Scientists have discovered a rare fossil called Kulindroplax, the missing link between two mollusc groups.
The researchers have unearthed the worm-like partly shelled Kulindroplax, which they have modelled in a 3D computer animation. Kulindroplax lived in the sea during the Silurian Period, approximately 425 million years ago, when most life lived in the oceans and the first plants were beginning to grow on land. The team found the Kulindroplax fossil, the only one of its kind in the world, in the Welsh borderland, and it is providing the evolutionary missing link between two groups of molluscs and shedding more light on the their early origins.
Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species. About 80% of all known mollusc species are gastropods (snails and slugs), including the cowry (a sea snail) pictured here.
A fossil unearthed in Great Britain may end a long-running debate about the mollusks, one of life's most diverse invertebrate groups: Which evolved first, shelled forms like clams and snails, or their shell-less, worm-like relatives?
There is good evidence for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 542 to 488.3 million years ago. However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated.
A small new fossil found in Great Britain provides the best fossil evidence yet that simpler worm-like mollusks evolved from their more anatomically complex shelled brethren, rather than the other way around.
The small new fossil, found in marine rocks along the English-Welsh border, provides the best fossil evidence yet that the simpler worm-like mollusks evolved from their more anatomically complex shelled brethren, rather than the other way around.
The discovery reinforces previous findings from molecular sequencing studies and 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. The researchers report their findings online Oct. 3 in the journal Nature.
The evolutionary relationships of worm-like mollusks, known as Aplacophora, has been a subject of controversy. Previously thought to be a product of the explosion of diversity during the early Cambrian period, they are now shown to have evolved probably 40-50 million years ago by losing shells like those on Kulindroplax.
Kulindroplax represents the first mollusk with an unambiguous combination of valves, or exterior shells, and a worm-like body, said Mark D. Sutton of Imperial College London, the paper's lead-author.
The researchers found the specimen of Kulindroplax more than 10 years ago in the Herefordshire fossil deposit, a rich assemblage of ancient marine life forms more than 400 million years old. About 2 cm wide and 4 cm long,Kulindroplax was buried in volcanic ash deposited on the sea floor. The researchers later reconstructed its three-dimensional shape using computer software, revealing both form and structure in fine detail. In addition to its seven shells, Kulindroplax had a dense covering of spicules over the rest of the body, which it probably used to gain purchase as it crawled on the muddy sea bed.
Kulindroplax is coined from the Greek words for a cylinder and a plate, referring to the rounded body with its series of shells.
Dr Sutton says: “Most people don’t realise that molluscs, 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 mollusc groups, which is also helping us to understand how molluscs have evolved on Earth.”
Scientists have discovered a rare fossil called Kulindroplax, the missing link between two mollusc groups, revealed in a 3D computer model
For over 20 years, scientists have debated the evolutionary relationship between two groups of molluscs called the aplacophorans, which are carnivorous, worm-like, sea-living creatures, and the chitons, which are molluscs that have shell plates for armour and live in the sea or on the seashore– both still live in Earth’s oceans today.
The researchers in today’s study analysed the 3D model of Kulindroplax and discovered that it had the worm-like body of the aplacophorans, but was partly shelled like the chitons. The combination of features in Kulindroplax confirmed to the team that that aplacophorans and chitons are closely related. Furthermore, the researchers believe that their 3D fossil is the missing link that shows how the worm-like aplacophorans evolved from chiton-like ancestors by losing their shells, providing fresh insight into the mollusc evolutionary tree.
The researchers discovered the Kulindroplax fossil, which is the size of a small caterpillar, in a deposit called the Herefordshire Lagerstätte. This deposit was formed when a cloud of volcanic ash settled through the Siluian seas and entombed a range of species, including Kulindroplax, as almost perfectly preserved fossils.
In order to develop their 3D animation, the team cut the Kulindroplax fossil into 1300 slices, taking digital images of each one, which were fed into a computer. The researchers in the study say this method provides unprecedented detail from the fossils, enabling them to analyse features that have been previously unseen.
The other authors of the paper are David J. Siveter of the University of Leicester, Derek J. Siveter of the University of Oxford and Julia D. Sigwart of Queen's University, Belfast.
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