Analysis of nuclear DNA sequences underlined the oddity: Xenoturbella were even thought to be highly degenerate molluscs until the revelation that molluscs are what Xenoturbella eat. Even stranger was the proposal that Xenoturbella and other acoels were most closely related to hemichordates (animals known as acorn worms and pterobranchs) and echinoderms (radially symmetrical marine animals such as sea urchins and starfish). This cast into question the timing of the evolution of several advanced characteristics, such as gill slits, that are shared by members of the deuterostome branch of Bilateria (to which hemichordates and echinoderms belong), but that are lacking in Xenoturbella. It even raised questions about the last common ancestor of Bilateria — perhaps Xenoturbella were not as simple as they looked, but had degenerated from a structurally more complex ancestor.
These questions are all but resolved by two studies in this week's issue. Cannon et al. (page 89)1 present a robust phylogenetic analysis based on the gene-transcript profiles of eleven species of Xenoturbella and other acoels. This shows that the combined group, known as Xenacoelomorpha, indeed lies at the very base of the bilaterian radiation. Rouse et al. (page 94)2 add four new species of Xenoturbella from the eastern Pacific Ocean to the one already known from the waters of Scotland and Scandinavia. The authors' anatomical and phylogenetic studies on these new forms add weight to the idea that these worms were the earliest to branch from other bilaterians. Zoologists can exhale, and their shy charges can resume their diet of molluscs in peace.
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