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On the Trail of Dinosaurs in “El Otro Big Bend”

Big Bend National Park has a rich fossil record, and its dinosaur fossils have been studied extensively. Dinosaur-bearing strata just across the Rio Grande in Mexico have not. Sul Ross' Dr. Thomas Shiller is changing that.

Big Bend National Park is a land of superlatives, and that extends to an area of primal interest: the Age of Reptiles.

Big Bend's unparalleled fossil record has enthralled paleontologists and dinosaur enthusiasts for a century. Discoveries here include the largest flying creature ever known – quetzalcoatlus, a reptile with a 35-foot wingspan. Big Bend outcrops hold the bones of alamosaurus – a 70-ton “titanosaur” that was the largest dinosaur to live in North America. In that lost world, giants clashed where mountain lions and rattlesnakes prowl today. Duck-billed hadrosaurs – “the cows of the Cretaceous” – grazed in herds or family groups. They were up to 30 feet long, but were prey for yet more massive animals – like tyrannosaurus, and the 50-foot-long crocodilian known as deinosuchus.

The park's fossils tell a vivid story. But as with all things Big Bend, Texas is only half the tale. Now, a Sul Ross State University professor is expanding our knowledge, with forays to study old rocks, and old bones, across the Rio Grande. 

Paleontology is increasingly a lab activity. But there are still old-school practitioners. 

Thomas Shiller does research near the isolated village of San Miguel, Coahuila. It's a two-and-a-half-hour drive from the border, and Shiller is an object of curiosity when he crosses at Boquillas to catch his ride.

“I always get some odd looks from the locals over there when I cross over with my big external-frame backpack and all of my geology gear,” Shiller said. “They don't quite know what to make of me. Then I come back into town two weeks later all haggard and sunburned, going the other way across.”

San Miguel is home to a few hundred people, one of whom – Homero Juarez – collects dinosaur bones. He shared his finds with a Mexican paleontologist, Hector Porras, who in turn connected Shiller and Juarez.

Big Bend's dinosaurs date to the Cretaceous Period, which ended 66 million years ago. And rocks from that period are a signature feature of the Big Bend landscape. Some, like the limestone of Santa Elena Canyon, were produced in a shallow sea. But in the late Cretaceous, the seaway receded – and mudstones and other rocks formed here in coastal environments. For paleontologists, one of the richest layers is known as the Aguja Formation.

“So imagine this kind of swampy environment,” Shiller said, “where you have all these dinosaurs living. There's lush vegetation. And then the uppermost part of the Aguja represents more of a coastal floodplain or river-type environment, which is another ideal environment for dinosaurs and other vertebrates. It's really interesting paleontologically, but also stratigraphically or geologically, because it's such a dynamic environment, such an ever-changing system.”

Dinosaur bones are plenty compelling in themselves. But to understand dinosaurs' evolution and interactions, to fill out the picture of that vanished past, scientists need to associate fossils with time periods and environments.

Shiller's first task was to identify the geologic strata yielding dinosaurs here.

“So when I went down to Mexico, I said, okay, I know there are dinosaurs – the guy down there is finding dinosaur bones, so this has to be Cretaceous,” he said. “Then I went down there and started looking at the patterns within the rocks, and I found, okay, here's the Boquillas formation. Overlying that is the Pen formation. And then, boom, overlying that is Aguja – looks just like the stuff I've seen for 10 years down in Big Bend National Park.”

This bi-national region of course has a shared geologic history. By mapping the connections, Shiller's work sheds new light on bones that Juarez and others had found. He was able to link a chasmosaurus – a triceratops-type dinosaur – found near San Miguel with dinosaurs in the Texas Big Bend.

And locating the Aguja Formation – and other fossil-rich strata – in Mexico opens up new vistas for study. Ironically, Shiller is geographically closer to these areas than his Mexican colleagues. But he hopes young Mexican paleontologists will come. 

And he sees a lifetime of work for himself there.

“It's such an unexplored frontier,” Shiller said. “We've just scratched the surface over there.If you look at the last hundred years or so of work that's been done just in Big Bend National Park in these same geologic units that are exposed over there, there's been just an incredible number of new dinosaurs and new animals described. Who knows? Over the next hundred years we might have an equal number of new things described over in Mexico.”

Drew Stuart is the producer for the Marfa Public Radio series Nature Notes.