About 131 million years ago, a pregnant ichthyosaur — a dolphin-like reptile of the dinosaur era — swam in seas that are now part of southern Chile. And then she died.
An accomplice in the killing: the breakup of the southern supercontinent of Gondwanaland.
South America, once unified with Africa and Antarctica, pulled away, and a new ocean basin called the Roca Verdes opened up.
“One of the hypotheses is that this is actually the opening of the early South Atlantic Ocean,” said Matthew Malkowski, a professor of geological sciences at the University of Texas at Austin.
The geological forces that pulled apart the continents also ruptured the Earth’s crust, causing volcanoes and earthquakes, and those earthquakes sometimes set off massive underwater landslides.
One day in the early Cretaceous period, one of those landslides collapsed down a submarine canyon in Roca Verdes, generating turbulent flows of sediment.
“Probably these landslides might have trapped the ichthyosaurs and threw them to the bottom of the canyon and covered them with sediment,” said Judith Pardo-Pérez, an associate professor at the University of Magallanes in Chile.
The burial in sediment of the 13-foot-long ichthyosaur preserved its corpse. Its bones transformed into fossils. The sediments hardened to rock. Over millions of years, tectonic forces closed up Roca Verdes and pushed it upward.
Until recently, a glacier covered the site, but in a warming world, the snow has melted, exposing the bones, which Dr. Pardo-Pérez discovered in 2009. The site is remote, cold and windy. Not until 2022 was a team of researchers able to excavate the largely intact skeleton, in five blocks, each weighing about 400 pounds, and then fly the blocks by helicopter to the Natural History Museum Río Seco in Punta Arenas, Chile.
They named their fossil skeleton discovery Fiona, after the princess-turned-ogre in the “Shrek” movies, because a glue used to protect it seeped into fissures in the rock, reacted with vegetation growing there and stained the skeleton.
“Extremely green, like fluorescent green,” Dr. Pardo-Pérez said. “After that, we washed it with liquids, and then she came back to the normal color.”
But the name stuck.
What was inside Fiona was also striking: an unborn baby, about 20 inches long, with its tail pointing toward the birth canal. Although they were reptiles, ichthyosaurs did not lay eggs, but gave birth to live young. Inside her rib cage was a jumble of small fish bones, possibly the leftovers of her last meal.
Dr. Pardo-Pérez and her colleagues described their latest findings about Fiona in February in the Journal of Vertebrate Paleontology.
The research highlights an interplay between geology and paleontology. The rocks provide clues about the environment that the animals lived in, while the fossil record offers a way to test ideas about how ocean currents changed direction as continents transformed.
Dr. Malkowski was working to study the evolution of the Roca Verdes basin and happened to connect with Dr. Pardo-Pérez. They realized that Dr. Malkowski’s geological study of the basin could help answer questions about Fiona and her habitat, including a precise dating of when she lived and died.
The basin was next to an arc of volcanoes that erupted fairly regularly. Volcanic rocks, unlike sedimentary rocks, can be accurately dated from the amount of radioactive elements in them.
“All that volcanic ash will erupt, move through the air and then settle out in the water,” Dr. Malkowski said. The layers of volcanic ash in the rock are “basically time stamps,” he said.
Dr. Malkowski and his colleagues at Boise State University, in Idaho, used a volcanic ash layer close to where the fossil was found in order to establish that Fiona lived and died 131 million years ago.
When Roca Verdes initially formed, the waters contained little oxygen. But by the time the ichthyosaurs arrived, the waters had probably become oxygenated, Dr. Malkowski said.
Ichthyosaurs went to the surface to breathe, but the things that they ate — fish — needed the presence of oxygen in the water to survive. That could suggest that a passageway had opened to allow ocean waters to flow through the fragmenting pieces of Gondwanaland.
“Our best way to investigate that is to go looking for marine records that cover these time spans,” Dr. Malkowski said.
There are more ichthyosaurs to be dug up, too — 87 others have been found in the same glacial field. Now that they are no longer protected by a covering of glacial snow, the harsh weather — wind, rain, repeated cycles of freezing and melting — is eroding them away. Dr. Pardo-Pérez’s next expedition is planned for January.
“It’s the most rich and well-preserved fossil site on the planet for that period of time,” she said.
And Fiona herself continues to surprise the researchers. After the team’s paper was published, a CT scan of the parts of the fossil that are still encased in rock revealed another complete fetus preserved inside her.
Kenneth Chang, a science reporter at The Times, covers NASA and the solar system, and research closer to Earth.
The post Fiona the Pregnant Sea Reptile’s Fossil Hints at the Birth of a New Ocean appeared first on New York Times.
