Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 146

pedicle. Animals skip across the surface of the sand, 88 and others bur-
row below its surface. There are few traces of microbial mats now—
just golden sand, beautiful ripples, and the traces and shells of
animals.
How did life change so fast between 600 and 500 million years
ago—for that is a very short time compared to the billions of years of
slow change in the Precambrian? We may be tempted to think of Fred
Hoyle's analogy again, of a whirlwind ripping through the Earth's
oceans to assemble complex life forms from a much simpler set of
components. But that misses the point once more, for life had been
accumulating a series of steps towards complexity: the development
of cells with organelles and nuclei, the acquisition of sex, the organi-
zation of many cells into multicellular organisms, and the division of
labour between those cells to specialize in reproduction, feeding, and
locomotion. Finally, there came the development of the first complex
groupings of organisms in the late Proterozoic, remnants of which
persist today in the forests of sponges in the deep oceans.
Associated with some of the later Ediacaran assemblages are the
first simple trace fossils. They are the first indications of animals
attempting to penetrate the seabed to search for food. Among these
early trace fossils are those called Treptichnus , a three-pronged burrow
that shows repeated penetration of the seabed. They first appear in
rocks about 550 million years old, but then suddenly became a very
important component of seabed deposits 541 million years ago.
These patterns had intrigued Jean Vannier, a scientist at the Univer-
sity of Lyon, France. Perplexed by the complexity of behaviour they
represented, and surmising that they might represent the trails of
priapulid worms from the abundance of these animals as fossils in
certain Cambrian deposits, Jean designed an experiment to test his
hypothesis. Priapulid worms form a very small group of animals at
present, with only 18 living species. But they are distinctive, in that