Earth Notes: Fossil Frog

Sep 4, 2019

The Triassic period was a time of giants on Earth: lumbering reptiles with armored plates, and fifteen-foot-long crocodiles. The fossils of these extinct beasts are preserved in the rainbow-colored rocks of the Petrified Forest National Park in northern Arizona. But the same rocks hold the secrets of tiny creatures too.

Chinle frog on phytosaur jaw
Credit Andrey Atuchin, Virginia Tech

Scientists recently discovered a frog so small it could have perched on the end of a pencil eraser. At more than 200 million years of age, it’s the oldest frog fossil found so far in North America.

At that time, the continents formed one huge landmass called Pangea. What’s now Arizona was then located just north of the equator. Fine grains of mud compressed into stone suggest this was a lush landscape crossed by lazy rivers—perfect for frogs.

But these animals’ bones are small and delicate. Not many survive to be found by paleontologists. So frog evolution is mostly a mystery for at least 60 million years of the Triassic and into the early Jurassic.

The newfound fossils in Petrified Forest begin to fill that gap. They’re four tiny hip bones, each the size of an eyelash. Paleontologist Michelle Stocker and her team discovered them by splitting stones and laying them open like books.

Many Triassic animals vanished in a mass extinction event, possibly triggered by volcanic eruptions. But these frogs survived, and adapted to the drier climate of the Jurassic. They belonged to a branch of amphibians that no longer exists today—but not so different from frogs today.

Next week on Earth Notes, hear about one of those modern descendants.