These fossils were deposited at the bottom of the large inland sea that covered most of western North America during the Cretaceous (about 80 million years ago).
(These fossils; were deposited; at the bottom of the large inland sea) (the large inland sea; covered most ago; T:during the Cretaceous)
These fossils the large inland sea
were deposited covered most ago
at the bottom of the large inland sea
T:during the Cretaceous