For the most
part the Earth's climate was warm and wet, with sea levels rising
as much as 1,970 feet (600 meters) above those of today. But once
Gondwana took up its polar position in the late Ordovician, massive
glaciers formed over Africa at the supercontinent's center. This
heralded a 20-million-year ice age during which shallow, life-rich
seas shrank away.
Life at the
start of the Ordovician remained confined to the seas with new
animals evolving in place of those that didn't survive the
Cambrian. Chief among them were the squidlike nautiloids, a type of
tentacled mollusk. The nautiloids lifted off from life on the
seabed as gas-filled chambers in their conical shells made them
buoyant. They were accomplished swimmers, propelling themselves by
jetting water through their body cavity. Equipped with grasping
tentacles, the nautiloids were effective predators.
Another group
of marine hunters were the mysterious conodonts, known mainly from
the tiny fossil teeth they left behind. The few complete fossils
that have been found suggest they were finned, eel-like creatures
with large eyes for locating prey. The conodonts are now thought to
have been true vertebrates; however, this line of backboned animals
later went extinct.
Fish started
becoming more widespread in the fossil record. They were small and
had downward-pointing, jawless mouths, indicating they lived by
sucking and filtering food from the seabed. Bony shields covered
the front of their bodies—the beginnings of a fashion for armor
plating among fish. Lampreys and hagfish are these fishes' living
descendants.
The archaic
sponge reef-dwellers of the Cambrian gave way to bryozoans—tiny,
group-living animals that built coral-like structures. Ordovician
reefs were also home to large sea lilies, relatives of sea stars.
Anchored to the bottom inside calcareous tubes, they collected food
particles with feathery arms that waved in the ocean
currents.
The
hard-bodied arthropods started eyeing opportunities on land. Edging
into freshwater and shallow lagoons, they likely included horseshoe
crabs, which, despite their name, are more closely related to
spiders and scorpions. A few species of these "living fossils"
still survive today, such as along the eastern seaboard of the
United States, where each spring horseshoe crabs crawl ashore to
spawn.
There is also
evidence that the first primitive plants began to appear on the
previously barren land.
These first
steps toward life on land were cut short by the freezing conditions
that gripped the planet toward the end of the Ordovician. This
resulted in the second largest mass extinction of all time, wiping
out at least half of all marine animal species about 443 million
years ago.