大鱼之始——曲靖潇湘动物群中发现志留纪最大的脊椎动物

The rise of big fish——the largest Silurian vertebrate from Xiaoxiang Fauna, Qujing

云南曲靖4.23亿年前的潇湘动物群在世界上独一无二地完好保存了大量志留纪有颌脊椎动物化石。2014年6月,中国科学家报道了潇湘动物群中的新成员,一种被命名为钝齿宏颌鱼(Megamastax amblyodus)的硬骨鱼,这是目前为止志留纪最大的脊椎动物。现有证据显示钝齿宏颌鱼属原始肉鳍鱼类,体长可达1.2 m,其牙齿适应捕食带硬壳的猎物,可能在食物链中占据较高的位置。这是潇湘动物群继梦幻鬼鱼和初始全颌鱼之后的又一重要发现,该发现挑战了早泥盆世埃姆斯期前不存在大型脊椎动物的传统看法,同时促使研究者重新思考已有的古大气/古海洋氧气含量变化理论模型,以及其与脊椎动物演化之间的关系。

The Silurian Xiaoxiang Fauna from Qujing, Yunnan is unique worldwide for its exquisitely-preserved jawed vertebrates either in diversity or in completeness. In June 2014, a new Silurian fish, Megamastax amblyodus, which is the largest Silurian vertebrate discovered to date with an estimated length up to 1.2 m, was reported from Xiaoxiang Fauna. The jaws of the new fish feature minute conical dentition on the occlusal margins and large blunt teeth that are fused to the coronoids, highlighting durophagous feeding. A global increase in oxygen levels may have facilitated the emergence and diversification of large jawed vertebrates in the mid-Paleozoic. Competing paleoatmospheric models predict the onset of near-modern oxygen levels during either the late Silurian （~420 Ma） or the Emsian （Early Devonian,~400 Ma）. The absence of pre-Emsian fishes more than half-a-metre in length was used as support for the latter model, with presumed low pre-Devonian oxygen levels imposing evolutionary constraints on maximum vertebrate body size. The new finding is consistent with previous studies documenting an increase in gnathostome diversity and functional disparity well before the advent of the Devonian along with predictions of a late Silurian oxygen rise.