By examining the collections of microfossils, you can
see that Pulleniatina is present in sediments only on
the Pacific side and only in the more recent layers
of sediments. Pulleniatina is an example of an organism
that had been isolated from other groups of foraminifera,
then diverged to become an entirely new species.
At some stage in Earth's geological history, a land
bridge formed, connecting Central and South America
and closing off the Caribbean Sea from the Pacific Ocean.
Pulleniatina was a species that developed in the new
conditions of the Pacific Ocean. It did not have an
opportunity to spread to the Caribbean side because
the land bridge was in the way.
Youve examined evidence that a land bridge formed,
separating the Caribbean Sea and the Pacific Ocean.
From the data youve collected, what can you infer
about the timing of the formation of the land bridge?
Use your chart showing the presence/absence of Pulleniatina
and the animation of plate movement to determine the
most likely date that the isthmus was formed.