The foreland basins filled with thick sediments eroded from nearby moun-
tains. Erosion from the mountains might have pumped nutrients into the sea,
fueling booms in marine plankton, thereby increasing the food supply for higher
creatures. Therefore, the number of genera of mollusks, brachiopods, and trilo-
bites (Fig. 70) dramatically increased, because organisms with abundant food are
more likely to thrive and diversify into different species.
Island arcs lying between the two colliding landmasses were scooped up
and plastered against continental edges as the two continents collided. The
oceanic crustal plate carrying the islands dived under Baltica in a process
known as subduction.The subduction rafted the islands into collision with the
Figure 70Fossil
brachiopods and trilobites
from the Bonanza King
Formation,Trail Canyon,
Death Valley National
Monument, Inyo County,
California.
(Photo by C. B. Hunt,
courtesy USGS)ORDOVICIAN VERTEBRATES