最古老的脊椎动物:海口虫或原牙形类?

THE EARLIEST ANCIENT VERTEBRATE FOSSIL: HAIKOUELLA, OR PROTOCONODONT?

产自南非晚奥陶世索姆页岩中的牙形动物化石被认为是一类脊椎动物的摄食器官 ,而产自中国云南早寒武世早期的云南虫 (Yunnanozoon)和海口虫 (Haikouella)具有矿化的单锥形牙齿构造 ,被认为是保存了软部的脊椎动物的祖先。经比较研究后提出了产自比澄江动物群更低层位 (梅树村阶 )中的原牙形类Protohertzina和Protocanthocordylodus等应代表保存了硬部的最早的脊索动物 ,甚至是脊椎动物或它们的共同祖先。

The chordata affinity of conodonts has been accepted with the discovery of conodont animals from the Ordovician Soom Shale in South Africa. Fortunately, in the wake of finding both Haikouella and Yunnanzoon with only simple cone like dental structures from the Chengjiang Fauna, it is timely to reconsider the Protoconodont bearers from the lowest Cambrian (Meishucunian Stage). Except for common Protohertzina spp.and relative genera, the compound type Protocanthocodylodus simoni Chen, 1999, with three discrete denticles is most remarkable for the complex morphological features. The morphology, histology and chemical composition of the abundant protoconodonts from the Meishucunian are well studied marking the onset of endoskeletonization in Metazoans. The probability is the Protoconodonts being the earliest representatives of chordata, even possibly vertebrate, or their common ancestor, in age of 545 Ma.