A West Texan on a beach vacation is in for at least a 500-mile drive, whether bound for the Gulf of Mexico, or the Gulf of California. We're truly high and dry today. But in the deep past, our region was submerged by a shallow sea — the Western Interior Seaway.
In “Dinosaurs and Other Ancient Animals of Big Bend,” from UT Press, authors Cindi Sirois Collins and Asher Elbein guide us into Big Bend National Park's singular fossil record. That includes a journey into the teeming waters of the West Texas oceans.
The shining walls of Santa Elena Canyon, the Sierra del Carmen's striated cliffs — these signature Big Bend landscapes possess an oceanic power. It's fitting. Their rocks formed in ocean waters, beginning 135 million years ago.
By 95 million years ago, the sea was receding. Crusty limestones, shales and mudstones known as the Boquillas and Pen formations were created in this near-shore environment. They contain remarkable fossils, Elbein said.
“These shallow seas seem to be quite rich in terms of life,” he said. “There are quite a lot of very large predators, which suggests there's an even larger prey base.”
Coral reefs sustain marine ecosystems today. Big Bend's ancient sea had its reefs but their architects weren't corals, but clams. In oxygen-starved waters, these clams — genus Inoceramus — grew nearly 7 feet long, Collins said.
“So these guys had to get bigger,” Collins said, “so that their gills and lungs would be bigger. They've been mistaken as dinosaur footprints.”
Amidst the clam reefs moved fish and crustaceans, sponges and sea urchins. And their predators.
This, of course, was the Age of Reptiles, and reptilian dominion extended to the seas. Paleontologists in Big Bend find fossils of mosasaurs, which Elbein compared to “giant, swimming komodo dragons.”
“You get things like Tylosaurus,” Elbein said, “which are hitting 30, 40, sometimes 50 feet, really immense animals, but also smaller mosasaurs like Clidastes, which are maybe the size of a tiger shark or great white shark.”
Mosasaurs were our ocean's apex predators. But smaller creatures weren't spared the menace of sharks. Those sharks included the 30-foot-long Ptychodus, which used its shell-crushing teeth to feed on clams and other mollusks, and Squalicorax, which dined on fish, birds – and small mosasaurs.
Ammonite shells are among the world's most famous fossils, and Big Bend has yielded its share. The look of the creatures that lived within these graceful, spiral shells isn't certain. But they were cephalopods, a class of mollusks that includes squids and nautiluses.
“Some of these animals seem to be living in the water column,” Elbein said. “It's possible some of them are creeping around like shelled octopuses on the floor of these clams reefs.”
There are fossils of diverse fish. But one stands out – Xiphactinus, the “bulldog tarpon.” Up to 20 feet long, with a jutting jaw and 4-inch fangs, it fed on turtles, squids, pterosaurs and smaller members of its own species. Sometimes, this fierce hunter literally bit off more than it could chew.
“My favorite fossil of that fish,” Collins said, “it's 13 foot, it swallowed a 6-foot relative, and they both died at the same time and were fossilized.”
By 80 million years ago, the sea was gone, and what's now Big Bend was a floodplain, busy with dinosaurs. But the legacy of the West Texas ocean endures in stunning limestone landscapes, and the stunning remains of vanished creatures.
