Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 145

penetrated the seabed to search for food, as many modern organisms
do. To do that, to be able to burrow, an animal needs a head and tail
end, and a bilateral symmetry like a worm, mollusc, or shrimp.
The Ediacarans represent a large step forward in the evolution of
ocean life, but they themselves were not to persist, and as far as we
know they did not leave descendants. Were they really a failed experi-
ment? Well, they persisted, albeit without showing very much in the
way of evolution, for some 60 million years during the late Protero-
zoic (and perhaps some elements persisted for longer, into the Cam-
brian). They were then swept away, and the ancient microbial mats
disrupted, by a new change in the ocean realm. Organisms with struc-
ture and behaviour far more complex than anything yet to appear on
the Earth were evolving, and they would invade the world of quilts
and microbes.
Tooth and Claw
A walk along the seashore 600 million years ago would be a very dif-
ferent experience from walking along the beach today. A few hours
after the tide has fallen you will find no trace of seashells on the shore.
No indication of life skipping across the surface of the beach, and no
indication of burrowing into the sand below. There is no point in
searching for lugworms here, no telltale worm-shaped patterns of
sediment being squeezed out like toothpaste above the sand: and
indeed, no fish are in the sea to be caught by fishermen. Nothing but
a pristine beach of golden sand, with perhaps here and there the signs
of microbial mats glistening at lowest tide.
One hundred million years later, and that walk along the beach
becomes very different—perhaps not so different from a beachcombing
walk of today. Now there are shells everywhere, and some of these even
resemble the shells we find on beaches today, such as the burrowing
brachiopod Lingula , with its two elongated valves and long protruding