Making an impression

WHAT came before the joyous explosion of animal life which ushered in the Cambrian period is, to quote Churchill, a riddle wrapped in a mystery inside an enigma. Fossil preservation favours shells, and before 541m years ago, there were none. Nor were there teeth, claws or spines. The animals of the Ediacaran, as the time immediately preceding the Cambrian is known, had only soft bodies. That makes their fossils rare. It also means many of the characteristics which zoologists use to classify species are hard to read, or missing altogether.

All of which makes this specimen, dubbed Haootia quadriformis, a valuable find. Its discoverers, Alexander Liu of Cambridge University and his colleagues, are pretty sure they know what type of animal it was: a cnidarian, related to modern jellyfish, sea anemones and coral. What is more, they think they can see the traces of its muscles. That removes all doubt that it was, indeed, an animal.

Dr Liu’s paper, just published in the Proceedings of the Royal Society, also neatly demonstrates the process by which an impression in the rocks (in this case the Fermeuse formation of Newfoundland, in Canada, which is 560m years old) is interpreted into a simulacrum of the living creature it derives from. The top-left picture is of the fossil itself, which is about 10cm across. A lot of striations, which Dr Liu believes are impressions of bundles of muscle fibres, are visible, but the specimen has been distorted by tectonic forces which have sheared the rock containing it over the millions of years since its formation. The researchers were able to work out the extent of the shear by examining nearby fossils of previously known species. That let them create an undistorted illustration of the specimen (top right), which put them in mind of a group of living cnidarians called staurozoa (bottom left) and provided a template for an artist’s impression of H. quadriformis (bottom right).

Staurozoa themselves are an oddity. Technically, they are jellyfish. But unlike others of their kind they live attached to rocks by stalks, rather than swimming free in the sea. A number of other Ediacaran fossils look like conventional jellyfish, so a new species belonging to the group is no surprise. It is intriguing, though, that the exceptional, sedentary lifestyle of the staurozoa had evolved so early.