Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, USA.
European Synchrotron Radiation Facility, BP 220, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France.
Nature. 2014 May 29;509(7502):608-11. doi: 10.1038/nature13195. Epub 2014 Apr 16.
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 by information gleaned mostly from both modern cartilaginous (chondrichthyan) and bony (osteichthyan) fishes. New fossil discoveries can profoundly affect our understanding of evolutionary history, by 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-dimensionally 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.
鳃内有一系列关节内骨骼支持物（内脏鳃弓）的演化，是有颌脊椎动物（颌口类）历史上的关键事件之一，因为它为随后颌骨的进化提供了形态学基础。然而，直到现在，人们对早期颌口类的鳃弓知之甚少，关于鳃弓进化的理论主要是基于现代软骨鱼（软骨鱼）和硬骨鱼（硬骨鱼）的信息推断而来。新的化石发现可以通过揭示以前从未见过的原始和衍生特征的组合，深刻影响我们对进化历史的理解。在这里，我们描述了一种 3.25 亿年前（三叠纪）的古生代鲨鱼状化石，据我们所知，这是最早被确定的软骨鱼类化石，其中完整的鳃骨架以其自然位置被三维保存。它的内脏弓排列非常类似于硬骨鱼，这表明这可能代表了有颌脊椎动物的共同祖先状态。因此，我们的发现重新解释了一些有颌脊椎动物的弓状特征的极性，并颠覆了经典假说，即现代鲨鱼保留了祖先的状态。这项研究强调了早期软骨鱼类在解决有颌脊椎动物进化历史中的重要性。
