Research On Conodont Evolution Across The Permian/Triassic Boundary In South China

Systematic and detailed biostratigraphic research is focused on conodonts from Permian-Triassic(P/T)transitional sections at Meishan(Changxing,Zhejiang Province),Shangsi (Guangyuan,Sichuan Province),Chaotian(Guangyuan,Sichuan Province),Dongpan(Guangxi Province),South China,and North Bukk Mountain,Hungary(Western Palaeotethys).41 species (including 8 new species:Hindeodus fusepyge,Hindeodus hump,Hindeodus semifusecarina, Hindeodus spurpyge,Neogondolella highcarina,Neogondolella laii,Neogondolella planarpyge, Neogondolella seniorlaii,1 new subspecies:Isarcicella staeschei huckriedi,belonging to the genera Neogondolella,Hindeodus and Isarcicella are identified.Six hindeodid zones and three parallel gondolellid zones from P/T transitional strata at the Meishan section are recognized.In ascending order,the hindeodid zones are as follows:Hindeodus latidentatus Zone(below bed 24a),Hindeodus praeparvus Zone(beds 24a-25),Hindeodus changxingensis Zone(beds 26-27b), Hindeodus parvus Zone(beds 27c-27d),Isarcicella staeschei staeschei Zone(beds 28-29a);the Neogondolella zones are:Neogondolella yini Zone(bed 24 and below),Neogondolella meishanensis Zone(beds 25-26),Neogondolella taylorae Zone(bed 27a and above).Five conodont zones are recognized in P/T transitional strata at the Shangsi section.In ascending order,these are as follows:Neogondolella yini Zone(bed 26 and below),Neogondolella meishanensis Zone(beds 27a-27c),Hindeodus parvus Zone(beds 28a-30d),Isarcicella staeschei staeschei Zone(bed 31a)and Isarcicella isarcica Zone(bed 31b and above).Four conodont zones from from P/T transitional strata at the Chaotian section are identified:Neogondolella yini Zone(bed D24 and below),Hindeodus parvus Zone(from bed F1 to the base of bed H55), Isarcicella staeschei staeschei Zone(top of bed H55to bed H58),Isarcicella isarcica Zone(bed H59 and above).Conodont elements are reported from P/T transitional strata at the Dongpan section for the first time.N.carinata,N.changxingensis,N.deflecta,N.dongpanensis sp.nov.,N. praetaylorae,N.postwangi and N.yini occur in the base of bed 3;the characteristic end-Permian species N.yini indicates that this bed belongs to the N.yini Zone.The conodonts N.carinata,N. changxingensis and N.dongpanensis are found in the top of bed 5,with no additional Neogondolella species following the mass extinction.Conodont species belonging to the H. praeparvus Zone(beds 1-7)and H.parvus Zone(bed 8 and above)from P/T transitional strata at the North Bukk Mountain section,Hungary,are also identified.Three successive faunal assemblages of conodonts are identified at the Meishan section. Stageâ… (beds 24a-26)is characterized by a marked abundance and diversity of Permian Neogondolella species.The genus Neogondolella comprises about 90%of the collections,with Hindeodus subsidiary.Two sub-stages are distinguished in this stage:the first sub-stage(beds 24a-24e)is characterized by a bloom of Neogondolella species;the second sub-stage(beds 25-26) is characterized by a combination of disappearances and appearances of some Permian Neogondolella species.In Stageâ…¡(beds 27a-28)characteristic Triassic forms appear,but still coexist with late Permian taxa.Quantitatively,Hindeodus and Isarcicella replace Neogondolella as the dominant genera.Stageâ…¢(beds 29a)is characterized by the disappearance of most transitional forms.This interval is highly(99%)dominated by Hindeodus and Isarcicella with only very few Neogondolella specimens.Conodont data from P/T transitional sections in South China show a clear replacement of Neogondolella by Hindeodus replaced Neogondolella at Meishan,Shangsi and Chaotian;this has not been recognized at Dongpan where the conodont data are insufficient.Two main extinctions of conodonts are identified at the Meishan section.The first extinction is also apparent at other South Chinese sections,but there are insufficient conodont data to identify the second extinction widely.The successive faunal assemblages of conodonts at the Meishan P/T transitional section reveal the effects of environmental change and the rapid evolution of conodonts through this interval.In addition to the two main extinctions,two small extinctions of conodonts are also found at the base of bed 26 and the base of bed 28,along with the appearance of some fresh forms.This reflects an instability of conodont communities following the first mass extinction. This is also evident in the conodont data from the Shangsi and Chaotian sections.Conodont faunas from the Dongpan section show slightly different changes.It can be concluded that, similar to other taxa,conodonts from South China were affected by changes that occurred during the P/T transitional interval.The diversity of early Triassic conodonts,which is similar to that of the end-Permian,reflects the response of different species to the extinction event.The rich conodont collections have also provided the possibility of proposing a new evolutionary lineage of Hindeodus-Isarcicella through the P/T transition:H.praeparvus is the proposed ancestor of some Isarcicella species and of H.parvus.H.parvus could be the ancestor of I.lobata and H.turgidus,with I,inflata derived from I,lobata.I.s.huckriedei subsp,nov. may have been derived from H.praeparvus,giving rise to I.s.staeschei then I.isarcica.H. changxingensis could have been derived from H.julfensis.Biological interpretations of conodont evolution across the P/T boundary(PTB)are also discussed in this paper.Based on the abundances of ramiform elements in the Meishan section and multi-element apparatus models,a 15-element apparatus for Neogondolella changxingensis in bed 24d is established as follows:paired P₁,P₂,M,S₁,S₂,S₃ and S₄,and an unpaired S₀.This apparatus has a characteristic breviform digyrate M element,characterised by a markedly high cusp,anterior and posterior processes that are approximately the same length,with 3 denticles on the anterior process and 4 on the posterior.Moreover,a 15-element apparatus for Hindeodus parvus in bed 27d is also established as follows:paired P₁,P₂,M;4 S_1/2;4 S_3/4;1 S₀.Using existing models of the three-dimensional apparatus the anatomical orientation of conodont elements,three-dimension apparatuses of Neogondolella changxingensis and Hindeodus parvus are reconstructed,and element function discussed.Morphometric study of polygonal microsculpture on some conodont oral surfaces provides a means of deducing conodont animal genome size.Specimens of the P₁ element of N. changxingensis and N.deflecta from the Upper Permian of the Meishan section have been studied.In all populations studied there is a small but significant increase of mean polygon size with element size,but no variation in polygon size related to position on the element.There is a difference in mean polygon size between the two species N.changxingensis and N.deflecta,but this appears merely to reflect the larger mean size of elements of the latter species.No evidence for periodicity of polygon size during ontogeny is detected.The 'dwarf' population of Neogondolella in Bed 24e appears to reflect high juvenile mortality.To further investigate conodont evolution across extinction events,gondolellid elements from the Tieqiao and Penglaitan sections,Liuzhou,Guangxi(Guadalupian- Lopingian Boundary (GLB)of Global Stratotype Section and Point(GSSP)),and Meishan,the section(GSSP PTB), have been studied to evaluate the relationship between polygon size,element length and the number of denticles on the carina.In most gondolellids element,there is a significant increase in denticle number with element length,implying that the number of denticles on the carina is a proxy for the physiological age of the conodont.From comparisons of denticle number and element length in populations across the GLB and PTB extinction events,it is evident that the conodont fauna following the GLB extinction event comprises younger individuals that matured earlier,were short-lived but grew relatively larger.The conodont fauna following the PTB extinction event also matured earlier,and comprised short-lived individuals that became relatively larger.This suggests that no Lilliput effect occurred in conodonts following these extinctions.Polygon size is larger and more extensive in range on gondolellid elements above each extinction boundary,suggesting an instability and increase in size of the conodont genome after the extincton events.A biological interpretation of conodont evolution across these extinction events(including the P/T extinction)is as follows:mutations drived by the unstable environment led to genome section duplications,accompanied by neotenic changes,a larger body size and short-lived individuals in the conodont fauna.