An Irish palaeontologist helps solve a mystery about a sea creature that lived 300 million years ago, writes Dick Ahlstrom.
An Irish palaeontologist based at Yale University in the US has helped to solve a 150-year-old scientific mystery about a tiny sea creature that went extinct 305 million years ago.
The researchers have finally been able to provide a family tree for the ancient marine worm after digging a 480 million-year-old fossil out of the rocks in Morocco.
The discovery was important enough to be published last month in the prestige journal, Nature, and relates to a group of invertebrate animals known as the machaeridians.
They were first described in 1857, but scientists could only guess at what they were, explains Prof Derek Briggs, the Frederick William Beinecke professor of geology and geophysics at Yale University in Connecticut.
Prof Briggs won the 2001 RDS Irish Times Boyle Medal for Scientific Excellence. He will also become the director of Yale's Peabody Museum of Natural History in July.
The assumption was the machaeridians were some type of worm or slug-like animal whose back was completely lined with rows of mineralised armour plates. The plates first started appearing 485 million years ago and are as plentiful in the marine fossil record as ordinary seashells are today. Yet no fossil remained to indicate what kind of creature sported this exotic armour.
This is always the difficulty with soft tissues, which do not readily fossilise, says Prof Briggs. "You have to satisfy a number of conditions to get soft-tissue fossils. You have to be buried quickly, you have to eliminate scavengers and you have to have a certain set of chemical circumstances in the surrounding water."
Yet occasionally these conditions are met and happily were for a machaeridian fossil recovered by post doctoral student Peter Van Roy in the Upper Tremadoc of Morocco.
Van Roy, who is currently an Irish Research Council for Science, Engineering and Technology research fellow in the School of Geological Sciences at University College Dublin, was at the time a graduate student at Ghent University.
Prof Briggs and his Danish graduate student Jakob Vinther became involved in helping to identify the exceptionally preserved creature and were astounded to discover it was a largely intact machaeridian. Before Van Roy's find, various researchers had suggested the machaeridians belonged to the molluscs, which include clams and snails, or perhaps the barnacle group that are included amongst the crustaceans. Other theories suggested they were echinoderms, which have family members including starfish and sea urchins or perhaps the segmented worms, the polychaete annelids which include aquatic bristle worms and also garden earthworms.
Van Roy found what he thought was an early mollusc. "Peter contacted Jakob and we began to realise this wasn't a mollusc at all it was a polychaete," Prof Briggs says. "They are big worms with little appendages on their sides used to wiggle along the seabed."
Each of these pseudo feet also sported long stiff bristles, bundled together at their ends. Similar structures are seen in modern animals such as the sea mouse and the polychaete Nereis which occurs around the shores of Ireland.
The machaeridians were the only animal in the group to produce the mineral armour, however, which ran in regular plates along its back. Some of these were dislodged before the animal became fossilised, allowing Prof Briggs to identify how the plates were connected. "It was a palaeontology mystery. Nobody knew what they belonged to, but now we know they were marine worms," he says.
Yet the discovery is more than just a puzzle solved. It helps to fill gaps in our knowledge of where the animal sits on the tree of life.
"We also have an idea about how these marine worms radiated" across the globe, Prof Briggs adds.
The machaeridians were a highly successful group, surviving over a 180 million year period. They came to a somewhat inglorious end however.
"They just kind of fizzled out," Prof Briggs says. Yet the new fossil discovery helps us understand a little bit more about their distant past.