Evolutionary Developmental Biology Of Articulated Trilobites From The Cambrian Series 2 Of Eastern Yunnan And Central Guizhou

Studies on trilobite ontogenies can provide direct evidence for our understanding of the origin and evolutionary developmental biology of early arthropods,and articulated trilobite exoskeletons can offer unique opportunities to explore the mechanism of trunk segmentation.Based on the ontogenies of two Cambrian Stage 3 trilobites,Duodingia duodingensis Chow and Zhangshania typica Li and Zhang,and their developmental processes,this study provides new insights into the early evolution of trilobites.Duodingia duodingensis yielded from the lower Cambrian(Series 2,Stage 3)Mingxinsi Formation in Weng'an,Guizhou displays a complete ontogeny from postembryonic to mature phases.The meraspid degree 0 contains three developmental stages recorded for the first time out of eodoscids.With grow the earliest known protaspis about 0.47 mm in length would increase up to 6.01 mm in holaspis.Striking morphological changes include forward extended glabellar furrow,contracted anterior glabellar lobes,narrowing fixigenae,hemicirculr to elliptical pygidium.Three possible developmental models are recognized: 1)successive development,in which the alternation of segment depletion and segments accretion processes;2)intraspecific polymorphism,in which the morph with three pygidial segments displays a synarthromerically development and the morph with four pygidial segments shows a protomerically development;3)phenotypic variation,in which pygidum with three or four segments just represents the variation within instars rather than successive growth,experiencing a prolonged equilibrium phase.New discovery of the early Cambrian yiliangellinine trilobite Z.typica from Kunming area further confirms that Zhangshan Member of Sichuan and Hongjingshao Formation of Yunnan are closely correlated with eath other.The almost complete ontogeny of Z.typica includes the main morphological changes as following: cranidum changed from circular to subtrapzoid,glabella changed from rodlike to conical,narrowing fixigenae,bifurcaged eye ridges,widened free checks,lengthened genal spines,lengthened posterior thoracic pleural spines,enlarged pygidium with a pair of lengthened pygidial spines.A prolonged meraspid equilibrium phase is recognized,when transitioning from the meraspis into the holaspis,two pygidial segments are added posteriorly.Holasid pygidium continues to incrase segments until sevent segments reached,representing a protarthrous development.Moreover,from the same beds that yielded the Z.typica,four new macroalgae are collected and described,Doulia rara gen.et sp.nov.,Eolaminaria simigladiola sp.nov.,Singulariphyca ramosa gen.et sp.nov.and Rugophyca longa gen.et sp.nov.,which fill the part of the long gap of missing macroalgal record between the Chengjiang Biota and the Kaili Biota.Comparing the preservational styles of these macroalgae and Z.typica,a time-averaged accumulation is favored and some organisms are quickly buried within their original habitats.