A Palaeontological Puzzle Solved?
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?