Restudy Of Three ’Artiopodan’ Euarthropods From The Chengjiang Biota

Arthropoda is the most dominant animal in species and individual numbers during the Cambrian explosion.The Cambrian Chengjiang Biota is one of the earliest and most well-preserved Lagerst?tten,which contains alomost all known animal phyla.It is not doubtful that Arthropoda is also the dominant animal phylum in this biota.A number of arthropod fossils from Chengjiang Biota preserve complete and exquisite morphological structures,such as nerves,eyes,and digestive tracts.These structures help us learn about the animals from more than 500 million years ago,as well as the differences between them and extant animals.In the meantime,the understandings of arthropod origin and early evolution are further deepened with the aid of these information.In the last decades,a great number of exquisite arthropod fossils were discovered,important morphological structures of which could be identified from the surface of specimens with or without any physical preparations,such as appendages.However,this is not always the case.Some specimens do not display valuable surface morphology,and are so rare and precious that it is improper to remove the exterior structures to expose those hidden structures.Also,the physical preparation with needles is not practical and effective for specimens with a small size,such as Primicaris larvaformis.In this thesis,three species of arthropods have been restudied systematically,including Sinoburius lunaris,Pygmaclypeatus daziensis and Primicaris larvaformis.The traditional methods can not play effective roles in studies of them.It is better to use them as supplementary methods.Therefore,I use micro-CT instead to scan the well preserved and mineralized fossils.Making use of the scanning-generated data,the specialized software creates the 3D models of the fossils,which reveal the hidden ventral morphological details,most of which are not reported before.Based on these new information,we can learn more about these arthropods’ morphology,and in turn modify the diagnoses and phylogenetic positons of them.With the application of micro-CT,we find some valuable morphological information about the three species.As far as Sinoburius lunaris concerned,it has one pair of short antennae and four pairs of biramous appendages under its head shield;the antennae bear antennal scales at the sub-basal parts;the first and second limbs have elongate and antenniform exopods,which are totally different from trunk limbs,and the first limb’s endopod is reduced,while the second limb’s is as same as the others;the thorax has seven freely articulated tergites,the fourth and seventh tergites each covering two pairs of biramous appendages and the others each covering one pair only;the biramous appendage consists of one gnathobasic basipod,one endopod of seven podomeres and one segmented exopod,the exopod composed of one shaft bearing lamellae;the pygidium covers four pairs of biramous appendages and one conic tail piece.The main morphological details of Pygmaclypeatus daziensis include: the anterior doublure of the head shield is relatively long and attached to the sub-trapezoidal hypostome;the head shield has one pair of stalked eyes,one pair of short antennae and three pairs of biramous appendages;the biramous appendage has one basipod,one endopod of seven podomeres each bearing one needle-like spine and one flap-like exopod fringed by lamellae bearing small setae;the thorax has six freely articulated tergites overlapping each other partly,each tergite covering one pair of biramous appendage;the pygidium is relatively big and olive-shaped,which covers five pairs of biramous appendages,and a segmented tail spine stretches out from under the pygidium.Primicaris larvaformis is less than 6 mm in length at most,so some structures are hard to be preserved.Due to the limitations of the CT techniques,such fossils are difficult to be differentiated from the surrounding fossil matrix,because they are so delicate that the density difference is not obvious enough.However,we make clear of the morphology of hypostome from the unsatisfactory scanning data.Still,we get some new knowledge about the biramous appendage and marginal ridge of Primicaris larvaformis through surface observations.In the end,we use TNT V1.5 to do the phylogenetic analyses of the species above,which lets us determine the positions of these species on the phylogenetic tree of the euarthropods from Chengjiang Biota.