Chang Mee-mann, Zhang Jiangyong, Miao Desui
Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China.
Nature. 2006 Jun 22;441(7096):972-4. doi: 10.1038/nature04730.
Widespread nowadays in freshwater and coastal seas of the cold and temporal zones, lampreys are a jawless vertebrate group that has been in existence for more than 300 million years but left a meagre fossil record. Only two fossil lamprey species, namely Mayomyzon pieckoensis and Hardistiella montanensis, have been recognized with certainty from North American Carboniferous marine deposits. Here we report a freshwater lamprey from the Early Cretaceous epoch (about 125 million years ago) of Inner Mongolia, China. The new taxon, Mesomyzon mengae, has a long snout, a well-developed sucking oral disk, a relatively long branchial apparatus showing branchial basket, seven gill pouches, gill arches and impressions of gill filaments, about 80 myomeres and several other characters that are previously unknown or ambiguous. Our finding not only indicates Mesomyzon's closer relationship to extant lampreys but also reveals the group's invasion into a freshwater environment no later than the Early Cretaceous. The new material furthers our understanding of ancient lampreys, bridges the gap between the Carboniferous ones and their recent relatives, and adds to our knowledge of the evolutionary history of lampreys.
七鳃鳗如今广泛分布于寒温带和温带的淡水及沿海水域，它们是一类无颌脊椎动物，已经存在了超过3亿年，但化石记录却很稀少。在北美石炭纪海相沉积中，仅明确识别出两种化石七鳃鳗物种，即派氏梅约七鳃鳗和蒙大拿哈氏七鳃鳗。在此，我们报告一种来自中国内蒙古早白垩世（约1.25亿年前）的淡水七鳃鳗。这个新分类单元，孟氏中生七鳃鳗，具有长吻、发育良好的吸附口盘、相对较长的鳃器，显示出鳃篮、七个鳃囊、鳃弓以及鳃丝的印记，约80个肌节以及其他一些先前未知或不明确的特征。我们的发现不仅表明中生七鳃鳗与现存七鳃鳗关系更为密切，还揭示了该类群不晚于早白垩世就已侵入淡水环境。新材料加深了我们对古代七鳃鳗的理解，弥合了石炭纪七鳃鳗与其近代亲属之间的差距，并增加了我们对七鳃鳗进化历史的认识。
