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.

You’ve examined evidence that a land bridge formed, separating the Caribbean Sea and the Pacific Ocean. From the data you’ve 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.