Isanichthys luchowensis ( Wang, 1974 )

Deesri, Uthumporn, Lauprasert, Komsorn, Suteethorn, Varavudh, Wongko, Kamonlak & Cavin, Lionel, 2014, A new species of the ginglymodian fish Isanichthys from the Late Jurassic Phu Kradung Formation, northeastern Thailand, Acta Palaeontologica Polonica 59 (2), pp. 313-331 : 327-328

Isanichthys luchowensis ( Wang, 1974)

Fig. 18 .

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 anteri- or 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.

Figures

Fig. 18. Skull of ginglymodian fish Isanichthys luchowensis sp. nov., cv 002; Early or middle Jurassic of Sichuan, China. A. Photograph. B. Semi-interpretative line drawing.