Morphology And Palaeoecology Of The Cambrian(Series 2)Ecdysozoan Worms In Kunming

Ecdysozoan worms are a group of periodically molting,vermiform,proboscis-bearing and limbless protostomes.With widespread distribution in the Palaeozoic period,they are especially abundant and diverse throughout the Cambrian,and have been mainly described as representatives of the palaeoscolecids and the priapulans.These vermiform creatures as active predators and bioturbators have played a significant role in early benthic ecosystems,and are of key importance to understanding the occurrence of Cambrian substrate revolution,ecosystem establishment and complexity,and ecdysozoan origin and evolution.However,researches on these soft bodied fossils mostly revolve around the description and classification due to insufficient preservation of fossil material,which may lead to descriptions on detailed morphology and interpretation of some taxa in controversial,and subsequent phylogeny of Ecdysozoa clade in high dispute.Consequently,detailed anatomical(e.g.proboscis and reproductive system)and ecological(e.g.lifestyle and interspecific interaction)studies of well-preserved individual taxa are necessary in order to evaluate the significance of ecdysozoan worms in the early evolution of ecdysozoans and the ecosystem.With new specimens of Mafangscolex and selkirkiids from the Xiaoshiba biota(Cambrian Stage 3)and ?Eximipriapulus from the Guanshan biota(Cambrian Stage 4)in Kunming,China,this study reconstructs the morphology(external and anatomical)and ecological behavior of the three taxa,trying to reveal some key aspects on the early evolutionary history of ecdysozoans.Specimens of Mafangscolex provide the first detailed account of a proboscis in Palaeoscoelcida sensu stricto.The eversible mouthpart of Mafangscolex comprises hexaradially symmetrical introvert scalids,a ring of coronal spines and quincuncially arranged pharyngeal armature,with a range of tooth morphologies.This configuration strikingly resembles the proboscis arrangement inferred for the ancestral ecdysozoan.The six-fold symmetry represents an important difference from the pentaradial priapulan proboscis armature.The possible retention of key aspects of the ancestral ecdysozoan body plan suggests that palaeoscolecids represent a useful window on the earliest stages of ecdysozoan evolution.Selkirkiid specimens are attributed to two genera,namely Paraselkirkia and Selkirkia.The proboscis of the former is re-divided,leaving the introvert with two subzones: the proximal subzone is armed with oblique rows of spines,differing from that of the latter with longitudinal arrangement.Selkirkia cf.transita is described here,expanding the spatiotemporal distribution of this genus.In situ oocytes and a symbiotic association with small brachiopods are described in Paraselkirkia,which with possible paired tubular ovaries located in the posterior half of the primary body cavity suggests an adaptation of semi-sedentary epibenthic lifestyle,and a life strategy with energy investment oriented towards quality rather than quantity.These findings provide key information on the reproduction of early ecdysozoans.Paraselkirkia resembles modern meiobenthic priapulans in the reproductive organs and strategy,thus indicating that the general organization of female tubular gonads in priapulan worms has remained virtually unchanged over half a billion years,and suggesting the possible critical role of ecology on the reproductive strategies of both modern and Cambrian worms.New fossil materials reveal a consistent occurrence of each priapulan worm ?Eximipriapulus within a hyolith conch.This represents the first direct evidence of a hermiting life strategy in the priapulan phylum,and within the Palaeozoic Era;it contributes to a growing picture that the Cambrian explosion was characterized by highly complex,predator-heavy ecosystems.