中华原始软骨硬鳞鱼之再描述及其地质时代

鲟形鱼类是一类脊椎骨尚未完全骨化、鳞片极度退化的原始辐鳍鱼类。本文重新系统描述了该类别中时代最古老的中华原始软骨硬鳞鱼(Eochondrosteus sinensis Lu,Li et Yang, 2005),补充并修订了其古生物学特征。该鱼类具有扇形的下鳃盖骨、多块条形鳃条骨、较大的喉板骨、具齿的上、下颌,且全身仅在尾鳍上叶具有鳞片等特征,可以区别于鲟形鱼类其他属种。初步的支序系统发育分析表明,Eochondrosteus可能是鲟形鱼类中最原始的类群,与所有其他鲟形鱼类的祖先类群构成一个姐妹群;同时其具有一些古鳕类才普遍拥有的特征,如相对较大的喉板骨和眶下骨等,有可能是原始辐鳍鱼类向鲟形鱼类进化过程中的过渡类型。这一鲟形鱼类化石产于甘肃省肃北县马鬃山镇西北约50 km的方山口组深灰色粉砂质页岩中。根据野外实地调查和岩石地层学、生物地层学对比,该鱼类的时代有可能是早三叠世,而非命名时认为的晚二叠世,但仍是迄今已发现的时代最古老之鲟形鱼类。

Redescription of Eochondrosteus sinensis (Acipenseriformes,Actinopterygii) and its geological age

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.