Polar bears on the tundra, elephants on the savanna, whales in oceans around the globe — mammals dominate the planet today. It’s a reign that began with the dinosaurs’ extinction. But the current fauna didn’t emerge overnight. Since our moment arrived, 66 million years ago, our warm-blooded cohort has gone through a dizzying array of changes.
So step aside T. Rex and Alamosaurus. Make room for the thunder beast and the hell pig. Meet the archaic mammals of the Big Bend.
For mammals, the dinosaur extinction was an instance of creative destruction. It opened up countless ecological niches, and in a wave of evolutionary innovation, a wild diversity of mammals emerged to fill those spaces.
“It seems to me, in that middle area in the Eocene, it was like God was playing with evolution — seeing if certain things worked or they didn’t work," said Cindi Sirois Collins. In "Dinosaurs and Other Ancient Animals of Big Bend,” from UT Press, she and co-author Asher Elbein introduce us to the region’s archaic mammals.
Mammals had stayed small and secretive for 150 million years, as reptiles ruled. Now, that changed, Elbein said.
“You're not that far into the Paleocene,” he said, “which is just a couple million years after the extinction of the dinosaurs, when you're starting to see mammals the size of bears, and eventually the size of rhinos, and then starting to get mammals the size of elephants.”
Those first large mammals included herbivores called pantodonts. Big Bend paleontologists have found the skull, and other fossil material, of a pantodont called Titanoides. With tusks and sharp claws, it weighed in at 330 pounds.
For company here it had some of the earliest hoofed animals — including the predatory Arctocyon, which hunted and foraged like a black bear.
West Texas was a swampy rainforest when the Age of Mammals began. During the Eocene Epoch, between 55 and 33 million years ago, it gradually transformed into prairies and forests.
Volcanoes rose. Beneath their smoking columns, the plains shook with stampeding herds of brontotheres, or “thunder beasts.” These rhino-relatives ultimately reached the size of modern elephants.
These Eocene plains and forests were also home to a fierce predator called Archaeotherium – the “hell pig.” The size of a cow, this aggressive hunter had the long cheekbones of a warthog, with bony knobs on its head.
It was a sort of chaos of mammals, with strange forms emerging and falling away. There were horse- and hippo-like creatures unrelated to the animals we know today. The common ancestor of dogs and cats appeared. There were dogs that stalked prey like modern big cats, Elbein said.
“But then you have cat-like things taking on a dog-like lifestyle,” he said. “It gets confusing very quickly. They're all sort of trying out each other's gimmick a little bit.”
And then there were mammals we might recognize – but would never associate with West Texas. The Big Bend was home to lemur-like primates, to caymans, hippos, and a diversity of rhinos, Collins said.
“You had three kinds of rhinos,” she said. “You had the running rhino, which had really long legs and a skinny body. Then you had the hippo-type rhino, with a really big body, and short squatty legs. Then you had Menoceras, like a regular rhinos.”
Warm, lush conditions had supported these diverse mammals. But as the planet cooled, many of the mammals that had flourished here blinked out, and the region’s fauna gradually became one we would recognize.
But traces of these strange, wondrous creature remain in the fossil record — a reminder of the richness of the Big Bend’s natural heritage.
