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.
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.
Tramping about on all fours and peaceably cropping land plants were bizarre looking horned dinosaurs with turtle-like beaks. For their weight they must have had the most powerful jaws of any backboned animal that ever lived. Armored species included one that looked like a horned toad the size of a dump truck. Another sported a huge lump of bone on the end of its tail, still another a thick lump of bone above a brain no bigger than the end of your little finger. The world’s largest crocodile, a 15-meter (50-foot) creature with 15-centimeter (6-inch) teeth, turned up in the park, but remarkably few bones of meat-eating dinosaurs have ever been found. One of the Earth’s great mysteries is why, at the top of their terrific form, the dominant dinosaurs died out? This happened in a geologically short time all around the globe. Perhaps it was because the highly specialized reptiles couldn’t cope with changes in their environment as the world climate grew cooler and more continental. No one really knows.
When the Age of Mammals began some 64 million years ago, Big Bend lay on an alluvial floodplain where summers were moist and winters were mild. Shallow rivers meandered between natural levees wooded with flowering plants, sycamore, stinking cedar, and tree ferns. Garfish swam in rivers and isolated ponds, while small lakes and swamps lingered on in meander cutoffs and abandoned river channels. It was a land where seasonal floods alternated with dry periods and where, during protracted dry spells, treeless areas invaded the forest.
The Fossil Record
Garfish and turtles in the Rio Grande give us a hint about life here 50 million years ago. Then they swam in waters haunted by crocodiles and visited by the modern horse’s earliest ancestor, Hyracotherium; the hippo-like Coryhodon; and Phenacodus, an early species of ungulate related to both hoofed and clawed mammals.
Fossils of these and other animals belie dry and barren Tornillo Flat’s earlier Eocene life as a lushly vegetated landscape, as depicted in this reconstruction.
At the time of this scene the dinosaurs (“terrible lizards”) had been extinct for 15 million years. Their fossils occur here too. Remains of the giant Pterosaur have been found. These “winged lizards” were flying reptiles whose 11-meter (36-foot) wingspan exceeded that of small jet fighters. The wing was a featherless membrane stretched out from the reptile’s body to the tip of its greatly enlarged fourth digit. Fossil remains of Brontosaurus, Allosaurus, Icthyosaur, and others have been found. Fossil Ammonites (see [page 67]), related to today’s sea-dwelling chambered nautilus, represent the even earlier period when today’s Big Bend was covered by a shallow inland sea. Sample fossils are displayed in a shelter off the Marathon entrance road near Tornillo Creek bridge.
