最早期脊椎动物的镶嵌演化

采自云南昆明市海口地区下寒武统筇竹寺组的昆明鱼为活埋标本 ,其软躯体构造保存极为精美 ;来自同一层位的海口鱼为正常死亡后埋藏 ,除身体最后部略有腐烂外 ,整体特征保存良好。这两种鱼皆呈鱼形 ,不仅具带鳃软骨的原始头颅和比无头类简单“人”字形肌节更为复杂的双“人”字形肌节 ,而且还发育了原始偶鳍和围心腔构造 ,表明它们已演化成高等脊索动物有头类 ( Craniata)或称脊椎动物 ( Vertebrata)。另一方面 ,它们的低骨化性 (如尚未发育出真正的脊椎骨 )和生殖系统演化的滞后性 (仍具多对生殖腺 )等原始特征 ,使它们呈现典型的镶嵌演化性。

Mosaic evolution of the oldest vertebrates

Soft bodied Myllokunmingia ,collected from the Qiongzhusi Formation,Lower Cambrian in Haikou area,Kunming City,Yunnan Province,is buried alive and exquisitely preserved,and Haikouichthys from the same bed is also well preserved,except for slight postmortem decay.Both animals are fish shaped,possess preliminary skull,zigzag myomeres,ventro lateral fins and well developed pericardial cavity,suggesting their fulfilling the key evolutionary steps from acraniates to craniates or vertebrates.On the other hand,both they are weakly skeletonized and yet to develop true vertebrae. Haikouichthys bears multi paired gonads,which is a symplesiomorphy to lancelet,indicating its mosaic evolution. Myllokunmingia is less advanced than Haikouichthys and believed so far to be the earliest known and the most primitive ancestor in Vertebrata.