West Texas Croc: Fossil Find Illumines a Dynamic Epoch in Earth's History
It's a time in the Earth's history that speaks clearly to our own.
The Eocene Epoch began – some 56 million years ago – with massive greenhouse gas emissions. Temperatures rose – the planet's life was transformed. But the epoch, which lasted 22 million years, was also a generative time. Its name means “dawn of the new” – in the animal life of the Eocene, we begin to see a world we recognize.
West Texas, in turns out, is offering potent insights into this dynamic time in Earth's history. The latest ambassador is Chinatichampsus wilsonorum – West Texas' own crocodilian.
Dr. Chris Kirk is a vertebrate paleontologist at UT-Austin. The center of his fieldwork has been the Dalquest Desert Research Station, on the Brewster-Presidio county line, south of Alpine. It's owned by Midwestern State University, in Wichita Falls, and dedicated to scientific study.
The area – and a geological formation there known as the Devil's Graveyard – abound in Eocene fossils. Kirk and his colleagues often focus on known fossil sites. But in 2010, they were exploring a previously un-surveyed canyon, one littered with sandstone boulders.
Kirk was joined on the outing by Sam Wilson, a UT anthropologist, and Wilson's daughter, Nellie.
“We'd been wandering around, going from boulder and boulder, just fanned out,” Kirk said. “This guy, Sam Wilson, came back and said, 'Hey, Nellie found something in one of these blocks. It looks really cool. You ought to come see it.'”
There, visible in the sandstone block, was part of a fossilized skull – with the distinctive profile of a crocodilian.
Kirk's specialty is mammals. But with hammer and chisel, he removed the partial cranium – and sought the input of Michelle Stocker. Then a UT grad student, Stocker now teaches at Virginia Tech. Among other areas of expertise, she knows Eocene reptiles.
Stocker initially tried to image the fossil with a CT scan, but decided she'd only be able to assess its anatomy if the surrounding rock were removed. Another colleague – a professional “preparator” – spent dozens of hours with pneumatic tools – removing a few grains of sand at a time.
Pinning down what kind of creature this was wasn't simple.
“We knew it was important,” Stocker said. “But we were still going back and forth on what exactly it was until a year and a half ago, because it's at this key part in the relationships of alligators and their relatives. So that was the big problem – telling which part of the family tree it actually belonged to.”
Based on analysis of the braincase, and other features, Stocker ultimately concluded the animal was a caiman.
Caimans are members of the order Crocodilia, and, along with alligators, make up the Alligatorid family. They're found today in Central and South America – and range in size from 4 feet long, to the 13-foot-long black caiman – the Amazon's largest predator.
Chinatichampsus wilsonorum was less distinct from its alligator kin than contemporary caimans. It's a genus new to science – and its name reflects the story of its discovery. “Wilsonorum” honors the father and daughter who found the fossil. “Champsus” is Greek for crocodile – and volcanic rocks at Dalquest originated in the Chinati volcano.
Our caiman was probably 6 feet long, Stocker said, and its teeth suggest it was a meat eater.
A tropical creature in West Texas? It's one sign of just how different things were here in the Eocene, Kirk said.
“It's not super surprising that you would find a caiman in the Eocene of North America,” Kirk said, “and it's also not surprising in the sense that in Central America today, you've got caimans. What is interesting is that we, in the Devil's Graveyard, are 1,200 kilometers from the most northern distribution of any caiman species. That is a function of temperature, and it's particularly a function of wetness.”
The caiman isn't the only evidence. Eocene sediments at Dalquest yield primate fossils. There are land snail fossils, and the fossil remains of palm trees – plants that don't tolerate freezing weather.
“It's a whole slew of information that goes into our environmental reconstruction,” Stocker said. “No matter what you're using to put it together, it's very different from what West Texas looks like today, obviously.”
As with human-driven climate change today, it was greenhouse gases – carbon dioxide and methane – that drove global warming at the beginning of the Eocene. Scientists debate the source of those gases – volcanism may have been a factor. And the rates are vastly different – the Eocene spike in greenhouse gases unfolded across tens of thousands of years. On our current course, we'll match it in just a few hundred.
Global temperatures increased by 10 to 15 degrees Fahrenheit in the early Eocene. Crocodilians flourished on Canada's Ellesmere Island – where polar bears romp today.
But the planet cooled as the epoch progressed. The Dalquest caiman dates to 42 million years ago. By that time, tropical forests had receded in much of North America. But West Texas, it seems, remained an outpost of those tropical conditions.
The land here had been bent and broken by the tectonic forces that raised the Rockies, and the volcanoes that created the Chisos, Davis and Chinati mountains were active. Great rivers flowed down from those volcanic highlands, surging through lush forests.
“If you squint your eyes in a lowland forest of Costa Rica or Guatemala, that helps you build up a picture of what you would see,” Kirk said. “When I'm thinking about this, I always like to populate the forest with animals I know were there. Now I've added a big, 6- to 7-foot caiman basking on the banks of those tropical rivers.”
The new fossil sheds light on how this land has changed over time. But it also sheds light on the history of a life form. More finds like Chinatichampsus could help scientists track the caiman's evolutionary journey – from West Texas, to the jungles of the Amazon.
“It depends on all the other specimens that are known for you to be able to make those interpretations,” Stocker said. “The more croc fossils that we find from West Texas will help us resolve this a little more. But we need more data.”