新型有原始肢突胴甲鱼的发现及胴甲鱼早期演化的初步探讨

本文记述的曲靖始突鱼(Procondylolepis qujingensis gen.et sp.nov.)是近几年在云南曲靖早泥盆世地层中发现的有肢突胴甲鱼一原始类型。它和已知胴甲鱼(包括早泥盆世无肢突的和中、晚泥盆世有“盔”状肢突的)不同的最大特点是在其肩带与胸鳍相接的肩关节处有原始的肢突和简单的关节窝;胸鳍甲近端的关节区很小。它展现出胴甲鱼类这一高度特化、长期使人迷惑不解的肢突,在胴甲鱼演化史上发展变化的梗概,填补了肢突从无到有中间的缺环,使人了解到胸鳍的具体结构。文中主要根据肢突的有无和特化程度等,对胴甲鱼早期演化史作了初步探讨,将胴甲鱼分为无肢突超目(Abrachicondylia)和有肢突超目(Brachicondylia)两大部分。始突鱼则代表有肢突超目一早期成员。

NEW FORM OF ANTIARCHI WITH PRIMITIVE BRACHIAL PROCESS FROM EARLY DEVONIAN OF YUNNAN

The present paper deals with a new antiarch——Procondylolepis qujingensis gen.et sp.nov.The specimens,including some detached shoulder joints,several pectoralfins and a few isolated pectoral fin plates were collected from the Lower Devonianof Cuifengshan,Qujing,Yunnan in the past few years.Associated with this antiarchare Yunnanolepis parvus and Phymolepis cuifengshanensis etc.The new genus is char-acterized by the presence of a primitive brachial process at the base of axillary fossa,by its rather complicated axillary fossa margin and its small articular area of pec-toral fin.It differs from Yunnanolepis and Phymolepis in having a V-shaped bonyplate representing an early stage of brachial process at the base of axillary fossa,inhaving the separate dorsal and ventral fossae which are articulated with pectoral finat margin of axillary fossa,and in presence of a quite smooth articulated surfaceat the medial margin of the axillary fossa.The pectoral fin is small,short unjointed,made up of four longitudinal series ofplates,and basically similar to that of Remigolepis of Late Devonian in pattern.Thenew form.however,differs distinctly from Remigolepis in some significant morphologicalcharacters:absence of a helmetlike brachial process;having a very small and convexarticular area of pectoral fin articulated with shoulder girdle;thick lateral and flatmedial margin of pectoral fin,which might imply the difference of motive means.Itis very significant for understanding the development of the shoulder girdle and pec-toral fin in various evolutionary stages and the changes in their evolutionary history,as the Antiarchi is considered to be one of the highly specialized form among the ver-tebrates in regards of its shoulder girdle and pectoral fin.The discovery of Procondylolepis(gen.nov.)has provided new evidences for thestudy of the origin,evolution and classification of Antiarchi.On the basis of characters of the shoulder joint and pectoral fin of Procondylo-lepis,it leads to the following conclusions:1.The pectoral fin armour appeared earlier than brachial process in the evolu-tionary history of the Antiarchi.2.Procondylolepis gen.nov.is a transitional form representing a stage of An-tiarchi evolution which intermediated between the non-brachial process form,as Yun-nanolepiformes in the Early Devonian and the brachial process form,as those in theMiddle and Late Devonian ages. 3.Among the so far known antiarchs of Early Devonian,the shoulder girdle ofProcondylolepis gen.nov.is close mostly to those of ancestoral antiarchs of Middleand Late Devonian morphologically.All the known antiarchs of Middle and LateDevonian may be descended from some forms related to Procondylolepis.4.The unjointed pectoral fin of antiarchs is a primitive characteristic.thedorsal articular plate of the antiarchs known from Middle and Late Devonian whichhas long been thought to be the first dorsal central plate(Cd_1)should be the firstlateral marginal plate(Ml_1).5.The motion pattern of the pectoral fin in Procondylolepis gen.nov.is differentfrom those of antiarchs of Middle and Late Devonian.6.Antiarchi is suggested to be classified into two Superorder,Abrachicondyliaand Brachicondylia.Procondylolepiformes(ord.nov.)is proposed under the Brachi-condylia.