Missing Link Mollusk In 3D
October 4, 2012

3D Reconstruction Reveals Mollusks Missing Link

[ Watch the Video: Virtual Reconstruction of Kulindroplax ℠Missing Link´ Mollusk ]

April Flowers for redOrbit.com - Your Universe Online

What came first, shelled forms like clams and snails, or their shell-less, worm-like relatives? A small new fossil recently found in Great Britain may finally end the long-running debate about mollusks, one of life's most diverse invertebrate groups.

An international team of scientists, including researchers from Imperial College London the University of Oxford, the University of Leicester, Yale University and Queen's University Belfast found the fossil, which lived in the sea during the Silurian Period, approximately 425 million years ago.

Scientists have been debating the relationship between the two groups of mollusks called aplacophorans, which are carnivorous, worm-like, sea-living creatures, and the chitons, which are mollusks that have shell plates for armor and live in the sea or on the seashore— both still live in Earth's oceans today.

Found in marine rocks along the English-Welsh border, the new fossil provides the best evidence yet that the simpler worm-like mollusks evolved from their more anatomically complex shelled cousins, rather than the other way around. It seems like the shell came first, after all.

The results of this study, published in Nature, reinforce previous findings from molecular sequencing studies. It helps clarify the evolutionary relationships of mollusks, a broad category that includes not only oysters and mussels but also slugs, squids and octopuses.

"This is a kind of missing link with a worm-like body, bearing a series of shells like those of a chiton or coat-of-mail shell," said Derek E. G. Briggs, director of the Yale Peabody Museum of Natural History and one of the paleontologists who studied the new fossil, Kulindroplax perissokomos.

Representing the first mollusk with an unambiguous combination of valves, or exterior shells, and a worm-like body, Kulindroplax was unearthed more than 10 years ago in the Herefordshire Lagerstatte fossil deposit. The deposit is a rich assemblage of ancient marine life forms more than 400 million years old preserved when a cloud of volcanic ash settled and entombed a range of species.

Dr Mark Sutton of Imperial College London says, "Most people don't realize that mollusks, which have been around for hundreds of millions of years, are an extremely rich and diverse branch of life on Earth. Just as tracing a long lost uncle is important for developing a more complete family tree, unearthing this extremely rare and ancient Kulindroplax fossil is helping us to understand the relationship between two mollusk groups, which is also helping us to understand how mollusks have evolved on Earth."

Kulindroplax is about 2 cm wide and 4 cm long. The Kulindroplax had seven shells and a dense covering of spicules over the rest of its body, mostly likely used to gain purchase as it crawled on the muddy sea bed. It was discovered buried in volcanic ash deposited on the sea floor. Using computer software, the scientific team was able to reconstruct its 3D shape, revealing both form and structure in detail. To achieve this, they sliced the fossil into 1300 segments, taking digital images of each one. What they discovered was that Kulindroplax has both the worm-like body of the aplacophorans, but was partly shelled like the chitons. This mix of features confirms for the researchers that the two groups are closely related, and that this new fossil is the missing link between them.