关键词: 原始软骨硬鳞鱼, 鲟形鱼类, 系统分类, 晚二叠世或早三叠世, 中国西北

Abstract:

Acipenseriformes are a group of early actinopterygian fish with an unfossified vertebral column, a smooth body without scales, or only with a few rows of bone scute. The group comprises of four families, two of them have living members. They are Acipenseridae and Polydontidae, which include the living fish Acipenser and Psephurus from Asia, Polydon from North America, and some fossil species such as the Early Cretaceous Protopsephurus and Late Cretaceous Paleopsephurus. The other two families are only fossil species, they are Chondrosteidae found in the Early Lias of Europe, and Peipiaosteidae from the Middle Jurassic-Early Cretaceous of Northeast Asia. Eochondrosteus sinensis Lu, Li et Yang, 2005, which does not belong to any of the aforementioned families, was preserved in 2005 as the oldest fossil acipenseriform fish. It was found in a black-grey silty shale previously identified as Late Permian in age, exposed 50 km west of Mazongshan Township, Gansu Province, Northwest China. Based on certain newly prepared specimens, this fish is redescribed herein. The revised diagnosis is as follows: small-sized acipenseriform fish; vertebral column unossified but with a row of basal dorsal and supraneural along its body; frontal (fr) rectangular in shape, nearly the size of the parietal (pa); posttemporal (pt) large, triangular in shape; dermosphenotic (dsp) small, connected with infraorbital (io); subopercular (sop) fan-shaped with an elongated anterior process, large in size, nearly double that of the opercular (op); more than two branchiostegal rays and one large size gular plate present; upper jaw (max) uprise in its middle part and a triangular premaxilla (pmx) with sharp teeth in its anterior ventral edge; dentary (den) long and slim, toothed in the anterior part; dorsal fin located behind pelvic fin, and caudal fin heterocercal, with both dorsal and ventral fulcra scales present; and fish body smooth and scaleless, with only a few rhombic scales present in the upper lobe of caudal fin. Eochondrosteus can be distinguished from other acipenseriform fish by the following composed characteristics: (1) a large subopercular, with an elongated anterior extension; (2) a premaxillary bone present and a toothed jaw; (3) a large gular bone; and (4) a smooth body, with scales present only in the upper lobe of caudal fin. A brief phylogenetic analysis was performed mainly based on the character matrix of Hilton and Forey with a few modifications, and a heuristic tree search was conducted using the TNT(V1.5) software. The parsimony cladograms obtained show that Eochondrosteus is on the root of the phylogenetic tree, as the sister group of all other acipenseriformes. The result suggests that it could be the most primitive acipenseriformes fish, although some bone characteristics of the fish skull still need to be added. The opinion on geological age from recent studies on non-mammalian fossil reptiles (trirachodontid Beishanodon youngi) and on fossil fish (Plesiofuro mingshuica and scanilepiform Beishanichthyes) from the ‘Beishan Hills’ of Gansu, is temporarily agreed here. All relevant studies suggest an Early Triassic age for the fossil-bearing beds. Therefore, the age of Eochondrosteus sinensis was changed from Late Permian, dated in 2005, to Early Triassic as those ‘Beishan Hills’ fossils are regarded as having the same stratigraphic horizon and even the same fossil locality as Eochondrosteus sinensis, according to the literature review and field investigation. Eochondrosteus sinensis remains as the oldest acipenseriform fish.

Key words: Eochondrosteus, acipenseriformes, phylogeny, Late Permian or Early Triassic, Northwest China