When you picture Antarctic wildlife, the first animals that probably come to mind are blubbery winter survivors such as penguins, seals, and whales. But this frigid continent was once covered with a temperate rainforest that was welcoming to warm-weather creatures—including a newly identified fossilized frog that lived in Antarctica some 40 million years ago.
The fossils represent “the first modern amphibian found in Antarctica,” according to a study published on Thursday in Scientific Reports. Measuring just a few millimeters across, the specimens are fragments of the ancient frog’s ornamented skull and a portion of its hip bone.
The results suggest that the animal belongs to the Calyptocephalellidae family—more popularly known as helmeted frogs—which includes five species that still live in forested pockets of the Chilean Andes in South America.
The bones were collected from Seymour Island, located near the tip of the Antarctic peninsula, during three joint Argentinian-Swedish expeditions from 2011 to 2013. Thomas Mörs, a paleontologist at the Swedish Museum of Natural History who led the study, called it an “unexpected, exciting discovery” in an email.
“My field project aimed to find mammal fossils, to get more information about the mammalian fauna that lived in Antarctica before the freeze,” Mörs said. “During these three expeditions, I did a lot of survey work on Seymour, but we also spent considerable time taking sediment samples from two localities that had earlier produced fossil mammalian teeth, among them tiny teeth of small marsupials.”
It wasn’t until Mörs and his colleagues sieved this sediment back at the laboratory that they spotted the utterly unique frog bones. The team then coated the specimens in gold so that they could be examined in high resolution using a scanning-electron microscope.
Reconstruction of Eocene helmeted frog on the Antarctic Peninsula. Image: Artwork by Pollyanna von Knorring, Swedish Museum of Natural History. Photo credits: Simon Pierre Barrette and José Grau de Puerto Montt, Wikimedia Commons (CC BY-SA 3.0), and Mats Wedin, Swedish Museum of Natural History.
While scientists have discovered the remains of amphibians that roamed Triassic Antarctica, back when it was part of the Pangaea supercontinent, there is “no record of amphibians, extinct or living groups, known from the 200 million years since,” Mörs said.
In other words, the fossils fill an enormous gap in our knowledge about the ecological shifts Antarctica experienced as it broke away from the southern supercontinent Gondwana. The new frog fossils date back to the Eocene period, when the continent was becoming increasingly isolated from South America and Australia in the Southern Ocean.
This shift led to colder temperatures, widespread glaciation, and the extinction of animals that could not adapt to the frozen conditions. Contrary to the metaphor of a frog slowly boiling in a pot, this Antarctic frog was eventually iced out.
Still, the mere existence of the amphibian suggests that Antarctica was, at one point, “a center of diversification” for these animals, according to the study. The find indicates that temperate forests and freshwater habitats were still thriving on the continent 40 million years ago. These ecosystems may have been similar to the woodland habitats that are still occupied by the extinct frog’s modern relatives.
Mörs and his colleagues hope to return to the region to search for other fossils from this bygone Antarctic landscape.
“On Seymour Island, the sedimentary record covers the whole Eocene and the Eocene-Oligocene Transition that marks the permanent glaciation of Antarctica,” he said. “It would be great to have more data for these [six million years] to get a better idea about the cooling process.”