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Earth & Life (http://www.geofinds.com), (2006-10-1), 1(1): 1--6.
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The conodonts of Bed 27c is characterized by the appearance of many Hindeodus species with parvus-type cusp, for example, H. parvus, which appeared in this bed for the first time and reached worldwide distribution. Besides, the components with S-shaped oral margin and those with adenticulate top face continued to develop in this bed. Though there were no mass extinction between beds 27b and 27c, appearance of Hindeodus with parvus-type cusp makes the boundary between beds 27b and 27c an important biotic evolutionary boundary. Bed 28 contains only 6 conodont species. Compared to Bed 27d, the conodont diversity of Bed 28 greatly dropped. Hindeodus components flourished in beds 27c and 27d suddenly disappeared in this bed. H. parvus is previously believed to last into Bed 28. But, in our opinion, they are not the same as those of Bed 27c-d. Besides, the components of Isarcicella and Neospathodus appeared characteristically for the first time. The change in conodonts from Bed 27d to Bed 28 indicates that the boundary between Beds 27d and 28 is a secondary biotic evolutionary boundary. The conodonts of the PTB interval are characterized by one continuance, one mass extinction event, one rapid recovery, one flourishing, and one small extinction event. The conodonts of Bed 24e mostly continued to live in Beds 25, which is the continuance. The Clarkina components of Bed 26 mostly disappeared at between beds 26 and 27a, which is the mass extinction. The conodonts of Beds 27a-b have a diversity degree similar to that of Bed 26, representing the rapid recovery. The conodont fauna of Bed 27a-b was dominated by Hindeoedus components. Compared to Bed 27b, the conodonts of Bed 27c greatly increased in species diversity and many species with big cusp appeared, which was a flourishing. Most components of Bed 27d disappeared at between beds 27d and 28, which is a small extinction event. Thus, the evolution of conodonts during the P-T boundary interval defines three biotic evolutionary boundaries: the most distinct one lies between beds 26 and 27a (Fig. 1, A), where Clarkina components of Bed 26 were mostly replaced by Hindeodus components. The second boundary of secondary order lies between beds 27b and 27c, above which Hindeodus with big cusp appeared. The third boundary of smaller magnitude lies between beds 27d and 28, where most Hindeodus components disappeared while Isarcicella and Neospathodus components appeared. 3. ConclusionsThis study shows that there is one mass extinction in conodonts at the end of the Permian (between beds 26 and 27a of Meishan) and it was later than the mass extinction of most marine animal groups, which occurred at between beds 24e and 25 of Meishan. The lagged appearance of the mass extinction of conodonts may be related to some special ecological features of this kind of enigmatic organisms. Conodonts are nektonic, soft-bodied, probably surface or upper water heterotrophic organisms and probably shared space with a large number of invertebrate and vertebrate animals that went extinct earlier at the end of the Permian. We do not know what characteristics of conodonts shielded them from the fate of associated organisms and what made them suffer a mass extinction at a later moment. The difference in mass extinction timing of conodonts and that of most other marine groups represent what intrinsic features of conodonts? More studies are needed to answer this question.
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