Geometry And Development Of Trunk Plates Of Cambrian Lobopodians

The Cambrian lobopodians,a group of marine "worms with legs",representing an ancestral stem-group of Onychophora,have been well demonstrated to belong to Ecdysozoa.Soft-bodied lobopodians from the Chengjiang fauna display paired trunk plates,which with distinct outlines are regularly distributed on both laterodorsal sides.Phosphatized lobopodian trunk plates extracted via acid dissolution are worldwide distributed in the Cambrian deports,and lots of this sort of sclerites have also been found from various localities in southern China.Because of mineralization,these fossil sclerites are normally able to show three-dimensional preservation of fine structures.Such details in most case are hard to be determined from soft-bodied fossils with Burgess Shale-type preservation.On this ground this study mainly focuses on phosphatized lobopodian plates,along with convincing information derived from the Burgess Shale-type material,to get additional data and evidence revealing the development and evolution of the Cambrian lobopodians.Based on the unique outlines,the isolated trunk plates dealt with here can be divided into several morphological styles,since they came from different parts of lobopodian trunks,where the dorsal plates varying in shape.Of each style the plates may represent individuals belonging to a series of growth stages based on careful observation,measurement,analysis and statistics to evaluate their essential feature related to development.The developmental mechanisms could be determined by examining the split of notes and the occurrence of holes of each plate.It seems evident that there are merely short spines and fine tubercles densely situated along the margin,where none of mushroom-shaped notes have been observed.Therefore at the beginning only a few mushroom-shaped notes appearing in the central portion of a plate,and subsequently more such notes would occur there.With growth some mushroom-shaped notes would remarkably increase in size,and then splitting into two pieces.This would finally led to three notes located around a wall junction where previously located only one.During succeeding growth,a new hole would appear in the center surrounded by the three notes.With further development additional nodes would become bigger and then splitting again.As a result a hole surrounded by four notes would emerge.Such growth and split of mushroom-shaped notes will continue and lead to occurrence of more holes surrounded by five,six or seven notes,including a few holes with eight or nine nodes.If we regard a node as an extreme of a hole wall,according to the number of nodes,a hole together with all around nodes could be seen as a polygon.In this way there are more hexagons than any other polygons in a plate;the pentagons and heptagons are less abundant;delta,square are rare;as an exceptional example,a hole with eight or even nine notes is extremely scarce.As an ecdysozoan,the development of lobopodians involving ecdysis has been convincingly clarified by either soft-bodied Microdictyon with duplicated dorsal plates from the Chengjiang fauna,or acid-liberated specimens also showing duplication of trunk plates.New material lately collected from northeastern Yunnan demonstrates that of the two conjoined plates both the smaller and the larger ones are concentrically stacked,this would keep the outline of the newly formed plate unchanged with growth.Despite the holes increase in size remarkably with growth,these holes along either margin of the old and new sclerites match quite well,exhibiting a one by one manner.This means at least there are no any holes emerging near the marginal area.On this basis we come to a conclusion:more holes would be added exclusively in the central area of a plate during development.With exact observation and geometrical analysis on detailed morphology and microstructure of lobopodian trunk plates,this study provides fresh information for a better understanding of the underlying mechanisms and development model associated with the Cambrian lobopodians.In general the new discovery also supplies critical evidence to the early evolution and phylogenetics of the intriguing onychophorans forebears.