Early Steps In The Evolution Of Scalidophora:Fossil Evidence From The Early Cambrian Of South China

Ecdysozoans are among the most abundant animals(e.g.arthropods)on Earth and have an extremely long evolutionary history with a rich Cambrian fossil record.This thesis focusses on one the major elements of the Cambrian ecdysozoan fauna,the scalidophoran worms.Our fossil material comes from: 1)Small Shelly Fossil(SSF)assemblages from the basal Cambrian(Fortunian Stage)Kuanchuanpu Formation(Shaanxi Province,China;ca 535 Ma)and 2)the early Cambrian(Stage 3)Chengjiang Lagerst?tte(Yunnan Province,China;ca518).Both localities yielded exceptionally preserved fossils that provide key information on the early evolutionary history of scalidophorans.Our results are summarized as follows:1)The finding of exuviae(including those turned inside out)provide solid fossil evidence that ancestor ecdysozoans animals grew by moulting at least 535 million years ago,before the diversification of arthropods.This confirms that the scalidophoran worms from the Kuanchuanpu Formation are the oldest known true ecdysozoans.We hypothesize that ecdysis may have been selected as the most adequate solution to overcome mechanical constraints(growing with a rigid exoskeleton)and opened up new evolutionary perspectives.2)We also show that the external surface of the cuticle of scalidophoran worms from the Kuanchuanpu Formation(ca 535 Ma)bears a fine,micrometre-sized,hexagonal network that is interpreted as the faithful replica of the boundary between underlying epithelial cells(mainly the column cells instead of gland or receptor cells).This interpretation is based on identical structures observed in extant priapulid worms.Cuticular reticulation in Ecdysozoa appears to be nothing than an initial by-product of cell division during cuticle growth.Although this basic process remained unchanged since at least the basal Cambrian,many different variants evolved through time.During the course of evolution,the increased complexity of cuticular features(e.g.sclerotization,mineralization)associated with possible epidermal differentiation gave rise independently in almost all groups(except nematodes)to a great variety of patterns of reticulation(e.g.in arthropods).3)Based on exceptionally preserved specimens from the Chengjiang Lagerst?tte(ca 518 Ma),the anatomy and lifestyle of Selkirkia are reexplored and more precisely the tube formation and relation to body.Fossil evidence indicate that the tube was probably secreted by trunk epithelial cells and renewed periodically via ecdysis.The tube of Selkirkia can be seen as a protective feature against physical damage and predation and may also have played a role in anchoring the animal to sediment.Possible brachiopod epibionts also give information on the epibenthic habitat of Selkirkia.Selkirkia may appear as the oldest unambiguous tube-dwelling ecdysozoan.4)A new phylogeny of Scalidophora is proposed based on augmented morphological data,reassessment of characters and various cladistic methods.Maximum parsimony(Heuristic TBR,TNT,and Tree Search),and maximum likelihood resolve topologies with extant priapulids forming a sister clade to Selkirkia and palaeoscolecids,and with Loricifera and Kinorhyncha branching off basally.By contrast,the unconstrained Bayesian runs converge on a basal Selkirkia sister group to Palaeoscolecida and Scalidophora.The strength of this model was tested by enforcing a backbone on the Bayesian analysis.The resulting topology bears overall similarity with the unweighted Tree Search topology and yields on average slightly better harmonic means than the unconstrained model.A better treatment of inapplicable states helped parsimony converge with Bayesian inference,assuming a model with basal Loricifera and Kinorhyncha.