A POSSIBLE COELOSCLERITOPHORAN/CHORDATE HYBRIDIZATION AND THE EDIACARAN ORIGIN OF TUNICATES

Tunicates (Urochordata) are chordates famous for their unusual ontogenetic metamorphosis. Typical species (“ascidians”) have a motile “tadpole” larval stage that preserves embryonic chordate synapomorphies but that later transforms into a saclike, siphonate, sessile adult bearing little resemblance to other adult chordates. Suspicion that the bizarre adult morphology arose through hybridization between a chordate and a member of some other phylum has recently received support from phylogenomic analyses of the tunicate Ciona with three to eight other metazoan taxa, pointing to tunicates arising from hybridization between a chordate and a protostome outside the clade defined by Nematoda and Arthropoda.

Coelosceritophorans are an enigmatic group of Cambrian animals united by the possession of distinctive, hollow, aragonitic sclerites. Because of the divergence in body form between radial, sessile, sponge-like chancelloriids and the creeping, bilateral, slug- to chiton-like halkieriids, some researchers view coeloscleritophorans to be a polyphyletic assemblage, with chancelloriids sometimes returned to their original assignment as sponges. However, analysis of sclerite microstructure and a report of transitional sclerite morphologies between those of halkieriids and chancelloriids have confirmed the homology of sclerites in these two groups. The recognition of halkieriids as either stem- or crown-group mollusks implies that a united, monophyletic Coeloscleritophora must contain Mollusca and perhaps other related phyla as well.

It is proposed herein that tunicate metamorphosis and the typical sessile adult phase might have arisen from hybridization between a chordate and a saclike coeloscleritophoran during the Ediacaran period. Testing this hypothesis requires a confluence of data from paleontologic and neontologic analysis. Further understanding of the morphology of coeloscleritophorans and the enigmatic Ediacaran Ausia may reveal important clues. Molecular phylogenetics will likely be employed to pinpoint the hybridization partner of the ancient chordate. Under this hypothesis and with currently available information, it is predicted that the chordate hybridization partner will resolve to be a sister to the Mollusca or to a clade that contains Mollusca.