Large rivers rinsed sand and decomposition debris into the Cretaceous sea.

A possible fossil osmeriform is Spaniodon, a piscivore from Late Cretaceous seas.

The fossil is shown being attacked by a fossil mosasaur, a large reptile predator of the shallow Cretaceous seas.

Large rivers carried sand and other eroded debris into the Cretaceous sea.

Mosasaurs lived in the Late Cretaceous seas of North Dakota.

Where missing, a lag of conglomerate may be present to indicate winnowing of sediments, which occurred by advancing Cretaceous sea.

The plesiosaur Dolichorhynchops osborni was another inhabitant of Wyoming's Cretaceous sea.

The invertebrates of Arkansas's Cretaceous sea included clams, echinoids, oysters, and snails.

It lived in flocks, nested on shorelines, and hunted for fish in Cretaceous seas.

These monitor lizards' marine relatives, the mosasaurs, were apex predators in late Cretaceous seas.