KS36-3, fragments of the squamation of a specimen with an almost complete skull except the snout region, which laid above the caudal region of KS36-2 (this part was destroyed during excavation) (Fig. 2A); KS34-281, portion of a skull showing in part the suspensorium; KS34-380, skull roof with part of the braincase.

Diagnosis.—Isanichthys with the body approximately 4.6 times longer than deep; total length approximately 3.8 times the head length; skull roof slightly sigmoidal in lateral view; preorbital region reduced; asymmetrical parietals; frontal almost quadrangular in shape; one pair of extrascapulars plus a small median one; infraorbital and suborbital bones weakly or not ornamented; few suborbitals (ca. 4 or 6) arranged in one row; dermal component of the sphenotic visible on the cheek; edentulous maxilla; dentary bears approximately 9 small cylindrical teeth; semi-tritorial dentition between the dermopalatines and the coronoid ossifications; median dorsal row of scales with spine; all fins with long basal and fringing fulcra.

Description

General features and proportions.—The body is proportionally shorter and deeper than in the type species, approximately 4.6 times longer than deep, and head length is proportionally shorter, being contained approximately 3.8 times in the total length. The head is 200 mm in average length (including the opercular series), and 175 mm in average depth. The ossifications of the head have no ganoin cover and most of the skull roof bones bear rugae and tubercles forming a reticulated pattern in the centers of ossifications and extending into radiating ridges to their margins (Fig. 5A).

Skull roof.—The frontal is the longest element of the dermal skull roof (Fig. 6). It is about twice as long as the parietal. The frontal is almost quandrangular, only slightly narrower anteriorly than posteriorly, with the maximum width at the level of the posterior margin of the orbit. The suture between both frontals is nearly straight. The suture between frontal and parietal on the left side is not aligned with the similar suture on the right side. The parietal is rectangular, approximately 1.5 times longer than wide. Both parietals are nearly equal in surface, but their shape is asymmetrical (Fig. 6). The parietal contacts the frontal anteriorly with an inter-digitating suture in some specimens (Fig. 6A), or with an irregular but smooth suture in other specimens (Fig. 6B). It sutures with the dermopterotic laterally and the lateral extrascapulars posteriorly. The posterior margin of the parietal is arched, with a concavity situated at mid-length in KS34-380. The lateral and medial margins are straight, except in KS34-281, which presents a digitated pattern on the posterior half of the medial suture. The dermopterotic is massive, elongated and with an anterolateral expansion extending lateral to the frontal. The dermopterotic is longer than the parietal but is approximately equal to it in maximum width. Its lateral margin contacts the dorsalmost suborbital and the dorsal extremity of the preopercle. There is only one extrascapular on each side of the skull, plus a small median one that is triangular in shape (Fig. 6B). The external surface of the extrascapulars is coarse, with ornamentation mainly present in the centres of the bones. The extrascapular has an irregular posterior margin and its anterior border abuts against the sigmoid border of dermopterotics and parietals (Figs. 3, 4). The occurrence of a single pair plus a small median extrascapular differs from the general pattern of showing two extrascapulars in most Jurassic ginglymodians and generally multiple pairs in Cretaceous ginglymodians including the lepisosteiforms (Cavin 2010). The anterior border of the frontal shows an indentation, in which rests the nasal. The nasal bone is a plate-like ossification visible on KS34-281 and KS36-2, which lies on the nasal processes of the premaxillae (Figs. 11, 13). The premaxilla apparently extends under the frontal as in other ginglymodians (Fig. 13), but it does not participate in the dermal skull roof cover as in gars and in some specimens of Pliodetes (Wenz 1999).

Braincase.—KS34-380 is a fairly well-preserved braincase, only slightly distorted laterally, lacking the ethmoid region and the parasphenoid. However, the posterior portion of the parasphenoid is visible in KS34-281 and forms a wing-like process ventrally (Fig. 5B). The occipital region forms an ossified block with the exoccipital contacting the prootic (Figs. 8A, B). The basioccipital is damaged but its general shape can be reconstructed. The lateral side shows a concavity at mid-length. Almost in the centre of the lateral wall of the basioccipital a foramen for the occipital artery opens (Figs. 7, 8A). The suture between the basioccipital and the exoccipital ossifications forms a wavy curve. The anterior border of the basioccipital is situated at the same level as the exoccipital (Fig. 8A). The exoccipital is a curved ossification, separated from its counterpart on the posterior face by a narrow gap located above the foramen magnum (Fig. 8C). The exoccipital comprises a laterally-oriented face, extending above the basioccipital, and a horizontally-oriented dorsal face (Fig. 8B). Both faces are separated by a rounded ridge that spreads out laterally. Several small foramina open in the mid-length of the rounded ridge, placed slightly on the posterodorsal surface of the ossification, are for the dorsal and ventral roots of spinal nerves or the occipital nerve. The anteroventral surface shows a large foramen for the vagus canal (X). The posterodorsal side of the exoccipital articulates with the first neural arch while the second neural arch, partly preserved, is separated by a gap from the occipital condyle (Fig. 8B). The intercalar is absent, a situation similar to that found in Macrosemimimus lennieri (described as “Lepidotes toombsi”) (Patterson 1975). The epioccipital is an irregular and complex bone. It forms a well-developed ventral limb that protrudes and contacts the exoccipital ventrally. This limb is developed as a very strong ridge with a vertically oriented base in lateral view that expands in posterior view. At the dorsoposterior edge of this ossification is a short process (Fig. 8C). The dorsal surface is irregular in shape and its anterior margin is marked by forked sutures with the posterior margin of the parietal (Fig. 6A). Both epioccipitals meet above the gap separating both exoccipitals. There is no supraoccipital. The prootic (Figs. 7, 8A, B) is well developed and contacts the exoccipital posteriorly. The bone has a crenulated posterior edge and bears a prominent straight ridge that runs vertically along the anterior margin. The dorsal and ventral ends of the ridge form sutural surfaces. The dorsal surface articulates with the sphenotic, while the ventral surface probably contacted the ascending ramus of the parasphenoid. There is a large foramen for the hyomandibular branch of the facial nerve (VII) in the centre of the bone. Three shallow grooves and two ridges radiating from the large foramen for the facial nerve mark the lateral face of the prootic. The ventral ridge is much more prominent than the dorsal one. Posterior to that large opening is a small foramen for the glossopharyngeal nerve (IX). The sphenotic (Figs. 7, 8A) is best preserved on the right side of KS34-380. It is sutured to the ventral side of the dermopterotic, and to the prootic posteriorly. This bone has an expanding curved anteromedial margin and a straight posterior margin. It forms a well-developed anteriorly inclined rounded crest that extends laterally to the level of the circumorbital ossifications. The sphenotic is also visible on both sides of KS36-2 (holotype) and KS36-3, in which the ossification develops a lateral blunt process that is visible on the cheek along the ventral margin of the dermosphenotic (Figs. 3, 4). A small dermal component of the sphenotic process is also visible in specimens of other ginglymodians (see Cavin et al. 2013 for a review). In gars the sphenotic is visible laterally and separated from the dermosphenotic (Grande 2010). In Isanichthys lertboosi the tip of the lateral process of the sphenotic is separated from the orbit by a gap, but it is closely associated with the dermosphenotic. In obaichthyids, sphenotic and dermosphenotic are fused together (Grande 2010).

In KS34-380, the basisphenoid, pterosphenoid and orbitosphenoid surround the orbital cavity, with only a gap on the ventral margin that was occupied by the parasphenoid, as confirmed by the presence of attachment areas on the ventral surface of the basisphenoid and orbitosphenoid (Fig. 8A). The basisphenoid is the smallest bone of the preserved series and is situated at the posteroventral edge of the orbit cavity. The posterior margin is covered with matrix, but we can estimate its shape. It is approximately triangular with a slightly curved anterodorsal margin, while the other margins are straight. Along the posterior margin runs a narrow groove that reaches the pterosphenoid. Although in KS34-380 the pterosphenoid has slightly shifted, it is clear that the pterosphenoid was sutured to the basisphenoid ventrally and to the orbitosphenoid anteriorly. The pterosphenoid is wing-shaped with its dorsal margin forming a rounded rim and its ventral margin marking a broad angle, almost at its mid-length. The lateral surface of the bone presents alternating ridges and grooves. The orbitosphenoid has an irregular shape and is located anteriorly to the orbit. Its posterior margin has a deep notch. It bears a blunt process on the posteroventral corner, which almost reaches the basisphenoid, while the posterodorsal extremity sutures with the pterosphenoid. Laterally, two grooves run anteriorly from the centre of ossification. The anterior margin forms a slightly concave line and the dorsal margin is sutured with the ventral side of the frontal. The basisphenoid is absent in gars and in Thaiichthys buddhabutrensis, while it is present in most other ginglymodians in which this region is known (Araripelepidotes, Scheenstia mantelli, Callipurbeckia minor, Lepidotes semiserratus).

Circumorbital and suborbital series.—The circumorbital ring is complete. It is composed of a large dermosphenotic forming the posterodorsal edge of the orbit, two supraorbitals above the orbit and six to eight infraorbitals located posteriorly, ventrally and anteriorly to the orbit. The number of infraorbitals is generally eight but KS34-281 contains six large infraorbitals only (Fig. 5A). Two infraorbitals anteriorly are not in contact with the orbit. The first, anteriormost, infraorbital is deep and irregular in shape with a dorsal margin shorter than the ventral margin, which is gently convex (Fig. 3B). The second infraorbital is approximately rectangular or ovoid in shape, slightly deeper than long. The third infraorbital, which forms the anteroventral corner of the orbit, and the remaining ones, situated below and behind the orbit, are deeper than long. On the left side of KS36-3, the infraorbitals are ornamented with strong knobs and grooves in the centres of all ossifications, except the large posteroventral one, which lacks ornamentation (Fig. 4A). The shape of the dermosphenotic varies: it is elongated and trapezoidal in KS36-2, large and rectangular in KS36-3 and narrow and rectangular in KS34-281. The supraorbitals consist of two large bones. The anteriormost is elongate and rectangular in shape, tapers anteriorly, and the posterior one is smaller and deeper than long. In the holotype (KS36-2), there are six suborbitals arranged in one row located between the dermosphenotic, the infraorbitals and the preopercle. In some specimens (KS36-3, KS34-281) four suborbitals only are present (Figs. 4, 5). The largest one, situated anteroventrally, is irregular in shape with its dorsal margin straight, and its ventral margin slightly undulating. It articulates posteriorly to the adjacent suborbital and dorsally to the infraorbitals, and contacts posteroventrally the blunt extremity of the preopercle. The dorsalmost suborbital is large and rectangular, longer than deep and articulates with the dermopterotic dorsally, the dermosphenotic anteriorly, the dorsal portion of the preopercle posteriorly and the adjacent suborbital ventrally. The remaining suborbitals are deeper than long (Fig. 4). On its right side, KS36-2 presents a small triangular suborbital located at the anterior end of the series (Fig. 3B).

Jaws.—There are some variations in the arrangement of jaw bones and in tooth morphology. The maxilla is best preserved on the left side of KS34-281. It is an elongate bone with a thin, slender anterior part and a plate-like, rounded posterior part. Its narrow anterior portion is edentulous (Figs. 5A, 10). The anterior articular process, prolonging the anterior thin part and corresponding to one third of the length of the bone, is inwardly curved. A supramaxilla rests on the dorsal margin of the posterior plate. Its depth is nearly half the depth of the maxilla, and it has a curved dorsal margin and a straight ventral margin that borders the maxilla. The nasal process of the premaxilla extends posterodorsally under the nasal and the frontal, and the alveolar portion extends laterally as blunt processes. Only one tooth is preserved on the premaxilla of KS34-281 but the exact number of teeth on the anterior transversal margin of the bone is unknown. The tooth is similar in shape and size to the teeth borne by the dentary and ectopterygoid. It is well developed, about 4 mm in height with an acrodine tip, which is about 0.5 mm high (Figs. 10, 11).

The lower jaw is massive. The visible ossifications are a dentary, one or two coronoids, a large angular, and a surangular located on the posterodorsal edge of the mandible and visible just anterior to the coronoid process in lateral view (Fig. 5A). The angular forms most of the posterior part of the mandible. It contacts the dentary along a suture, whose pattern varies within the set of available specimens. An elongated posterior process of the dentary is present on the holotype. The dentary is robust and slightly tapering at its anterior extremity, which bears a row of approximately 13 conical teeth with bulbous and pointed acrodine caps. Each tooth is composed of a high cylindrical base, approximately 4 mm in height and 1 mm in width, topped by a bulbous cap that is shallow (0.5 mm) compared to its base. All teeth are held on a single row along the anterior portion of the dorsal rim of the dentary. A second row of teeth, similar in both shape and size to the dentary teeth, lies behind the anterior marginal teeth. These teeth are regarded as coronoid teeth. A second coronoid (or the posterior part of the anterior one) is visible as a thin blade of tooth-bearing bone wedged in a notch of the dentary, anteriorly to the coronoid process and visible on both sides of KS34-281 (Figs. 9, 10).

The holotype, KS36-2 (Figs. 12, 13), shows a different arrangement of its jaws. The premaxillae have no preserved teeth. Posteriorly, the narrow and elongate nasal process of the premaxilla extends under the frontal, and anteriorly the bone extends transversely to form the alveolar processes (Fig. 13A). On this specimen, the lower jaw is broken but we observe a piece of mandible shifted close to the skull. This piece of bone bears two rows of crushing teeth. The teeth, approximately 5 mm high, have a cylindrical base with a bulbous acrodine cap, which is 2.5 mm high, 2 mm wide (this morphology corresponds to the “moderately tritorial dentition” as defined by Cavin 2010). The teeth are not attached to the dentary, and we regard them as coronoid teeth (Fig. 13B). Based on the location of the articulation between the lower and upper jaws, it seems that this fish had a wide gape.

Hyopalatine series.—The suspensorium is visible on both sides of the holotype (KS36-2), but it is poorly preserved and identification of the ossifications remains uncertain. The hyomandibula is partially exposed; the anteroventral part only is visible, while the rest is hidden by the opercle and subopercle. Based on the surface of the exposed region, it seems that the bone is a massive and relative large ossification. The metapterygoid (Fig. 3A) is irregular in shape, with its posterior part expanding and with a tapering anterior extremity. It contacts the hyomandibula posteriorly and the entopterygoid anteriorly, but we cannot see if there was a contact with the quadrate. The entopterygoid is a triangular bone wedged between the metapterygoid dorsally and the ectopterygoid anteriorly. The ectopterygoid is preserved as a crescent-shaped ossification suturing posteriorly with the entopterygoid. The anterior portion of the ectopterygoid bears at least 7 small cylindrical-based teeth along its ventral border. Each of these teeth is tipped with a bulbous cap. In KS36-2, the dermopalatine is covered with teeth (Fig. 12). The teeth are variable in size and irregularly arranged, but two main rows of crushing teeth appear to be present. In KS34-281, the right ectopterygoid is also visible as a thin bone lying under the infraorbitals (Figs. 9, 10). It bears 6–7 conical teeth restricted to its anterior margin. The arrangement of the palatine dentition is unknown in other specimens.

Opercular series.—The opercular series is complete, formed by the preopercle, opercle, subopercle, and interopercle.

In the holotype, KS36-2, the preopercle (Figs. 3–5) is preserved only on the right side. It is narrow with a poorly developed ventral limb, and is slightly bent forward. The ossification is crescent-shaped, with no marked angle between both limbs. In KS34-281, the preopercle of the right side shows two limbs forming a more closed angle than in the holotype, and the horizontal limb is shorter than the vertical (Fig. 5B), but it is unclear if this arrangement is genuine or caused by preservation. The vertical branch has parallel margins that do not converge dorsally, and the ossification ends anteriorly as a blunt spine. The extremity of the horizontal branch slightly widens in its anterior part before the blunt spine. The preopercular sensory canal extends enclosed within the thickened anterior margin of the ossification. The opercle (Fig. 3) is roughly quadrangular and slightly deeper than long, but in one specimen (KS34-281) the ossification is more rounded (Fig. 5A). The anterior border is straight, while the posterior border is strongly convex and widely overlaps the supracleithrum. The dorsal border reaches the dermal skull roof. The surface is smooth in KS36-2 (holotype) but in some specimens, such as KS36-3, the external surface is ornamented with fine grooves, and in KS34-281 rough tubercles are present. The subopercle is well developed with a vertical limb reaching half the depth of the opercle. The anterior margin of the bone is vertical and straight, and contacts the posterior border of the interopercle. The ventral margin of the bone is convex. The interopercle is visible as a triangular bone wedged between the subopercle and the posteroventral edge of the preopercle (Fig. 3B). The anterior tip tapers and it runs toward the ventral extremity of the preopercle. In KS36- 3, however, the interopercle is visible as a narrow bone along the ventral margin of preopercle (Fig. 4A).

Hyoid arch and branchiostegal rays.—Six thin and elongated branchiostegal rays are preserved on the left side of holotype, KS36-2 (Fig. 3A); the posteriormost one is the shortest but the widest. The four anterior rays articulate with the anterior ceratohyal and the two posterior ones with the posterior ceratohyal. The anterior ceratohyal is hourglass-shaped, with a slightly convex posterior border. There is a weak groove running on the median region of the lateral surface of the bone. The posterior ceratohyal is roughly triangular in shape, with a regularly curved margin contacting the anterior ceratohyal.

Pectoral girdle.—The posttemporal is visible as a crescent-shaped ossification with a notch along its posterolateral margin for the exit of the sensory canal (Fig. 4). The supracleithrum is an ovoid ossification and its dorsal border articulates with the posterior margin of the posttemporal. The path of the sensory canal is indicated by three pores that open adjacent to the exit of the sensory canal. The cleithrum is a long and curved ossification, proportionally narrow on its exposed part, which is overlapped by the opercular series. The lateral face of the cleithrum bears traces of enamel. The posterior margin of the bone is curved. In KS34-281, the ventral margin of the cleithrum shows a concavity for the insertion of the pectoral fin (Fig. 5A). The horizontal limb of the cleithrum is rather short and deep, with its lateral face marked by a smooth ridge. One gently curved postcleithrum is visible.

Cephalic sensory canal.—The anterior part of the lateral line is indicated by a series of pores located on the dorsal margin of the supracleithrum, then by a pore located in the concavity of the middle part of the posttemporal and by one on the anteroventral edge of this ossification. The occipital sensory canal (forming the supratemporal commissure) is indicated by a series of pores that open along the posterior edges of extrascapular ossifications. The supraorbital canal marks an angle at the level of the posterodorsal corner of the orbit, and several pores along the lateral margin of the frontal and along the lateral margin of the parietal. The canal passes through the nasal along the longitudinal axis of the bone. There is one pore visible on the anterior tip of the nasal in specimens KS36-2 and KS34-281. The middle pit line is indicated by a groove and pores opening between parietal and dermopterotic close to the posterior edge of these bones. The supratemporal sensory canal extends along the dermopterotic through the dermosphenotic, which marks the connection with the infraorbital sensory canal. The infraorbital sensory canal runs in the centre of the infraorbital ossifications, and gives off openings located near the ventral and posterior margin of the infraorbitals located respectively ventrally and posteroventrally to the orbit. There is no evidence of a sensory canal running within the supraorbital bones. The preopercular sensory canal presents several pores along the horizontal and vertical branches of the ossification: three elongate pores open in the curvature of the bone near its ventral margin and two smaller pores open more dorsally (KS36-3), and two pores are visible close to the dorsal margin of the preopercle in KS34-281. The mandibular sensory canal gives off two or three pores in the angular, and approximately six pores in the dentary arranged in two lines in KS34-281, the dorsal one corresponding to the oral canal.

Pectoral fins.—The pectoral fin of KS36-2 is composed of three basal fulcra-the anterior one, unpaired, is followed by two elongate and paired ones-and two fringing fulcra (only two basal fulcra are visible in Fig. 15A because of the angle). The unsegmented basal portions of ten rays are visible in KS36-2.

Pelvic fins.—The pelvic fin consists of two basal fulcra, the anterior of which is unpaired and the second paired (both halves of the paired fulcrum are visible in Fig. 15B), five thin fringing fulcra and five rays. The rays are very long, representing 47% of the head length (KS36-2; fin length 90 mm, head length 190 mm)

Unpaired fins.—The dorsal fin is visible on KS36-2 and more completely on KS36-3 (Fig. 16) while the anal fin is lacking in all specimens. The dorsal fin is composed generally of four basal fulcra (the anterior one is unpaired and the other are paired on KS36-2), approximately eight fringing fulcra and 12 rays (KS36-3, Fig. 16C). The fringing fulcra are slightly curved and very elongated, the first one being equal to half the length of the first ray. The first third of the length of the rays is not segmented. A first longitudinal division of the ray occurs at mid-length, and a second division occurs approximately at the third quarter of the length of the rays. Although the very tips of the rays are usually not preserved, the outline of the fin observed in KS36-3 seems to indicate that the distal margin of the fin was straight.

As for the other fins, the caudal fin is poorly preserved (Fig. 17). The dorsal lobe is preceded by an indeterminate number of fulcra, whose arrangement cannot be exactly described. The ventral lobe is preceded by at least two basal fulcra and 12 small and thin fringing fulcra. There are probably 25 rays, 12 in the ventral lobe separated by a deep fork in the squamation from the 13 in the dorsal lobe. Each ray divides three times. The ventral lobe of the caudal fin equals approximately 60% of the head length and we can infer that the dorsal lobe, poorly preserved on the available material, was similar in size (the tail is usually externally homocercal in ginglymodians).

Squamation.—Based on the general outline, we can estimate the number of scales along the lateral line as approximately 53, 12 scales along a row from the anterior extremity of the dorsal fin to the lateral line and 25 scales in the mid-line between the head and the dorsal fin. The shape of the scales varies depending on position on the body. The anterior margin of individual scales bears a peg articulation as in most ginglymodians (see Cavin et al. [2009] and López-Arbarello [2012] for a discussion of this character). The lateral line scales are marked with a small pore on their surface. The posterior extremity of the scales forms a blunt spine, sometimes more acute, especially in the posterior part of the trunk. In KS36-2, the median dorsal scales are longer than broad, rhomboidal in shape and have a slightly concave surface. There are very elongate spines in KS36-3.

Holotype: TF 7764, a nearly complete individual, only known specimen.

Type locality: Phu Nam Jun, Thailand.

Type horizon: Upper part of the Phu Kradung Formation, probably Berriasian.

Emended diagnosis.—Isanichthys with the body elongated and slender, with the total legnth 5.5 times the estimated maximum depth, and 4.5 times the head length (including the opercular series); 8–9 suborbitals arranged in a mosaic pattern; no dermal component of the sphenotic; infraorbital and suborbital bones strongly ornamented; median dorsal row of scales without posterior spine; two pairs of extrascapulars; toothed maxilla; dentary bearing approximately 12 small cylindrical teeth.

Remarks.—Isanichthys palustris Cavin and Suteethorn, 2006 was identified on the basis of a single specimen from the Phu Nam Jun locality, Thailand. Although from the same formation, i.e., the Phu Kradung Formation, the Phu Nam Jun locality is stratigraphically higher in the formation, close to its top, and consequently the fish assemblage of this locality is likely younger than the assemblage from the Phu Noi locality. The description of I. lertboosi led to a re-examination of the specific characters of the type species.

Material.—BMNH P.6838, subcomplete individual; BMNH P.6840, isolated skull bones; BMNH P. 6841 (holotype) isolated but articulated skull bones and scales; BMNH P. 45620, head; from Middle Jurassic of the United Kingdom (for details see Cavin 2010).

Emended diagnosis.—Isanichthys with most of the dermal ossifications of the skull ornamented with tubercles of ganoin; skull roof proportionally short, being approximately 1.5 times longer than wide; parietals asymmetrical; preorbital region reduced; ca. 12 suborbitals arranged in a mosaic pattern; semi-tritorial dentition; two pairs of extrascapulars; toothed maxilla.

Remarks.—Isanichthys latifrons (Woodward, 1893) was originally described as Lepidotes latifrons by Woodward (1893) on the basis of material from the Oxford Clay of Northamptonshire, UK. Woodward (1893) compared the skull of this species to the Early Jurassic “Dapedius” (= Dapedium), to which it was thought to be closely related. Later, Jain (1983) discussed some characters of this species.

Isanichthys latifrons possesses characters of the genus Isanichthys such as the strongly ornamented bones of the skull roof, the frontal only slightly narrower anteriorly than posteriorly, the frontal less than 2.5 times longer than the parietal, two supraorbitals, the cheek region completely covered by bones, the preopercle slightly curved, the maxilla forming a thin blade with a posterior rounded plate-like part, a posttemporal fossa and an oral sensory canal, ratio of skull length to orbit length greater than 6, and the quadrate situated below the orbit. Moreover, based on the proportions of the skull and the reconstruction figured by Jain (1983), the anterior infraorbital would have been deep, and the anterior supraorbital would have closed the orbit anteriorly and should have contacted two infraorbitals, three other characters diagnostic of Isanichthys. In contrast to the other Isanichthys species, I. latifrons bears small tubercles of ganoin of its dermal skull bones, and other specific characters mentioned in the diagnosis above.

Holotype: CDUT cv 002, a nearly complete individual, only known specimen.

Type locality: Upper Shaximiao Formation, Sichuan, China.

Type horizon: Early to Middle Jurassic.

Emended diagnosis.—Isanichthys with preorbital region reduced; no tritorial dentition; few infraorbitals (7) and few suborbitals (6) arranged in one row; dentary and premaxilla bearing strong teeth with vertical shaft and conical cap; no conspicuous dorsal median ridge scale.

Remarks.—“Lepidotes” luchowensis was described by Wang (1974) from the Upper Shaximiao Formation in Sichuan, a series of continental Early to Middle Jurassic deposits (Chang and Jin 1996). We have studied the only known specimen (CDUT cv 002), and we review only those osteological characters which are important in the comparison with the Thai material, or characters for which we disagree with the original description by Wang (1974). The dermal bones, especially the circumorbital ossifications, bear strong, densely arranged tubercles. The frontal is almost complete, except the anterior extremity and the medial border, which are broken. It is roughly rectangular and only slightly narrower anteriorly than posteriorly. The parietal and extrascapular are not preserved. The dorsal margin of the orbit is bordered by a supraorbital anteriorly, which contacts an infraorbital. Posterior to the anterior supraorbital is a large impression in the matrix, which is deeper posteriorly than anteriorly and has curved dorsal and ventral margins. This impression would have contained the dermosphenotic, and possibly a posterior supraorbital as figured by Wang (1974), but this arrangement is uncertain. The infraorbital series consists of seven infraorbitals with the anteriormost one not contacting the orbit. There are six suborbitals arranged in one row (with two small ossifications arranged transversely to the row, but the whole pattern cannot be regarded as a mosaic pattern). The oral process of the premaxilla is well developed and bears at least six strong conical teeth with a bulbous cap. The nasal process of the premaxilla is covered by the nasal. Posteriorly to the series of premaxillary teeth, two more teeth are visible, and these are borne on a poorly preserved ossification. We identify this bone as the maxilla. Posterior to this element is a better preserved elongated rod of bone, which was interpreted as a supramaxilla by Wang (1974), and that we identify as the continuation of the maxilla. Ventral to the premaxilla is a broad ossification, with a quadrangular anterior extremity and a broadened posterior part that Wang identified as a maxilla. Four, widely spaced teeth are present in the anterior half of the bone. Because of its shape and position—the ossification extends ventrally to the premaxilla, and it extends posteriorly beneath the maxilla and coronoid process—we consider that this element represents part of the suspensorium, probably the anterior extremity of the ectopterygoid and the dermopalatine (but no suture is visible in this element). The anterior part of the dentary bears ten conical teeth with a bulbous cap on its anterior rim. The anterior part of a deep coronoid process, probably formed by the dentary, is preserved. The imprints of the posterior part of the mandible show the limit of the dentary, which develops an elongated posterior process that does not reach the posterior margin of the lower jaw, and shows a deep impression corresponding to the sensory canal that runs along the centre of the bone and opens with three large pores at its anterior part. The preopercle is narrow and gently curved. The anterior part of the horizontal limb is missing, but we can estimate the shape from the trace of bone. The opercle is approximately rectangular in shape and deeper than long. The subopercle is well developed with a vertical limb reaching half the depth of the opercle, and the interopercle is visible as a large triangular bone. The posttemporal, supracleithrum, cleithrum, and postcleithrum are preserved. The cleithrum has a narrow vertical limb and a broader horizontal limb. The anterior end is missing but we can determine the limit of the bone from the imprint. A portion of the branchial lamina extending under the subopercle is visible dorsally. A band of denticles extends along the ridge that separates the branchial from the lateral surface of the cleithrum. This band is broader than in Semionotus elegans (Newberry, 1888) (Olsen and McCune 1991), Propterus Agassiz, 1834 (Bartram 1977) but reminiscent of the eight to ten parallel bands of denticles in Lepidotes gloriae Thies, 1989.

Only a few characters visible in the type specimen of Isanichthys luchowensis are diagnostic for the genus Isanichthys. These are: the strongly ornamented bones of skull roof, the frontal only slightly narrower anteriorly than posteriorly, the cheek region completely covered by bones, the slightly curved preopercle, the ratio of skull length to orbit length greater than 6, the quadrate situated below the orbit. The grouping of I. luchowensis with the other species of that genus also rests on the optimization of some characters, which are unknown in the Chinese species, and consequently this identification should be regarded as tentative (see phylogenetic analysis below). Characters distinguishing this species are mentioned in the diagnosis above.