tened bodies of many animals was the limited food supply available during the
Proterozoic.A high ratio of surface area to volume collected sunlight for algae,
which lived symbiotically within their host’s body. This helped to nourish
their hosts while the worms supported the algae for mutual benefit.
A rounded worm from 565 million years ago was symmetrical from side
to side and had what appeared to be a head. It contained three tissue layers, with
an ectoderm, mesoderm, and endoderm wrapped around a gut cavity.The body
apparently housed a primitive heart, a blood and vascular system, and a means
of respiration. The worm left behind fossil burrows, fecal pellets, and scratches
in sediments made by the very beginnings of limbs. It possibly gave rise to ani-
mals with mouths known as deuterostomes, including chordates, sea urchins,
starfish, and marine worms. This ancestral creature might also have led to the
protostomes, including mollusks, insects, spiders, leeches, and earthworms.
Tiny marine worms called acoels might be the closest living representa-
tives of the first bilaterally symmetrical organisms on Earth. They are flat-
worms grouped along with parasites such as tapeworms and liver flukes and
represent a living relic of the transition between radially symmetrical animals
such as jellyfish and more complex bilateral organisms such as vertebrates and
arthropods.The animals appear to be bilateral survivors from before the Cam-
brian, a period of unprecendented growth, providing a living window on early
metazoan life. Acoels probably branched off from an ancestral animal after the
radial jellyfish but before the three major bilateral groups, the vertebrates, mol-
lusks,and arthropods, began to diverge. They therefore offer a living link
between primitive and more complex animals.
The earliest fossils of plants appear to be almost entirely composed of algae
that built stromatolite structures (Fig. 33). The stromatolite colonies were pro-
duced by layers of cells that excreted a gluelike substance used to bind sediments
together.These were topped by photosynthetic organisms that multiplied using
sunlight and supplied the lower layers with nutrients. During the late Protero-
zoic, around 800 million years ago, stromatolites underwent a marked decline in
diversity possibly due to the appearance of algae-eating animals.
THE EDIACARAN FAUNA
Earth underwent many profound physical changes near the end of the Pro-
terozoic, prompting a rapid radiation of species. At this time, a supercontinent
called Rodinia (Fig. 34), Russian for “motherland,” was located on the equa-
tor. It subsequently rifted apart, producing intense hydrothermal activity that
caused fundamental environmental changes. With these profound changes in
Earth, the increased marine habitat area spawned the greatest explosion of
new species the world has ever known. As a result, the seas contained large
PROTEROZOIC METAZOANS