Nature Notes; The Western Interior Seaway

Sixty-foot-long lizards hunted the waters, while reptiles with wingspans of nearly 40 feet soared above. It was the Age of Dinosaurs, and much of what would become West Texas was submerged in a warm inland sea.

The Western Interior Seaway shaped the land in which we live, and evidence of this ancient sea and its life still can be found from Midland to Terlingua.

One hundred million years ago, in the middle Cretaceous Period, a tectonic collision was reshaping North America. Out in the Pacific Ocean, the Farallon Plate was driving itself beneath the continent. Volcanoes formed, and mountains were raised. In the center of North America, the land slumped and dropped.

Sea levels were higher in the Cretaceous. Waters began to spread across North America, from the Arctic Ocean in the north and the Gulf of Mexico in the south. Ninety million years ago, a single seaway had formed. The continent was cut into two land masses – “Laramidia” to the west, and “Appalachia” to the east.

This Inland Seaway persisted for more than 20 million years. Traces of it can be found in the Texas landscape today.

Calcium, carbon and oxygen precipitated out of the waters, to form limestone. That limestone can be seen from the Hill Country to the mountains and canyons of the Big Bend. Cretaceous limestone strata are buried in parts of the Llano Estacado.

And animal life at this time? The earliest birds had made their appearance, but mammals were rare.

Michael Nickell is museum scientist at the Sibley Nature Center in Midland.

“[There were] certainly not mammals in the sense that we think of mammals today,” Nickell said, but “small mammals, possibly egg-laying type mammals, maybe some marsupial-type mammals. So the mammals back then would have still had probably some vestiges of reptilian characteristics.”

Reptiles ruled. Dinosaurs roamed the land, while other reptiles dominated the air and the waters of the inland sea.

“Here in the oceans of course, we had several different kinds of marine reptiles,” Nickell said. “Things like plesiosaurs, these long-necked, flipper-type, kind of flat-body type reptiles that probably mostly ate fish. There were also mosasaurs, which were giant marine lizards, and they were very, very carnivorous. In the air there were several species of flying reptiles – the pterosaurs. And of course down in Big Bend you probably had the biggest pterosaur of all, the quetzalcuatlus. That animal, as far as height, would have been comparable in size to a modern-day giraffe.”

The first Quetzalcoatlus fossil was found in Big Bend National Park in 1971. Mosasaur and Plesiosaur fossils have been found to the north, from Dallas to Kansas. In West Texas, the majority of fossils from the inland sea are of invertebrate life.

“Specifically in the Llano Estacado, there are several different species of ammonites,” Nickell said, “and ammonites are a coiled-up cephalopod, related to squids and octopus, but most of them had a coiled shell. I think the closest thing you could compare an ammonite to would be the modern-day chamber nautilus.

“Those were significant predators as far as invertebrates in the Cretaceous seas,” he said. “We also had many species of bi-valves, clam- and oyster-type things, which we call gastropods. There are also several species of echinoderms, spiny-skinned animals like sea urchins and sea biscuits and things like that.”

Sea life surged and declined. Volcanoes sent ash into the ocean waters. That changed the pH levels of the sea, producing “blooms” of small organisms.

But a die-off of a different order came at the end of the Cretaceous Period, about 66 million years ago.

Three quarters of plant and animal life were wiped out in this Cretaceous extinction event. An asteroid collision off the Yucatan Peninsula was partially to blame. The end of the big reptiles opened new space for mammals – and set the stage for world in which we now live.

As the Rocky Mountains rose, the waters receded. It was 65 million years ago when this inland sea finally had withdrawn. But from interstate road cuts to the cliffs of Santa Elena Canyon, the sea’s history can be read in the West Texas landscape.

Nature Notes is underwritten by the Dixon Water Foundation and is produced by Marfa Public Radio in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Andrew Stuart.

Marfa Public Radio logo sibley nature center