Sunday, June 19, 2011

Spirals of the Abyss

In shales formed from deep-sea muds, all over the world, geologists keep stumbling upon the intriguing trace fossil Spirorhaphe. A perfect spiral, a foot or more in diameter, is imprinted upon the rock face like a bronze-age ornament. These spirals date back at least to the Ordovician period, some 460 million years ago, and continue through the geological record almost to the present day. But what are they? The organism responsible for these fantastic feeding traces was believed to be extinct, and its identity forever lost to science.

Then, in 1962, when a camera was lowered into the Kermadec Trench in the southwestern Pacific, beautiful, modern-day Spirorhaphe traces were finally revealed to scientists (Bourne and Heezen 1965). One of the pictures even seemed to capture the trace-maker in action. It looked like an acorn worm, a representative of one of those enigmatic groups that fit only uncomfortably into the System of Animals and require phyla of their own. And it was huge: with a diameter of 5 cm it was quite a monster compared with most of its shallower-water brethren.

As more pictures were taken from the deep sea, these spirals turned out to be relatively common. The famous photographic volume “The Face of the Deep” (Heezen and Hollister 1971) contains several examples. But it was not until 2005 that a good video recording of the actual trace making was announced, together with the spectacular capture of the organism. The story was sensational enough to make it to the pages of Nature, but without reference to the fossil record (Holland et al. 2005).

Almost since the conception of animal life, this slimy worm has been sitting in the eternally dark and cold depths of the sea, silently spinning its spirals at a speed of 5 millimeters per minute. Hundreds of thousands of millennia passed. Life ventured onto land. Dinosaurs came and went, mammals and birds conquered the dry world. For the deep-sea acorn worm, nothing of this mattered much. It sat down there where the sun never shines, surviving, hardly moving.

How appropriate that it builds a perfect spiral, the symbol of eternity.