SCALIDOPHORAN ANIMALS AT THE EDIACARAN-CAMBRIAN TRANSITION

Scalidophorans (kinorhynchs + loriciferans + priapulids) are worm-like animals that can potentially illuminate the early evolution of ecdysozoans and panarthropods. However, their phylogenetic relationships and fossil record are poorly resolved, partly because they are minor phyla among living animals and they generally lack biomineralized skeletons to be readily fossilized. Morphology-based phylogenetic analyses support the monophyly of scalidophorans and nematoids (nematods + nematomorphs), together constituting the cycloneuralians which are a sister group of panarthropods. Some molecular phylogenetic studies support a monophyletic cycloneuralian clade, but others resolve nematoids as more closely related to panarthropods than to scalidophorans. The monophyly of scalidophorans has also been challenged by molecular phylogenetic analyses that remove loriciferans from the scalidophorans and instead favor a loriciferan-nematomorph or a loriciferan-panarthropod relationship. Although molecular clocks suggest an Ediacaran divergence of scalidophorans phyla, scalidophoran fossils are completely lacking until the Ediacaran-Cambrian transition. The trace fossil Treptichnus, which first appears in the terminal Ediacaran, has been interpreted as a possible priapulid burrow, but until recently scalidophoran body fossils are few in rocks older than Cambrian Stage 3. Our recent investigation of basal Cambrian (Fortunian) rocks in South China reveals a suite of phosphatized scalidophoran microfossils that resemble modern priapulids, kinorhynchs, and loriciferans. Some of these fossils are armored with sclerites, which when dispersed resemble previously described form taxa of small shelly fossils. These new fossils help us to resolve the sequence of character acquisition and hence the phylogenetic relationship among scalidophorans, cycloneuralians, and ecdysozoans.