A new rearrangement of the animal kingdom has expanded a little-known offshoot into a major branch, and opened a gap between relatively simple and highly complex life.

Ocean-dwelling flatworms called acoelomorphs were thought to represent one of life’s early stages, bridging primitive animals with radially symmetrical bodies, like jellyfish and sea anemones, and animals with bilaterally symmetrical bodies, from butterflies to humans.

But acoelomorphs now appear related to Xenoturbella, a bilaterally symmetrical ocean worm.

“It was nice to have this intermediate group between jellyfish and the other complicated animals. We’ve lost that,” said Max Telford, a University College London geneticist and co-author of the new analysis, published Feb. 9 in Nature.

The identity of both worms is a matter of ongoing interpretation among scientists who map life’s tree. Until the early 1990s, both were lumped with the Platyhelmine, or “true flatworm,” phylum. Then genetic data reclassified Xenoturbella as molluscs and acoelomorphs as descended from that unknown, ur–complex animal.

Over the last decade, the Xenoturbella interpretation was revealed as a mistake. It wasn’t a mollusk, but ate them: Researchers had confused DNA from shellfish prey with genes from Xenoturbella itself. Thus corrected, Xenoturbella was recognized as a phylum unto itself, clearly grouped with other bilaterally symmetrical animals.

Now Telford’s team has turned a corrective lens on the acoelomorphs. Like other phylogenists, the team analyzed DNA from species in every phylum, searching for patterns of relationships and then constructing a tree to fit the data. But Telford and fellow researchers made one key adjustment, tweaking their algorithms to account for the unusually high mutation rates of acoelomorphs.

“They’ve evolved much more quickly than other animals,” said Telford. “We suspected that this effect caused previous studies to put them a long way from animals with normal rates of evolution.”

This rate-adjusted analysis pulled the acoelomorphs out from the base of the tree of life, and set them down on a far branch alongside Xenoturbella. Together, the two Xenoturbella species and hundreds of acoelomorph species constitute a newly classified phylum, which the researchers named Xenacoelomorpha.

Xenacoelomorpha is on roughly the same branch of life’s tree as sea stars, sea cucumbers and acorn worms — not the most complicated animals, but far more complex than what the acoelomorphs were thought to be.

As for what might replace acoelomorphs as a candidate for root-animal status, Telford doesn’t know.

Another question raised by the findings is how and why acoelomorphs evolved in such fashion. Their ancestors were likely creatures with guts, fluid-filled cavities and gill slits. Acoelomorphs have none of these features, and are so physically primitive as to resemble researchers’ notions of how an ancient animal ought to appear.

“Now we’ve got these very simple worms nested right in the middle of the complex animals,” said Telford. “How did they end up so simple? They must have lost a lot of complexity.”

Videos: 1) Acoelomorph flatworm/Max Telford. 2) Xenoturbella/Max Telford.

See Also:

Citation: “Acoelomorph Flatworms Are Deuterostomes Related to Xenoturbella.” By Herve Philippe, Henner Brinkmann, Richard R. Copley, Leonid L. Moroz, Hiroaki Nakano, Albert J. Poustka, Andreas Wallberg, Kevin J. Peterson & Maximilian J. Telford. Nature, Vol. 470 No. 7333, Feb. 9, 2011.