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A Palaeontological Puzzle Solved?

Keith Thomson

The Technical Approach

One of the early major contributors to making Palaeospondylus a bigger puzzle than it needed to be was Oxford professor William Sollas. He perfected a serial grinding technique and apparatus for using thin sections of fossils to create enlarged wax-plate reconstructions. Fossil after fossil was dissected in this way by Sollas and his daughter Igerna at Cambridge, including a graptolite, a Triassic dicynodont reptile and a Cretaceous sea urchin. And, in 1903, Palaeospondylus. They revealed a wealth of anatomical detail not visible with the microscope alone. But Sollas also led everyone down a wrong track. Unable to homologize the head structures he found with those of other fishes, he gave each its own new name—in Greek. Ampyx, tauidion, hemidome and gammation are some of the most memorable. The result was that everyone then accepted that the anatomy really was uniquely different from that of other vertebrates.

A couple of years ago I discovered that all of Sollas's material was still in the Oxford University Museum. He had sectioned several specimens at about 0.25-millimeter intervals and photographed the cut and polished surfaces using superb lenses, so that each was preserved as a glass-plate negative. Naturally I had the idea that modern computer manipulations of his sections would give us a better reconstruction of the anatomy. My assistant Bethia Thomas set to work.

But I was wrong. Sollas had captured just about every detail that was available. However, postdoctoral fellow Mark Sutton had been working with my colleague Derek Siveter on magnificent computer reconstructions of far more minutely sectioned Silurian invertebrates. So I asked Mark if he would like to have a go at Palaeospondylus, making new sections at only 30-micron intervals. The Natural History Museum in London gamely offered up three specimens to be sacrificed—eventually to reappear in virtual form. This time the result was amazing. One of the specimens was preserved relatively uncrushed and undistorted, and at last we had a new look at its structure. There were the tauidion, gammation et cetera in all their glory. But still the question remained: how to interpret such a set of structures apparently unlike those in any known fish?

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