Ancient Fossils From Britain Shed Light On The Evolutionary History of Molluscs

Today, Britain's landscape is not particularly extreme. While picturesque in many places, it does not have record-breaking climbs or vast jungles possessed by other countries or continents. Yet its geological past tells a different story. The most north-western strip of Scotland was once a piece of an ancient Precambrian super-continent crushed and annealed to the Palaeozoic Hibernian south. This entire landmass collided with what would become England during the assembly of the last great super-continent, Pangaea.

The Gobi Desert is very similar to Britain when
it was part of the red continent of Pangaea

The island has been covered by shallow tropical seas dotted with volcanic island arcs whose remains now lie under the hills and forests of Leicestershire. 400 million years ago it was part of a mountain range known as the Caledonides, whose highest peaks dwarfed Mount Everest. Once it was a desert of red sand, derived from the dust of those primeval ranges, baking under a burning, orange sun. Later it slipped under the waves of a chalk ocean, filled with giant, marine predators. The rocks of this already ancient country were scoured by the glaciers which dominated Northern Europe during the last great Ice Age, and then as ancient woodlands which now lie just outside of human memory, under the moors of Essex, the fens of Norfolk and the peat bogs of the Grampian mountains.

Despite all this the British Isles have contributed little to the world of palaeontology. While Britain is small in size, it has a rich geological mix which should have produced many marvels. Yet some do exist. The Rhynie Chert in Scotland preserves the remains of the oldest terrestrial ecosystem on the planet. While further north, rocks have revealed the complete armour from the largest sea scorpion ever found.

A single fossil, recovered from Silurian rocks on the England/Wales border has cleared up a major discrepancy on the evolutionary history of molluscs: namely whether such creatures first evolved with or without a shell. The fossil is a of a creature, Kulindroplax perissokomox, buried in volcanic ash 400 million years ago, and present in our collections for over 10 years. 2 centimetres in diameter and 4 centimetres in length, Kulindroplax has the body-shape of a worm.

What makes it important is that it was covered in a series of scale-shaped, interlocking plates which ran along the length of its back. Using scanning and modelling software, researchers led by Mark. D. Sutton from Imperial College, London, were able to look closer at the fossil. In addition to the seven plates, Kulindroplax had a covering of small pointed rods on the underside of its body which would have allowed it to gain extra purchase on the muddy sea floor which was its home.

The 400 million year old fossil of Kulindroplax perissokomox.
The coloured sections show the position of the seven interlocking plates

As a result, this creature combines the physiology of two groups of molluscs: the shell-lacking aplacophorans which possess a worm-like body and the rest, such as bivalves, which have a hard shell covering. This creature, therefore, lies in between these two groups. 'This is a kind of missing link with a worm-like body, bearing a series of shells like those of a chitin or coat-of-mail shell,' said Derek E. G. Briggs, director of the Yale Peabody Museum of Natural History.

It seems that the hard shells evolved first and that soft bodied molluscs, rather than making an appearance during the Cambrian explosion, as was previously thought, are actually rather more recent, coming into the picture only 50 million years ago with the loss of their shells. Today molluscs are an incredibly diverse group with a rich evolutionary history stretching back to even before the Cambrian period. Their ancient shells now form the rocks of the Earth and the buildings around us today; Kulindroplax plays a major part in that story.