About 131 million years ago, a pregnant ichthyosaur, a dolphin-like reptile from the dinosaur era, swam in the seas that are now part of southern Chile before dying. The breakup of the southern supercontinent Gondwanaland contributed to her demise.
As South America separated from Africa and Antarctica, a new ocean basin called Roca Verdes opened up. Matthew Malkowski, a professor of geological sciences at the University of Texas at Austin, noted that this event may represent the opening of the early South Atlantic Ocean.
The geological forces that separated the continents also caused ruptures in the Earth’s crust, leading to volcanic activity and earthquakes, which sometimes triggered massive underwater landslides. During the early Cretaceous period, one such landslide occurred in Roca Verdes, generating turbulent sediment flows.
Judith Pardo-Pérez, an associate professor at the University of Magallanes in Chile, suggested that these landslides likely trapped the ichthyosaurs, burying them under sediment. The 13-foot-long ichthyosaur's remains were preserved as its bones transformed into fossils, with the surrounding sediments hardening into rock.
Over millions of years, tectonic forces closed Roca Verdes and pushed it upward. A glacier covered the site until recent warming trends melted the snow, exposing the bones discovered by Dr. Pardo-Pérez in 2009. In 2022, researchers excavated the largely intact skeleton, transporting it in five blocks, each weighing about 400 pounds, to the Natural History Museum RÃo Seco in Punta Arenas, Chile.
The fossil skeleton was named Fiona, inspired by the character from the “Shrek” movies, due to a glue used for protection that stained the skeleton a fluorescent green. After cleaning, the skeleton returned to its normal color.
Notably, Fiona contained an unborn baby ichthyosaur, approximately 20 inches long, positioned toward the birth canal. Unlike other reptiles, ichthyosaurs gave birth to live young. Inside Fiona's rib cage were small fish bones, possibly remnants of her last meal.
Dr. Pardo-Pérez and her colleagues published their findings about Fiona in February in the Journal of Vertebrate Paleontology, emphasizing the relationship between geology and paleontology. The geological context provides insights into the environment of the ichthyosaurs, while the fossil record aids in understanding how ocean currents evolved as continents shifted.
Dr. Malkowski's research on the Roca Verdes basin connected with Dr. Pardo-Pérez's work, allowing for precise dating of Fiona's existence. The basin's proximity to a volcanic arc enabled accurate dating through the radioactive elements in volcanic rocks.
Using a volcanic ash layer near the fossil site, Dr. Malkowski and his colleagues established that Fiona lived and died 131 million years ago. Initially, Roca Verdes had low oxygen levels, but by the time of the ichthyosaurs, the waters likely became oxygenated, suggesting a passageway for ocean waters as Gondwanaland fragmented.
Future investigations will focus on marine records from this period. Additionally, 87 other ichthyosaurs have been discovered in the same glacial field, which are now at risk due to erosion from harsh weather conditions. Dr. Pardo-Pérez plans her next expedition for January.
Fiona continues to yield surprises; a recent CT scan of parts of the fossil still encased in rock revealed another complete fetus preserved inside her.