216-Million-Year-Old Microfossils Represent North America’s Earliest Frog
Paleontologists in Arizona have identified microfossils of what are thought to be the oldest known frog relative in North America.
The newly-discovered microfossils represent the first known and earliest equatorial remains of a salientian — the group containing living frogs and their most-closely related fossil relatives — from the Late Triassic, roughly 216 million years ago.
They came from the Chinle Formation of Arizona and are composed of several small pieces of hip bone, called an ilium.
“This new find highlights just how much there is still to learn about the Late Triassic ecosystem, and how much we find when we just look a little closer,” said Dr. Michelle Stocker, a researcher in the Department of Geosciences at Virginia Tech.
“We’re familiar with the charismatic archosaurs from the Chinle Formation, but we know that based on other ecosystems, they should make up a small percentage of the animals that lived together.”
“With this new focus we’re able to fill in a lot of those missing smaller components with new discoveries.”
Coming from multiple individuals, the hip bones are long and hollow, with a hip socket offset rather than centered.
“The bones of the frogs show how tiny they were: just a bit over half-an-inch long. The Chinle frog could fit on the end of your finger,” Dr. Stocker said.
Even though the fossils are part of the Chinle frog family, they are not yet naming the specific fossils.
“We refrain from naming this Chinle frog because we are continuing to process microvertebrate matrix that will likely yield additional skull and postcranial material that has the potential to be even more informative,” Dr. Stocker said.
The Chinle frog shares more features with living frogs and Prosalirus, an Early Jurassic frog found in sediments from the present-day Navajo Nation, than to Triadobatrachus, an Early Triassic frog found in modern day Madagascar in Africa.
“These are the oldest frogs from near the equator,” Dr. Stocker said.
“The oldest frogs overall are roughly 250 million years old from Madagascar and Poland, but those specimens are from higher latitudes and not equatorial.”
“Now we know that tiny frogs were present approximately 215 million years ago from North America, we may be able to find other members of the modern vertebrate communities in the Triassic period,” said team member Dr. Sterling Nesbitt, also from the Department of Geosciences at Virginia Tech.
The discovery is reported in a paper in the journal Biology Letters.
Michelle R. Stocker et al. The earliest equatorial record of frogs from the Late Triassic of Arizona. Biology Letters, published online February 27, 2019; doi: 10.1098/rsbl.2018.0922