The Triassic seems like an alien time. Spanning 251 to 201 million years ago, it was a period when crocodile cousins ruled, dinosaurs drooled, and life would seem to be very different from today. And yet the Triassic was when many forms of life familiar to us today - such as turtles and mammals - originated. Amphibians are part of that picture, too, and some tiny bones from Petrified Forest National Park help pin down the early days of frogs.
Evolutionary trees based on genetic data from living frogs hinted that the last common ancestor of all today’s frogs lived sometime before 200 million years ago. The problem, paleontologist Michelle Stocker and colleagues point out, is that the first 60 million years of this time window are pretty sparse when it comes to amphibian fossils. Thanks to some distinctive hip bones found in the Late Triassic strata of Arizona, though, Stocker and coauthors have identified an early frog that helps fill in the gap.
Not enough of the frog has been found yet to give it a name, but Stocker and colleagues have identified the animal as part of an ancient frog group called the Salientia. This is a major group that contains all living frogs and toads, as well as their closest fossil relatives. And while the finds made in Petrified Forest aren’t the oldest fossil frogs yet uncovered, they’re important because they represent ancient equatorial habitats.
In the time of these amphibians, over 200 million years ago, the world’s continents were bound up as Pangea and what’s now Petrified Forest was closer to the planet’s midline. Combined with other finds from younger, Jurassic rocks, the new finds suggest that frogs were present in habitats around the equator during the Triassic and persisted in these places even as the climate became drier and more desert-like in the early Jurassic.
But there’s an even more salient takeaway from the new discovery. Museum displays, documentaries, and news coverage often gives the impression that the Mesozoic was all about living large. These were the eras of giants. But that’s only part of the picture. It’s not that all life was larger during the Age of Reptiles. It’s that the range of animal sizes was wider than it is now, and there’s still a great deal for us to learn about the small lives that often get overshadowed by megafauna. “These fossils highlight the importance of the targeted collection of microfossils,” Stocker and colleagues write. If we’re going to dream big and take in the full vista of Mesozoic life, we need to look for the tiny, too.