T
his chapter examines the early vertebrate life-forms and the geology
of the Ordovician period. The Ordovician, from 500 to 435 million
years ago, was named for the ancient Ordovices tribe of Wales, Great
Britain. Ordovician marine sediments are recognized on all continents of the
Northern Hemisphere, in the Andes Mountains of South America, and in
Australia. However, they are absent in Antarctica, Africa, and India. Ordovi-
cian terrestrial deposits are not easily recognized because of the lack of fos-
silized land organisms.
The multitudes of species that exploded onto the scene in the early Cam-
brian advanced significantly in the warm Ordovician seas (Fig. 60). Corals began
building extensive carbonate reefs in the Ordovician. The first fish appeared in
the ocean.The existence of freshwater jawless fish suggests that unicellular plants,
including red and green algae, inhabited lakes and streams on land.
THE JAWLESS FISH
Beginning about 520 million years ago, the first vertebrates appeared on the
scene.They had an internal skeleton made of bone or cartilage, one of life’s
ORDOVICIAN
VERTEBRATES
THE AGE OF SPINAL LIFE-FORMS