Reconstruction Of The Middle Triassic Conodont Apparatus From East Yunnan,China And Its Significances

Conodonts have lived for nearly 300 million years since the Miaolingian(Cambrian),surviving up to three mass extinctions,including the witness of the largest mass extinction event in earth history during the Permian-Triassic(P-Tr)interval,but becoming extinct at the end of the Late Triassic.Conodonts are eel-shaped animals,which unique hard tissues are represented by the conodont elements distribute at the mouth and pharynx region.Due to their worldwide distribution and rapid evolution,they are very important index fossil to establish high resolution chronostratigraphic framework in both Triassic and Paleozoic,for instance,the beginning of the Mesozoic and the definition of global Permian-Triassic Boundary is defined by the first appearance of the conodont Hindeodus parvus.After long term studies of the conodont animals,clusters and natural assemblages,we are starting to understood the conodont apparatuses,existing a large number of different conodont morphotypes that need to be evaluated to completely understand their complexity,functional and ecological importance.Therefore,further study are necessary to evaluate the composition of the different conodont apparatuses,and the interrelationships among the different taxa.Moreover,its study can provide the theoretical principle for selecting some specific conodont elements as the index fossil to biostratigraphic correlations,and it is also important to understand the affinities of conodont animal.Abundant conodont clusters and natural assemblages of Nicoraella were collected in Luoping biota,Yunnan.They possess the intact conodont elements and relatively original three-dimensional spatial structure,which provides some important materials to reveal the element compositions and the architecture of the“feeding apparatus”of the Nicoraella conodont animal.Using scanning electron microscopy(SEM)and synchrotron X-ray tomography(SRXTM)methods we have explored the the conodont apparatus composition and their morphologies,and reconstruct their spatial architecture in the apparatus model.In addition,we specially observe the microstructures on rock surfaces that bearing the conodont clusters and assemblages,and further discussed their micro-taphonomic environment.The relevant results are as follows:1.Confirming the age of conodont clusters and natural assemblages.The conodont index fossil of the Nicoraella kockeli were identified in the Dawazi and Shangshikan sections,and the P₁elements of this type elements were also abundant in the natural assemblages and clusters,so it is determined that these conodonts belong to Ni.kockeli zone,and the age is Pelsonian substage,Anisian,Middle Triassic.In Luxi area,Ni.germanicus,Ni.kockeli,Neogondolella?bifurcata and other forms are diagnosed in Member II,Guanling Formation of both the Baishuitang and Liziqing sections.So,the conodont assemblage indicates a Bithynian?Pelsonian age,Middle Triassic.These conodont clusters,marine reptiles and other fossils also appear in the Ni.kockeli zone in Luxi,thus suggesting the age of the relevant strata are equal to the Luoping area,belonging to the Pelsonian substage,Anisian,Middle Triassic.2.Conodont clusters and natural assemblages are preserved in a relatively low energy,anoxia,and constrained sedimentary environments.The rocks containing the conodont clusters and natural assemblages are mainly composed of calcite and dolomite grains,with the size of 1?4?m(micrite)and 8?15?m(microspar).These particles are typical products in the low energy environment,which is basically consistent with the characteristics of the results to the field sedimentary macrostructure and the element geochemical analysis.In addition,there are lots of clay minerals and pyrites in matrix,which indicates a low energy and anoxic environment.3.Determining the number and elements morphology of the Nicoraella apparatus.The Nicoraella apparatus include a total number of 15 elements,comprising of 7morphological types:an unpaired alate S₀element(hibbardelliform),a paired breviform digrate S₁(cypridodelliform)and S₂(enantiognathiform)elements,a paired bipennate S₃and S₄elements(hindeodelliform),a paired inwardly curved breviform digyrate M elements(cypridodelliform),and a paired P₁and P₂elements that they may be carminate or blade elements(nicoraelliform and xianiognathiform).It demonstrates that the arrangement of enantiognathiform and cypridodelliform elements in the apparatus,are in the positions of S₂and S₁respectively.4.Restoration the spatial architecture of the Nicoraella apparatus and possible mechanisms of the apparatus movements.Nicoraella apparatus possesses the bilateral symmetrical characteristic.The S₀element is located at the middle in the Nicoraella appratus;flanked successively abaxially by paired of S_1-4and M elements at the sinistral and dextral sides,forming a bilateral symmetrical basket-like structure;paired P₂and P₁elements are occluded,and the P₂elements locate below the distal part of the S_1-4elements and the P₁elements settle caudad from the S-M elements.During the feeding,M elements firstly draw back inward,then the sinistral and dextral S_1-4elements evert outside to open the apparatus,meanwhile S₀elements rotate caudad to transfer food particles to P elements,subsequently P elements function as scissors to cut foods to small pieces,thus the recurrent movements to finish a feeding process.5.The reconstructed Nicoraella apparatus possess traits of Cyclostomes and some vertebrate fish,such as the bilateral symmetrical S₁-S₄elements similar to the two rows of keratin teeth of hagfish,the S₀element analogous to teeth in lamprey piston cartilage tongue,and P₁and P₂elements equivalent to the pharyngeal teeth in some fish.The affinity of the conodonts,probably place between the Cyclostomes and Ostracoderms,possibly relying them as the most“primitive”vertebrates with skeleton.Conodonts may place at the bottom of the food chain,but they are an important link in energy transformation and transmission in the ecosystem.6.The composition of conodont apparatus in the order of Ozarkodinida has a strong similarity,and the more closer relationship the more closer morphological similarities.(1)The total number of 15 elements are part of apparatus,consisting of 11S-M elements and 4 P elements;(2)The elements of the corresponding positions in apparatus are morphological similarity or slightly different;(3)Spatial structural similarity,especially high similarity of the different apparatuses in the same family(eg.Superfamily Gondolelloidea),suggest that they may have higher functional similarity.Contrarily,the more distant relationships of the apparatuses constitution,the more different elements shapes,numbers and spatial arrangements are.