Revision of two relic actinopterygians from the Middle or Upper Jurassic Karabastau Formation,Karatau Range,Kazakhstan

Three hundred and forty-six articulated fossil specimens of two species from the Middle or Upper Jurassic Karabastau Formation of the Karatau Range (Kazakhstan, Asia) were studied to revise two little-known palaeonisciform fish: Pteroniscus turkestanensis and Morrolepis aniscowitchi. Detailed morphological analysis shows that P. turkestanensis, Daqingshaniscus longiventralis and Uighuroniscidae form a closely related group. They are far more distantly related to the Palaeoniscidae than previously inferred. The first detailed scanning electron microscopy of the unique scale cover of M. aniscowitchi is presented. Morrolepis is found to be devoid of denticles on the surface of the bones, scales and lepidotrichia—so far considered to be a key coccolepidid characteristic. However, it bears exceptionally robust lateral line scales. Comparison of the axial skeletons of M. aniscowitchi and Morrolepis andrewsi reveals their close affinities within Coccolepididae. The axial skeleton, despite its rare preservation in palaeonisciforms, may be taxonomically informative, at least at the family level. The Karatau palaeonisciforms, being among the youngest examples of basal actinopterygians (persisting in Asia through the late Mesozoic), possess a set of conservative morphological characters that suggest they were relictual taxa by Jurassic times, thus highlighting some freshwater systems as refuges for plesiomorphic taxa.