On the evolution and histology of some Cambrian protoconodonts,paraconodonts and primitive euconodonts

Previous reports on the morphological evolution of protoconodonts and paracono-donts are reviewed. The evolutionary trends exhibited by species of the protoconodont genus Gapparodus and the paraconodont genus Westergaadodina are discussed. Based on the pre-sent study on histology, genus Paibiconus is protoconodont, while genus Yongshunella is para-conodont (fig. 1(a)). However, the nature of Huayuanodontus has not been fully understood so far. Recently, the transitional form New Gen. Sensu Miller, 1980 of late Late Cambrian has been found in Hunan Province, South China. It is considered as the Sister-Group of Proconodontus. Moreover, another kind of transitional form of middle Late Cambrian and late Late Cambrian between Prooneotodus rotundatus and Proconodontus, which looks like Prooneotodus rotunda-tus in gross morphology, but has anterior and posterior costae, has also been found in Hunan, South China. This kind of transitional form is believed to be probably the real intermediate link between Prooneotodus rotundatus and Proconodontus, and Prooneotodus rotundatus is most probably the ancestor of Proconodontus. The two transitional forms noted above are all eu-conodonts by the study of their histology. Unlike the euconodonts of Post-Cambrian, the evolu-tionary relationships among the species of protoconodonts, paraconodonts and the primitive euconodonts could not be made clear without the histological study on them.