A Palaeozoic shark with osteichthyan-like branchial arches

摘要

The evolution of serially arranged, jointed endoskeletal supports internal to the gills-the visceral branchial arches-represents one of the key events in early jawed vertebrate (gnathostome) history, because it provided the morphological basis for the subsequent evolution of jaws. However, until now little was known about visceral arches in early gnathostomes, and theories about gill arch evolution were driven information gleaned mostly from both modern cartilaginous (chondrichthyan) and bony (osteichthyan) fishes. New fossil discoveries can profoundly affect our understanding of evolutionary history, revealing hitherto unseen combinations of primitive and derived characters. Here we describe a 325 million year (Myr)-old Palaeozoic shark-like fossil that represents, to our knowledge, the earliest identified chondrichthyan in which the complete gill skeleton is three-dirnensionaUy preserved in its natural position. Its visceral arch arrangement is remarkably osteichthyan-like, suggesting that this may represent the common ancestral condition for crown gnathostomes. Our findings thus reinterpret the polarity of some arch features of the crown jawed vertebrates and invert the classic hypothesis, in which modern sharks retain the ancestral condition. This study underscores the importance of early chondrichthyans in resolving the evolutionary history of jawed vertebrates.%软骨鱼（包括鲨鱼、魔鬼鱼rays和鳐鱼skates) 之所以这么叫，是因为它们的骨架主要由软骨 组成，而不是由硬度大得多的骨头组成。今天 现存的很多鲨鱼看起来一般都像化石鲨鱼，该 事实造成这样一个印象：它们是"活化石"， 在其解剖结构中保留了一些非常原始的状态。 越来越多的证据证明，事实远不是这样的，而 这项研究又为此提供了新的证据。作者描述 了一个非常早的化石鲨鱼的鳃骨架，它与硬rn骨鱼的鳃骨架有明显相似之处，说明现代鲨 鱼（鳃骨架）的排列反映了演化创新而不是形 态停滞。