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Earth & Life (http://www.geofinds.com), (2006-10-1), 1(1): 1--6.
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As seen from Fig.1, Bed 24e contains only 4 species of conodonts. Three of them persisted into Bed 25. The survival rate (ratio of survival species to total species) is 75%. Thus, there are no big extinction between beds 24e and 25. The conodont species diversity in Bed 25 increased to 7 species, further indicating continuous development of conodonts from Bed 24e to Bed 25. Jin et al.’s study (2000) shows that most marine animal groups suffered a sudden mass extinction at between beds 24e and 25. Why conodonts did not suffer such an extinction? Did conodont animals have some special ecological particularity, which enable them to survive the crisis that had killed most other animals? If they have, what are them? Bed 26 contains 12 conodont species, subspecies and potential new species. Compared to Bed 25, species diversity increased in Bed 26. Two species (C. yini and H. inflatus) of Bed 25 extended into Bed 26. The Clarkina component with deflected platform end (C. reductus) appeared in Bed 25 for the first time. In Bed 26, however, Clarkina components with deflected platform end developed into 4 species and subspecies (C. deflecta and C. paradeflecta). In addition, Clarkina components with branched carina end appeared in Bed 26 for the first time. Beds 24e, 25, and 26 are composed of limestone, white claystone formed from ash, and black claystone, respectively. No mass extinction in conodonts is related to changes in lithology or sedimentary environments. Eleven of the 12 Clarkina species of Bed 26 disappeared at between beds 26 and 27a. Only one species (C. paradeflecta) persisted into Bed 27a. The extinction ratio is as high as 92%, representing a mass extinction in conodonts. Besides C. paradeflecta, one Hindeodus species (H. inflatus) of Bed 26 lasted into Bed 27a. Beds 27a and 27b contain 14 conodont species and subspecies. Twelve of them are Hindeodus components, 11 of which are new species. The most important components are H. changxingensis and H. difformis. The two Clarkina species include a new species (C. elliptica), and an old species lasting from Bed 26. The conodonts of Bed 27a-b are mostly new species, representing rapid recovery after the crisis. The Hindeodus components of Beds 27a-b include three types: those with erect cusp but adenticualte top face, exampled by H. changxingensis; those with indistinct cusp but S-shaped oral margin, exampled by H. zhejiangensis; and those with a not very big cusp but erect denticles, as exampled by H. limus. The most important characteristic feature of the conodonts of these two beds is the absence of Hindeodus with parvus-type cusp (wider than denticles by more than 2~3 times). Three of the 6 conodont species and subspecies of Bed 27b persisted into Bed 27c, with a survival ratio of 50%, indicating that there was no big extinction event at between beds 27b and 27c. Beds 27c and 27d contain 18 conodont species, greatly increased in diversity than Beds 27a-b. Bed 27c contains 13 conodont species and subspecies, more than two times of that of Bed 27b, showing marked increase in conodont diversity from Bed 27b to Bed 27c. |
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