Morphology,Taxonomy And Affinity Of Small Shelly Fossil Maikhanelids From The Early Cambrian Xixiang Lagerst?tte

The “Cambrian explosion” have produced a large number of earliest skeletal fossils(Small shelly fossils),early univalved mollusk fossils have been found in the vicinity of the Precambrian-Cambrian boundary The phylogenic relationship and classification phylogical are uncertain,the division of genera and species is also controversial.Among them,maikhanellids are distinctly different from other univalved mollusk fossils with their distinctive outer surface ornamented.Zhangjiagou section,in northern margin of the Yangtze Platform,a large number of maikhanellids were collected.Based on this,Morphology,Taxonomy and Affinity were studied.The maikhanellids collected from Zhangjiagou section in Xixiang Lagerst?tte are divided into four genus: Purella Missarzhevsky,Maikhanella Zhegallo,Ramenta Jiang and Media Feng.The maikhanellids fossils found in the Zhangjiagou section of have new morphological characteristics.Described in this paper a new species,Purella pulchram sp.nov.,The new species enrich the genus characteristics of maikhanellids and provide new material support for study the phylogenetic evolution of maikhanellids and their relationship with other univalved mollusk.Fossil measurements and statistics show that the size of maikhanellids apex is positively correlated with the shell size.The different size and location of the apex seems to reflect more the characteristics of different growth stages of the shell.The unique outer surface ornamented is a typical feature of maikhanellids,which constitutes an important basis for the taxonomy of maikhanellids.The surface ornamented of maikhanellids shows obvious transition,which reflects the relationship among the genus.Ramenta is a controversial genus of maikhanellids,which has been as Canopoconus and Maikhanella.This paper holds that Ramenta has obvious genus characteristics and its shell decoration shows the transitional nature between Maikhanella and Purella,and should be a valid genus.Mediata is characterized by a distinct dorsal ridged on its back.Its type species Mediata kunyangensis were considered to be the same species as Ramenta spuerata due to poor fossil preservation.A large number of Mediata xixiangensis have been collected in the study area.Their morphological characteristics are obvious.Mediatais considered as a valid genus in this paper.Maikhanellides is one of the representative fossil groups in the Xixiang Lagerst?tte.Although these genera are obviously different in shape and some other aspects of morphology,they are all characterized by possessing a distinct bare apex,with the rest of outer sufface ornamented by closely distributed projective granular structures.Purella is the the representative fossils of Meishucun stage of the Yangtze Platform in China and the Tommotian Stage of Siberia Platform in Russia.The study of maikhanellids,can not only make a comparative and judgement on the stratigraphic age of the Lower Cambrian in the world,but also play an important role in understanding the evolution of Lower Cambrian univalved molluscs.