For example, geologists know that back in the dim dark distances of Earth time, Big Bend lay repeatedly at the bottom of the sea. Convulsions within the Earth repeatedly raised these sea floors to the tops of mountains, and time after time these mountains wore away. One of the ancient ocean beds can be seen at Persimmon Gap where, in remnants of the park’s oldest mountains, fossil sponges, brachiopods, and other simple marine organisms lie exposed. For hundreds of millions of years the three-lobed trilobite was among the most prolific animals in the world, but it had long been extinct when the last great ocean, the so-called Cretaceous sea, washed across Big Bend. Most of Mexico lay submerged and a sort of mid-continent seaway cut North America in two about on the line of the Rocky Mountains. In its early stages this sea harbored ancestral clams, oysters, snails, corals, and a coiled shellfish called an ammonite. You can see these animals preserved in the limestone walls of Santa Elena Canyon. Giant clam shells a meter (3 feet) across and fossil fishes preserved in the round between Boquillas and Mariscal Mountain tell us what lived in later seas. Sea turtles, sharks, and a 9-meter (30-foot) marine lizard that swam in the open ocean have left their remains in the yellowish badlands near the park’s western entrance. Late in this oceanic period the Rocky Mountains began to rise to the north of Big Bend, the Sierra Madre to the south. The park’s own Santiago, del Carmen, and Mariscal ranges, and the first upward thrust of the Chisos also occurred at this time. As the mountains rose and started to wear away, delta deposits began to build out farther and farther into the seaway, forming barrier bars and tidal shelves where turtles, snails, oysters, and sharks lived and died. Gradually the near-shore, subtidal environment changed to a tidal flat. This in turn changed to marsh, to beach, to brackish and freshwater lagoons, and finally to an estuary and river floodplain environment. Such was the Big Bend world to which the dinosaurs came. They had been ruling the Earth for eons, but they did not reach the park until the Cretaceous sea withdrew. [Illustration: Ammonite impressions (top) and fossil clams reveal the era in which the Big Bend area repeatedly lay beneath a shallow sea. Fossil finds show that today’s mountaintops were once sea bottoms.] [Illustration: Fossil clams.] Sloshing about in the freshwater were amphibious and semi-aquatic species. Probably commonest was the duckbill dinosaur, an enormous reptile that walked on huge hind legs. It had as many as 2,000 teeth. Specially adapted for grubbing up and munching freshwater plants, these flat grinders occurred in batteries in the duckbill’s jaw. As a tooth wore out, another popped into place.