Radiodont Arthropods From The Cambrian Chengjiang Biota,South China

Arthropoda,the most extensive phylum in the present animal kingdom,is most abundant and diverse,as well as widely distributed metazoan.Their dominance can be traced all the way back to their evolutionary origins — Cambrian period.Studies on the origin and early evolution of arthropods play a key role in reconstructing the evolutionary pathway of metazoan and revealing the essence of the Cambrian explosion.Radiodonta occupies a pivotal evolutionary position as stem-euarthropods and has revealved the origins of euarthropod arthropodization,compound eyes and biramous limbs.As the apex Palaeozoic nektonic predators,in the meanwhile,radiodonts also filled important ecological niches in early animal ecosystems.They commonly occurred in the Cambrian exceptionally preserved Lagerst?tten emblemized by the Chengjiang Lagerst?tte(Yunnan,South China).This deposit has yielded abundant and diverse radiodonts with lots of detailed information,which provides excellent empirical material for our understanding the morphology,taxonomy,ontogeny,phylogeny and biogeography of radiodonts.Firstly,based on detailed investigation and comparison on ca.3000 radiodont specimens from the Chengjiang Lagerst?tte of South China,8 genera and 9 species are systematically described,including Anomalocaris ankylosskelos sp.nov.,Anomalocaris cf.canadensis Whiteaves,1892,Lenisicaris lupata gen.et sp.nov.,Houcaris saron(Hou,Bergstr?m and Ahlberg,1995)comb.nov.,Amplectobelua symbrachiata Hou,Bergstr?m and Ahlberg,1995,Lyrarapax unguispinus Cong et al.,2014,Ramskoeldia consimilis Cong et al.,2018,Peytoia sinensis sp.nov.and Cambroraster sp.These radiodonts are recognized as representatives of all four radiodont families,namely Anomalocarididae,Tamisiocarididae,Amplectobeluidae and Hurdiidae respectively.H.saron(previously Anomalocaris saron)is reassigned to the Family Tamisiocarididae due to its diagnostic features of this family.Thus it represents the first tamisiocaridid taxa from South China.P.sinensis and Cambroraster sp.constitute the first discovery of conclusive hurdiid fossil records in Chengjiang.Therefore the diverse Chengjiang radiodonts presumably reflects the first radiation of Radiodonta during metazoan macroevolution in South China.Secondly,we erect two new taxa within Radiodonta,namely Suborder Anomalocarida nov.and Hurdiida nov.,largely based on the morphology of frontal appendages and endites.Anomalocarida contains the Family Anomalocarididae,Tamisiocarididae and Amplectobeluidae,of which frontal appendage is herein defined as “E-appendage”.This appendage is characterized by slender appendage morphology,possessing well-developed arthrodial membrane,presence of paired spine-form seta-type endites(STEs)and presence of auxiliary spines on both proximal and distal edge of endites.Hurdiida exclusively comprises the Family Hurdiidae.Its appendage is here referred to as “H-appendage” that typified by robust and short morphology,bearing less podomeres,presence of unpaired plate-like spine-type endites(SPEs)with auxiliary spines on distal edge only.The STEs of “E-appendage” are produced by trichogen and connected with the epidermis through an articulatory socket in the cuticle.On the contrary,SPEs of “H-appendage” are the immovable acellular projection of cuticle involving fixed evaginations of epicuticle and exocuticle.Given that,the two types of endites derived from different layer of integument,and thus represent non-homologous structures.The frontal appendage of Ursulinacaris represents a special type of “H-appendage”owing to the presence of paired plate-like SPEs.Thirdly,a phylogenetic analysis is conducted by using maximum parsimony and Bayesian inference based on new dataset coding 38 taxa and 75 characters.The monophyly of Anomalocarida and Hurdiida is well supported,and the two clade are recovered as sister group.The strict consensus of the most parsimonious trees shows that Tamisiocarididae has closer relationship with Anomalocarididae and Amplectobeluidae,but more distant affinity with Hurdiidae.Our phylogenetic analysis recoverd Tamisiocarididae as a monophyletic group,where Houcaris saron occupies the most basal position.Ursulinacaris is the sister group of the clade comprising other hurdiids,owing to the presence of paired spine-type endites.Fourthly,ontogenetic research of abundant frontal appendage specimens of Amplectobelua symbrachiata from the early Cambrian Chengjiang Biota reveals an isometry-epimorphic postembryonic development of frontal appendages based on the morphologically statistical analysis.This statistical study provides the first detailed description of post-embryonic development in a lower stem-group arthropods,and further suggests that the juvenile and adult Am.symbrachiata may occupy the same ecological niche.Given Am.symbrachiata is recovered as the most distant relative of crown-group arthropods,we suggest that the isometric-epimorphosis post-embryonic development of anterior-most limbs may represent the ancestral condition in Arthropoda.Finally,global documents of radiodonts in all over the world are summarized.This,coupled with network analysis,shows that tamisiocaridids,anomalocaridids and amplectobeluids mainly occurred in South China and Laurentia within tropics/subtropics belt,whereas hurdiids occurred within both low and middle-high latitude region(including Baltica,Avalonia and eastGondwana).It thus suggests that hurdiids were eurythermic animals,but a possible climatic control on the distribution of Anomalocaridida.In addition,the known distribution of radiodonts in well-documented Konservat-Lagerst?tten representing various sedimentary settings indicates that radiodonts were able to adapt to a broad spectrum of environments.The brief global radiodont biostratigraphy indicates that the radiation of this animal group experienced an explosive in Cambrian Series 2,which is characterized by high diversity of radiodonts and predominance of Anomalocaridida.During Miaolingian Series,the Hurdiida is abundant and highly diverse.After Drumian Stage,radiodonts suffered an important setback and were in serious decline,and ultimately went extinct after Deconian Emsian Stage.This radiation pattern of radiodonts is consistent with the traditional bottom-heavy model of Cambrian biological evolution.