The First Record of a Ray-finned Fish (Actinopterygii, Palaeoniscidae) from the Lower to Middle Permian of Tak Fa Formation in Phetchabun, Central Thailand

Abstract

The actinopterygian fossil was found from the Permian carbonate succession of Tak Fa Formation, Saraburi Group, in Bueng Sam Phan District, Phetchabun Province, central northeast Thailand. The partially preserved body part with ganoid scales is identified as belonging to a ray-finned fish (subclass Actinopterygii). The squamation is composed of smooth-surfaced articulated scales with a hard-diamond shape. The posterior part of the specimen showed some translucent ganoine covering mineralized bone layers. The microstructure of squamation is characterized by the presence of 3–6 ridges running parallel to the antero-ventral margin of scales on the bone layer. This suggests that the Paleozoic actinopterygian specimens from Phetchabun likely belong to Palaeoniscum sp. The age of the fauna is assigned to the Early to Middle Permian based on the presence of microfossils and invertebrates recovered from nearby areas. The paleoenvironments of the area corresponded to lagoons or subtidal zones with a low-energy depositional condition according to lithological, stratigraphical, and petrographic examinations of limestones interbedded with shales. This study represents the first record of palaeoniscids from Southeast Asia during the Early to Middle Permian and supported the idea that the Indochina terrane was a part of epeiric seas at that time.