New fossil may end mollusk evolution debate

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Photo by Michael McHugh.

An unassuming, golf ball-sized rock unearthed from volcanic ash in the United Kingdom has flipped the scientific community upside down.

A paper released in the scientific journal “Nature” Oct. 3 detailed the newly discovered evolutionary implications of the fossil Kulindroplax perissokomos. Yale and United Kingdom researchers found the fossil approximately a decade ago in the Herefordshire fossil deposit in the Welsh borderlands. The Kulindroplax fossil could resolve the long-lasting dispute about the evolutionary relationships of shelled chitons and shell-less Aplacophorans, two classes of mollusk.

Bristol University’s Jakob Vinther, who helped with the research, said traditional hypotheses state that shelled mollusks evolved from their shell-less counterparts. However, evidence collected over recent years supports the contrary argument, which suggests the worm-like shape of Aplacophorans is a derivative of something much like a chiton. Kulindroplax delivers the physical evidence necessary to prove that the worm-like body of the Aplacophorans evolved from shelled chitons.

“To understand where mollusks fit in the big tree of life you need to understand what the most primitive mollusks looks like,” said Imperial College London professor Mark D. Sutton, the lead author of the paper.

Kulindroplax has characteristics of both the shelled chitons and shell-less Aplacophorans, Sutton said. Whereas chitons have shells, spicules and a large, flat foot for mobility, Aplacophorans have no shell and no foot, but instead possess a round, wormlike body.

To determine Kulindroplax’s morphology, researchers had to create a three-dimensional image of the fossil. Scientists had to repeatedly grind away small portions of the fossil and, after washing the remaining structure, photograph the exposed portion of the fossil at micron-length distances. These photos were then compiled into a digital three-dimensional reconstruction of the entire Kulindroplax specimen. This virtual image is now all that remains of the fossil, which was completely ground away.

Through this reconstruction, the researchers were able to deduce that Kulindroplax had a series of seven shells, spicules, no foot and a wormlike body. Therefore, Kulindroplax is an intermediate between shelled and shell-less mollusks, said co-researcher Derek E.G. Briggs, Director of the Yale Peabody Museum of Natural History.

Vinther said the research done on Kulindroplax has sufficiently shut down the argument that chitons evolved from Aplocophorans.

“With this discovery we really put the nail in the coffin of the traditional hypothesis, and radically,” Vinther said.

The Kulindroplax fossil evidence, Briggs said, confirms genetic evidence from molecular sequences in DNA and proteins that also refutes the traditional argument, Briggs said. He added that he would be surprised if the scientific community did not accept Kulindroplax as definitive evidence of the evolutionary history of mollusks.

Yale University ecology and evolutionary professor Thomas Near said that Kulindroplax provides a line of evidence for the evolution of mollusks that has never existed before — evidence that turns the traditionally accepted evolutionary path of the mollusk on its head. A re-evaluation of the data, he added, would have to argue against Kulindroplax’s classification as an Aplacophoran.

“This is the missing link that the model predicts,” said Briggs. “It’s confirming what we thought based on molecular evolution — the controversy is resolved.”

Or, at least, it is resolved for now. Briggs said there is always the possibility that new evidence, such as different gene sequences, could come along and derail what is currently accepted as the truth. Since evolutionary biology is an inference-based science, these disputes are never completely settled, Sutton said. He said he hopes, however, that the strength of the newly discovered fossil evidence will shift people from the opposing camp into his own.

Researchers from the University of Leicester and Queens University Belfast also contributed to the paper.

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